Linux下端口扫描程序nmap介绍
时间:2016-05-08 20:09 来源:linux.it.net.cn 作者:IT
摘要
nmap是一个网络探测和安全扫描程序，系统管理者和个人可以使用这个软件扫描大型的网络，获取那台主机正在运行以及提供什么服务等信息。nmap支持很多扫描技术，例如：UDP、TCP connect()、TCP SYN(半开扫描)、ftp代理(bounce攻击)、反向标志、ICMP、FIN、ACK扫描、圣诞树(Xmas Tree)、SYN扫描和null扫描。从扫描类型一节可以得到细节。nmap还提供了一些高级的特征，例如：通过TCP/IP协议栈特征探测操作系统类型，秘密扫描，动态延时和重传计算，并行扫描，通过并行ping扫描探测关闭的主机，诱饵扫描，避开端口过滤检测，直接RPC扫描(无须端口影射)，碎片扫描，以及灵活的目标和端口设定.
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1.名称
nmap-网络探测和安全扫描工具
2.语法
nmap [Scan Type(s)] [Options]
3.描述
nmap是一个网络探测和安全扫描程序，系统管理者和个人可以使用这个软件扫描大型的网络，获取那台主机正在运行以及提供什么服务等信息。nmap支持很多扫描技术，例如：UDP、TCP connect()、TCP SYN(半开扫描)、ftp代理(bounce攻击)、反向标志、ICMP、FIN、ACK扫描、圣诞树(Xmas Tree)、SYN扫描和null扫描。从扫描类型一节可以得到细节。nmap还提供了一些高级的特征，例如：通过TCP/IP协议栈特征探测操作系统类型，秘密扫描，动态延时和重传计算，并行扫描，通过并行ping扫描探测关闭的主机，诱饵扫描，避开端口过滤检测，直接RPC扫描(无须端口影射)，碎片扫描，以及灵活的目标和端口设定。
为了提高nmap在non-root状态下的性能，软件的设计者付出了很大的努力。很不幸，一些内核界面(例如raw socket)需要在root状态下使用。所以应该尽可能在root使用nmap。
nmap运行通常会得到被扫描主机端口的列表。nmap总会给出well known端口的服务名(如果可能)、端口号、状态和协议等信息。每个端口的状态有：open、filtered、unfiltered。open状态意味着目标主机能够在这个端口使用accept()系统调用接受连接。filtered状态表示：防火墙、包过滤和其它的网络安全软件掩盖了这个端口，禁止 nmap探测其是否打开。unfiltered表示：这个端口关闭，并且没有防火墙/包过滤软件来隔离nmap的探测企图。通常情况下，端口的状态基本都是unfiltered状态，只有在大多数被扫描的端口处于filtered状态下，才会显示处于unfiltered状态的端口。
根据使用的功能选项，nmap也可以报告远程主机的下列特征：使用的操作系统、TCP序列、运行绑定到每个端口上的应用程序的用户名、DNS名、主机地址是否是欺骗地址、以及其它一些东西。
4.功能选项
功能选项可以组合使用。一些功能选项只能够在某种扫描模式下使用。nmap会自动识别无效或者不支持的功能选项组合，并向用户发出警告信息。
如果你是有经验的用户，可以略过结尾的示例一节。可以使用nmap -h快速列出功能选项的列表。
4.1 扫描类型
-sT
TCP connect()扫描：这是最基本的TCP扫描方式。connect()是一种系统调用，由操作系统提供，用来打开一个连接。如果目标端口有程序监听， connect()就会成功返回，否则这个端口是不可达的。这项技术最大的优点是，你勿需root权限。任何UNIX用户都可以自由使用这个系统调用。这种扫描很容易被检测到，在目标主机的日志中会记录大批的连接请求以及错误信息。
-sS
TCP同步扫描(TCP SYN)：因为不必全部打开一个TCP连接，所以这项技术通常称为半开扫描(half-open)。你可以发出一个TCP同步包(SYN)，然后等待回应。如果对方返回SYN|ACK(响应)包就表示目标端口正在监听；如果返回RST数据包，就表示目标端口没有监听程序；如果收到一个SYN|ACK包，源主机就会马上发出一个RST(复位)数据包断开和目标主机的连接，这实际上有我们的操作系统内核自动完成的。这项技术最大的好处是，很少有系统能够把这记入系统日志。不过，你需要root权限来定制SYN数据包。
-sF -sF -sN
秘密FIN数据包扫描、圣诞树(Xmas Tree)、空(Null)扫描模式：即使SYN扫描都无法确定的情况下使用。一些防火墙和包过滤软件能够对发送到被限制端口的SYN数据包进行监视，而且有些程序比如synlogger和courtney能够检测那些扫描。这些高级的扫描方式可以逃过这些干扰。这些扫描方式的理论依据是：关闭的端口需要对你的探测包回应RST包，而打开的端口必需忽略有问题的包(参考RFC 793第64页)。FIN扫描使用暴露的FIN数据包来探测，而圣诞树扫描打开数据包的FIN、URG和PUSH标志。不幸的是，微软决定完全忽略这个标准，另起炉灶。所以这种扫描方式对Windows95/NT无效。不过，从另外的角度讲，可以使用这种方式来分别两种不同的平台。如果使用这种扫描方式可以发现打开的端口，你就可以确定目标注意运行的不是Windows系统。如果使用-sF、-sX或者-sN扫描显示所有的端口都是关闭的，而使用SYN扫描显示有打开的端口，你可以确定目标主机可能运行的是Windwos系统。现在这种方式没有什么太大的用处，因为nmap有内嵌的操作系统检测功能。还有其它几个系统使用和windows同样的处理方式，包括Cisco、BSDI、HP/UX、MYS、IRIX。在应该抛弃数据包时，以上这些系统都会从打开的端口发出复位数据包。
-sP
ping扫描：有时你只是想知道此时网络上哪些主机正在运行。通过向你指定的网络内的每个IP地址发送ICMP echo请求数据包，nmap就可以完成这项任务。如果主机正在运行就会作出响应。不幸的是，一些站点例如：microsoft.com阻塞ICMP echo请求数据包。然而，在默认的情况下nmap也能够向80端口发送TCP ack包，如果你收到一个RST包，就表示主机正在运行。nmap使用的第三种技术是：发送一个SYN包，然后等待一个RST或者SYN/ACK包。对于非root用户，nmap使用connect()方法。
在默认的情况下(root用户)，nmap并行使用ICMP和ACK技术。
注意，nmap在任何情况下都会进行ping扫描，只有目标主机处于运行状态，才会进行后续的扫描。如果你只是想知道目标主机是否运行，而不想进行其它扫描，才会用到这个选项。
-sU
UDP扫描：如果你想知道在某台主机上提供哪些UDP(用户数据报协议,RFC768)服务，可以使用这种扫描方法。nmap首先向目标主机的每个端口发出一个0字节的UDP包，如果我们收到端口不可达的ICMP消息，端口就是关闭的，否则我们就假设它是打开的。
有些人可能会想UDP扫描是没有什么意思的。但是，我经常会想到最近出现的solaris rpcbind缺陷。rpcbind隐藏在一个未公开的UDP端口上，这个端口号大于32770。所以即使端口111(portmap的众所周知端口号) 被防火墙阻塞有关系。但是你能发现大于30000的哪个端口上有程序正在监听吗?使用UDP扫描就能！cDc Back Orifice的后门程序就隐藏在Windows主机的一个可配置的UDP端口中。不考虑一些通常的安全缺陷，一些服务例如:snmp、tftp、NFS 使用UDP协议。不幸的是，UDP扫描有时非常缓慢，因为大多数主机限制ICMP错误信息的比例(在RFC1812中的建议)。例如，在Linux内核中 (在net/ipv4/icmp.h文件中)限制每4秒钟只能出现80条目标不可达的ICMP消息，如果超过这个比例，就会给1/4秒钟的处罚。 solaris的限制更加严格，每秒钟只允许出现大约2条ICMP不可达消息，这样，使扫描更加缓慢。nmap会检测这个限制的比例，减缓发送速度，而不是发送大量的将被目标主机丢弃的无用数据包。
不过Micro$oft忽略了RFC1812的这个建议，不对这个比例做任何的限制。所以我们可以能够快速扫描运行Win95/NT的主机上的所有65K个端口。
-sA
ACK扫描：这项高级的扫描方法通常用来穿过防火墙的规则集。通常情况下，这有助于确定一个防火墙是功能比较完善的或者是一个简单的包过滤程序，只是阻塞进入的SYN包。
这种扫描是向特定的端口发送ACK包(使用随机的应答/序列号)。如果返回一个RST包，这个端口就标记为unfiltered状态。如果什么都没有返回，或者返回一个不可达ICMP消息，这个端口就归入filtered类。注意，nmap通常不输出unfiltered的端口，所以在输出中通常不显示所有被探测的端口。显然，这种扫描方式不能找出处于打开状态的端口。
-sW
对滑动窗口的扫描：这项高级扫描技术非常类似于ACK扫描，除了它有时可以检测到处于打开状态的端口，因为滑动窗口的大小是不规则的，有些操作系统可以报告其大小。这些系统至少包括：某些版本的AIX、Amiga、BeOS、BSDI、Cray、Tru64 UNIX、DG/UX、OpenVMS、Digital UNIX、OpenBSD、OpenStep、QNX、Rhapsody、SunOS 4.x、Ultrix、VAX、VXWORKS。从nmap-hackers邮件3列表的文档中可以得到完整的列表。
-sR
RPC扫描。这种方法和nmap的其它不同的端口扫描方法结合使用。选择所有处于打开状态的端口向它们发出SunRPC程序的NULL命令，以确定它们是否是RPC端口，如果是，就确定是哪种软件及其版本号。因此你能够获得防火墙的一些信息。诱饵扫描现在还不能和RPC扫描结合使用。
-b
FTP反弹攻击(bounce attack):FTP协议(RFC 959)有一个很有意思的特征，它支持代理FTP连接。也就是说，我能够从evil.com连接到FTP服务器target.com，并且可以要求这台 FTP服务器为自己发送Internet上任何地方的文件！1985年，RFC959完成时，这个特征就能很好地工作了。然而，在今天的Internet 中，我们不能让人们劫持FTP服务器，让它向Internet上的任意节点发送数据。如同Hobbit在1995年写的文章中所说的，这个协议"能够用来做投递虚拟的不可达邮件和新闻，进入各种站点的服务器,填满硬盘，跳过防火墙，以及其它的骚扰活动，而且很难进行追踪"。我们可以使用这个特征，在一台代理FTP服务器扫描TCP端口。因此，你需要连接到防火墙后面的一台FTP服务器，接着进行端口扫描。如果在这台FTP服务器中有可读写的目录，你还可以向目标端口任意发送数据(不过nmap不能为你做这些)。
传递给-b功能选项的参数是你要作为代理的FTP服务器。语法格式为：
-b username:password@server:port。
除了server以外，其余都是可选的。如果你想知道什么服务器有这种缺陷，可以参考我在Phrack 51发表的文章。还可以在nmap的站点得到这篇文章的最新版本。
4.2 通用选项
这些内容不是必需的，但是很有用。
-P0
在扫描之前，不必ping主机。有些网络的防火墙不允许ICMP echo请求穿过，使用这个选项可以对这些网络进行扫描。microsoft.com就是一个例子，因此在扫描这个站点时，你应该一直使用-P0或者-PT 80选项。
-PT
扫描之前，使用TCP ping确定哪些主机正在运行。nmap不是通过发送ICMP echo请求包然后等待响应来实现这种功能，而是向目标网络(或者单一主机)发出TCP ACK包然后等待回应。如果主机正在运行就会返回RST包。只有在目标网络/主机阻塞了ping包，而仍旧允许你对其进行扫描时，这个选项才有效。对于非 root用户，我们使用connect()系统调用来实现这项功能。使用-PT <端口号>来设定目标端口。默认的端口号是80，因为这个端口通常不会被过滤。
-PS
对于root用户，这个选项让nmap使用SYN包而不是ACK包来对目标主机进行扫描。如果主机正在运行就返回一个RST包(或者一个SYN/ACK包)。
-PI
设置这个选项，让nmap使用真正的ping(ICMP echo请求)来扫描目标主机是否正在运行。使用这个选项让nmap发现正在运行的主机的同时，nmap也会对你的直接子网广播地址进行观察。直接子网广播地址一些外部可达的IP地址，把外部的包转换为一个内向的IP广播包，向一个计算机子网发送。这些IP广播包应该删除，因为会造成拒绝服务攻击(例如 smurf)。
-PB
这是默认的ping扫描选项。它使用ACK(-PT)和ICMP(-PI)两种扫描类型并行扫描。如果防火墙能够过滤其中一种包，使用这种方法，你就能够穿过防火墙。
-O
这个选项激活对TCP/IP指纹特征(fingerprinting)的扫描，获得远程主机的标志。换句话说，nmap使用一些技术检测目标主机操作系统网络协议栈的特征。nmap使用这些信息建立远程主机的指纹特征，把它和已知的操作系统指纹特征数据库做比较，就可以知道目标主机操作系统的类型。
-I
这个选项打开nmap的反向标志扫描功能。Dave Goldsmith 1996年向bugtap发出的邮件注意到这个协议，ident协议(rfc 1413)允许使用TCP连接给出任何进程拥有者的用户名，即使这个进程并没有初始化连接。例如，你可以连接到HTTP端口，接着使用identd确定这个服务器是否由root用户运行。这种扫描只能在同目标端口建立完全的TCP连接时(例如：-sT扫描选项)才能成功。使用-I选项是，远程主机的 identd精灵进程就会查询在每个打开的端口上监听的进程的拥有者。显然，如果远程主机没有运行identd程序，这种扫描方法无效。
-f
这个选项使nmap使用碎片IP数据包发送SYN、FIN、XMAS、NULL。使用碎片数据包增加包过滤、入侵检测系统的难度，使其无法知道你的企图。不过，要慎重使用这个选项！有些程序在处理这些碎片包时会有麻烦，我最喜欢的嗅探器在接受到碎片包的头36个字节时，就会发生 segmentation faulted。因此，在nmap中使用了24个字节的碎片数据包。虽然包过滤器和防火墙不能防这种方法，但是有很多网络出于性能上的考虑，禁止数据包的分片。
注意这个选项不能在所有的平台上使用。它在Linux、FreeBSD、OpenBSD以及其它一些UNIX系统能够很好工作。
-v
冗余模式。强烈推荐使用这个选项，它会给出扫描过程中的详细信息。使用这个选项，你可以得到事半功倍的效果。使用-d选项可以得到更加详细的信息。
-h
快速参考选项。
-oN
把扫描结果重定向到一个可读的文件logfilename中。
-oM
把扫描结果重定向到logfilename文件中，这个文件使用主机可以解析的语法。你可以使用-oM -来代替logfilename，这样输出就被重定向到标准输出stdout。在这种情况下，正常的输出将被覆盖，错误信息荏苒可以输出到标准错误 stderr。要注意，如果同时使用了-v选项，在屏幕上会打印出其它的信息。
-oS thIs l0gz th3 r3suLtS of YouR ScanZ iN a s| THe fiL3 U sPecfy 4s an arGuMEnT! U kAn gIv3 the 4rgument -
(wItHOUt qUOteZ) to sh00t output iNT0 stDouT!@!! 莫名其妙，下面是我猜着翻译的，相形字？
把扫描结果重定向到一个文件logfilename中，这个文件使用一种"黑客方言"的语法形式(作者开的玩笑?)。同样，使用-oS -就会把结果重定向到标准输出上。
-resume
某个网络扫描可能由于control-C或者网络损失等原因被中断，使用这个选项可以使扫描接着以前的扫描进行。logfilename是被取消扫描的日志文件，它必须是可读形式或者机器可以解析的形式。而且接着进行的扫描不能增加新的选项，只能使用与被中断的扫描相同的选项。nmap会接着日志文件中的最后一次成功扫描进行新的扫描。
-iL
从inputfilename文件中读取扫描的目标。在这个文件中要有一个主机或者网络的列表，由空格键、制表键或者回车键作为分割符。如果使用-iL -，nmap就会从标准输入stdin读取主机名字。你可以从指定目标一节得到更加详细的信息。
-iR
让nmap自己随机挑选主机进行扫描。
-p <端口范围>
这个选项让你选择要进行扫描的端口号的范围。例如，-p 23表示：只扫描目标主机的23号端口。-p 20-30,139,60000-表示：扫描20到30号端口，139号端口以及所有大于60000的端口。在默认情况下，nmap扫描从1到1024号以及nmap-services文件(如果使用RPM软件包，一般在/usr/share/nmap/目录中)中定义的端口列表。
-F
快速扫描模式，只扫描在nmap-services文件中列出的端口。显然比扫描所有65535个端口要快。
-D
使用诱饵扫描方法对目标网络/主机进行扫描。如果nmap使用这种方法对目标网络进行扫描，那么从目标主机/网络的角度来看，扫描就象从其它主机 (decoy1,等)发出的。从而，即使目标主机的IDS(入侵检测系统)对端口扫描发出报警，它们也不可能知道哪个是真正发起扫描的地址，哪个是无辜的。这种扫描方法可以有效地对付例如路由跟踪、response-dropping等积极的防御机制，能够很好地隐藏你的IP地址。
每个诱饵主机名使用逗号分割开，你也可以使用ME选项，它代表你自己的主机，和诱饵主机名混杂在一起。如果你把ME放在第六或者更靠后的位置，一些端口扫描检测软件几乎根本不会显示你的IP地址。如果你不使用ME选项，nmap会把你的IP地址随机夹杂在诱饵主机之中。
注意:你用来作为诱饵的主机应该正在运行或者你只是偶尔向目标发送SYN数据包。很显然，如果在网络上只有一台主机运行，目标将很轻松就会确定是哪台主机进行的扫描。或许，你还要直接使用诱饵的IP地址而不是其域名，这样诱饵网络的域名服务器的日志上就不会留下关于你的记录。
还要注意：一些愚蠢的端口扫描检测软件会拒绝路由试图进行端口扫描的主机。因而，你需要让目标主机和一些诱饵断开连接。如果诱饵是目标主机的网关或者就是其自己时，会给目标主机造成很大问题。所以你需要慎重使用这个选项。
诱饵扫描既可以在起始的ping扫描也可以在真正的扫描状态下使用。它也可以和-O选项组合使用。
使用太多的诱饵扫描能够减缓你的扫描速度甚至可能造成扫描结果不正确。同时，有些ISP会把你的欺骗包过滤掉。虽然现在大多数的ISP不会对此进行限制。
-S <IP_Address>
在一些情况下，nmap可能无法确定你的源地址(nmap会告诉你)。在这种情况下，可以使用这个选项给出你的IP地址。
在欺骗扫描时，也使用这个选项。使用这个选项可以让目标认为是其它的主机对自己进行扫描。
-e
告诉nmap使用哪个接口发送和接受数据包。nmap能够自动对此接口进行检测，如果无效就会告诉你。
-g
设置扫描的源端口。一些天真的防火墙和包过滤器的规则集允许源端口为DNS(53)或者FTP-DATA(20)的包通过和实现连接。显然，如果攻击者把源端口修改为20或者53，就可以摧毁防火墙的防护。在使用UDP扫描时，先使用53号端口；使用TCP扫描时，先使用20号端口。注意只有在能够使用这个端口进行扫描时，nmap才会使用这个端口。例如，如果你无法进行TCP扫描，nmap会自动改变源端口，即使你使用了-g选项。
对于一些扫描，使用这个选项会造成性能上的微小损失，因为我有时会保存关于特定源端口的一些有用的信息。
-r
告诉nmap不要打乱被扫描端口的顺序。
--randomize_hosts
使nmap在扫描之前，打乱每组扫描中的主机顺序，nmap每组可以扫描最多2048台主机。这样，可以使扫描更不容易被网络监视器发现，尤其和--scan_delay 选项组合使用，更能有效避免被发现。
-M
设置进行TCP connect()扫描时，最多使用多少个套接字进行并行的扫描。使用这个选项可以降低扫描速度，避免远程目标宕机。
4.3 适时选项
通常，nmap在运行时，能够很好地根据网络特点进行调整。扫描时，nmap会尽量减少被目标检测到的机会，同时尽可能加快扫描速度。然而，nmap默认的适时策略有时候不太适合你的目标。使用下面这些选项，可以控制nmap的扫描timing：
-T
设置nmap的适时策略。Paranoid:为了避开IDS的检测使扫描速度极慢，nmap串行所有的扫描，每隔至少5分钟发送一个包； Sneaky：也差不多，只是数据包的发送间隔是15秒；Polite：不增加太大的网络负载，避免宕掉目标主机，串行每个探测，并且使每个探测有0.4 秒种的间隔；Normal:nmap默认的选项，在不是网络过载或者主机/端口丢失的情况下尽可能快速地扫描；Aggressive:设置5分钟的超时限制，使对每台主机的扫描时间不超过5分钟，并且使对每次探测回应的等待时间不超过1.5秒钟；b>Insane:只适合快速的网络或者你不在意丢失某些信息，每台主机的超时限制是75秒，对每次探测只等待0.3秒钟。你也可是使用数字来代替这些模式，例如：-T 0等于-T Paranoid，-T 5等于-T Insane。
这些适时模式不能下面的适时选项组合使用。
--host_timeout
设置扫描一台主机的时间，以毫秒为单位。默认的情况下，没有超时限制。
--max_rtt_timeout
设置对每次探测的等待时间，以毫秒为单位。如果超过这个时间限制就重传或者超时。默认值是大约9000毫秒。
--min_rtt_timeout
当目标主机的响应很快时，nmap就缩短每次探测的超时时间。这样会提高扫描的速度，但是可能丢失某些响应时间比较长的包。使用这个选项，可以让nmap对每次探测至少等待你指定的时间，以毫秒为单位。
--initial_rtt_timeout
设置初始探测的超时值。一般这个选项只在使用-P0选项扫描有防火墙保护的主机才有用。默认值是6000毫秒。
--max_parallelism
设置最大的并行扫描数量。--max_parallelism 1表示同时只扫描一个端口。这个选项对其它的并行扫描也有效，例如ping sweep, RPC scan。
--scan_delay
设置在两次探测之间，nmap必须等待的时间。这个选项主要用于降低网络的负载。
4.4 目标设定
在nmap的所有参数中，只有目标参数是必须给出的。其最简单的形式是在命令行直接输入一个主机名或者一个IP地址。如果你希望扫描某个IP地址的一个子网，你可以在主机名或者IP地址的后面加上/掩码。掩码在0(扫描整个网络)到32(只扫描这个主机)。使用/24扫描C类地址，/16扫描B类地址。
除此之外，nmap还有更加强大的表示方式让你更加灵活地指定IP地址。例如，如果要扫描这个B类网络128.210.*.*，你可以使用下面三种方式来指定这些地址:128.210.*.*、128.21-.0-255.0-255或者128.210.0.0/16这三种形式是等价的。
5.例子
本节将由浅入深地举例说明如何使用nmap。
nmap -v target.example.com
扫描主机target.example.com的所有TCP端口。-v打开冗余模式。
nmap -sS -O target.example.com/24
发起对target.example.com所在网络上的所有255个IP地址的秘密SYN扫描。同时还探测每台主机操作系统的指纹特征。需要root权限。
nmap -sX -p 22,53,110,143,4564 128.210.*.1-127
对B类IP地址128.210中255个可能的8位子网的前半部分发起圣诞树扫描。确定这些系统是否打开了sshd、DNS、pop3d、imapd和4564端口。注意圣诞树扫描对Micro$oft的系统无效，因为其协议栈的TCP层有缺陷。
nmap -v --randomize_hosts -p 80 *.*.2.3-5
只扫描指定的IP范围，有时用于对这个Internet进行取样分析。nmap将寻找Internet上所有后两个字节是.2.3、.2.4、.2.5的 IP地址上的WEB服务器。如果你想发现更多有意思的主机，你可以使用127-222，因为在这个范围内有意思的主机密度更大。
host -l company.com | cut -d -f 4 | ./nmap -v -iL -
列出company.com网络的所有主机，让nmap进行扫描。注意：这项命令在GNU/Linux下使用。如果在其它平台，你可能要使用 其它的命令/选项。
下面是man文档
NMAP(1) Nmap Reference Guide NMAP(1)
NAME
nmap - Network exploration tool and security / port scanner
SYNOPSIS
nmap [Scan Type...] [Options] {target specification}
DESCRIPTION
Nmap (“Network Mapper”) is an open source tool for network exploration
and security auditing. It was designed to rapidly scan large networks,
although it works fine against single hosts. Nmap uses raw IP packets
in novel ways to determine what hosts are available on the network,
what services (application name and version) those hosts are offering,
what operating systems (and OS versions) they are running, what type of
packet filters/firewalls are in use, and dozens of other
characteristics. While Nmap is commonly used for security audits, many
systems and network administrators find it useful for routine tasks
such as network inventory, managing service upgrade schedules, and
monitoring host or service uptime.
The output from Nmap is a list of scanned targets, with supplemental
information on each depending on the options used. Key among that
information is the “interesting ports table”.. That table lists the
port number and protocol, service name, and state. The state is either
open, filtered, closed, or unfiltered. Open. means that an
application on the target machine is listening for connections/packets
on that port. Filtered. means that a firewall, filter, or other
network obstacle is blocking the port so that Nmap cannot tell whether
it is open or closed. Closed. ports have no application listening on
them, though they could open up at any time. Ports are classified as
unfiltered. when they are responsive to Nmap´s probes, but Nmap cannot
determine whether they are open or closed. Nmap reports the state
combinations open|filtered. and closed|filtered. when it cannot
determine which of the two states describe a port. The port table may
also include software version details when version detection has been
requested. When an IP protocol scan is requested (-sO), Nmap provides
information on supported IP protocols rather than listening ports.
In addition to the interesting ports table, Nmap can provide further
information on targets, including reverse DNS names, operating system
guesses, device types, and MAC addresses.
A typical Nmap scan is shown in Example 1. The only Nmap arguments used
in this example are -A, to enable OS and version detection, script
scanning, and traceroute; -T4 for faster execution; and then the two
target hostnames.
Example 1. A representative Nmap scan
# nmap -A -T4 scanme.nmap.org
Starting Nmap ( http://nmap.org )
Interesting ports on scanme.nmap.org (64.13.134.52):
Not shown: 994 filtered ports
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 4.3 (protocol 2.0)
25/tcp closed smtp
53/tcp open domain ISC BIND 9.3.4
70/tcp closed gopher
80/tcp open http Apache httpd 2.2.2 ((Fedora))
|_ HTML title: Go ahead and ScanMe!
113/tcp closed auth
Device type: general purpose
Running: Linux 2.6.X
OS details: Linux 2.6.20-1 (Fedora Core 5)
TRACEROUTE (using port 80/tcp)
HOP RTT ADDRESS
[Cut first seven hops for brevity]
8 10.59 so-4-2-0.mpr3.pao1.us.above.net (64.125.28.142)
9 11.00 metro0.sv.svcolo.com (208.185.168.173)
10 9.93 scanme.nmap.org (64.13.134.52)
Nmap done: 1 IP address (1 host up) scanned in 17.00 seconds
The newest version of Nmap can be obtained from http://nmap.org. The
newest version of this man page is available at
http://nmap.org/book/man.html. It is also included as a chapter of
Nmap Network Scanning: The Official Nmap Project Guide to Network
Discovery and Security Scanning (see http://nmap.org/book/).
OPTIONS SUMMARY
This options summary is printed when Nmap is run with no arguments, and
the latest version is always available at
http://nmap.org/data/nmap.usage.txt. It helps people remember the most
common options, but is no substitute for the in-depth documentation in
the rest of this manual. Some obscure options aren´t even included
here.
Nmap 5.21 ( http://nmap.org )
Usage: nmap [Scan Type(s)] [Options] {target specification}
TARGET SPECIFICATION:
Can pass hostnames, IP addresses, networks, etc.
Ex: scanme.nmap.org, microsoft.com/24, 192.168.0.1; 10.0.0-255.1-254
-iL <inputfilename>: Input from list of hosts/networks
-iR <num hosts>: Choose random targets
--exclude <host1[,host2][,host3],...>: Exclude hosts/networks
--excludefile <exclude_file>: Exclude list from file
HOST DISCOVERY:
-sL: List Scan - simply list targets to scan
-sP: Ping Scan - go no further than determining if host is online
-PN: Treat all hosts as online -- skip host discovery
-PS/PA/PU/PY[portlist]: TCP SYN/ACK, UDP or SCTP discovery to given ports
-PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes
-PO[protocol list]: IP Protocol Ping
-n/-R: Never do DNS resolution/Always resolve [default: sometimes]
--dns-servers <serv1[,serv2],...>: Specify custom DNS servers
--system-dns: Use OS´s DNS resolver
--traceroute: Trace hop path to each host
SCAN TECHNIQUES:
-sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans
-sU: UDP Scan
-sN/sF/sX: TCP Null, FIN, and Xmas scans
--scanflags <flags>: Customize TCP scan flags
-sI <zombie host[:probeport]>: Idle scan
-sY/sZ: SCTP INIT/COOKIE-ECHO scans
-sO: IP protocol scan
-b <FTP relay host>: FTP bounce scan
PORT SPECIFICATION AND SCAN ORDER:
-p <port ranges>: Only scan specified ports
Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080
-F: Fast mode - Scan fewer ports than the default scan
-r: Scan ports consecutively - don´t randomize
--top-ports <number>: Scan <number> most common ports
--port-ratio <ratio>: Scan ports more common than <ratio>
SERVICE/VERSION DETECTION:
-sV: Probe open ports to determine service/version info
--version-intensity <level>: Set from 0 (light) to 9 (try all probes)
--version-light: Limit to most likely probes (intensity 2)
--version-all: Try every single probe (intensity 9)
--version-trace: Show detailed version scan activity (for debugging)
SCRIPT SCAN:
-sC: equivalent to --script=default
--script=<Lua scripts>: <Lua scripts> is a comma separated list of
directories, script-files or script-categories
--script-args=<n1=v1,[n2=v2,...]>: provide arguments to scripts
--script-trace: Show all data sent and received
--script-updatedb: Update the script database.
OS DETECTION:
-O: Enable OS detection
--osscan-limit: Limit OS detection to promising targets
--osscan-guess: Guess OS more aggressively
TIMING AND PERFORMANCE:
Options which take <time> are in milliseconds, unless you append ´s´
(seconds), ´m´ (minutes), or ´h´ (hours) to the value (e.g. 30m).
-T<0-5>: Set timing template (higher is faster)
--min-hostgroup/max-hostgroup <size>: Parallel host scan group sizes
--min-parallelism/max-parallelism <time>: Probe parallelization
--min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>: Specifies
probe round trip time.
--max-retries <tries>: Caps number of port scan probe retransmissions.
--host-timeout <time>: Give up on target after this long
--scan-delay/--max-scan-delay <time>: Adjust delay between probes
--min-rate <number>: Send packets no slower than <number> per second
--max-rate <number>: Send packets no faster than <number> per second
FIREWALL/IDS EVASION AND SPOOFING:
-f; --mtu <val>: fragment packets (optionally w/given MTU)
-D <decoy1,decoy2[,ME],...>: Cloak a scan with decoys
-S <IP_Address>: Spoof source address
-e <iface>: Use specified interface
-g/--source-port <portnum>: Use given port number
--data-length <num>: Append random data to sent packets
--ip-options <options>: Send packets with specified ip options
--ttl <val>: Set IP time-to-live field
--spoof-mac <mac address/prefix/vendor name>: Spoof your MAC address
--badsum: Send packets with a bogus TCP/UDP/SCTP checksum
--adler32: Use deprecated Adler32 instead of CRC32C for SCTP checksums
OUTPUT:
-oN/-oX/-oS/-oG <file>: Output scan in normal, XML, s|<rIpt kIddi3,
and Grepable format, respectively, to the given filename.
-oA <basename>: Output in the three major formats at once
-v: Increase verbosity level (use twice or more for greater effect)
-d[level]: Set or increase debugging level (Up to 9 is meaningful)
--reason: Display the reason a port is in a particular state
--open: Only show open (or possibly open) ports
--packet-trace: Show all packets sent and received
--iflist: Print host interfaces and routes (for debugging)
--log-errors: Log errors/warnings to the normal-format output file
--append-output: Append to rather than clobber specified output files
--resume <filename>: Resume an aborted scan
--stylesheet <path/URL>: XSL stylesheet to transform XML output to HTML
--webxml: Reference stylesheet from Nmap.Org for more portable XML
--no-stylesheet: Prevent associating of XSL stylesheet w/XML output
MISC:
-6: Enable IPv6 scanning
-A: Enables OS detection and Version detection, Script scanning and Traceroute
--datadir <dirname>: Specify custom Nmap data file location
--send-eth/--send-ip: Send using raw ethernet frames or IP packets
--privileged: Assume that the user is fully privileged
--unprivileged: Assume the user lacks raw socket privileges
-V: Print version number
-h: Print this help summary page.
EXAMPLES:
nmap -v -A scanme.nmap.org
nmap -v -sP 192.168.0.0/16 10.0.0.0/8
nmap -v -iR 10000 -PN -p 80
SEE THE MAN PAGE (http://nmap.org/book/man.html) FOR MORE OPTIONS AND EXAMPLES
TARGET SPECIFICATION
Everything on the Nmap command-line that isn´t an option (or option
argument) is treated as a target host specification. The simplest case
is to specify a target IP address or hostname for scanning.
Sometimes you wish to scan a whole network of adjacent hosts. For this,
Nmap supports CIDR-style. addressing. You can append /numbits to an
IPv4 address or hostname and Nmap will scan every IP address for which
the first numbits are the same as for the reference IP or hostname
given. For example, 192.168.10.0/24 would scan the 256 hosts between
192.168.10.0 (binary: 11000000 10101000 00001010 00000000) and
192.168.10.255 (binary: 11000000 10101000 00001010 11111111),
inclusive. 192.168.10.40/24 would scan exactly the same targets. Given
that the host scanme.nmap.org. is at the IP address 64.13.134.52, the
specification scanme.nmap.org/16 would scan the 65,536 IP addresses
between 64.13.0.0 and 64.13.255.255. The smallest allowed value is /0,
which scans the whole Internet. The largest value is /32, which scans
just the named host or IP address because all address bits are fixed.
CIDR notation is short but not always flexible enough. For example, you
might want to scan 192.168.0.0/16 but skip any IPs ending with .0 or
.255 because they may be used as subnet network and broadcast
addresses. Nmap supports this through octet range addressing. Rather
than specify a normal IP address, you can specify a comma-separated
list of numbers or ranges for each octet. For example,
192.168.0-255.1-254 will skip all addresses in the range that end in .0
or .255, and 192.168.3-5,7.1 will scan the four addresses 192.168.3.1,
192.168.4.1, 192.168.5.1, and 192.168.7.1. Either side of a range may
be omitted; the default values are 0 on the left and 255 on the right.
Using - by itself is the same as 0-255, but remember to use 0- in the
first octet so the target specification doesn´t look like a
command-line option. Ranges need not be limited to the final octets:
the specifier 0-255.0-255.13.37 will perform an Internet-wide scan for
all IP addresses ending in 13.37. This sort of broad sampling can be
useful for Internet surveys and research.
IPv6 addresses can only be specified by their fully qualified IPv6
address or hostname. CIDR and octet ranges aren´t supported for IPv6
because they are rarely useful.
Nmap accepts multiple host specifications on the command line, and they
don´t need to be the same type. The command nmap scanme.nmap.org
192.168.0.0/8 10.0.0,1,3-7.- does what you would expect.
While targets are usually specified on the command lines, the following
options are also available to control target selection:
-iL inputfilename (Input from list) .
Reads target specifications from inputfilename. Passing a huge list
of hosts is often awkward on the command line, yet it is a common
desire. For example, your DHCP server might export a list of 10,000
current leases that you wish to scan. Or maybe you want to scan all
IP addresses except for those to locate hosts using unauthorized
static IP addresses. Simply generate the list of hosts to scan and
pass that filename to Nmap as an argument to the -iL option.
Entries can be in any of the formats accepted by Nmap on the
command line (IP address, hostname, CIDR, IPv6, or octet ranges).
Each entry must be separated by one or more spaces, tabs, or
newlines. You can specify a hyphen (-) as the filename if you want
Nmap to read hosts from standard input rather than an actual file.
The input file may contain comments that start with # and extend to
the end of the line.
-iR num hosts (Choose random targets) .
For Internet-wide surveys and other research, you may want to
choose targets at random. The num hosts argument tells Nmap how
many IPs to generate. Undesirable IPs such as those in certain
private, multicast, or unallocated address ranges are automatically
skipped. The argument 0 can be specified for a never-ending scan.
Keep in mind that some network administrators bristle at
unauthorized scans of their networks and may complain. Use this
option at your own risk! If you find yourself really bored one
rainy afternoon, try the command nmap -sS -PS80 -iR 0 -p 80 to
locate random web servers for browsing.
--exclude host1[,host2[,...]] (Exclude hosts/networks) .
Specifies a comma-separated list of targets to be excluded from the
scan even if they are part of the overall network range you
specify. The list you pass in uses normal Nmap syntax, so it can
include hostnames, CIDR netblocks, octet ranges, etc. This can be
useful when the network you wish to scan includes untouchable
mission-critical servers, systems that are known to react adversely
to port scans, or subnets administered by other people.
--excludefile exclude_file (Exclude list from file) .
This offers the same functionality as the --exclude option, except
that the excluded targets are provided in a newline, space, or tab
delimited exclude_file rather than on the command line.
The exclude file may contain comments that start with # and extend
to the end of the line.
HOST DISCOVERY
One of the very first steps in any network reconnaissance mission is to
reduce a (sometimes huge) set of IP ranges into a list of active or
interesting hosts. Scanning every port of every single IP address is
slow and usually unnecessary. Of course what makes a host interesting
depends greatly on the scan purposes. Network administrators may only
be interested in hosts running a certain service, while security
auditors may care about every single device with an IP address. An
administrator may be comfortable using just an ICMP ping to locate
hosts on his internal network, while an external penetration tester may
use a diverse set of dozens of probes in an attempt to evade firewall
restrictions.
Because host discovery needs are so diverse, Nmap offers a wide variety
of options for customizing the techniques used. Host discovery is
sometimes called ping scan, but it goes well beyond the simple ICMP
echo request packets associated with the ubiquitous ping tool. Users
can skip the ping step entirely with a list scan (-sL) or by disabling
ping (-PN), or engage the network with arbitrary combinations of
multi-port TCP SYN/ACK, UDP, SCTP INIT and ICMP probes. The goal of
these probes is to solicit responses which demonstrate that an IP
address is actually active (is being used by a host or network device).
On many networks, only a small percentage of IP addresses are active at
any given time. This is particularly common with private address space
such as 10.0.0.0/8. That network has 16 million IPs, but I have seen it
used by companies with less than a thousand machines. Host discovery
can find those machines in a sparsely allocated sea of IP addresses.
If no host discovery options are given, Nmap sends an ICMP echo
request, a TCP SYN packet to port 443, and TCP ACK packet to port 80,
and an ICMP timestamp request. These defaults are equivalent to the -PE
-PS443 -PA80 -PP options. An exception to this is that an ARP scan is
used for any targets which are on a local ethernet network. For
unprivileged Unix shell users, the default probes are a SYN packet to
ports 80 and 443 using the connect system call.. This host discovery
is often sufficient when scanning local networks, but a more
comprehensive set of discovery probes is recommended for security
auditing.
The -P* options (which select ping types) can be combined. You can
increase your odds of penetrating strict firewalls by sending many
probe types using different TCP ports/flags and ICMP codes. Also note
that ARP discovery (-PR). is done by default against targets on a
local ethernet network even if you specify other -P* options, because
it is almost always faster and more effective.
By default, Nmap does host discovery and then performs a port scan
against each host it determines is online. This is true even if you
specify non-default host discovery types such as UDP probes (-PU). Read
about the -sP option to learn how to perform only host discovery, or
use -PN to skip host discovery and port scan all target hosts. The
following options control host discovery:
-sL (List Scan) .
The list scan is a degenerate form of host discovery that simply
lists each host of the network(s) specified, without sending any
packets to the target hosts. By default, Nmap still does
reverse-DNS resolution on the hosts to learn their names. It is
often surprising how much useful information simple hostnames give
out. For example, fw.chi is the name of one company´s Chicago
firewall. Nmap also reports the total number of IP addresses at
the end. The list scan is a good sanity check to ensure that you
have proper IP addresses for your targets. If the hosts sport
domain names you do not recognize, it is worth investigating
further to prevent scanning the wrong company´s network.
Since the idea is to simply print a list of target hosts, options
for higher level functionality such as port scanning, OS detection,
or ping scanning cannot be combined with this. If you wish to
disable ping scanning while still performing such higher level
functionality, read up on the -PN (skip ping) option.
-sP (Skip port scan) .
This option tells Nmap not to do a port scan after host discovery,
and only print out the available hosts that responded to the scan.
This is often known as a “ping scan”, but you can also request that
traceroute and NSE host scripts be run. This is by default one step
more intrusive than the list scan, and can often be used for the
same purposes. It allows light reconnaissance of a target network
without attracting much attention. Knowing how many hosts are up is
more valuable to attackers than the list provided by list scan of
every single IP and host name.
Systems administrators often find this option valuable as well. It
can easily be used to count available machines on a network or
monitor server availability. This is often called a ping sweep, and
is more reliable than pinging the broadcast address because many
hosts do not reply to broadcast queries.
The -sP option sends an ICMP echo request, TCP SYN to port 443, TCP
ACK to port 80, and an ICMP timestamp request by default. When
executed by an unprivileged user, only SYN packets are sent (using
a connect call) to ports 80 and 443 on the target. When a
privileged user tries to scan targets on a local ethernet network,
ARP requests are used unless --send-ip was specified. The -sP
option can be combined with any of the discovery probe types (the
-P* options, excluding -PN) for greater flexibility. If any of
those probe type and port number options are used, the default
probes are overridden. When strict firewalls are in place between
the source host running Nmap and the target network, using those
advanced techniques is recommended. Otherwise hosts could be missed
when the firewall drops probes or their responses.
-PN (No ping) .
This option skips the Nmap discovery stage altogether. Normally,
Nmap uses this stage to determine active machines for heavier
scanning. By default, Nmap only performs heavy probing such as port
scans, version detection, or OS detection against hosts that are
found to be up. Disabling host discovery with -PN causes Nmap to
attempt the requested scanning functions against every target IP
address specified. So if a class B sized target address space (/16)
is specified on the command line, all 65,536 IP addresses are
scanned. Proper host discovery is skipped as with the list scan,
but instead of stopping and printing the target list, Nmap
continues to perform requested functions as if each target IP is
active. To skip ping scan and port scan, while still allowing NSE
to run, use the two options -PN -sP together.
For machines on a local ethernet network, ARP scanning will still
be performed (unless --send-ip is specified) because Nmap needs MAC
addresses to further scan target hosts. This option flag used to be
P0 (uses zero), but was renamed to avoid confusion with protocol
ping´s PO (uses the letter O) flag.
-PS port list (TCP SYN Ping) .
This option sends an empty TCP packet with the SYN flag set. The
default destination port is 80 (configurable at compile time by
changing DEFAULT_TCP_PROBE_PORT_SPEC in nmap.h). Alternate ports
can be specified as a parameter. The syntax is the same as for the
-p except that port type specifiers like T: are not allowed.
Examples are -PS22 and -PS22-25,80,113,1050,35000. Note that there
can be no space between -PS and the port list. If multiple probes
are specified they will be sent in parallel.
The SYN flag suggests to the remote system that you are attempting
to establish a connection. Normally the destination port will be
closed, and a RST (reset) packet sent back. If the port happens to
be open, the target will take the second step of a TCP
three-way-handshake. by responding with a SYN/ACK TCP packet. The
machine running Nmap then tears down the nascent connection by
responding with a RST rather than sending an ACK packet which would
complete the three-way-handshake and establish a full connection.
The RST packet is sent by the kernel of the machine running Nmap in
response to the unexpected SYN/ACK, not by Nmap itself.
Nmap does not care whether the port is open or closed. Either the
RST or SYN/ACK response discussed previously tell Nmap that the
host is available and responsive.
On Unix boxes, only the privileged user root. is generally able to
send and receive raw TCP packets.. For unprivileged users, a
workaround is automatically employed. whereby the connect system
call is initiated against each target port. This has the effect of
sending a SYN packet to the target host, in an attempt to establish
a connection. If connect returns with a quick success or an
ECONNREFUSED failure, the underlying TCP stack must have received a
SYN/ACK or RST and the host is marked available. If the connection
attempt is left hanging until a timeout is reached, the host is
marked as down. This workaround is also used for IPv6 connections,
as raw IPv6 packet building support is not yet available in Nmap..
-PA port list (TCP ACK Ping) .
The TCP ACK ping is quite similar to the just-discussed SYN ping.
The difference, as you could likely guess, is that the TCP ACK flag
is set instead of the SYN flag. Such an ACK packet purports to be
acknowledging data over an established TCP connection, but no such
connection exists. So remote hosts should always respond with a RST
packet, disclosing their existence in the process.
The -PA option uses the same default port as the SYN probe (80) and
can also take a list of destination ports in the same format. If an
unprivileged user tries this, or an IPv6 target is specified, the
connect workaround discussed previously is used. This workaround is
imperfect because connect is actually sending a SYN packet rather
than an ACK.
The reason for offering both SYN and ACK ping probes is to maximize
the chances of bypassing firewalls. Many administrators configure
routers and other simple firewalls to block incoming SYN packets
except for those destined for public services like the company web
site or mail server. This prevents other incoming connections to
the organization, while allowing users to make unobstructed
outgoing connections to the Internet. This non-stateful approach
takes up few resources on the firewall/router and is widely
supported by hardware and software filters. The Linux
Netfilter/iptables. firewall software offers the --syn convenience
option to implement this stateless approach. When stateless
firewall rules such as this are in place, SYN ping probes (-PS) are
likely to be blocked when sent to closed target ports. In such
cases, the ACK probe shines as it cuts right through these rules.
Another common type of firewall uses stateful rules that drop
unexpected packets. This feature was initially found mostly on
high-end firewalls, though it has become much more common over the
years. The Linux Netfilter/iptables system supports this through
the --state option, which categorizes packets based on connection
state. A SYN probe is more likely to work against such a system, as
unexpected ACK packets are generally recognized as bogus and
dropped. A solution to this quandary is to send both SYN and ACK
probes by specifying -PS and -PA.
-PU port list (UDP Ping) .
Another host discovery option is the UDP ping, which sends a UDP
packet to the given ports. For most ports, the packet will be
empty, though for a few a protocol-specific payload will be sent
that is more likely to get a response.. See the file payload.cc.
for exactly which ports have payloads. The --data-length. option
sends a fixed-length random payload for all ports.
The port list takes the same format as with the previously
discussed -PS and -PA options. If no ports are specified, the
default is 40125. This default can be configured at compile-time by
changing DEFAULT_UDP_PROBE_PORT_SPEC. in nmap.h.. A highly
uncommon port is used by default because sending to open ports is
often undesirable for this particular scan type.
Upon hitting a closed port on the target machine, the UDP probe
should elicit an ICMP port unreachable packet in return. This
signifies to Nmap that the machine is up and available. Many other
types of ICMP errors, such as host/network unreachables or TTL
exceeded are indicative of a down or unreachable host. A lack of
response is also interpreted this way. If an open port is reached,
most services simply ignore the empty packet and fail to return any
response. This is why the default probe port is 40125, which is
highly unlikely to be in use. A few services, such as the Character
Generator (chargen) protocol, will respond to an empty UDP packet,
and thus disclose to Nmap that the machine is available.
The primary advantage of this scan type is that it bypasses
firewalls and filters that only screen TCP. For example, I once
owned a Linksys BEFW11S4 wireless broadband router. The external
interface of this device filtered all TCP ports by default, but UDP
probes would still elicit port unreachable messages and thus give
away the device.
-PY port list (SCTP INIT Ping) .
This option sends an SCTP packet containing a minimal INIT chunk.
The default destination port is 80 (configurable at compile time by
changing DEFAULT_SCTP_PROBE_PORT_SPEC in nmap.h). Alternate ports
can be specified as a parameter. The syntax is the same as for the
-p except that port type specifiers like S: are not allowed.
Examples are -PY22 and -PY22,80,179,5060. Note that there can be no
space between -PY and the port list. If multiple probes are
specified they will be sent in parallel.
The INIT chunk suggests to the remote system that you are
attempting to establish an association. Normally the destination
port will be closed, and an ABORT chunk will be sent back. If the
port happens to be open, the target will take the second step of an
SCTP four-way-handshake. by responding with an INIT-ACK chunk. If
the machine running Nmap has a functional SCTP stack, then it tears
down the nascent association by responding with an ABORT chunk
rather than sending a COOKIE-ECHO chunk which would be the next
step in the four-way-handshake. The ABORT packet is sent by the
kernel of the machine running Nmap in response to the unexpected
INIT-ACK, not by Nmap itself.
Nmap does not care whether the port is open or closed. Either the
ABORT or INIT-ACK response discussed previously tell Nmap that the
host is available and responsive.
On Unix boxes, only the privileged user root. is generally able to
send and receive raw SCTP packets.. Using SCTP INIT Pings is
currently not possible for unprivileged users.. The same
limitation applies to IPv6, which is currently not supported for
SCTP INIT Ping..
-PE; -PP; -PM (ICMP Ping Types) .
In addition to the unusual TCP, UDP and SCTP host discovery types
discussed previously, Nmap can send the standard packets sent by
the ubiquitous ping program. Nmap sends an ICMP type 8 (echo
request) packet to the target IP addresses, expecting a type 0
(echo reply) in return from available hosts.. Unfortunately for
network explorers, many hosts and firewalls now block these
packets, rather than responding as required by RFC 1122[2]. For
this reason, ICMP-only scans are rarely reliable enough against
unknown targets over the Internet. But for system administrators
monitoring an internal network, they can be a practical and
efficient approach. Use the -PE option to enable this echo request
behavior.
While echo request is the standard ICMP ping query, Nmap does not
stop there. The ICMP standards (RFC 792[3]. and RFC 950[4]. “a
host SHOULD NOT implement these messages”. Timestamp and address
mask queries can be sent with the -PP and -PM options,
respectively. A timestamp reply (ICMP code 14) or address mask
reply (code 18) discloses that the host is available. These two
queries can be valuable when administrators specifically block echo
request packets while forgetting that other ICMP queries can be
used for the same purpose.
-PO protocol list (IP Protocol Ping) .
The newest host discovery option is the IP protocol ping, which
sends IP packets with the specified protocol number set in their IP
header. The protocol list takes the same format as do port lists in
the previously discussed TCP, UDP and SCTP host discovery options.
If no protocols are specified, the default is to send multiple IP
packets for ICMP (protocol 1), IGMP (protocol 2), and IP-in-IP
(protocol 4). The default protocols can be configured at
compile-time by changing DEFAULT_PROTO_PROBE_PORT_SPEC. in nmap.h.
Note that for the ICMP, IGMP, TCP (protocol 6), UDP (protocol 17)
and SCTP (protocol 132), the packets are sent with the proper
protocol headers. while other protocols are sent with no
additional data beyond the IP header (unless the --data-length.
option is specified).
This host discovery method looks for either responses using the
same protocol as a probe, or ICMP protocol unreachable messages
which signify that the given protocol isn´t supported on the
destination host. Either type of response signifies that the target
host is alive.
-PR (ARP Ping) .
One of the most common Nmap usage scenarios is to scan an ethernet
LAN. On most LANs, especially those using private address ranges
specified by RFC 1918[5], the vast majority of IP addresses are
unused at any given time. When Nmap tries to send a raw IP packet
such as an ICMP echo request, the operating system must determine
the destination hardware (ARP) address corresponding to the target
IP so that it can properly address the ethernet frame. This is
often slow and problematic, since operating systems weren´t written
with the expectation that they would need to do millions of ARP
requests against unavailable hosts in a short time period.
ARP scan puts Nmap and its optimized algorithms in charge of ARP
requests. And if it gets a response back, Nmap doesn´t even need to
worry about the IP-based ping packets since it already knows the
host is up. This makes ARP scan much faster and more reliable than
IP-based scans. So it is done by default when scanning ethernet
hosts that Nmap detects are on a local ethernet network. Even if
different ping types (such as -PE or -PS) are specified, Nmap uses
ARP instead for any of the targets which are on the same LAN. If
you absolutely don´t want to do an ARP scan, specify --send-ip.
--traceroute (Trace path to host) .
Traceroutes are performed post-scan using information from the scan
results to determine the port and protocol most likely to reach the
target. It works with all scan types except connect scans (-sT) and
idle scans (-sI). All traces use Nmap´s dynamic timing model and
are performed in parallel.
Traceroute works by sending packets with a low TTL (time-to-live)
in an attempt to elicit ICMP Time Exceeded messages from
intermediate hops between the scanner and the target host. Standard
traceroute implementations start with a TTL of 1 and increment the
TTL until the destination host is reached. Nmap´s traceroute starts
with a high TTL and then decrements the TTL until it reaches zero.
Doing it backwards lets Nmap employ clever caching algorithms to
speed up traces over multiple hosts. On average Nmap sends 5–10
fewer packets per host, depending on network conditions. If a
single subnet is being scanned (i.e. 192.168.0.0/24) Nmap may only
have to send a single packet to most hosts.
-n (No DNS resolution) .
Tells Nmap to never do reverse DNS resolution on the active IP
addresses it finds. Since DNS can be slow even with Nmap´s built-in
parallel stub resolver, this option can slash scanning times.
-R (DNS resolution for all targets) .
Tells Nmap to always do reverse DNS resolution on the target IP
addresses. Normally reverse DNS is only performed against
responsive (online) hosts.
--system-dns (Use system DNS resolver) .
By default, Nmap resolves IP addresses by sending queries directly
to the name servers configured on your host and then listening for
responses. Many requests (often dozens) are performed in parallel
to improve performance. Specify this option to use your system
resolver instead (one IP at a time via the getnameinfo call). This
is slower and rarely useful unless you find a bug in the Nmap
parallel resolver (please let us know if you do). The system
resolver is always used for IPv6 scans.
--dns-servers server1[,server2[,...]] (Servers to use for reverse DNS
queries) .
By default, Nmap determines your DNS servers (for rDNS resolution)
from your resolv.conf file (Unix) or the Registry (Win32).
Alternatively, you may use this option to specify alternate
servers. This option is not honored if you are using --system-dns
or an IPv6 scan. Using multiple DNS servers is often faster,
especially if you choose authoritative servers for your target IP
space. This option can also improve stealth, as your requests can
be bounced off just about any recursive DNS server on the Internet.
This option also comes in handy when scanning private networks.
Sometimes only a few name servers provide proper rDNS information,
and you may not even know where they are. You can scan the network
for port 53 (perhaps with version detection), then try Nmap list
scans (-sL) specifying each name server one at a time with
--dns-servers until you find one which works.
PORT SCANNING BASICS
While Nmap has grown in functionality over the years, it began as an
efficient port scanner, and that remains its core function. The simple
command nmap target scans more than 1660 TCP ports on the host target.
While many port scanners have traditionally lumped all ports into the
open or closed states, Nmap is much more granular. It divides ports
into six states: open, closed, filtered, unfiltered, open|filtered, or
closed|filtered.
These states are not intrinsic properties of the port itself, but
describe how Nmap sees them. For example, an Nmap scan from the same
network as the target may show port 135/tcp as open, while a scan at
the same time with the same options from across the Internet might show
that port as filtered.
The six port states recognized by Nmap
An application is actively accepting TCP connections, UDP datagrams
or SCTP associations on this port. Finding these is often the
primary goal of port scanning. Security-minded people know that
each open port is an avenue for attack. Attackers and pen-testers
want to exploit the open ports, while administrators try to close
or protect them with firewalls without thwarting legitimate users.
Open ports are also interesting for non-security scans because they
show services available for use on the network.
A closed port is accessible (it receives and responds to Nmap probe
packets), but there is no application listening on it. They can be
helpful in showing that a host is up on an IP address (host
discovery, or ping scanning), and as part of OS detection. Because
closed ports are reachable, it may be worth scanning later in case
some open up. Administrators may want to consider blocking such
ports with a firewall. Then they would appear in the filtered
state, discussed next.
Nmap cannot determine whether the port is open because packet
filtering prevents its probes from reaching the port. The filtering
could be from a dedicated firewall device, router rules, or
host-based firewall software. These ports frustrate attackers
because they provide so little information. Sometimes they respond
with ICMP error messages such as type 3 code 13 (destination
unreachable: communication administratively prohibited), but
filters that simply drop probes without responding are far more
common. This forces Nmap to retry several times just in case the
probe was dropped due to network congestion rather than filtering.
This slows down the scan dramatically.
The unfiltered state means that a port is accessible, but Nmap is
unable to determine whether it is open or closed. Only the ACK
scan, which is used to map firewall rulesets, classifies ports into
this state. Scanning unfiltered ports with other scan types such as
Window scan, SYN scan, or FIN scan, may help resolve whether the
port is open.
Nmap places ports in this state when it is unable to determine
whether a port is open or filtered. This occurs for scan types in
which open ports give no response. The lack of response could also
mean that a packet filter dropped the probe or any response it
elicited. So Nmap does not know for sure whether the port is open
or being filtered. The UDP, IP protocol, FIN, NULL, and Xmas scans
classify ports this way.
This state is used when Nmap is unable to determine whether a port
is closed or filtered. It is only used for the IP ID idle scan.
PORT SCANNING TECHNIQUES
As a novice performing automotive repair, I can struggle for hours
trying to fit my rudimentary tools (hammer, duct tape, wrench, etc.) to
the task at hand. When I fail miserably and tow my jalopy to a real
mechanic, he invariably fishes around in a huge tool chest until
pulling out the perfect gizmo which makes the job seem effortless. The
art of port scanning is similar. Experts understand the dozens of scan
techniques and choose the appropriate one (or combination) for a given
task. Inexperienced users and script kiddies,. on the other hand, try
to solve every problem with the default SYN scan. Since Nmap is free,
the only barrier to port scanning mastery is knowledge. That certainly
beats the automotive world, where it may take great skill to determine
that you need a strut spring compressor, then you still have to pay
thousands of dollars for it.
Most of the scan types are only available to privileged users.. This
is because they send and receive raw packets,. which requires root
access on Unix systems. Using an administrator account on Windows is
recommended, though Nmap sometimes works for unprivileged users on that
platform when WinPcap has already been loaded into the OS. Requiring
root privileges was a serious limitation when Nmap was released in
1997, as many users only had access to shared shell accounts. Now, the
world is different. Computers are cheaper, far more people have
always-on direct Internet access, and desktop Unix systems (including
Linux and Mac OS X) are prevalent. A Windows version of Nmap is now
available, allowing it to run on even more desktops. For all these
reasons, users have less need to run Nmap from limited shared shell
accounts. This is fortunate, as the privileged options make Nmap far
more powerful and flexible.
While Nmap attempts to produce accurate results, keep in mind that all
of its insights are based on packets returned by the target machines
(or firewalls in front of them). Such hosts may be untrustworthy and
send responses intended to confuse or mislead Nmap. Much more common
are non-RFC-compliant hosts that do not respond as they should to Nmap
probes. FIN, NULL, and Xmas scans are particularly susceptible to this
problem. Such issues are specific to certain scan types and so are
discussed in the individual scan type entries.
This section documents the dozen or so port scan techniques supported
by Nmap. Only one method may be used at a time, except that UDP scan
(-sU) and any one of the SCTP scan types (-sY, -sZ) may be combined
with any one of the TCP scan types. As a memory aid, port scan type
options are of the form -sC, where C is a prominent character in the
scan name, usually the first. The one exception to this is the
deprecated FTP bounce scan (-b). By default, Nmap performs a SYN Scan,
though it substitutes a connect scan if the user does not have proper
privileges to send raw packets (requires root access on Unix) or if
IPv6 targets were specified. Of the scans listed in this section,
unprivileged users can only execute connect and FTP bounce scans.
-sS (TCP SYN scan) .
SYN scan is the default and most popular scan option for good
reasons. It can be performed quickly, scanning thousands of ports
per second on a fast network not hampered by restrictive firewalls.
SYN scan is relatively unobtrusive and stealthy, since it never
completes TCP connections. It also works against any compliant TCP
stack rather than depending on idiosyncrasies of specific platforms
as Nmap´s FIN/NULL/Xmas, Maimon and idle scans do. It also allows
clear, reliable differentiation between the open, closed, and
filtered states.
This technique is often referred to as half-open scanning, because
you don´t open a full TCP connection. You send a SYN packet, as if
you are going to open a real connection and then wait for a
response. A SYN/ACK indicates the port is listening (open), while a
RST (reset) is indicative of a non-listener. If no response is
received after several retransmissions, the port is marked as
filtered. The port is also marked filtered if an ICMP unreachable
error (type 3, code 1, 2, 3, 9, 10, or 13) is received.
-sT (TCP connect scan) .
TCP connect scan is the default TCP scan type when SYN scan is not
an option. This is the case when a user does not have raw packet
privileges or is scanning IPv6 networks. Instead of writing raw
packets as most other scan types do, Nmap asks the underlying
operating system to establish a connection with the target machine
and port by issuing the connect system call. This is the same
high-level system call that web browsers, P2P clients, and most
other network-enabled applications use to establish a connection.
It is part of a programming interface known as the Berkeley Sockets
API. Rather than read raw packet responses off the wire, Nmap uses
this API to obtain status information on each connection attempt.
When SYN scan is available, it is usually a better choice. Nmap has
less control over the high level connect call than with raw
packets, making it less efficient. The system call completes
connections to open target ports rather than performing the
half-open reset that SYN scan does. Not only does this take longer
and require more packets to obtain the same information, but target
machines are more likely to log the connection. A decent IDS will
catch either, but most machines have no such alarm system. Many
services on your average Unix system will add a note to syslog, and
sometimes a cryptic error message, when Nmap connects and then
closes the connection without sending data. Truly pathetic services
crash when this happens, though that is uncommon. An administrator
who sees a bunch of connection attempts in her logs from a single
system should know that she has been connect scanned.
-sU (UDP scans) .
While most popular services on the Internet run over the TCP
protocol, UDP[6] services are widely deployed. DNS, SNMP, and DHCP
(registered ports 53, 161/162, and 67/68) are three of the most
common. Because UDP scanning is generally slower and more difficult
than TCP, some security auditors ignore these ports. This is a
mistake, as exploitable UDP services are quite common and attackers
certainly don´t ignore the whole protocol. Fortunately, Nmap can
help inventory UDP ports.
UDP scan is activated with the -sU option. It can be combined with
a TCP scan type such as SYN scan (-sS) to check both protocols
during the same run.
UDP scan works by sending a UDP packet to every targeted port. For
some common ports such as 53 and 161, a protocol-specific payload
is sent, but for most ports the packet is empty.. The
--data-length option can be used to send a fixed-length random
payload to every port. If an ICMP port unreachable error (type 3,
code 3) is returned, the port is closed. Other ICMP unreachable
errors (type 3, codes 1, 2, 9, 10, or 13) mark the port as
filtered. Occasionally, a service will respond with a UDP packet,
proving that it is open. If no response is received after
retransmissions, the port is classified as open|filtered. This
means that the port could be open, or perhaps packet filters are
blocking the communication. Version detection (-sV) can be used to
help differentiate the truly open ports from the filtered ones.
A big challenge with UDP scanning is doing it quickly. Open and
filtered ports rarely send any response, leaving Nmap to time out
and then conduct retransmissions just in case the probe or response
were lost. Closed ports are often an even bigger problem. They
usually send back an ICMP port unreachable error. But unlike the
RST packets sent by closed TCP ports in response to a SYN or
connect scan, many hosts rate limit. ICMP port unreachable
messages by default. Linux and Solaris are particularly strict
about this. For example, the Linux 2.4.20 kernel limits destination
unreachable messages to one per second (in net/ipv4/icmp.c).
Nmap detects rate limiting and slows down accordingly to avoid
flooding the network with useless packets that the target machine
will drop. Unfortunately, a Linux-style limit of one packet per
second makes a 65,536-port scan take more than 18 hours. Ideas for
speeding your UDP scans up include scanning more hosts in parallel,
doing a quick scan of just the popular ports first, scanning from
behind the firewall, and using --host-timeout to skip slow hosts.
-sY (SCTP INIT scan) .
SCTP[7] is a relatively new alternative to the TCP and UDP
protocols, combining most characteristics of TCP and UDP, and also
adding new features like multi-homing and multi-streaming. It is
mostly being used for SS7/SIGTRAN related services but has the
potential to be used for other applications as well. SCTP INIT scan
is the SCTP equivalent of a TCP SYN scan. It can be performed
quickly, scanning thousands of ports per second on a fast network
not hampered by restrictive firewalls. Like SYN scan, INIT scan is
relatively unobtrusive and stealthy, since it never completes SCTP
associations. It also allows clear, reliable differentiation
between the open, closed, and filtered states.
This technique is often referred to as half-open scanning, because
you don´t open a full SCTP association. You send an INIT chunk, as
if you are going to open a real association and then wait for a
response. An INIT-ACK chunk indicates the port is listening (open),
while an ABORT chunk is indicative of a non-listener. If no
response is received after several retransmissions, the port is
marked as filtered. The port is also marked filtered if an ICMP
unreachable error (type 3, code 1, 2, 3, 9, 10, or 13) is received.
-sN; -sF; -sX (TCP NULL, FIN, and Xmas scans) .
These three scan types (even more are possible with the --scanflags
option described in the next section) exploit a subtle loophole in
the TCP RFC[8] to differentiate between open and closed ports. Page
65 of RFC 793 says that “if the [destination] port state is CLOSED
.... an incoming segment not containing a RST causes a RST to be
sent in response.” Then the next page discusses packets sent to
open ports without the SYN, RST, or ACK bits set, stating that:
“you are unlikely to get here, but if you do, drop the segment, and
return.”
When scanning systems compliant with this RFC text, any packet not
containing SYN, RST, or ACK bits will result in a returned RST if
the port is closed and no response at all if the port is open. As
long as none of those three bits are included, any combination of
the other three (FIN, PSH, and URG) are OK. Nmap exploits this with
three scan types:
Null scan (-sN)
Does not set any bits (TCP flag header is 0)
FIN scan (-sF)
Sets just the TCP FIN bit.
Xmas scan (-sX)
Sets the FIN, PSH, and URG flags, lighting the packet up like a
Christmas tree.
These three scan types are exactly the same in behavior except for
the TCP flags set in probe packets. If a RST packet is received,
the port is considered closed, while no response means it is
open|filtered. The port is marked filtered if an ICMP unreachable
error (type 3, code 1, 2, 3, 9, 10, or 13) is received.
The key advantage to these scan types is that they can sneak
through certain non-stateful firewalls and packet filtering
routers. Another advantage is that these scan types are a little
more stealthy than even a SYN scan. Don´t count on this though—most
modern IDS products can be configured to detect them. The big
downside is that not all systems follow RFC 793 to the letter. A
number of systems send RST responses to the probes regardless of
whether the port is open or not. This causes all of the ports to be
labeled closed. Major operating systems that do this are Microsoft
Windows, many Cisco devices, BSDI, and IBM OS/400. This scan does
work against most Unix-based systems though. Another downside of
these scans is that they can´t distinguish open ports from certain
filtered ones, leaving you with the response open|filtered.
-sA (TCP ACK scan) .
This scan is different than the others discussed so far in that it
never determines open (or even open|filtered) ports. It is used to
map out firewall rulesets, determining whether they are stateful or
not and which ports are filtered.
The ACK scan probe packet has only the ACK flag set (unless you use
--scanflags). When scanning unfiltered systems, open and closed
ports will both return a RST packet. Nmap then labels them as
unfiltered, meaning that they are reachable by the ACK packet, but
whether they are open or closed is undetermined. Ports that don´t
respond, or send certain ICMP error messages back (type 3, code 1,
2, 3, 9, 10, or 13), are labeled filtered.
-sW (TCP Window scan) .
Window scan is exactly the same as ACK scan except that it exploits
an implementation detail of certain systems to differentiate open
ports from closed ones, rather than always printing unfiltered when
a RST is returned. It does this by examining the TCP Window field
of the RST packets returned. On some systems, open ports use a
positive window size (even for RST packets) while closed ones have
a zero window. So instead of always listing a port as unfiltered
when it receives a RST back, Window scan lists the port as open or
closed if the TCP Window value in that reset is positive or zero,
respectively.
This scan relies on an implementation detail of a minority of
systems out on the Internet, so you can´t always trust it. Systems
that don´t support it will usually return all ports closed. Of
course, it is possible that the machine really has no open ports.
If most scanned ports are closed but a few common port numbers
(such as 22, 25, 53) are filtered, the system is most likely
susceptible. Occasionally, systems will even show the exact
opposite behavior. If your scan shows 1000 open ports and three
closed or filtered ports, then those three may very well be the
truly open ones.
-sM (TCP Maimon scan) .
The Maimon scan is named after its discoverer, Uriel Maimon.. He
described the technique in Phrack Magazine issue #49 (November
1996).. Nmap, which included this technique, was released two
issues later. This technique is exactly the same as NULL, FIN, and
Xmas scans, except that the probe is FIN/ACK. According to RFC
793[8] (TCP), a RST packet should be generated in response to such
a probe whether the port is open or closed. However, Uriel noticed
that many BSD-derived systems simply drop the packet if the port is
open.
--scanflags (Custom TCP scan) .
Truly advanced Nmap users need not limit themselves to the canned
scan types offered. The --scanflags option allows you to design
your own scan by specifying arbitrary TCP flags.. Let your
creative juices flow, while evading intrusion detection systems.
whose vendors simply paged through the Nmap man page adding
specific rules!
The --scanflags argument can be a numerical flag value such as 9
(PSH and FIN), but using symbolic names is easier. Just mash
together any combination of URG, ACK, PSH, RST, SYN, and FIN. For
example, --scanflags URGACKPSHRSTSYNFIN sets everything, though
it´s not very useful for scanning. The order these are specified in
is irrelevant.
In addition to specifying the desired flags, you can specify a TCP
scan type (such as -sA or -sF). That base type tells Nmap how to
interpret responses. For example, a SYN scan considers no-response
to indicate a filtered port, while a FIN scan treats the same as
open|filtered. Nmap will behave the same way it does for the base
scan type, except that it will use the TCP flags you specify
instead. If you don´t specify a base type, SYN scan is used.
-sZ (SCTP COOKIE ECHO scan) .
SCTP COOKIE ECHO scan is a more advanced SCTP scan. It takes
advantage of the fact that SCTP implementations should silently
drop packets containing COOKIE ECHO chunks on open ports, but send
an ABORT if the port is closed. The advantage of this scan type is
that it is not as obvious a port scan than an INIT scan. Also,
there may be non-stateful firewall rulesets blocking INIT chunks,
but not COOKIE ECHO chunks. Don´t be fooled into thinking that this
will make a port scan invisible; a good IDS will be able to detect
SCTP COOKIE ECHO scans too. The downside is that SCTP COOKIE ECHO
scans cannot differentiate between open and filtered ports, leaving
you with the state open|filtered in both cases.
-sI zombie host[:probeport] (idle scan) .
This advanced scan method allows for a truly blind TCP port scan of
the target (meaning no packets are sent to the target from your
real IP address). Instead, a unique side-channel attack exploits
predictable IP fragmentation ID sequence generation on the zombie
host to glean information about the open ports on the target. IDS
systems will display the scan as coming from the zombie machine you
specify (which must be up and meet certain criteria). This
fascinating scan type is too complex to fully describe in this
reference guide, so I wrote and posted an informal paper with full
details at http://nmap.org/book/idlescan.html.
Besides being extraordinarily stealthy (due to its blind nature),
this scan type permits mapping out IP-based trust relationships
between machines. The port listing shows open ports from the
perspective of the zombie host. So you can try scanning a target
using various zombies that you think might be trusted. (via
router/packet filter rules).
You can add a colon followed by a port number to the zombie host if
you wish to probe a particular port on the zombie for IP ID
changes. Otherwise Nmap will use the port it uses by default for
TCP pings (80).
-sO (IP protocol scan) .
IP protocol scan allows you to determine which IP protocols (TCP,
ICMP, IGMP, etc.) are supported by target machines. This isn´t
technically a port scan, since it cycles through IP protocol
numbers rather than TCP or UDP port numbers. Yet it still uses the
-p option to select scanned protocol numbers, reports its results
within the normal port table format, and even uses the same
underlying scan engine as the true port scanning methods. So it is
close enough to a port scan that it belongs here.
Besides being useful in its own right, protocol scan demonstrates
the power of open-source software. While the fundamental idea is
pretty simple, I had not thought to add it nor received any
requests for such functionality. Then in the summer of 2000,
Gerhard Rieger. conceived the idea, wrote an excellent patch
implementing it, and sent it to the nmap-hackers mailing list.. I
incorporated that patch into the Nmap tree and released a new
version the next day. Few pieces of commercial software have users
enthusiastic enough to design and contribute their own
improvements!
Protocol scan works in a similar fashion to UDP scan. Instead of
iterating through the port number field of a UDP packet, it sends
IP packet headers and iterates through the eight-bit IP protocol
field. The headers are usually empty, containing no data and not
even the proper header for the claimed protocol. The exceptions are
TCP, UDP, ICMP, SCTP, and IGMP. A proper protocol header for those
is included since some systems won´t send them otherwise and
because Nmap already has functions to create them. Instead of
watching for ICMP port unreachable messages, protocol scan is on
the lookout for ICMP protocol unreachable messages. If Nmap
receives any response in any protocol from the target host, Nmap
marks that protocol as open. An ICMP protocol unreachable error
(type 3, code 2) causes the protocol to be marked as closed Other
ICMP unreachable errors (type 3, code 1, 3, 9, 10, or 13) cause the
protocol to be marked filtered (though they prove that ICMP is open
at the same time). If no response is received after
retransmissions, the protocol is marked open|filtered
-b FTP relay host (FTP bounce scan) .
An interesting feature of the FTP protocol (RFC 959[9]) is support
for so-called proxy FTP connections. This allows a user to connect
to one FTP server, then ask that files be sent to a third-party
server. Such a feature is ripe for abuse on many levels, so most
servers have ceased supporting it. One of the abuses this feature
allows is causing the FTP server to port scan other hosts. Simply
ask the FTP server to send a file to each interesting port of a
target host in turn. The error message will describe whether the
port is open or not. This is a good way to bypass firewalls because
organizational FTP servers are often placed where they have more
access to other internal hosts than any old Internet host would.
Nmap supports FTP bounce scan with the -b option. It takes an
argument of the form username:password@server:port. Server is the
name or IP address of a vulnerable FTP server. As with a normal
URL, you may omit username:password, in which case anonymous login
credentials (user: anonymous password:-wwwuser@) are used. The port
number (and preceding colon) may be omitted as well, in which case
the default FTP port (21) on server is used.
This vulnerability was widespread in 1997 when Nmap was released,
but has largely been fixed. Vulnerable servers are still around, so
it is worth trying when all else fails. If bypassing a firewall is
your goal, scan the target network for open port 21 (or even for
any FTP services if you scan all ports with version detection),
then try a bounce scan using each. Nmap will tell you whether the
host is vulnerable or not. If you are just trying to cover your
tracks, you don´t need to (and, in fact, shouldn´t) limit yourself
to hosts on the target network. Before you go scanning random
Internet addresses for vulnerable FTP servers, consider that
sysadmins may not appreciate you abusing their servers in this way.
PORT SPECIFICATION AND SCAN ORDER
In addition to all of the scan methods discussed previously, Nmap
offers options for specifying which ports are scanned and whether the
scan order is randomized or sequential. By default, Nmap scans the most
common 1,000 ports for each protocol.
-p port ranges (Only scan specified ports) .
This option specifies which ports you want to scan and overrides
the default. Individual port numbers are OK, as are ranges
separated by a hyphen (e.g. 1-1023). The beginning and/or end
values of a range may be omitted, causing Nmap to use 1 and 65535,
respectively. So you can specify -p- to scan ports from 1 through
65535. Scanning port zero. is allowed if you specify it
explicitly. For IP protocol scanning (-sO), this option specifies
the protocol numbers you wish to scan for (0–255).
When scanning both TCP and UDP ports, you can specify a particular
protocol by preceding the port numbers by T: or U:. The qualifier
lasts until you specify another qualifier. For example, the
argument -p U:53,111,137,T:21-25,80,139,8080 would scan UDP ports
53, 111,and 137, as well as the listed TCP ports. Note that to scan
both UDP and TCP, you have to specify -sU and at least one TCP scan
type (such as -sS, -sF, or -sT). If no protocol qualifier is given,
the port numbers are added to all protocol lists. Ports can also
be specified by name according to what the port is referred to in
the nmap-services. You can even use the wildcards * and ? with the
names. For example, to scan FTP and all ports whose names begin
with “http”, use -p ftp,http*. Be careful about shell expansions
and quote the argument to -p if unsure.
Ranges of ports can be surrounded by square brackets to indicate
ports inside that range that appear in nmap-services. For example,
the following will scan all ports in nmap-services equal to or
below 1024: -p [-1024]. Be careful with shell expansions and quote
the argument to -p if unsure.
-F (Fast (limited port) scan) .
Specifies that you wish to scan fewer ports than the default.
Normally Nmap scans the most common 1,000 ports for each scanned
protocol. With -F, this is reduced to 100.
Nmap needs an nmap-services file with frequency information in
order to know which ports are the most common. If port frequency
information isn´t available, perhaps because of the use of a custom
nmap-services file, -F means to scan only ports that are named in
the services file (normally Nmap scans all named ports plus ports
1–1024).
-r (Don´t randomize ports) .
By default, Nmap randomizes the scanned port order (except that
certain commonly accessible ports are moved near the beginning for
efficiency reasons). This randomization is normally desirable, but
you can specify -r for sequential (sorted from lowest to highest)
port scanning instead.
--port-ratio <decimal number between 0 and 1>
Scans all ports in nmap-services file with a ratio greater than the
number specified as the argument.
--top-ports <integer of 1 or greater>
Scans the N highest-ratio ports found in nmap-services file.
SERVICE AND VERSION DETECTION
Point Nmap at a remote machine and it might tell you that ports 25/tcp,
80/tcp, and 53/udp are open. Using its nmap-services. database of
about 2,200 well-known services,. Nmap would report that those ports
probably correspond to a mail server (SMTP), web server (HTTP), and
name server (DNS) respectively. This lookup is usually accurate—the
vast majority of daemons listening on TCP port 25 are, in fact, mail
servers. However, you should not bet your security on this! People can
and do run services on strange ports..
Even if Nmap is right, and the hypothetical server above is running
SMTP, HTTP, and DNS servers, that is not a lot of information. When
doing vulnerability assessments (or even simple network inventories) of
your companies or clients, you really want to know which mail and DNS
servers and versions are running. Having an accurate version number
helps dramatically in determining which exploits a server is vulnerable
to. Version detection helps you obtain this information.
After TCP and/or UDP ports are discovered using one of the other scan
methods, version detection interrogates those ports to determine more
about what is actually running. The nmap-service-probes. database
contains probes for querying various services and match expressions to
recognize and parse responses. Nmap tries to determine the service
protocol (e.g. FTP, SSH, Telnet, HTTP), the application name (e.g. ISC
BIND, Apache httpd, Solaris telnetd), the version number, hostname,
device type (e.g. printer, router), the OS family (e.g. Windows, Linux)
and sometimes miscellaneous details like whether an X server is open to
connections, the SSH protocol version, or the KaZaA user name). Of
course, most services don´t provide all of this information. If Nmap
was compiled with OpenSSL support, it will connect to SSL servers to
deduce the service listening behind that encryption layer.. When RPC
services are discovered, the Nmap RPC grinder. (-sR). is
automatically used to determine the RPC program and version numbers.
Some UDP ports are left in the open|filtered state after a UDP port
scan is unable to determine whether the port is open or filtered.
Version detection will try to elicit a response from these ports (just
as it does with open ports), and change the state to open if it
succeeds. open|filtered TCP ports are treated the same way. Note that
the Nmap -A option enables version detection among other things. A
paper documenting the workings, usage, and customization of version
detection is available at http://nmap.org/book/vscan.html.
When Nmap receives responses from a service but cannot match them to
its database, it prints out a special fingerprint and a URL for you to
submit if to if you know for sure what is running on the port. Please
take a couple minutes to make the submission so that your find can
benefit everyone. Thanks to these submissions, Nmap has about 3,000
pattern matches for more than 350 protocols such as SMTP, FTP, HTTP,
etc..
Version detection is enabled and controlled with the following options:
-sV (Version detection) .
Enables version detection, as discussed above. Alternatively, you
can use -A, which enables version detection among other things.
--allports (Don´t exclude any ports from version detection) .
By default, Nmap version detection skips TCP port 9100 because some
printers simply print anything sent to that port, leading to dozens
of pages of HTTP GET requests, binary SSL session requests, etc.
This behavior can be changed by modifying or removing the Exclude
directive in nmap-service-probes, or you can specify --allports to
scan all ports regardless of any Exclude directive.
--version-intensity intensity (Set version scan intensity) .
When performing a version scan (-sV), Nmap sends a series of
probes, each of which is assigned a rarity value between one and
nine. The lower-numbered probes are effective against a wide
variety of common services, while the higher numbered ones are
rarely useful. The intensity level specifies which probes should be
applied. The higher the number, the more likely it is the service
will be correctly identified. However, high intensity scans take
longer. The intensity must be between 0 and 9. The default is 7.
When a probe is registered to the target port via the
nmap-service-probes ports directive, that probe is tried regardless
of intensity level. This ensures that the DNS probes will always be
attempted against any open port 53, the SSL probe will be done
against 443, etc.
--version-light (Enable light mode) .
This is a convenience alias for --version-intensity 2. This light
mode makes version scanning much faster, but it is slightly less
likely to identify services.
--version-all (Try every single probe) .
An alias for --version-intensity 9, ensuring that every single
probe is attempted against each port.
--version-trace (Trace version scan activity) .
This causes Nmap to print out extensive debugging info about what
version scanning is doing. It is a subset of what you get with
--packet-trace.
-sR (RPC scan) .
This method works in conjunction with the various port scan methods
of Nmap. It takes all the TCP/UDP ports found open and floods them
with SunRPC program NULL commands in an attempt to determine
whether they are RPC ports, and if so, what program and version
number they serve up. Thus you can effectively obtain the same info
as rpcinfo -p even if the target´s portmapper is behind a firewall
(or protected by TCP wrappers). Decoys do not currently work with
RPC scan.. This is automatically enabled as part of version scan
(-sV) if you request that. As version detection includes this and
is much more comprehensive, -sR is rarely needed.
OS DETECTION
One of Nmap´s best-known features is remote OS detection using TCP/IP
stack fingerprinting. Nmap sends a series of TCP and UDP packets to the
remote host and examines practically every bit in the responses. After
performing dozens of tests such as TCP ISN sampling, TCP options
support and ordering, IP ID sampling, and the initial window size
check, Nmap compares the results to its nmap-os-db. database of more
than a thousand known OS fingerprints and prints out the OS details if
there is a match. Each fingerprint includes a freeform textual
description of the OS, and a classification which provides the vendor
name (e.g. Sun), underlying OS (e.g. Solaris), OS generation (e.g. 10),
and device type (general purpose, router, switch, game console, etc).
If Nmap is unable to guess the OS of a machine, and conditions are good
(e.g. at least one open port and one closed port were found), Nmap will
provide a URL you can use to submit the fingerprint if you know (for
sure) the OS running on the machine. By doing this you contribute to
the pool of operating systems known to Nmap and thus it will be more
accurate for everyone.
OS detection enables some other tests which make use of information
that is gathered during the process anyway. One of these is TCP
Sequence Predictability Classification. This measures approximately how
hard it is to establish a forged TCP connection against the remote
host. It is useful for exploiting source-IP based trust relationships
(rlogin, firewall filters, etc) or for hiding the source of an attack.
This sort of spoofing is rarely performed any more, but many machines
are still vulnerable to it. The actual difficulty number is based on
statistical sampling and may fluctuate. It is generally better to use
the English classification such as “worthy challenge” or “trivial
joke”. This is only reported in normal output in verbose (-v) mode.
When verbose mode is enabled along with -O, IP ID sequence generation
is also reported. Most machines are in the “incremental” class, which
means that they increment the ID field in the IP header for each packet
they send. This makes them vulnerable to several advanced information
gathering and spoofing attacks.
Another bit of extra information enabled by OS detection is a guess at
a target´s uptime. This uses the TCP timestamp option (RFC 1323[10]) to
guess when a machine was last rebooted. The guess can be inaccurate due
to the timestamp counter not being initialized to zero or the counter
overflowing and wrapping around, so it is printed only in verbose mode.
A paper documenting the workings, usage, and customization of OS
detection is available at http://nmap.org/book/osdetect.html.
OS detection is enabled and controlled with the following options:
-O (Enable OS detection) .
Enables OS detection, as discussed above. Alternatively, you can
use -A to enable OS detection along with other things.
--osscan-limit (Limit OS detection to promising targets) .
OS detection is far more effective if at least one open and one
closed TCP port are found. Set this option and Nmap will not even
try OS detection against hosts that do not meet this criteria. This
can save substantial time, particularly on -PN scans against many
hosts. It only matters when OS detection is requested with -O or
-A.
--osscan-guess; --fuzzy (Guess OS detection results) .
When Nmap is unable to detect a perfect OS match, it sometimes
offers up near-matches as possibilities. The match has to be very
close for Nmap to do this by default. Either of these (equivalent)
options make Nmap guess more aggressively. Nmap will still tell you
when an imperfect match is printed and display its confidence level
(percentage) for each guess.
--max-os-tries (Set the maximum number of OS detection tries against a
target) .
When Nmap performs OS detection against a target and fails to find
a perfect match, it usually repeats the attempt. By default, Nmap
tries five times if conditions are favorable for OS fingerprint
submission, and twice when conditions aren´t so good. Specifying a
lower --max-os-tries value (such as 1) speeds Nmap up, though you
miss out on retries which could potentially identify the OS.
Alternatively, a high value may be set to allow even more retries
when conditions are favorable. This is rarely done, except to
generate better fingerprints for submission and integration into
the Nmap OS database.
NMAP SCRIPTING ENGINE (NSE)
The Nmap Scripting Engine (NSE) is one of Nmap´s most powerful and
flexible features. It allows users to write (and share) simple scripts
(using the Lua programming language[11],
Tasks we had in mind when creating the system include network
discovery, more sophisticated version detection, vulnerability
detection. NSE can even be used for vulnerability exploitation.
To reflect those different uses and to simplify the choice of which
scripts to run, each script contains a field associating it with one or
more categories. Currently defined categories are safe, intrusive,
malware, version, discovery, vuln, auth, and default. These are all
described at http://nmap.org/book/nse-usage.html#nse-categories.
Scripts are not run in a sandbox and thus could accidentally or
maliciously damage your system or invade your privacy. Never run
scripts from third parties unless you trust the authors or have
carefully audited the scripts yourself.
The Nmap Scripting Engine is described in detail at
http://nmap.org/book/nse.html
and is controlled by the following options:
-sC .
Performs a script scan using the default set of scripts. It is
equivalent to --script=default. Some of the scripts in this
category are considered intrusive and should not be run against a
target network without permission.
--script filename|category|directory|expression|all[,...] .
Runs a script scan using the comma-separated list of filenames,
script categories, and directories. Each element in the list may
also be a Boolean expression describing a more complex set of
scripts. Each element is interpreted first as an expression, then
as a category, and finally as a file or directory name. The special
argument all makes every script in Nmap´s script database eligible
to run. The all argument should be used with caution as NSE may
contain dangerous scripts including exploits, brute force
authentication crackers, and denial of service attacks.
File and directory names may be relative or absolute. Absolute
names are used directly. Relative paths are looked for in the
following places until found:
--datadir
$NMAPDIR
~/.nmap (not searched on Windows)
NMAPDATADIR
the current directory
A scripts subdirectory is also tried in each of these.
When a directory name is given, Nmap loads every file in the
directory whose name ends with .nse. All other files are ignored
and directories are not searched recursively. When a filename is
given, it does not have to have the .nse extension; it will be
added automatically if necessary. Nmap scripts are stored in a
scripts subdirectory of the Nmap data directory by default (see
http://nmap.org/book/data-files.html).
For efficiency, scripts are indexed in a database stored in
scripts/script.db,. which lists the category or categories in
which each script belongs. When referring to scripts from
script.db by name, you can use a shell-style ‘*’ wildcard.
nmap --script "http-*"
Loads all scripts whose name starts with http-, such as
http-auth.nse and http-open-proxy.nse. The argument to --script
had to be in quotes to protect the wildcard from the shell.
More complicated script selection can be done using the and, or,
and not operators to build Boolean expressions. The operators have
the same precedence[12] as in Lua: not is the highest, followed by
and and then or. You can alter precedence by using parentheses.
Because expressions contain space characters it is necessary to
quote them.
nmap --script "not intrusive"
Loads every script except for those in the intrusive category.
nmap --script "default or safe"
This is functionally equivalent to nmap --script
"default,safe". It loads all scripts that are in the default
category or the safe category or both.
nmap --script "default and safe"
Loads those scripts that are in both the default and safe
categories.
nmap --script "(default or safe or intrusive) and not http-*"
Loads scripts in the default, safe, or intrusive categories,
except for those whose names start with http-.
--script-args name1=value1,name2={name3=value3},name4={value4,value5} .
Lets you provide arguments to NSE scripts. Arguments are a
comma-separated list of name=value pairs. Names and values may be
strings not containing whitespace or the characters ‘{’, ‘}’, ‘=’,
or ‘,’. To include one of these characters in a string, enclose the
string in single or double quotes. Within a quoted string, ‘/’
escapes a quote. A backslash is only used to escape quotation marks
in this special case; in all other cases a backslash is interpreted
literally. Values may also be tables enclosed in {}, just as in
Lua. A table may contain simple string values or more name-value
pairs, including nested tables. An example of script arguments:
--script-args
auth={user=foo,pass=´,{}=bar´},userdb=C:/Path/To/File. The online
NSE Documentation Portal at http://nmap.org/nsedoc/ lists the
arguments that each script accepts.
--script-trace .
This option does what --packet-trace does, just one ISO layer
higher. If this option is specified all incoming and outgoing
communication performed by a script is printed. The displayed
information includes the communication protocol, the source, the
target and the transmitted data. If more than 5% of all transmitted
data is not printable, then the trace output is in a hex dump
format. Specifying --packet-trace enables script tracing too.
--script-updatedb .
This option updates the script database found in scripts/script.db
which is used by Nmap to determine the available default scripts
and categories. It is only necessary to update the database if you
have added or removed NSE scripts from the default scripts
directory or if you have changed the categories of any script. This
option is generally used by itself: nmap --script-updatedb.
TIMING AND PERFORMANCE
One of my highest Nmap development priorities has always been
performance. A default scan (nmap hostname) of a host on my local
network takes a fifth of a second. That is barely enough time to blink,
but adds up when you are scanning hundreds or thousands of hosts.
Moreover, certain scan options such as UDP scanning and version
detection can increase scan times substantially. So can certain
firewall configurations, particularly response rate limiting. While
Nmap utilizes parallelism and many advanced algorithms to accelerate
these scans, the user has ultimate control over how Nmap runs. Expert
users carefully craft Nmap commands to obtain only the information they
care about while meeting their time constraints.
Techniques for improving scan times include omitting non-critical
tests, and upgrading to the latest version of Nmap (performance
enhancements are made frequently). Optimizing timing parameters can
also make a substantial difference. Those options are listed below.
Some options accept a time parameter. This is specified in milliseconds
by default, though you can append ‘s’, ‘m’, or ‘h’ to the value to
specify seconds, minutes, or hours. So the --host-timeout arguments
900000, 900s, and 15m all do the same thing.
--min-hostgroup numhosts; --max-hostgroup numhosts (Adjust parallel
scan group sizes) .
Nmap has the ability to port scan or version scan multiple hosts in
parallel. Nmap does this by dividing the target IP space into
groups and then scanning one group at a time. In general, larger
groups are more efficient. The downside is that host results can´t
be provided until the whole group is finished. So if Nmap started
out with a group size of 50, the user would not receive any reports
(except for the updates offered in verbose mode) until the first 50
hosts are completed.
By default, Nmap takes a compromise approach to this conflict. It
starts out with a group size as low as five so the first results
come quickly and then increases the groupsize to as high as 1024.
The exact default numbers depend on the options given. For
efficiency reasons, Nmap uses larger group sizes for UDP or
few-port TCP scans.
When a maximum group size is specified with --max-hostgroup, Nmap
will never exceed that size. Specify a minimum size with
--min-hostgroup and Nmap will try to keep group sizes above that
level. Nmap may have to use smaller groups than you specify if
there are not enough target hosts left on a given interface to
fulfill the specified minimum. Both may be set to keep the group
size within a specific range, though this is rarely desired.
These options do not have an effect during the host discovery phase
of a scan. This includes plain ping scans (-sP). Host discovery
always works in large groups of hosts to improve speed and
accuracy.
The primary use of these options is to specify a large minimum
group size so that the full scan runs more quickly. A common choice
is 256 to scan a network in Class C sized chunks. For a scan with
many ports, exceeding that number is unlikely to help much. For
scans of just a few port numbers, host group sizes of 2048 or more
may be helpful.
--min-parallelism numprobes; --max-parallelism numprobes (Adjust probe
parallelization) .
These options control the total number of probes that may be
outstanding for a host group. They are used for port scanning and
host discovery. By default, Nmap calculates an ever-changing ideal
parallelism based on network performance. If packets are being
dropped, Nmap slows down and allows fewer outstanding probes. The
ideal probe number slowly rises as the network proves itself
worthy. These options place minimum or maximum bounds on that
variable. By default, the ideal parallelism can drop to one if the
network proves unreliable and rise to several hundred in perfect
conditions.
The most common usage is to set --min-parallelism to a number
higher than one to speed up scans of poorly performing hosts or
networks. This is a risky option to play with, as setting it too
high may affect accuracy. Setting this also reduces Nmap´s ability
to control parallelism dynamically based on network conditions. A
value of ten might be reasonable, though I only adjust this value
as a last resort.
The --max-parallelism option is sometimes set to one to prevent
Nmap from sending more than one probe at a time to hosts. The
--scan-delay option, discussed later, is another way to do this.
--min-rtt-timeout time, --max-rtt-timeout time, --initial-rtt-timeout
time (Adjust probe timeouts) .
Nmap maintains a running timeout value for determining how long it
will wait for a probe response before giving up or retransmitting
the probe. This is calculated based on the response times of
previous probes.
If the network latency shows itself to be significant and variable,
this timeout can grow to several seconds. It also starts at a
conservative (high) level and may stay that way for a while when
Nmap scans unresponsive hosts.
Specifying a lower --max-rtt-timeout and --initial-rtt-timeout than
the defaults can cut scan times significantly. This is particularly
true for pingless (-PN) scans, and those against heavily filtered
networks. Don´t get too aggressive though. The scan can end up
taking longer if you specify such a low value that many probes are
timing out and retransmitting while the response is in transit.
If all the hosts are on a local network, 100 milliseconds is a
reasonable aggressive --max-rtt-timeout value. If routing is
involved, ping a host on the network first with the ICMP ping
utility, or with a custom packet crafter such as hping2. that is
more likely to get through a firewall. Look at the maximum round
trip time out of ten packets or so. You might want to double that
for the --initial-rtt-timeout and triple or quadruple it for the
--max-rtt-timeout. I generally do not set the maximum RTT below
100 ms, no matter what the ping times are. Nor do I exceed 1000 ms.
--min-rtt-timeout is a rarely used option that could be useful when
a network is so unreliable that even Nmap´s default is too
aggressive. Since Nmap only reduces the timeout down to the minimum
when the network seems to be reliable, this need is unusual and
should be reported as a bug to the nmap-dev mailing list..
--max-retries numtries (Specify the maximum number of port scan probe
retransmissions) .
When Nmap receives no response to a port scan probe, it could mean
the port is filtered. Or maybe the probe or response was simply
lost on the network. It is also possible that the target host has
rate limiting enabled that temporarily blocked the response. So
Nmap tries again by retransmitting the initial probe. If Nmap
detects poor network reliability, it may try many more times before
giving up on a port. While this benefits accuracy, it also lengthen
scan times. When performance is critical, scans may be sped up by
limiting the number of retransmissions allowed. You can even
specify --max-retries 0 to prevent any retransmissions, though that
is only recommended for situations such as informal surveys where
occasional missed ports and hosts are acceptable.
The default (with no -T template) is to allow ten retransmissions.
If a network seems reliable and the target hosts aren´t rate
limiting, Nmap usually only does one retransmission. So most target
scans aren´t even affected by dropping --max-retries to a low value
such as three. Such values can substantially speed scans of slow
(rate limited) hosts. You usually lose some information when Nmap
gives up on ports early, though that may be preferable to letting
the --host-timeout expire and losing all information about the
target.
--host-timeout time (Give up on slow target hosts) .
Some hosts simply take a long time to scan. This may be due to
poorly performing or unreliable networking hardware or software,
packet rate limiting, or a restrictive firewall. The slowest few
percent of the scanned hosts can eat up a majority of the scan
time. Sometimes it is best to cut your losses and skip those hosts
initially. Specify --host-timeout with the maximum amount of time
you are willing to wait. For example, specify 30m to ensure that
Nmap doesn´t waste more than half an hour on a single host. Note
that Nmap may be scanning other hosts at the same time during that
half an hour, so it isn´t a complete loss. A host that times out is
skipped. No port table, OS detection, or version detection results
are printed for that host.
--scan-delay time; --max-scan-delay time (Adjust delay between probes)
.
This option causes Nmap to wait at least the given amount of time
between each probe it sends to a given host. This is particularly
useful in the case of rate limiting.. Solaris machines (among many
others) will usually respond to UDP scan probe packets with only
one ICMP message per second. Any more than that sent by Nmap will
be wasteful. A --scan-delay of 1s will keep Nmap at that slow rate.
Nmap tries to detect rate limiting and adjust the scan delay
accordingly, but it doesn´t hurt to specify it explicitly if you
already know what rate works best.
When Nmap adjusts the scan delay upward to cope with rate limiting,
the scan slows down dramatically. The --max-scan-delay option
specifies the largest delay that Nmap will allow. A low
--max-scan-delay can speed up Nmap, but it is risky. Setting this
value too low can lead to wasteful packet retransmissions and
possible missed ports when the target implements strict rate
limiting.
Another use of --scan-delay is to evade threshold based intrusion
detection and prevention systems (IDS/IPS)..
--min-rate number; --max-rate number (Directly control the scanning
rate) .
Nmap´s dynamic timing does a good job of finding an appropriate
speed at which to scan. Sometimes, however, you may happen to know
an appropriate scanning rate for a network, or you may have to
guarantee that a scan will be finished by a certain time. Or
perhaps you must keep Nmap from scanning too quickly. The
--min-rate and --max-rate options are designed for these
situations.
When the --min-rate option is given Nmap will do its best to send
packets as fast as or faster than the given rate. The argument is a
positive real number representing a packet rate in packets per
second. For example, specifying --min-rate 300 means that Nmap will
try to keep the sending rate at or above 300 packets per second.
Specifying a minimum rate does not keep Nmap from going faster if
conditions warrant.
Likewise, --max-rate limits a scan´s sending rate to a given
maximum. Use --max-rate 100, for example, to limit sending to 100
packets per second on a fast network. Use --max-rate 0.1 for a slow
scan of one packet every ten seconds. Use --min-rate and --max-rate
together to keep the rate inside a certain range.
These two options are global, affecting an entire scan, not
individual hosts. They only affect port scans and host discovery
scans. Other features like OS detection implement their own timing.
There are two conditions when the actual scanning rate may fall
below the requested minimum. The first is if the minimum is faster
than the fastest rate at which Nmap can send, which is dependent on
hardware. In this case Nmap will simply send packets as fast as
possible, but be aware that such high rates are likely to cause a
loss of accuracy. The second case is when Nmap has nothing to send,
for example at the end of a scan when the last probes have been
sent and Nmap is waiting for them to time out or be responded to.
It´s normal to see the scanning rate drop at the end of a scan or
in between hostgroups. The sending rate may temporarily exceed the
maximum to make up for unpredictable delays, but on average the
rate will stay at or below the maximum.
Specifying a minimum rate should be done with care. Scanning faster
than a network can support may lead to a loss of accuracy. In some
cases, using a faster rate can make a scan take longer than it
would with a slower rate. This is because Nmap´s
adaptive retransmission algorithms will detect the network
congestion caused by an excessive scanning rate and increase the
number of retransmissions in order to improve accuracy. So even
though packets are sent at a higher rate, more packets are sent
overall. Cap the number of retransmissions with the --max-retries
option if you need to set an upper limit on total scan time.
--defeat-rst-ratelimit .
Many hosts have long used rate limiting. to reduce the number of
ICMP error messages (such as port-unreachable errors) they send.
Some systems now apply similar rate limits to the RST (reset)
packets they generate. This can slow Nmap down dramatically as it
adjusts its timing to reflect those rate limits. You can tell Nmap
to ignore those rate limits (for port scans such as SYN scan which
don´t treat non-responsive ports as open) by specifying
--defeat-rst-ratelimit.
Using this option can reduce accuracy, as some ports will appear
non-responsive because Nmap didn´t wait long enough for a
rate-limited RST response. With a SYN scan, the non-response
results in the port being labeled filtered rather than the closed
state we see when RST packets are received. This option is useful
when you only care about open ports, and distinguishing between
closed and filtered ports isn´t worth the extra time.
-T paranoid|sneaky|polite|normal|aggressive|insane (Set a timing
template) .
While the fine-grained timing controls discussed in the previous
section are powerful and effective, some people find them
confusing. Moreover, choosing the appropriate values can sometimes
take more time than the scan you are trying to optimize. So Nmap
offers a simpler approach, with six timing templates. You can
specify them with the -T option and their number (0–5) or their
name. The template names are paranoid (0), sneaky (1), polite (2),
normal (3), aggressive (4), and insane (5). The first two are for
IDS evasion. Polite mode slows down the scan to use less bandwidth
and target machine resources. Normal mode is the default and so -T3
does nothing. Aggressive mode speeds scans up by making the
assumption that you are on a reasonably fast and reliable network.
Finally insane mode. assumes that you are on an extraordinarily
fast network or are willing to sacrifice some accuracy for speed.
These templates allow the user to specify how aggressive they wish
to be, while leaving Nmap to pick the exact timing values. The
templates also make some minor speed adjustments for which
fine-grained control options do not currently exist. For example,
-T4. prohibits the dynamic scan delay from exceeding 10 ms for TCP
ports and -T5 caps that value at 5 ms. Templates can be used in
combination with fine-grained controls, and the fine-grained
controls will you specify will take precedence over the timing
template default for that parameter. I recommend using -T4 when
scanning reasonably modern and reliable networks. Keep that option
even when you add fine-grained controls so that you benefit from
those extra minor optimizations that it enables.
If you are on a decent broadband or ethernet connection, I would
recommend always using -T4. Some people love -T5 though it is too
aggressive for my taste. People sometimes specify -T2 because they
think it is less likely to crash hosts or because they consider
themselves to be polite in general. They often don´t realize just
how slow -T polite. really is. Their scan may take ten times
longer than a default scan. Machine crashes and bandwidth problems
are rare with the default timing options (-T3) and so I normally
recommend that for cautious scanners. Omitting version detection is
far more effective than playing with timing values at reducing
these problems.
While -T0. and -T1. may be useful for avoiding IDS alerts, they
will take an extraordinarily long time to scan thousands of
machines or ports. For such a long scan, you may prefer to set the
exact timing values you need rather than rely on the canned -T0 and
-T1 values.
The main effects of T0 are serializing the scan so only one port is
scanned at a time, and waiting five minutes between sending each
probe. T1 and T2 are similar but they only wait 15 seconds and 0.4
seconds, respectively, between probes. T3 is Nmap´s default
behavior, which includes parallelization.. -T4 does the equivalent
of --max-rtt-timeout 1250 --initial-rtt-timeout 500 --max-retries 6
and sets the maximum TCP scan delay to 10 milliseconds. T5 does
the equivalent of --max-rtt-timeout 300 --min-rtt-timeout 50
--initial-rtt-timeout 250 --max-retries 2 --host-timeout 15m as
well as setting the maximum TCP scan delay to 5 ms.
FIREWALL/IDS EVASION AND SPOOFING
Many Internet pioneers envisioned a global open network with a
universal IP address space allowing virtual connections between any two
nodes. This allows hosts to act as true peers, serving and retrieving
information from each other. People could access all of their home
systems from work, changing the climate control settings or unlocking
the doors for early guests. This vision of universal connectivity has
been stifled by address space shortages and security concerns. In the
early 1990s, organizations began deploying firewalls for the express
purpose of reducing connectivity. Huge networks were cordoned off from
the unfiltered Internet by application proxies, network address
translation, and packet filters. The unrestricted flow of information
gave way to tight regulation of approved communication channels and the
content that passes over them.
Network obstructions such as firewalls can make mapping a network
exceedingly difficult. It will not get any easier, as stifling casual
reconnaissance is often a key goal of implementing the devices.
Nevertheless, Nmap offers many features to help understand these
complex networks, and to verify that filters are working as intended.
It even supports mechanisms for bypassing poorly implemented defenses.
One of the best methods of understanding your network security posture
is to try to defeat it. Place yourself in the mind-set of an attacker,
and deploy techniques from this section against your networks. Launch
an FTP bounce scan, idle scan, fragmentation attack, or try to tunnel
through one of your own proxies.
In addition to restricting network activity, companies are increasingly
monitoring traffic with intrusion detection systems (IDS). All of the
major IDSs ship with rules designed to detect Nmap scans because scans
are sometimes a precursor to attacks. Many of these products have
recently morphed into intrusion prevention systems (IPS). that
actively block traffic deemed malicious. Unfortunately for network
administrators and IDS vendors, reliably detecting bad intentions by
analyzing packet data is a tough problem. Attackers with patience,
skill, and the help of certain Nmap options can usually pass by IDSs
undetected. Meanwhile, administrators must cope with large numbers of
false positive results where innocent activity is misdiagnosed and
alerted on or blocked.
Occasionally people suggest that Nmap should not offer features for
evading firewall rules or sneaking past IDSs. They argue that these
features are just as likely to be misused by attackers as used by
administrators to enhance security. The problem with this logic is that
these methods would still be used by attackers, who would just find
other tools or patch the functionality into Nmap. Meanwhile,
administrators would find it that much harder to do their jobs.
Deploying only modern, patched FTP servers is a far more powerful
defense than trying to prevent the distribution of tools implementing
the FTP bounce attack.
There is no magic bullet (or Nmap option) for detecting and subverting
firewalls and IDS systems. It takes skill and experience. A tutorial is
beyond the scope of this reference guide, which only lists the relevant
options and describes what they do.
-f (fragment packets); --mtu (using the specified MTU) .
The -f option causes the requested scan (including ping scans) to
use tiny fragmented IP packets. The idea is to split up the TCP
header over several packets to make it harder for packet filters,
intrusion detection systems, and other annoyances to detect what
you are doing. Be careful with this! Some programs have trouble
handling these tiny packets. The old-school sniffer named Sniffit
segmentation faulted immediately upon receiving the first fragment.
Specify this option once, and Nmap splits the packets into eight
bytes or less after the IP header. So a 20-byte TCP header would be
split into three packets. Two with eight bytes of the TCP header,
and one with the final four. Of course each fragment also has an IP
header. Specify -f again to use 16 bytes per fragment (reducing the
number of fragments).. Or you can specify your own offset size
with the --mtu option. Don´t also specify -f if you use --mtu. The
offset must be a multiple of eight. While fragmented packets won´t
get by packet filters and firewalls that queue all IP fragments,
such as the CONFIG_IP_ALWAYS_DEFRAG option in the Linux kernel,
some networks can´t afford the performance hit this causes and thus
leave it disabled. Others can´t enable this because fragments may
take different routes into their networks. Some source systems
defragment outgoing packets in the kernel. Linux with the iptables.
connection tracking module is one such example. Do a scan while a
sniffer such as Wireshark. is running to ensure that sent packets
are fragmented. If your host OS is causing problems, try the
--send-eth. option to bypass the IP layer and send raw ethernet
frames.
Fragmentation is only supported for Nmap´s raw packet features,
which includes TCP and UDP port scans (except connect scan and FTP
bounce scan) and OS detection. Features such as version detection
and the Nmap Scripting Engine generally don´t support fragmentation
because they rely on your host´s TCP stack to communicate with
target services.
-D decoy1[,decoy2][,ME][,...] (Cloak a scan with decoys) .
Causes a decoy scan to be performed, which makes it appear to the
remote host that the host(s) you specify as decoys are scanning the
target network too. Thus their IDS might report 5–10 port scans
from unique IP addresses, but they won´t know which IP was scanning
them and which were innocent decoys. While this can be defeated
through router path tracing, response-dropping, and other active
mechanisms, it is generally an effective technique for hiding your
IP address.
Separate each decoy host with commas, and you can optionally use
ME. as one of the decoys to represent the position for your real
IP address. If you put ME in the sixth position or later, some
common port scan detectors (such as Solar Designer´s. excellent
Scanlogd). are unlikely to show your IP address at all. If you
don´t use ME, Nmap will put you in a random position. You can also
use RND. to generate a random, non-reserved IP address, or
RND:number to generate number addresses.
Note that the hosts you use as decoys should be up or you might
accidentally SYN flood your targets. Also it will be pretty easy to
determine which host is scanning if only one is actually up on the
network. You might want to use IP addresses instead of names (so
the decoy networks don´t see you in their nameserver logs).
Decoys are used both in the initial ping scan (using ICMP, SYN,
ACK, or whatever) and during the actual port scanning phase. Decoys
are also used during remote OS detection (-O). Decoys do not work
with version detection or TCP connect scan. When a scan delay is in
effect, the delay is enforced between each batch of spoofed probes,
not between each individual probe. Because decoys are sent as a
batch all at once, they may temporarily violate congestion control
limits.
It is worth noting that using too many decoys may slow your scan
and potentially even make it less accurate. Also, some ISPs will
filter out your spoofed packets, but many do not restrict spoofed
IP packets at all.
-S IP_Address (Spoof source address) .
In some circumstances, Nmap may not be able to determine your
source address (Nmap will tell you if this is the case). In this
situation, use -S with the IP address of the interface you wish to
send packets through.
Another possible use of this flag is to spoof the scan to make the
targets think that someone else is scanning them. Imagine a company
being repeatedly port scanned by a competitor! The -e option and
-PN are generally required for this sort of usage. Note that you
usually won´t receive reply packets back (they will be addressed to
the IP you are spoofing), so Nmap won´t produce useful reports.
-e interface (Use specified interface) .
Tells Nmap what interface to send and receive packets on. Nmap
should be able to detect this automatically, but it will tell you
if it cannot.
--source-port portnumber; -g portnumber (Spoof source port number) .
One surprisingly common misconfiguration is to trust traffic based
only on the source port number. It is easy to understand how this
comes about. An administrator will set up a shiny new firewall,
only to be flooded with complains from ungrateful users whose
applications stopped working. In particular, DNS may be broken
because the UDP DNS replies from external servers can no longer
enter the network. FTP is another common example. In active FTP
transfers, the remote server tries to establish a connection back
to the client to transfer the requested file.
Secure solutions to these problems exist, often in the form of
application-level proxies or protocol-parsing firewall modules.
Unfortunately there are also easier, insecure solutions. Noting
that DNS replies come from port 53 and active FTP from port 20,
many administrators have fallen into the trap of simply allowing
incoming traffic from those ports. They often assume that no
attacker would notice and exploit such firewall holes. In other
cases, administrators consider this a short-term stop-gap measure
until they can implement a more secure solution. Then they forget
the security upgrade.
Overworked network administrators are not the only ones to fall
into this trap. Numerous products have shipped with these insecure
rules. Even Microsoft has been guilty. The IPsec filters that
shipped with Windows 2000 and Windows XP contain an implicit rule
that allows all TCP or UDP traffic from port 88 (Kerberos). In
another well-known case, versions of the Zone Alarm personal
firewall up to 2.1.25 allowed any incoming UDP packets with the
source port 53 (DNS) or 67 (DHCP).
Nmap offers the -g and --source-port options (they are equivalent)
to exploit these weaknesses. Simply provide a port number and Nmap
will send packets from that port where possible. Nmap must use
different port numbers for certain OS detection tests to work
properly, and DNS requests ignore the --source-port flag because
Nmap relies on system libraries to handle those. Most TCP scans,
including SYN scan, support the option completely, as does UDP
scan.
--data-length number (Append random data to sent packets) .
Normally Nmap sends minimalist packets containing only a header. So
its TCP packets are generally 40 bytes and ICMP echo requests are
just 28. Some UDP ports. and IP protocols. get a custom payload
by default. This option tells Nmap to append the given number of
random bytes to most of the packets it sends, and not to use any
protocol-specific payloads. (Use --data-length 0 for no random or
protocol-specific payloads.. OS detection (-O) packets are not
affected. because accuracy there requires probe consistency, but
most pinging and portscan packets support this. It slows things
down a little, but can make a scan slightly less conspicuous.
--ip-options S|R [route]|L [route]|T|U ... ; --ip-options hex string
(Send packets with specified ip options) .
The IP protocol[13] offers several options which may be placed in
packet headers. Unlike the ubiquitous TCP options, IP options are
rarely seen due to practicality and security concerns. In fact,
many Internet routers block the most dangerous options such as
source routing. Yet options can still be useful in some cases for
determining and manipulating the network route to target machines.
For example, you may be able to use the record route option to
determine a path to a target even when more traditional
traceroute-style approaches fail. Or if your packets are being
dropped by a certain firewall, you may be able to specify a
different route with the strict or loose source routing options.
The most powerful way to specify IP options is to simply pass in
values as the argument to --ip-options. Precede each hex number
with /x then the two digits. You may repeat certain characters by
following them with an asterisk and then the number of times you
wish them to repeat. For example, /x01/x07/x04/x00*36/x01 is a hex
string containing 36 NUL bytes.
Nmap also offers a shortcut mechanism for specifying options.
Simply pass the letter R, T, or U to request record-route,.
record-timestamp,. or both options together, respectively. Loose
or strict source routing. may be specified with an L or S followed
by a space and then a space-separated list of IP addresses.
If you wish to see the options in packets sent and received,
specify --packet-trace. For more information and examples of using
IP options with Nmap, see
http://seclists.org/nmap-dev/2006/q3/0052.html.
--ttl value (Set IP time-to-live field) .
Sets the IPv4 time-to-live field in sent packets to the given
value.
--randomize-hosts (Randomize target host order) .
Tells Nmap to shuffle each group of up to 16384 hosts before it
scans them. This can make the scans less obvious to various network
monitoring systems, especially when you combine it with slow timing
options. If you want to randomize over larger group sizes, increase
PING_GROUP_SZ. in nmap.h. and recompile. An alternative solution
is to generate the target IP list with a list scan (-sL -n -oN
filename), randomize it with a Perl script, then provide the whole
list to Nmap with -iL..
--spoof-mac MAC address, prefix, or vendor name (Spoof MAC address) .
Asks Nmap to use the given MAC address for all of the raw ethernet
frames it sends. This option implies --send-eth. to ensure that
Nmap actually sends ethernet-level packets. The MAC given can take
several formats. If it is simply the number 0, Nmap chooses a
completely random MAC address for the session. If the given string
is an even number of hex digits (with the pairs optionally
separated by a colon), Nmap will use those as the MAC. If fewer
than 12 hex digits are provided, Nmap fills in the remainder of the
six bytes with random values. If the argument isn´t a zero or hex
string, Nmap looks through nmap-mac-prefixes to find a vendor name
containing the given string (it is case insensitive). If a match is
found, Nmap uses the vendor´s OUI (three-byte prefix). and fills
out the remaining three bytes randomly. Valid --spoof-mac argument
examples are Apple, 0, 01:02:03:04:05:06, deadbeefcafe, 0020F2, and
Cisco. This option only affects raw packet scans such as SYN scan
or OS detection, not connection-oriented features such as version
detection or the Nmap Scripting Engine.
--badsum (Send packets with bogus TCP/UDP checksums) .
Asks Nmap to use an invalid TCP, UDP or SCTP checksum for packets
sent to target hosts. Since virtually all host IP stacks properly
drop these packets, any responses received are likely coming from a
firewall or IDS that didn´t bother to verify the checksum. For more
details on this technique, see http://nmap.org/p60-12.html
--adler32 (Use deprecated Adler32 instead of CRC32C for SCTP checksums)
.
Asks Nmap to use the deprecated Adler32 algorithm for calculating
the SCTP checksum. If --adler32 is not given, CRC-32C (Castagnoli)
is used. RFC 2960[14] originally defined Adler32 as checksum
algorithm for SCTP; RFC 4960[7] later redefined the SCTP checksums
to use CRC-32C. Current SCTP implementations should be using
CRC-32C, but in order to elicit responses from old, legacy SCTP
implementations, it may be preferrable to use Adler32.
OUTPUT
Any security tool is only as useful as the output it generates. Complex
tests and algorithms are of little value if they aren´t presented in an
organized and comprehensible fashion. Given the number of ways Nmap is
used by people and other software, no single format can please
everyone. So Nmap offers several formats, including the interactive
mode for humans to read directly and XML for easy parsing by software.
In addition to offering different output formats, Nmap provides options
for controlling the verbosity of output as well as debugging messages.
Output types may be sent to standard output or to named files, which
Nmap can append to or clobber. Output files may also be used to resume
aborted scans.
Nmap makes output available in five different formats. The default is
called interactive output,. and it is sent to standard output
(stdout).. There is also normal output,. which is similar to
interactive except that it displays less runtime information and
warnings since it is expected to be analyzed after the scan completes
rather than interactively.
XML output. is one of the most important output types, as it can be
converted to HTML, easily parsed by programs such as Nmap graphical
user interfaces, or imported into databases.
The two remaining output types are the simple grepable output. which
includes most information for a target host on a single line, and
sCRiPt KiDDi3 0utPUt. for users who consider themselves |<-r4d.
While interactive output is the default and has no associated
command-line options, the other four format options use the same
syntax. They take one argument, which is the filename that results
should be stored in. Multiple formats may be specified, but each format
may only be specified once. For example, you may wish to save normal
output for your own review while saving XML of the same scan for
programmatic analysis. You might do this with the options -oX
myscan.xml -oN myscan.nmap. While this chapter uses the simple names
like myscan.xml for brevity, more descriptive names are generally
recommended. The names chosen are a matter of personal preference,
though I use long ones that incorporate the scan date and a word or two
describing the scan, placed in a directory named after the company I´m
scanning.
While these options save results to files, Nmap still prints
interactive output to stdout as usual. For example, the command nmap
-oX myscan.xml target prints XML to myscan.xml and fills standard
output with the same interactive results it would have printed if -oX
wasn´t specified at all. You can change this by passing a hyphen
character as the argument to one of the format types. This causes Nmap
to deactivate interactive output, and instead print results in the
format you specified to the standard output stream. So the command nmap
-oX - target will send only XML output to stdout.. Serious errors may
still be printed to the normal error stream, stderr..
Unlike some Nmap arguments, the space between the logfile option flag
(such as -oX) and the filename or hyphen is mandatory. If you omit the
flags and give arguments such as -oG- or -oXscan.xml, a backwards
compatibility feature of Nmap will cause the creation of normal format
output files named G- and Xscan.xml respectively.
All of these arguments support strftime-like. conversions in the
filename. %H, %M, %S, %m, %d, %y, and %Y are all exactly the same as
in strftime. %T is the same as %H%M%S, %R is the same as %H%M, and %D
is the same as %m%d%y. A % followed by any other character just yields
that character (%% gives you a percent symbol). So -oX ´scan-%T-%D.xml´
will use an XML file in the form of scan-144840-121307.xml.
Nmap also offers options to control scan verbosity and to append to
output files rather than clobbering them. All of these options are
described below.
Nmap Output Formats
-oN filespec (normal output) .
Requests that normal output be directed to the given filename. As
discussed above, this differs slightly from interactive output.
-oX filespec (XML output) .
Requests that XML output be directed to the given filename. Nmap
includes a document type definition (DTD) which allows XML parsers
to validate Nmap XML output. While it is primarily intended for
programmatic use, it can also help humans interpret Nmap XML
output. The DTD defines the legal elements of the format, and often
enumerates the attributes and values they can take on. The latest
version is always available from http://nmap.org/data/nmap.dtd.
XML offers a stable format that is easily parsed by software. Free
XML parsers are available for all major computer languages,
including C/C++, Perl, Python, and Java. People have even written
bindings for most of these languages to handle Nmap output and
execution specifically. Examples are Nmap::Scanner[15] and
Nmap::Parser[16] in Perl CPAN. In almost all cases that a
non-trivial application interfaces with Nmap, XML is the preferred
format.
The XML output references an XSL stylesheet which can be used to
format the results as HTML. The easiest way to use this is simply
to load the XML output in a web browser such as Firefox or IE. By
default, this will only work on the machine you ran Nmap on (or a
similarly configured one) due to the hard-coded nmap.xsl filesystem
path. Use the --webxml or --stylesheet options to create portable
XML files that render as HTML on any web-connected machine.
-oS filespec (ScRipT KIdd|3 oUTpuT) .
Script kiddie output is like interactive output, except that it is
post-processed to better suit the l33t HaXXorZ who previously
looked down on Nmap due to its consistent capitalization and
spelling. Humor impaired people should note that this option is
making fun of the script kiddies before flaming me for supposedly
“helping them”.
-oG filespec (grepable output) .
This output format is covered last because it is deprecated. The
XML output format is far more powerful, and is nearly as convenient
for experienced users. XML is a standard for which dozens of
excellent parsers are available, while grepable output is my own
simple hack. XML is extensible to support new Nmap features as they
are released, while I often must omit those features from grepable
output for lack of a place to put them.
Nevertheless, grepable output is still quite popular. It is a
simple format that lists each host on one line and can be trivially
searched and parsed with standard Unix tools such as grep, awk,
cut, sed, diff, and Perl. Even I usually use it for one-off tests
done at the command line. Finding all the hosts with the SSH port
open or that are running Solaris takes only a simple grep to
identify the hosts, piped to an awk or cut command to print the
desired fields.
Grepable output consists of comments (lines starting with a pound
(#)). and target lines. A target line includes a combination of
six labeled fields, separated by tabs and followed with a colon.
The fields are Host, Ports, Protocols, Ignored State, OS, Seq
Index, IP ID, and Status.
The most important of these fields is generally Ports, which gives
details on each interesting port. It is a comma separated list of
port entries. Each port entry represents one interesting port, and
takes the form of seven slash (/) separated subfields. Those
subfields are: Port number, State, Protocol, Owner, Service, SunRPC
info, and Version info.
As with XML output, this man page does not allow for documenting
the entire format. A more detailed look at the Nmap grepable output
format is available from
http://nmap.org/book/output-formats-grepable-output.html.
-oA basename (Output to all formats) .
As a convenience, you may specify -oA basename to store scan
results in normal, XML, and grepable formats at once. They are
stored in basename.nmap, basename.xml, and basename.gnmap,
respectively. As with most programs, you can prefix the filenames
with a directory path, such as ~/nmaplogs/foocorp/ on Unix or
c:/hacking/sco on Windows.
Verbosity and debugging options
-v (Increase verbosity level) .
Increases the verbosity level, causing Nmap to print more
information about the scan in progress. Open ports are shown as
they are found and completion time estimates are provided when Nmap
thinks a scan will take more than a few minutes. Use it twice or
more for even greater verbosity.
Most changes only affect interactive output, and some also affect
normal and script kiddie output. The other output types are meant
to be processed by machines, so Nmap can give substantial detail by
default in those formats without fatiguing a human user. However,
there are a few changes in other modes where output size can be
reduced substantially by omitting some detail. For example, a
comment line in the grepable output that provides a list of all
ports scanned is only printed in verbose mode because it can be
quite long.
-d [level] (Increase or set debugging level) .
When even verbose mode doesn´t provide sufficient data for you,
debugging is available to flood you with much more! As with the
verbosity option (-v), debugging is enabled with a command-line
flag (-d) and the debug level can be increased by specifying it
multiple times.. Alternatively, you can set a debug level by
giving an argument to -d. For example, -d9 sets level nine. That is
the highest effective level and will produce thousands of lines
unless you run a very simple scan with very few ports and targets.
Debugging output is useful when a bug is suspected in Nmap, or if
you are simply confused as to what Nmap is doing and why. As this
feature is mostly intended for developers, debug lines aren´t
always self-explanatory. You may get something like: Timeout vals:
srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==> srtt: 14987 rttvar:
14987 to: 100000. If you don´t understand a line, your only
recourses are to ignore it, look it up in the source code, or
request help from the development list (nmap-dev).. Some lines are
self explanatory, but the messages become more obscure as the debug
level is increased.
--reason (Host and port state reasons) .
Shows the reason each port is set to a specific state and the
reason each host is up or down. This option displays the type of
the packet that determined a port or hosts state. For example, A
RST packet from a closed port or an echo reply from an alive host.
The information Nmap can provide is determined by the type of scan
or ping. The SYN scan and SYN ping (-sS and -PS) are very detailed,
but the TCP connect scan (-sT) is limited by the implementation of
the connect system call. This feature is automatically enabled by
the debug option (-d). and the results are stored in XML log files
even if this option is not specified.
--stats-every time (Print periodic timing stats) .
Periodically prints a timing status message after each interval of
time. The time is a specification of the kind described in the
section called “TIMING AND PERFORMANCE”; so for example, use
--stats-every 10s to get a status update every 10 seconds. Updates
are printed to interactive output (the screen) and XML output.
--packet-trace (Trace packets and data sent and received) .
Causes Nmap to print a summary of every packet sent or received.
This is often used for debugging, but is also a valuable way for
new users to understand exactly what Nmap is doing under the
covers. To avoid printing thousands of lines, you may want to
specify a limited number of ports to scan, such as -p20-30. If you
only care about the goings on of the version detection subsystem,
use --version-trace instead. If you only care about script tracing,
specify --script-trace. With --packet-trace, you get all of the
above.
--open (Show only open (or possibly open) ports) .
Sometimes you only care about ports you can actually connect to
(open ones), and don´t want results cluttered with closed,
filtered, and closed|filtered ports. Output customization is
normally done after the scan using tools such as grep, awk, and
Perl, but this feature was added due to overwhelming requests.
Specify --open to only see open, open|filtered, and unfiltered
ports. These three ports are treated just as they normally are,
which means that open|filtered and unfiltered may be condensed into
counts if there are an overwhelming number of them.
--iflist (List interfaces and routes) .
Prints the interface list and system routes as detected by Nmap.
This is useful for debugging routing problems or device
mischaracterization (such as Nmap treating a PPP connection as
ethernet).
--log-errors (Log errors/warnings to normal mode output file) .
Warnings and errors printed by Nmap usually go only to the screen
(interactive output), leaving any normal-format output files
(usually specified with -oN) uncluttered. When you do want to see
those messages in the normal output file you specified, add this
option. It is useful when you aren´t watching the interactive
output or when you want to record errors while debugging a problem.
The error and warning messages will still appear in interactive
mode too. This won´t work for most errors related to bad
command-line arguments because Nmap may not have initialized its
output files yet. In addition, some Nmap error and warning messages
use a different system which does not yet support this option.
An alternative to --log-errors is redirecting interactive output
(including the standard error stream) to a file. Most Unix shells
make this approach easy, though it can be difficult on Windows.
Miscellaneous output options
--append-output (Append to rather than clobber output files) .
When you specify a filename to an output format flag such as -oX or
-oN, that file is overwritten by default. If you prefer to keep the
existing content of the file and append the new results, specify
the --append-output option. All output filenames specified in that
Nmap execution will then be appended to rather than clobbered. This
doesn´t work well for XML (-oX) scan data as the resultant file
generally won´t parse properly until you fix it up by hand.
--resume filename (Resume aborted scan) .
Some extensive Nmap runs take a very long time—on the order of
days. Such scans don´t always run to completion. Restrictions may
prevent Nmap from being run during working hours, the network could
go down, the machine Nmap is running on might suffer a planned or
unplanned reboot, or Nmap itself could crash. The administrator
running Nmap could cancel it for any other reason as well, by
pressing ctrl-C. Restarting the whole scan from the beginning may
be undesirable. Fortunately, if normal (-oN) or grepable (-oG) logs
were kept, the user can ask Nmap to resume scanning with the target
it was working on when execution ceased. Simply specify the
--resume option and pass the normal/grepable output file as its
argument. No other arguments are permitted, as Nmap parses the
output file to use the same ones specified previously. Simply call
Nmap as nmap --resume logfilename. Nmap will append new results to
the data files specified in the previous execution. Resumption does
not support the XML output format because combining the two runs
into one valid XML file would be difficult.
--stylesheet path or URL (Set XSL stylesheet to transform XML output) .
Nmap ships with an XSL stylesheet named nmap.xsl for viewing or
translating XML output to HTML. The XML output includes an
xml-stylesheet directive which points to nmap.xml where it was
initially installed by Nmap (or in the current working directory on
Windows). Simply load Nmap´s XML output in a modern web browser and
it should retrieve nmap.xsl from the filesystem and use it to
render results. If you wish to use a different stylesheet, specify
it as the argument to --stylesheet. You must pass the full pathname
or URL. One common invocation is --stylesheet
http://nmap.org/data/nmap.xsl. This tells a browser to load the
latest version of the stylesheet from Nmap.Org. The --webxml option
does the same thing with less typing and memorization. Loading the
XSL from Nmap.Org makes it easier to view results on a machine that
doesn´t have Nmap (and thus nmap.xsl) installed. So the URL is
often more useful, but the local filesystem location of nmap.xsl is
used by default for privacy reasons.
--webxml (Load stylesheet from Nmap.Org) .
This convenience option is simply an alias for --stylesheet
http://nmap.org/data/nmap.xsl.
--no-stylesheet (Omit XSL stylesheet declaration from XML) .
Specify this option to prevent Nmap from associating any XSL
stylesheet with its XML output. The xml-stylesheet directive is
omitted.
MISCELLANEOUS OPTIONS
This section describes some important (and not-so-important) options
that don´t really fit anywhere else.
-6 (Enable IPv6 scanning) .
Since 2002, Nmap has offered IPv6 support for its most popular
features. In particular, ping scanning (TCP-only), connect
scanning, and version detection all support IPv6. The command
syntax is the same as usual except that you also add the -6 option.
Of course, you must use IPv6 syntax if you specify an address
rather than a hostname. An address might look like
3ffe:7501:4819:2000:210:f3ff:fe03:14d0, so hostnames are
recommended. The output looks the same as usual, with the IPv6
address on the “interesting ports” line being the only IPv6 give
away.
While IPv6 hasn´t exactly taken the world by storm, it gets
significant use in some (usually Asian) countries and most modern
operating systems support it. To use Nmap with IPv6, both the
source and target of your scan must be configured for IPv6. If your
ISP (like most of them) does not allocate IPv6 addresses to you,
free tunnel brokers are widely available and work fine with Nmap. I
use the free IPv6 tunnel broker. service at
http://www.tunnelbroker.net. Other tunnel brokers are listed at
Wikipedia[17]. 6to4 tunnels are another popular, free approach.
-A (Aggressive scan options) .
This option enables additional advanced and aggressive options. I
haven´t decided exactly which it stands for yet. Presently this
enables OS detection (-O), version scanning (-sV), script scanning
(-sC) and traceroute (--traceroute). More features may be added in
the future. The point is to enable a comprehensive set of scan
options without people having to remember a large set of flags.
However, because script scanning with the default set is considered
intrusive, you should not use -A against target networks without
permission. This option only enables features, and not timing
options (such as -T4) or verbosity options (-v) that you might want
as well.
--datadir directoryname (Specify custom Nmap data file location) .
Nmap obtains some special data at runtime in files named
nmap-service-probes, nmap-services, nmap-protocols, nmap-rpc,
nmap-mac-prefixes, and nmap-os-db. If the location of any of these
files has been specified (using the --servicedb or --versiondb
options), that location is used for that file. After that, Nmap
searches these files in the directory specified with the --datadir
option (if any). Any files not found there, are searched for in the
directory specified by the NMAPDIR environmental variable.
~/.nmap. for real and effective UIDs (POSIX systems only) or
location of the Nmap executable (Win32 only), and then a
compiled-in location such as /usr/local/share/nmap or
/usr/share/nmap . As a last resort, Nmap will look in the current
directory.
--servicedb services file (Specify custom services file) .
Asks Nmap to use the specified services file rather than the
nmap-services data file that comes with Nmap. Using this option
also causes a fast scan (-F) to be used. See the description for
--datadir for more information on Nmap´s data files.
--versiondb service probes file (Specify custom service probes file) .
Asks Nmap to use the specified service probes file rather than the
nmap-service-probes data file that comes with Nmap. See the
description for --datadir for more information on Nmap´s data
files.
--send-eth (Use raw ethernet sending) .
Asks Nmap to send packets at the raw ethernet (data link) layer
rather than the higher IP (network) layer. By default, Nmap chooses
the one which is generally best for the platform it is running on.
Raw sockets (IP layer). are generally most efficient for Unix
machines, while ethernet frames are required for Windows operation
since Microsoft disabled raw socket support. Nmap still uses raw IP
packets on Unix despite this option when there is no other choice
(such as non-ethernet connections).
--send-ip (Send at raw IP level) .
Asks Nmap to send packets via raw IP sockets rather than sending
lower level ethernet frames. It is the complement to the --send-eth
option discussed previously.
--privileged (Assume that the user is fully privileged) .
Tells Nmap to simply assume that it is privileged enough to perform
raw socket sends, packet sniffing, and similar operations that
usually require root privileges. on Unix systems. By default Nmap
quits if such operations are requested but geteuid is not zero.
--privileged is useful with Linux kernel capabilities and similar
systems that may be configured to allow unprivileged users to
perform raw-packet scans. Be sure to provide this option flag
before any flags for options that require privileges (SYN scan, OS
detection, etc.). The NMAP_PRIVILEGED. environmental variable may
be set as an equivalent alternative to --privileged.
--unprivileged (Assume that the user lacks raw socket privileges) .
This option is the opposite of --privileged. It tells Nmap to treat
the user as lacking network raw socket and sniffing privileges.
This is useful for testing, debugging, or when the raw network
functionality of your operating system is somehow broken. The
NMAP_UNPRIVILEGED. environmental variable may be set as an
equivalent alternative to --unprivileged.
--release-memory (Release memory before quitting) .
This option is only useful for memory-leak debugging. It causes
Nmap to release allocated memory just before it quits so that
actual memory leaks are easier to spot. Normally Nmap skips this as
the OS does this anyway upon process termination.
--interactive (Start in interactive mode) .
Starts Nmap in interactive mode, which offers an interactive Nmap
prompt allowing easy launching of multiple scans (either
synchronously or in the background). This is useful for people who
scan from multi-user systems as they often want to test their
security without letting everyone else on the system know exactly
which systems they are scanning. Use --interactive to activate this
mode and then type h for help. This option is rarely used because
proper shells are usually more familiar and feature-complete. This
option includes a bang (!) operator for executing shell commands,
which is one of many reasons not to install Nmap setuid root..
-V; --version (Print version number) .
Prints the Nmap version number and exits.
-h; --help (Print help summary page) .
Prints a short help screen with the most common command flags.
Running Nmap without any arguments does the same thing.
RUNTIME INTERACTION
During the execution of Nmap, all key presses are captured. This allows
you to interact with the program without aborting and restarting it.
Certain special keys will change options, while any other keys will
print out a status message telling you about the scan. The convention
is that lowercase letters increase the amount of printing, and
uppercase letters decrease the printing. You may also press ‘?’ for
help.
v / V
Increase / decrease the verbosity level
d / D
Increase / decrease the debugging Level
p / P
Turn on / off packet tracing
?
Print a runtime interaction help screen
Anything else
Print out a status message like this:
Stats: 0:00:08 elapsed; 111 hosts completed (5 up), 5 undergoing
Service Scan
Service scan Timing: About 28.00% done; ETC: 16:18 (0:00:15
remaining)
EXAMPLES
Here are some Nmap usage examples, from the simple and routine to a
little more complex and esoteric. Some actual IP addresses and domain
names are used to make things more concrete. In their place you should
substitute addresses/names from your own network.. While I don´t think
port scanning other networks is or should be illegal, some network
administrators don´t appreciate unsolicited scanning of their networks
and may complain. Getting permission first is the best approach.
For testing purposes, you have permission to scan the host
scanme.nmap.org. This permission only includes scanning via Nmap and
not testing exploits or denial of service attacks. To conserve
bandwidth, please do not initiate more than a dozen scans against that
host per day. If this free scanning target service is abused, it will
be taken down and Nmap will report Failed to resolve given hostname/IP:
scanme.nmap.org. These permissions also apply to the hosts
scanme2.nmap.org, scanme3.nmap.org, and so on, though those hosts do
not currently exist.
nmap -v scanme.nmap.org
This option scans all reserved TCP ports on the machine scanme.nmap.org
. The -v option enables verbose mode.
nmap -sS -O scanme.nmap.org/24
Launches a stealth SYN scan against each machine that is up out of the
256 IPs on “class C” sized network where Scanme resides. It also tries
to determine what operating system is running on each host that is up
and running. This requires root privileges because of the SYN scan and
OS detection.
nmap -sV -p 22,53,110,143,4564 198.116.0-255.1-127
Launches host enumeration and a TCP scan at the first half of each of
the 255 possible eight-bit subnets in the 198.116 class B address
space. This tests whether the systems run SSH, DNS, POP3, or IMAP on
their standard ports, or anything on port 4564. For any of these ports
found open, version detection is used to determine what application is
running.
nmap -v -iR 100000 -PN -p 80
Asks Nmap to choose 100,000 hosts at random and scan them for web
servers (port 80). Host enumeration is disabled with -PN since first
sending a couple probes to determine whether a host is up is wasteful
when you are only probing one port on each target host anyway.
nmap -PN -p80 -oX logs/pb-port80scan.xml -oG logs/pb-port80scan.gnmap
216.163.128.20/20
This scans 4096 IPs for any web servers (without pinging them) and
saves the output in grepable and XML formats.
NMAP BOOK
While this reference guide details all material Nmap options, it can´t
fully demonstrate how to apply those features to quickly solve
real-world tasks. For that, we released Nmap Network Scanning: The
Official Nmap Project Guide to Network Discovery and Security Scanning.
Topics include subverting firewalls and intrusion detection systems,
optimizing Nmap performance, and automating common networking tasks
with the Nmap Scripting Engine. Hints and instructions are provided for
common Nmap tasks such as taking network inventory, penetration
testing, detecting rogue wireless access points, and quashing network
worm outbreaks. Examples and diagrams show actual communication on the
wire. More than half of the book is available free online. See
http://nmap.org/book for more information.
BUGS
Like its author, Nmap isn´t perfect. But you can help make it better by
sending bug reports or even writing patches. If Nmap doesn´t behave the
way you expect, first upgrade to the latest version available from
http://nmap.org. If the problem persists, do some research to determine
whether it has already been discovered and addressed. Try searching for
the error message on our search page at http://insecure.org/search.html
or at Google. Also try browsing the nmap-dev archives at
http://seclists.org/.. Read this full manual page as well. If nothing
comes of this, mail a bug report to nmap-dev@insecure.org. Please
include everything you have learned about the problem, as well as what
version of Nmap you are running and what operating system version it is
running on. Problem reports and Nmap usage questions sent to
nmap-dev@insecure.org are far more likely to be answered than those
sent to Fyodor directly. If you subscribe to the nmap-dev list before
posting, your message will bypass moderation and get through more
quickly. Subscribe at
http://cgi.insecure.org/mailman/listinfo/nmap-dev.
Code patches to fix bugs are even better than bug reports. Basic
instructions for creating patch files with your changes are available
at http://nmap.org/data/HACKING. Patches may be sent to nmap-dev
(recommended) or to Fyodor directly.
AUTHOR
Gordon “Fyodor” Lyon fyodor@insecure.org (http://insecure.org)
Hundreds of people have made valuable contributions to Nmap over the
years. These are detailed in the CHANGELOG. file which is distributed
with Nmap and also available from http://nmap.org/changelog.html.
LEGAL NOTICES
Nmap Copyright and Licensing
The Nmap Security Scanner is (C) 1996–2009 Insecure.Com LLC. Nmap is
also a registered trademark of Insecure.Com LLC. This program is free
software; you may redistribute and/or modify it under the terms of the
GNU General Public License as published by the Free Software
Foundation; Version 2 with the clarifications and exceptions described
below. This guarantees your right to use, modify, and redistribute this
software under certain conditions. If you wish to embed Nmap technology
into proprietary software, we sell alternative licenses (contact
sales@insecure.com). Dozens of software vendors already license Nmap
technology such as host discovery, port scanning, OS detection, and
version detection.
Note that the GPL places important restrictions on “derived works”, yet
it does not provide a detailed definition of that term. To avoid
misunderstandings, we consider an application to constitute a
“derivative work” for the purpose of this license if it does any of the
following:
· Integrates source code from Nmap
· Reads or includes Nmap copyrighted data files, such as nmap-os-db
or nmap-service-probes.
· Executes Nmap and parses the results (as opposed to typical shell
or execution-menu apps, which simply display raw Nmap output and so
are not derivative works.)
· Integrates/includes/aggregates Nmap into a proprietary executable
installer, such as those produced by InstallShield.
· Links to a library or executes a program that does any of the
above.
The term “Nmap” should be taken to also include any portions or derived
works of Nmap. This list is not exclusive, but is meant to clarify our
interpretation of derived works with some common examples. Our
interpretation applies only to Nmap—we don´t speak for other people´s
GPL works.
If you have any questions about the GPL licensing restrictions on using
Nmap in non-GPL works, we would be happy to help. As mentioned above,
we also offer alternative license to integrate Nmap into proprietary
applications and appliances. These contracts have been sold to many
security vendors, and generally include a perpetual license as well as
providing for priority support and updates as well as helping to fund
the continued development of Nmap technology. Please email
sales@insecure.com for further information.
As a special exception to the GPL terms, Insecure.Com LLC grants
permission to link the code of this program with any version of the
OpenSSL library which is distributed under a license identical to that
listed in the included COPYING.OpenSSL file, and distribute linked
combinations including the two.. You must obey the GNU GPL in all
respects for all of the code used other than OpenSSL. If you modify
this file, you may extend this exception to your version of the file,
but you are not obligated to do so.
If you received these files with a written license agreement or
contract stating terms other than the terms above, then that
alternative license agreement takes precedence over these comments.
Creative Commons License for this Nmap Guide
This Nmap Reference Guide is (C) 2005–2009 Insecure.Com LLC. It is
hereby placed under version 3.0 of the Creative Commons Attribution
License[18]. This allows you redistribute and modify the work as you
desire, as long as you credit the original source. Alternatively, you
may choose to treat this document as falling under the same license as
Nmap itself (discussed previously).
Source Code Availability and Community Contributions
Source is provided to this software because we believe users have a
right to know exactly what a program is going to do before they run it.
This also allows you to audit the software for security holes (none
have been found so far).
Source code also allows you to port Nmap to new platforms, fix bugs,
and add new features. You are highly encouraged to send your changes to
nmap-dev@insecure.org for possible incorporation into the main
distribution. By sending these changes to Fyodor or one of the
Insecure.Org development mailing lists, it is assumed that you are
offering the Nmap Project (Insecure.Com LLC) the unlimited,
non-exclusive right to reuse, modify, and relicense the code. Nmap will
always be available Open Source,. but this is important because the
inability to relicense code has caused devastating problems for other
Free Software projects (such as KDE and NASM). We also occasionally
relicense the code to third parties as discussed above. If you wish to
specify special license conditions of your contributions, just say so
when you send them.
No Warranty.
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License v2.0 for more details at
http://www.gnu.org/licenses/gpl-2.0.html, or in the COPYING file
included with Nmap.
It should also be noted that Nmap has occasionally been known to crash
poorly written applications, TCP/IP stacks, and even operating
systems.. While this is extremely rare, it is important to keep in
mind. Nmap should never be run against mission critical systems unless
you are prepared to suffer downtime. We acknowledge here that Nmap may
crash your systems or networks and we disclaim all liability for any
damage or problems Nmap could cause.
Inappropriate Usage
Because of the slight risk of crashes and because a few black hats like
to use Nmap for reconnaissance prior to attacking systems, there are
administrators who become upset and may complain when their system is
scanned. Thus, it is often advisable to request permission before doing
even a light scan of a network.
Nmap should never be installed with special privileges (e.g. suid root)
for security reasons..
Third-Party Software
This product includes software developed by the Apache Software
Foundation[19]. A modified version of the Libpcap portable packet
capture library[20]. is distributed along with Nmap. The Windows
version of Nmap utilized the Libpcap-derived WinPcap library[21].
instead. Regular expression support is provided by the PCRE
library[22],. which is open-source software, written by Philip Hazel..
Certain raw networking functions use the Libdnet[23]. networking
library, which was written by Dug Song.. A modified version is
distributed with Nmap. Nmap can optionally link with the OpenSSL
cryptography toolkit[24]. for SSL version detection support. The Nmap
Scripting Engine uses an embedded version of the Lua programming
language[25].. All of the third-party software described in this
paragraph is freely redistributable under BSD-style software licenses.
United States Export Control.
Nmap only uses encryption when compiled with the optional OpenSSL
support and linked with OpenSSL. When compiled without OpenSSL support,
Insecure.Com LLC believes that Nmap is not subject to U.S. Export
Administration Regulations (EAR)[26] export control. As such, there is
no applicable ECCN (explort control classification number) and
exportation does not require any special license, permit, or other
governmental authorization.
When compiled with OpenSSL support or distributed as source code,
Insecure.Com LLC believes that Nmap falls under U.S. ECCN 5D002[27]
(“Information Security Software”). We distribute Nmap under the TSU
exception for publicly available encryption software defined in EAR
740.13(e)[28].
NOTES
1. Nmap Network Scanning: The Official Nmap Project Guide to Network
Discovery and Security Scanning
http://nmap.org/book/
2. RFC 1122
http://www.rfc-editor.org/rfc/rfc1122.txt
3. RFC 792
http://www.rfc-editor.org/rfc/rfc792.txt
4. RFC 950
http://www.rfc-editor.org/rfc/rfc950.txt
5. RFC 1918
http://www.rfc-editor.org/rfc/rfc1918.txt
6. UDP
http://www.rfc-editor.org/rfc/rfc768.txt
7. SCTP
http://www.rfc-editor.org/rfc/rfc4960.txt
8. TCP RFC
http://www.rfc-editor.org/rfc/rfc793.txt
9. RFC 959
http://www.rfc-editor.org/rfc/rfc959.txt
10. RFC 1323
http://www.rfc-editor.org/rfc/rfc1323.txt
11. Lua programming language
http://lua.org
12. precedence
http://www.lua.org/manual/5.1/manual.html#2.5.3
13. IP protocol
http://www.rfc-editor.org/rfc/rfc791.txt
14. RFC 2960
http://www.rfc-editor.org/rfc/rfc2960.txt
15. Nmap::Scanner
http://sourceforge.net/projects/nmap-scanner/
16. Nmap::Parser
http://nmapparser.wordpress.com/
17. listed at Wikipedia
http://en.wikipedia.org/wiki/List_of_IPv6_tunnel_brokers
18. Creative Commons Attribution License
http://creativecommons.org/licenses/by/3.0/
19. Apache Software Foundation
http://www.apache.org
20. Libpcap portable packet capture library
http://www.tcpdump.org
21. WinPcap library
http://www.winpcap.org
22. PCRE library
http://www.pcre.org
23. Libdnet
http://libdnet.sourceforge.net
24. OpenSSL cryptography toolkit
http://www.openssl.org
25. Lua programming language
http://www.lua.org
26. Export Administration Regulations (EAR)
http://www.access.gpo.gov/bis/ear/ear_data.html
27. 5D002
http://www.access.gpo.gov/bis/ear/pdf/ccl5-pt2.pdf
28. EAR 740.13(e)
http://www.access.gpo.gov/bis/ear/pdf/740.pdf
Nmap 01/26/2010 NMAP(1)
(责任编辑：IT)
| 摘要 nmap是一个网络探测和安全扫描程序，系统管理者和个人可以使用这个软件扫描大型的网络，获取那台主机正在运行以及提供什么服务等信息。nmap支持很多扫描技术，例如：UDP、TCP connect()、TCP SYN(半开扫描)、ftp代理(bounce攻击)、反向标志、ICMP、FIN、ACK扫描、圣诞树(Xmas Tree)、SYN扫描和null扫描。从扫描类型一节可以得到细节。nmap还提供了一些高级的特征，例如：通过TCP/IP协议栈特征探测操作系统类型，秘密扫描，动态延时和重传计算，并行扫描，通过并行ping扫描探测关闭的主机，诱饵扫描，避开端口过滤检测，直接RPC扫描(无须端口影射)，碎片扫描，以及灵活的目标和端口设定. -------------------------------------------------------------------------------- 1.名称 nmap-网络探测和安全扫描工具 2.语法 nmap [Scan Type(s)] [Options] 3.描述 nmap是一个网络探测和安全扫描程序，系统管理者和个人可以使用这个软件扫描大型的网络，获取那台主机正在运行以及提供什么服务等信息。nmap支持很多扫描技术，例如：UDP、TCP connect()、TCP SYN(半开扫描)、ftp代理(bounce攻击)、反向标志、ICMP、FIN、ACK扫描、圣诞树(Xmas Tree)、SYN扫描和null扫描。从扫描类型一节可以得到细节。nmap还提供了一些高级的特征，例如：通过TCP/IP协议栈特征探测操作系统类型，秘密扫描，动态延时和重传计算，并行扫描，通过并行ping扫描探测关闭的主机，诱饵扫描，避开端口过滤检测，直接RPC扫描(无须端口影射)，碎片扫描，以及灵活的目标和端口设定。 为了提高nmap在non-root状态下的性能，软件的设计者付出了很大的努力。很不幸，一些内核界面(例如raw socket)需要在root状态下使用。所以应该尽可能在root使用nmap。 nmap运行通常会得到被扫描主机端口的列表。nmap总会给出well known端口的服务名(如果可能)、端口号、状态和协议等信息。每个端口的状态有：open、filtered、unfiltered。open状态意味着目标主机能够在这个端口使用accept()系统调用接受连接。filtered状态表示：防火墙、包过滤和其它的网络安全软件掩盖了这个端口，禁止 nmap探测其是否打开。unfiltered表示：这个端口关闭，并且没有防火墙/包过滤软件来隔离nmap的探测企图。通常情况下，端口的状态基本都是unfiltered状态，只有在大多数被扫描的端口处于filtered状态下，才会显示处于unfiltered状态的端口。 根据使用的功能选项，nmap也可以报告远程主机的下列特征：使用的操作系统、TCP序列、运行绑定到每个端口上的应用程序的用户名、DNS名、主机地址是否是欺骗地址、以及其它一些东西。 4.功能选项 功能选项可以组合使用。一些功能选项只能够在某种扫描模式下使用。nmap会自动识别无效或者不支持的功能选项组合，并向用户发出警告信息。 如果你是有经验的用户，可以略过结尾的示例一节。可以使用nmap -h快速列出功能选项的列表。 4.1 扫描类型 -sT TCP connect()扫描：这是最基本的TCP扫描方式。connect()是一种系统调用，由操作系统提供，用来打开一个连接。如果目标端口有程序监听， connect()就会成功返回，否则这个端口是不可达的。这项技术最大的优点是，你勿需root权限。任何UNIX用户都可以自由使用这个系统调用。这种扫描很容易被检测到，在目标主机的日志中会记录大批的连接请求以及错误信息。 -sS TCP同步扫描(TCP SYN)：因为不必全部打开一个TCP连接，所以这项技术通常称为半开扫描(half-open)。你可以发出一个TCP同步包(SYN)，然后等待回应。如果对方返回SYN|ACK(响应)包就表示目标端口正在监听；如果返回RST数据包，就表示目标端口没有监听程序；如果收到一个SYN|ACK包，源主机就会马上发出一个RST(复位)数据包断开和目标主机的连接，这实际上有我们的操作系统内核自动完成的。这项技术最大的好处是，很少有系统能够把这记入系统日志。不过，你需要root权限来定制SYN数据包。 -sF -sF -sN 秘密FIN数据包扫描、圣诞树(Xmas Tree)、空(Null)扫描模式：即使SYN扫描都无法确定的情况下使用。一些防火墙和包过滤软件能够对发送到被限制端口的SYN数据包进行监视，而且有些程序比如synlogger和courtney能够检测那些扫描。这些高级的扫描方式可以逃过这些干扰。这些扫描方式的理论依据是：关闭的端口需要对你的探测包回应RST包，而打开的端口必需忽略有问题的包(参考RFC 793第64页)。FIN扫描使用暴露的FIN数据包来探测，而圣诞树扫描打开数据包的FIN、URG和PUSH标志。不幸的是，微软决定完全忽略这个标准，另起炉灶。所以这种扫描方式对Windows95/NT无效。不过，从另外的角度讲，可以使用这种方式来分别两种不同的平台。如果使用这种扫描方式可以发现打开的端口，你就可以确定目标注意运行的不是Windows系统。如果使用-sF、-sX或者-sN扫描显示所有的端口都是关闭的，而使用SYN扫描显示有打开的端口，你可以确定目标主机可能运行的是Windwos系统。现在这种方式没有什么太大的用处，因为nmap有内嵌的操作系统检测功能。还有其它几个系统使用和windows同样的处理方式，包括Cisco、BSDI、HP/UX、MYS、IRIX。在应该抛弃数据包时，以上这些系统都会从打开的端口发出复位数据包。 -sP ping扫描：有时你只是想知道此时网络上哪些主机正在运行。通过向你指定的网络内的每个IP地址发送ICMP echo请求数据包，nmap就可以完成这项任务。如果主机正在运行就会作出响应。不幸的是，一些站点例如：microsoft.com阻塞ICMP echo请求数据包。然而，在默认的情况下nmap也能够向80端口发送TCP ack包，如果你收到一个RST包，就表示主机正在运行。nmap使用的第三种技术是：发送一个SYN包，然后等待一个RST或者SYN/ACK包。对于非root用户，nmap使用connect()方法。 在默认的情况下(root用户)，nmap并行使用ICMP和ACK技术。 注意，nmap在任何情况下都会进行ping扫描，只有目标主机处于运行状态，才会进行后续的扫描。如果你只是想知道目标主机是否运行，而不想进行其它扫描，才会用到这个选项。 -sU UDP扫描：如果你想知道在某台主机上提供哪些UDP(用户数据报协议,RFC768)服务，可以使用这种扫描方法。nmap首先向目标主机的每个端口发出一个0字节的UDP包，如果我们收到端口不可达的ICMP消息，端口就是关闭的，否则我们就假设它是打开的。 有些人可能会想UDP扫描是没有什么意思的。但是，我经常会想到最近出现的solaris rpcbind缺陷。rpcbind隐藏在一个未公开的UDP端口上，这个端口号大于32770。所以即使端口111(portmap的众所周知端口号) 被防火墙阻塞有关系。但是你能发现大于30000的哪个端口上有程序正在监听吗?使用UDP扫描就能！cDc Back Orifice的后门程序就隐藏在Windows主机的一个可配置的UDP端口中。不考虑一些通常的安全缺陷，一些服务例如:snmp、tftp、NFS 使用UDP协议。不幸的是，UDP扫描有时非常缓慢，因为大多数主机限制ICMP错误信息的比例(在RFC1812中的建议)。例如，在Linux内核中 (在net/ipv4/icmp.h文件中)限制每4秒钟只能出现80条目标不可达的ICMP消息，如果超过这个比例，就会给1/4秒钟的处罚。 solaris的限制更加严格，每秒钟只允许出现大约2条ICMP不可达消息，这样，使扫描更加缓慢。nmap会检测这个限制的比例，减缓发送速度，而不是发送大量的将被目标主机丢弃的无用数据包。 不过Micro$oft忽略了RFC1812的这个建议，不对这个比例做任何的限制。所以我们可以能够快速扫描运行Win95/NT的主机上的所有65K个端口。 -sA ACK扫描：这项高级的扫描方法通常用来穿过防火墙的规则集。通常情况下，这有助于确定一个防火墙是功能比较完善的或者是一个简单的包过滤程序，只是阻塞进入的SYN包。 这种扫描是向特定的端口发送ACK包(使用随机的应答/序列号)。如果返回一个RST包，这个端口就标记为unfiltered状态。如果什么都没有返回，或者返回一个不可达ICMP消息，这个端口就归入filtered类。注意，nmap通常不输出unfiltered的端口，所以在输出中通常不显示所有被探测的端口。显然，这种扫描方式不能找出处于打开状态的端口。 -sW 对滑动窗口的扫描：这项高级扫描技术非常类似于ACK扫描，除了它有时可以检测到处于打开状态的端口，因为滑动窗口的大小是不规则的，有些操作系统可以报告其大小。这些系统至少包括：某些版本的AIX、Amiga、BeOS、BSDI、Cray、Tru64 UNIX、DG/UX、OpenVMS、Digital UNIX、OpenBSD、OpenStep、QNX、Rhapsody、SunOS 4.x、Ultrix、VAX、VXWORKS。从nmap-hackers邮件3列表的文档中可以得到完整的列表。 -sR RPC扫描。这种方法和nmap的其它不同的端口扫描方法结合使用。选择所有处于打开状态的端口向它们发出SunRPC程序的NULL命令，以确定它们是否是RPC端口，如果是，就确定是哪种软件及其版本号。因此你能够获得防火墙的一些信息。诱饵扫描现在还不能和RPC扫描结合使用。 -b FTP反弹攻击(bounce attack):FTP协议(RFC 959)有一个很有意思的特征，它支持代理FTP连接。也就是说，我能够从evil.com连接到FTP服务器target.com，并且可以要求这台 FTP服务器为自己发送Internet上任何地方的文件！1985年，RFC959完成时，这个特征就能很好地工作了。然而，在今天的Internet 中，我们不能让人们劫持FTP服务器，让它向Internet上的任意节点发送数据。如同Hobbit在1995年写的文章中所说的，这个协议"能够用来做投递虚拟的不可达邮件和新闻，进入各种站点的服务器,填满硬盘，跳过防火墙，以及其它的骚扰活动，而且很难进行追踪"。我们可以使用这个特征，在一台代理FTP服务器扫描TCP端口。因此，你需要连接到防火墙后面的一台FTP服务器，接着进行端口扫描。如果在这台FTP服务器中有可读写的目录，你还可以向目标端口任意发送数据(不过nmap不能为你做这些)。 传递给-b功能选项的参数是你要作为代理的FTP服务器。语法格式为： -b username:password@server:port。 除了server以外，其余都是可选的。如果你想知道什么服务器有这种缺陷，可以参考我在Phrack 51发表的文章。还可以在nmap的站点得到这篇文章的最新版本。 4.2 通用选项 这些内容不是必需的，但是很有用。 -P0 在扫描之前，不必ping主机。有些网络的防火墙不允许ICMP echo请求穿过，使用这个选项可以对这些网络进行扫描。microsoft.com就是一个例子，因此在扫描这个站点时，你应该一直使用-P0或者-PT 80选项。 -PT 扫描之前，使用TCP ping确定哪些主机正在运行。nmap不是通过发送ICMP echo请求包然后等待响应来实现这种功能，而是向目标网络(或者单一主机)发出TCP ACK包然后等待回应。如果主机正在运行就会返回RST包。只有在目标网络/主机阻塞了ping包，而仍旧允许你对其进行扫描时，这个选项才有效。对于非 root用户，我们使用connect()系统调用来实现这项功能。使用-PT <端口号>来设定目标端口。默认的端口号是80，因为这个端口通常不会被过滤。 -PS 对于root用户，这个选项让nmap使用SYN包而不是ACK包来对目标主机进行扫描。如果主机正在运行就返回一个RST包(或者一个SYN/ACK包)。 -PI 设置这个选项，让nmap使用真正的ping(ICMP echo请求)来扫描目标主机是否正在运行。使用这个选项让nmap发现正在运行的主机的同时，nmap也会对你的直接子网广播地址进行观察。直接子网广播地址一些外部可达的IP地址，把外部的包转换为一个内向的IP广播包，向一个计算机子网发送。这些IP广播包应该删除，因为会造成拒绝服务攻击(例如 smurf)。 -PB 这是默认的ping扫描选项。它使用ACK(-PT)和ICMP(-PI)两种扫描类型并行扫描。如果防火墙能够过滤其中一种包，使用这种方法，你就能够穿过防火墙。 -O 这个选项激活对TCP/IP指纹特征(fingerprinting)的扫描，获得远程主机的标志。换句话说，nmap使用一些技术检测目标主机操作系统网络协议栈的特征。nmap使用这些信息建立远程主机的指纹特征，把它和已知的操作系统指纹特征数据库做比较，就可以知道目标主机操作系统的类型。 -I 这个选项打开nmap的反向标志扫描功能。Dave Goldsmith 1996年向bugtap发出的邮件注意到这个协议，ident协议(rfc 1413)允许使用TCP连接给出任何进程拥有者的用户名，即使这个进程并没有初始化连接。例如，你可以连接到HTTP端口，接着使用identd确定这个服务器是否由root用户运行。这种扫描只能在同目标端口建立完全的TCP连接时(例如：-sT扫描选项)才能成功。使用-I选项是，远程主机的 identd精灵进程就会查询在每个打开的端口上监听的进程的拥有者。显然，如果远程主机没有运行identd程序，这种扫描方法无效。 -f 这个选项使nmap使用碎片IP数据包发送SYN、FIN、XMAS、NULL。使用碎片数据包增加包过滤、入侵检测系统的难度，使其无法知道你的企图。不过，要慎重使用这个选项！有些程序在处理这些碎片包时会有麻烦，我最喜欢的嗅探器在接受到碎片包的头36个字节时，就会发生 segmentation faulted。因此，在nmap中使用了24个字节的碎片数据包。虽然包过滤器和防火墙不能防这种方法，但是有很多网络出于性能上的考虑，禁止数据包的分片。 注意这个选项不能在所有的平台上使用。它在Linux、FreeBSD、OpenBSD以及其它一些UNIX系统能够很好工作。 -v 冗余模式。强烈推荐使用这个选项，它会给出扫描过程中的详细信息。使用这个选项，你可以得到事半功倍的效果。使用-d选项可以得到更加详细的信息。 -h 快速参考选项。 -oN 把扫描结果重定向到一个可读的文件logfilename中。 -oM 把扫描结果重定向到logfilename文件中，这个文件使用主机可以解析的语法。你可以使用-oM -来代替logfilename，这样输出就被重定向到标准输出stdout。在这种情况下，正常的输出将被覆盖，错误信息荏苒可以输出到标准错误 stderr。要注意，如果同时使用了-v选项，在屏幕上会打印出其它的信息。 -oS thIs l0gz th3 r3suLtS of YouR ScanZ iN a s| THe fiL3 U sPecfy 4s an arGuMEnT! U kAn gIv3 the 4rgument - (wItHOUt qUOteZ) to sh00t output iNT0 stDouT!@!! 莫名其妙，下面是我猜着翻译的，相形字？ 把扫描结果重定向到一个文件logfilename中，这个文件使用一种"黑客方言"的语法形式(作者开的玩笑?)。同样，使用-oS -就会把结果重定向到标准输出上。 -resume 某个网络扫描可能由于control-C或者网络损失等原因被中断，使用这个选项可以使扫描接着以前的扫描进行。logfilename是被取消扫描的日志文件，它必须是可读形式或者机器可以解析的形式。而且接着进行的扫描不能增加新的选项，只能使用与被中断的扫描相同的选项。nmap会接着日志文件中的最后一次成功扫描进行新的扫描。 -iL 从inputfilename文件中读取扫描的目标。在这个文件中要有一个主机或者网络的列表，由空格键、制表键或者回车键作为分割符。如果使用-iL -，nmap就会从标准输入stdin读取主机名字。你可以从指定目标一节得到更加详细的信息。 -iR 让nmap自己随机挑选主机进行扫描。 -p <端口范围> 这个选项让你选择要进行扫描的端口号的范围。例如，-p 23表示：只扫描目标主机的23号端口。-p 20-30,139,60000-表示：扫描20到30号端口，139号端口以及所有大于60000的端口。在默认情况下，nmap扫描从1到1024号以及nmap-services文件(如果使用RPM软件包，一般在/usr/share/nmap/目录中)中定义的端口列表。 -F 快速扫描模式，只扫描在nmap-services文件中列出的端口。显然比扫描所有65535个端口要快。 -D 使用诱饵扫描方法对目标网络/主机进行扫描。如果nmap使用这种方法对目标网络进行扫描，那么从目标主机/网络的角度来看，扫描就象从其它主机 (decoy1,等)发出的。从而，即使目标主机的IDS(入侵检测系统)对端口扫描发出报警，它们也不可能知道哪个是真正发起扫描的地址，哪个是无辜的。这种扫描方法可以有效地对付例如路由跟踪、response-dropping等积极的防御机制，能够很好地隐藏你的IP地址。 每个诱饵主机名使用逗号分割开，你也可以使用ME选项，它代表你自己的主机，和诱饵主机名混杂在一起。如果你把ME放在第六或者更靠后的位置，一些端口扫描检测软件几乎根本不会显示你的IP地址。如果你不使用ME选项，nmap会把你的IP地址随机夹杂在诱饵主机之中。 注意:你用来作为诱饵的主机应该正在运行或者你只是偶尔向目标发送SYN数据包。很显然，如果在网络上只有一台主机运行，目标将很轻松就会确定是哪台主机进行的扫描。或许，你还要直接使用诱饵的IP地址而不是其域名，这样诱饵网络的域名服务器的日志上就不会留下关于你的记录。 还要注意：一些愚蠢的端口扫描检测软件会拒绝路由试图进行端口扫描的主机。因而，你需要让目标主机和一些诱饵断开连接。如果诱饵是目标主机的网关或者就是其自己时，会给目标主机造成很大问题。所以你需要慎重使用这个选项。 诱饵扫描既可以在起始的ping扫描也可以在真正的扫描状态下使用。它也可以和-O选项组合使用。 使用太多的诱饵扫描能够减缓你的扫描速度甚至可能造成扫描结果不正确。同时，有些ISP会把你的欺骗包过滤掉。虽然现在大多数的ISP不会对此进行限制。 -S <IP_Address> 在一些情况下，nmap可能无法确定你的源地址(nmap会告诉你)。在这种情况下，可以使用这个选项给出你的IP地址。 在欺骗扫描时，也使用这个选项。使用这个选项可以让目标认为是其它的主机对自己进行扫描。 -e 告诉nmap使用哪个接口发送和接受数据包。nmap能够自动对此接口进行检测，如果无效就会告诉你。 -g 设置扫描的源端口。一些天真的防火墙和包过滤器的规则集允许源端口为DNS(53)或者FTP-DATA(20)的包通过和实现连接。显然，如果攻击者把源端口修改为20或者53，就可以摧毁防火墙的防护。在使用UDP扫描时，先使用53号端口；使用TCP扫描时，先使用20号端口。注意只有在能够使用这个端口进行扫描时，nmap才会使用这个端口。例如，如果你无法进行TCP扫描，nmap会自动改变源端口，即使你使用了-g选项。 对于一些扫描，使用这个选项会造成性能上的微小损失，因为我有时会保存关于特定源端口的一些有用的信息。 -r 告诉nmap不要打乱被扫描端口的顺序。 --randomize_hosts 使nmap在扫描之前，打乱每组扫描中的主机顺序，nmap每组可以扫描最多2048台主机。这样，可以使扫描更不容易被网络监视器发现，尤其和--scan_delay 选项组合使用，更能有效避免被发现。 -M 设置进行TCP connect()扫描时，最多使用多少个套接字进行并行的扫描。使用这个选项可以降低扫描速度，避免远程目标宕机。 4.3 适时选项 通常，nmap在运行时，能够很好地根据网络特点进行调整。扫描时，nmap会尽量减少被目标检测到的机会，同时尽可能加快扫描速度。然而，nmap默认的适时策略有时候不太适合你的目标。使用下面这些选项，可以控制nmap的扫描timing： -T 设置nmap的适时策略。Paranoid:为了避开IDS的检测使扫描速度极慢，nmap串行所有的扫描，每隔至少5分钟发送一个包； Sneaky：也差不多，只是数据包的发送间隔是15秒；Polite：不增加太大的网络负载，避免宕掉目标主机，串行每个探测，并且使每个探测有0.4 秒种的间隔；Normal:nmap默认的选项，在不是网络过载或者主机/端口丢失的情况下尽可能快速地扫描；Aggressive:设置5分钟的超时限制，使对每台主机的扫描时间不超过5分钟，并且使对每次探测回应的等待时间不超过1.5秒钟；b>Insane:只适合快速的网络或者你不在意丢失某些信息，每台主机的超时限制是75秒，对每次探测只等待0.3秒钟。你也可是使用数字来代替这些模式，例如：-T 0等于-T Paranoid，-T 5等于-T Insane。 这些适时模式不能下面的适时选项组合使用。 --host_timeout 设置扫描一台主机的时间，以毫秒为单位。默认的情况下，没有超时限制。 --max_rtt_timeout 设置对每次探测的等待时间，以毫秒为单位。如果超过这个时间限制就重传或者超时。默认值是大约9000毫秒。 --min_rtt_timeout 当目标主机的响应很快时，nmap就缩短每次探测的超时时间。这样会提高扫描的速度，但是可能丢失某些响应时间比较长的包。使用这个选项，可以让nmap对每次探测至少等待你指定的时间，以毫秒为单位。 --initial_rtt_timeout 设置初始探测的超时值。一般这个选项只在使用-P0选项扫描有防火墙保护的主机才有用。默认值是6000毫秒。 --max_parallelism 设置最大的并行扫描数量。--max_parallelism 1表示同时只扫描一个端口。这个选项对其它的并行扫描也有效，例如ping sweep, RPC scan。 --scan_delay 设置在两次探测之间，nmap必须等待的时间。这个选项主要用于降低网络的负载。 4.4 目标设定 在nmap的所有参数中，只有目标参数是必须给出的。其最简单的形式是在命令行直接输入一个主机名或者一个IP地址。如果你希望扫描某个IP地址的一个子网，你可以在主机名或者IP地址的后面加上/掩码。掩码在0(扫描整个网络)到32(只扫描这个主机)。使用/24扫描C类地址，/16扫描B类地址。 除此之外，nmap还有更加强大的表示方式让你更加灵活地指定IP地址。例如，如果要扫描这个B类网络128.210.*.*，你可以使用下面三种方式来指定这些地址:128.210.*.*、128.21-.0-255.0-255或者128.210.0.0/16这三种形式是等价的。 5.例子 本节将由浅入深地举例说明如何使用nmap。 nmap -v target.example.com 扫描主机target.example.com的所有TCP端口。-v打开冗余模式。 nmap -sS -O target.example.com/24 发起对target.example.com所在网络上的所有255个IP地址的秘密SYN扫描。同时还探测每台主机操作系统的指纹特征。需要root权限。 nmap -sX -p 22,53,110,143,4564 128.210.*.1-127 对B类IP地址128.210中255个可能的8位子网的前半部分发起圣诞树扫描。确定这些系统是否打开了sshd、DNS、pop3d、imapd和4564端口。注意圣诞树扫描对Micro$oft的系统无效，因为其协议栈的TCP层有缺陷。 nmap -v --randomize_hosts -p 80 *.*.2.3-5 只扫描指定的IP范围，有时用于对这个Internet进行取样分析。nmap将寻找Internet上所有后两个字节是.2.3、.2.4、.2.5的 IP地址上的WEB服务器。如果你想发现更多有意思的主机，你可以使用127-222，因为在这个范围内有意思的主机密度更大。 host -l company.com | cut -d -f 4 | ./nmap -v -iL - 列出company.com网络的所有主机，让nmap进行扫描。注意：这项命令在GNU/Linux下使用。如果在其它平台，你可能要使用 其它的命令/选项。 下面是man文档 NMAP(1) Nmap Reference Guide NMAP(1) NAME nmap - Network exploration tool and security / port scanner SYNOPSIS nmap [Scan Type...] [Options] {target specification} DESCRIPTION Nmap (“Network Mapper”) is an open source tool for network exploration and security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) those hosts are offering, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics. While Nmap is commonly used for security audits, many systems and network administrators find it useful for routine tasks such as network inventory, managing service upgrade schedules, and monitoring host or service uptime. The output from Nmap is a list of scanned targets, with supplemental information on each depending on the options used. Key among that information is the “interesting ports table”.. That table lists the port number and protocol, service name, and state. The state is either open, filtered, closed, or unfiltered. Open. means that an application on the target machine is listening for connections/packets on that port. Filtered. means that a firewall, filter, or other network obstacle is blocking the port so that Nmap cannot tell whether it is open or closed. Closed. ports have no application listening on them, though they could open up at any time. Ports are classified as unfiltered. when they are responsive to Nmap´s probes, but Nmap cannot determine whether they are open or closed. Nmap reports the state combinations open|filtered. and closed|filtered. when it cannot determine which of the two states describe a port. The port table may also include software version details when version detection has been requested. When an IP protocol scan is requested (-sO), Nmap provides information on supported IP protocols rather than listening ports. In addition to the interesting ports table, Nmap can provide further information on targets, including reverse DNS names, operating system guesses, device types, and MAC addresses. A typical Nmap scan is shown in Example 1. The only Nmap arguments used in this example are -A, to enable OS and version detection, script scanning, and traceroute; -T4 for faster execution; and then the two target hostnames. Example 1. A representative Nmap scan # nmap -A -T4 scanme.nmap.org Starting Nmap ( http://nmap.org ) Interesting ports on scanme.nmap.org (64.13.134.52): Not shown: 994 filtered ports PORT STATE SERVICE VERSION 22/tcp open ssh OpenSSH 4.3 (protocol 2.0) 25/tcp closed smtp 53/tcp open domain ISC BIND 9.3.4 70/tcp closed gopher 80/tcp open http Apache httpd 2.2.2 ((Fedora)) |_ HTML title: Go ahead and ScanMe! 113/tcp closed auth Device type: general purpose Running: Linux 2.6.X OS details: Linux 2.6.20-1 (Fedora Core 5) TRACEROUTE (using port 80/tcp) HOP RTT ADDRESS [Cut first seven hops for brevity] 8 10.59 so-4-2-0.mpr3.pao1.us.above.net (64.125.28.142) 9 11.00 metro0.sv.svcolo.com (208.185.168.173) 10 9.93 scanme.nmap.org (64.13.134.52) Nmap done: 1 IP address (1 host up) scanned in 17.00 seconds The newest version of Nmap can be obtained from http://nmap.org. The newest version of this man page is available at http://nmap.org/book/man.html. It is also included as a chapter of Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning (see http://nmap.org/book/). OPTIONS SUMMARY This options summary is printed when Nmap is run with no arguments, and the latest version is always available at http://nmap.org/data/nmap.usage.txt. It helps people remember the most common options, but is no substitute for the in-depth documentation in the rest of this manual. Some obscure options aren´t even included here. Nmap 5.21 ( http://nmap.org ) Usage: nmap [Scan Type(s)] [Options] {target specification} TARGET SPECIFICATION: Can pass hostnames, IP addresses, networks, etc. Ex: scanme.nmap.org, microsoft.com/24, 192.168.0.1; 10.0.0-255.1-254 -iL <inputfilename>: Input from list of hosts/networks -iR <num hosts>: Choose random targets --exclude <host1[,host2][,host3],...>: Exclude hosts/networks --excludefile <exclude_file>: Exclude list from file HOST DISCOVERY: -sL: List Scan - simply list targets to scan -sP: Ping Scan - go no further than determining if host is online -PN: Treat all hosts as online -- skip host discovery -PS/PA/PU/PY[portlist]: TCP SYN/ACK, UDP or SCTP discovery to given ports -PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes -PO[protocol list]: IP Protocol Ping -n/-R: Never do DNS resolution/Always resolve [default: sometimes] --dns-servers <serv1[,serv2],...>: Specify custom DNS servers --system-dns: Use OS´s DNS resolver --traceroute: Trace hop path to each host SCAN TECHNIQUES: -sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans -sU: UDP Scan -sN/sF/sX: TCP Null, FIN, and Xmas scans --scanflags <flags>: Customize TCP scan flags -sI <zombie host[:probeport]>: Idle scan -sY/sZ: SCTP INIT/COOKIE-ECHO scans -sO: IP protocol scan -b <FTP relay host>: FTP bounce scan PORT SPECIFICATION AND SCAN ORDER: -p <port ranges>: Only scan specified ports Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080 -F: Fast mode - Scan fewer ports than the default scan -r: Scan ports consecutively - don´t randomize --top-ports <number>: Scan <number> most common ports --port-ratio <ratio>: Scan ports more common than <ratio> SERVICE/VERSION DETECTION: -sV: Probe open ports to determine service/version info --version-intensity <level>: Set from 0 (light) to 9 (try all probes) --version-light: Limit to most likely probes (intensity 2) --version-all: Try every single probe (intensity 9) --version-trace: Show detailed version scan activity (for debugging) SCRIPT SCAN: -sC: equivalent to --script=default --script=<Lua scripts>: <Lua scripts> is a comma separated list of directories, script-files or script-categories --script-args=<n1=v1,[n2=v2,...]>: provide arguments to scripts --script-trace: Show all data sent and received --script-updatedb: Update the script database. OS DETECTION: -O: Enable OS detection --osscan-limit: Limit OS detection to promising targets --osscan-guess: Guess OS more aggressively TIMING AND PERFORMANCE: Options which take <time> are in milliseconds, unless you append ´s´ (seconds), ´m´ (minutes), or ´h´ (hours) to the value (e.g. 30m). -T<0-5>: Set timing template (higher is faster) --min-hostgroup/max-hostgroup <size>: Parallel host scan group sizes --min-parallelism/max-parallelism <time>: Probe parallelization --min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>: Specifies probe round trip time. --max-retries <tries>: Caps number of port scan probe retransmissions. --host-timeout <time>: Give up on target after this long --scan-delay/--max-scan-delay <time>: Adjust delay between probes --min-rate <number>: Send packets no slower than <number> per second --max-rate <number>: Send packets no faster than <number> per second FIREWALL/IDS EVASION AND SPOOFING: -f; --mtu <val>: fragment packets (optionally w/given MTU) -D <decoy1,decoy2[,ME],...>: Cloak a scan with decoys -S <IP_Address>: Spoof source address -e <iface>: Use specified interface -g/--source-port <portnum>: Use given port number --data-length <num>: Append random data to sent packets --ip-options <options>: Send packets with specified ip options --ttl <val>: Set IP time-to-live field --spoof-mac <mac address/prefix/vendor name>: Spoof your MAC address --badsum: Send packets with a bogus TCP/UDP/SCTP checksum --adler32: Use deprecated Adler32 instead of CRC32C for SCTP checksums OUTPUT: -oN/-oX/-oS/-oG <file>: Output scan in normal, XML, s|<rIpt kIddi3, and Grepable format, respectively, to the given filename. -oA <basename>: Output in the three major formats at once -v: Increase verbosity level (use twice or more for greater effect) -d[level]: Set or increase debugging level (Up to 9 is meaningful) --reason: Display the reason a port is in a particular state --open: Only show open (or possibly open) ports --packet-trace: Show all packets sent and received --iflist: Print host interfaces and routes (for debugging) --log-errors: Log errors/warnings to the normal-format output file --append-output: Append to rather than clobber specified output files --resume <filename>: Resume an aborted scan --stylesheet <path/URL>: XSL stylesheet to transform XML output to HTML --webxml: Reference stylesheet from Nmap.Org for more portable XML --no-stylesheet: Prevent associating of XSL stylesheet w/XML output MISC: -6: Enable IPv6 scanning -A: Enables OS detection and Version detection, Script scanning and Traceroute --datadir <dirname>: Specify custom Nmap data file location --send-eth/--send-ip: Send using raw ethernet frames or IP packets --privileged: Assume that the user is fully privileged --unprivileged: Assume the user lacks raw socket privileges -V: Print version number -h: Print this help summary page. EXAMPLES: nmap -v -A scanme.nmap.org nmap -v -sP 192.168.0.0/16 10.0.0.0/8 nmap -v -iR 10000 -PN -p 80 SEE THE MAN PAGE (http://nmap.org/book/man.html) FOR MORE OPTIONS AND EXAMPLES TARGET SPECIFICATION Everything on the Nmap command-line that isn´t an option (or option argument) is treated as a target host specification. The simplest case is to specify a target IP address or hostname for scanning. Sometimes you wish to scan a whole network of adjacent hosts. For this, Nmap supports CIDR-style. addressing. You can append /numbits to an IPv4 address or hostname and Nmap will scan every IP address for which the first numbits are the same as for the reference IP or hostname given. For example, 192.168.10.0/24 would scan the 256 hosts between 192.168.10.0 (binary: 11000000 10101000 00001010 00000000) and 192.168.10.255 (binary: 11000000 10101000 00001010 11111111), inclusive. 192.168.10.40/24 would scan exactly the same targets. Given that the host scanme.nmap.org. is at the IP address 64.13.134.52, the specification scanme.nmap.org/16 would scan the 65,536 IP addresses between 64.13.0.0 and 64.13.255.255. The smallest allowed value is /0, which scans the whole Internet. The largest value is /32, which scans just the named host or IP address because all address bits are fixed. CIDR notation is short but not always flexible enough. For example, you might want to scan 192.168.0.0/16 but skip any IPs ending with .0 or .255 because they may be used as subnet network and broadcast addresses. Nmap supports this through octet range addressing. Rather than specify a normal IP address, you can specify a comma-separated list of numbers or ranges for each octet. For example, 192.168.0-255.1-254 will skip all addresses in the range that end in .0 or .255, and 192.168.3-5,7.1 will scan the four addresses 192.168.3.1, 192.168.4.1, 192.168.5.1, and 192.168.7.1. Either side of a range may be omitted; the default values are 0 on the left and 255 on the right. Using - by itself is the same as 0-255, but remember to use 0- in the first octet so the target specification doesn´t look like a command-line option. Ranges need not be limited to the final octets: the specifier 0-255.0-255.13.37 will perform an Internet-wide scan for all IP addresses ending in 13.37. This sort of broad sampling can be useful for Internet surveys and research. IPv6 addresses can only be specified by their fully qualified IPv6 address or hostname. CIDR and octet ranges aren´t supported for IPv6 because they are rarely useful. Nmap accepts multiple host specifications on the command line, and they don´t need to be the same type. The command nmap scanme.nmap.org 192.168.0.0/8 10.0.0,1,3-7.- does what you would expect. While targets are usually specified on the command lines, the following options are also available to control target selection: -iL inputfilename (Input from list) . Reads target specifications from inputfilename. Passing a huge list of hosts is often awkward on the command line, yet it is a common desire. For example, your DHCP server might export a list of 10,000 current leases that you wish to scan. Or maybe you want to scan all IP addresses except for those to locate hosts using unauthorized static IP addresses. Simply generate the list of hosts to scan and pass that filename to Nmap as an argument to the -iL option. Entries can be in any of the formats accepted by Nmap on the command line (IP address, hostname, CIDR, IPv6, or octet ranges). Each entry must be separated by one or more spaces, tabs, or newlines. You can specify a hyphen (-) as the filename if you want Nmap to read hosts from standard input rather than an actual file. The input file may contain comments that start with # and extend to the end of the line. -iR num hosts (Choose random targets) . For Internet-wide surveys and other research, you may want to choose targets at random. The num hosts argument tells Nmap how many IPs to generate. Undesirable IPs such as those in certain private, multicast, or unallocated address ranges are automatically skipped. The argument 0 can be specified for a never-ending scan. Keep in mind that some network administrators bristle at unauthorized scans of their networks and may complain. Use this option at your own risk! If you find yourself really bored one rainy afternoon, try the command nmap -sS -PS80 -iR 0 -p 80 to locate random web servers for browsing. --exclude host1[,host2[,...]] (Exclude hosts/networks) . Specifies a comma-separated list of targets to be excluded from the scan even if they are part of the overall network range you specify. The list you pass in uses normal Nmap syntax, so it can include hostnames, CIDR netblocks, octet ranges, etc. This can be useful when the network you wish to scan includes untouchable mission-critical servers, systems that are known to react adversely to port scans, or subnets administered by other people. --excludefile exclude_file (Exclude list from file) . This offers the same functionality as the --exclude option, except that the excluded targets are provided in a newline, space, or tab delimited exclude_file rather than on the command line. The exclude file may contain comments that start with # and extend to the end of the line. HOST DISCOVERY One of the very first steps in any network reconnaissance mission is to reduce a (sometimes huge) set of IP ranges into a list of active or interesting hosts. Scanning every port of every single IP address is slow and usually unnecessary. Of course what makes a host interesting depends greatly on the scan purposes. Network administrators may only be interested in hosts running a certain service, while security auditors may care about every single device with an IP address. An administrator may be comfortable using just an ICMP ping to locate hosts on his internal network, while an external penetration tester may use a diverse set of dozens of probes in an attempt to evade firewall restrictions. Because host discovery needs are so diverse, Nmap offers a wide variety of options for customizing the techniques used. Host discovery is sometimes called ping scan, but it goes well beyond the simple ICMP echo request packets associated with the ubiquitous ping tool. Users can skip the ping step entirely with a list scan (-sL) or by disabling ping (-PN), or engage the network with arbitrary combinations of multi-port TCP SYN/ACK, UDP, SCTP INIT and ICMP probes. The goal of these probes is to solicit responses which demonstrate that an IP address is actually active (is being used by a host or network device). On many networks, only a small percentage of IP addresses are active at any given time. This is particularly common with private address space such as 10.0.0.0/8. That network has 16 million IPs, but I have seen it used by companies with less than a thousand machines. Host discovery can find those machines in a sparsely allocated sea of IP addresses. If no host discovery options are given, Nmap sends an ICMP echo request, a TCP SYN packet to port 443, and TCP ACK packet to port 80, and an ICMP timestamp request. These defaults are equivalent to the -PE -PS443 -PA80 -PP options. An exception to this is that an ARP scan is used for any targets which are on a local ethernet network. For unprivileged Unix shell users, the default probes are a SYN packet to ports 80 and 443 using the connect system call.. This host discovery is often sufficient when scanning local networks, but a more comprehensive set of discovery probes is recommended for security auditing. The -P* options (which select ping types) can be combined. You can increase your odds of penetrating strict firewalls by sending many probe types using different TCP ports/flags and ICMP codes. Also note that ARP discovery (-PR). is done by default against targets on a local ethernet network even if you specify other -P* options, because it is almost always faster and more effective. By default, Nmap does host discovery and then performs a port scan against each host it determines is online. This is true even if you specify non-default host discovery types such as UDP probes (-PU). Read about the -sP option to learn how to perform only host discovery, or use -PN to skip host discovery and port scan all target hosts. The following options control host discovery: -sL (List Scan) . The list scan is a degenerate form of host discovery that simply lists each host of the network(s) specified, without sending any packets to the target hosts. By default, Nmap still does reverse-DNS resolution on the hosts to learn their names. It is often surprising how much useful information simple hostnames give out. For example, fw.chi is the name of one company´s Chicago firewall. Nmap also reports the total number of IP addresses at the end. The list scan is a good sanity check to ensure that you have proper IP addresses for your targets. If the hosts sport domain names you do not recognize, it is worth investigating further to prevent scanning the wrong company´s network. Since the idea is to simply print a list of target hosts, options for higher level functionality such as port scanning, OS detection, or ping scanning cannot be combined with this. If you wish to disable ping scanning while still performing such higher level functionality, read up on the -PN (skip ping) option. -sP (Skip port scan) . This option tells Nmap not to do a port scan after host discovery, and only print out the available hosts that responded to the scan. This is often known as a “ping scan”, but you can also request that traceroute and NSE host scripts be run. This is by default one step more intrusive than the list scan, and can often be used for the same purposes. It allows light reconnaissance of a target network without attracting much attention. Knowing how many hosts are up is more valuable to attackers than the list provided by list scan of every single IP and host name. Systems administrators often find this option valuable as well. It can easily be used to count available machines on a network or monitor server availability. This is often called a ping sweep, and is more reliable than pinging the broadcast address because many hosts do not reply to broadcast queries. The -sP option sends an ICMP echo request, TCP SYN to port 443, TCP ACK to port 80, and an ICMP timestamp request by default. When executed by an unprivileged user, only SYN packets are sent (using a connect call) to ports 80 and 443 on the target. When a privileged user tries to scan targets on a local ethernet network, ARP requests are used unless --send-ip was specified. The -sP option can be combined with any of the discovery probe types (the -P* options, excluding -PN) for greater flexibility. If any of those probe type and port number options are used, the default probes are overridden. When strict firewalls are in place between the source host running Nmap and the target network, using those advanced techniques is recommended. Otherwise hosts could be missed when the firewall drops probes or their responses. -PN (No ping) . This option skips the Nmap discovery stage altogether. Normally, Nmap uses this stage to determine active machines for heavier scanning. By default, Nmap only performs heavy probing such as port scans, version detection, or OS detection against hosts that are found to be up. Disabling host discovery with -PN causes Nmap to attempt the requested scanning functions against every target IP address specified. So if a class B sized target address space (/16) is specified on the command line, all 65,536 IP addresses are scanned. Proper host discovery is skipped as with the list scan, but instead of stopping and printing the target list, Nmap continues to perform requested functions as if each target IP is active. To skip ping scan and port scan, while still allowing NSE to run, use the two options -PN -sP together. For machines on a local ethernet network, ARP scanning will still be performed (unless --send-ip is specified) because Nmap needs MAC addresses to further scan target hosts. This option flag used to be P0 (uses zero), but was renamed to avoid confusion with protocol ping´s PO (uses the letter O) flag. -PS port list (TCP SYN Ping) . This option sends an empty TCP packet with the SYN flag set. The default destination port is 80 (configurable at compile time by changing DEFAULT_TCP_PROBE_PORT_SPEC in nmap.h). Alternate ports can be specified as a parameter. The syntax is the same as for the -p except that port type specifiers like T: are not allowed. Examples are -PS22 and -PS22-25,80,113,1050,35000. Note that there can be no space between -PS and the port list. If multiple probes are specified they will be sent in parallel. The SYN flag suggests to the remote system that you are attempting to establish a connection. Normally the destination port will be closed, and a RST (reset) packet sent back. If the port happens to be open, the target will take the second step of a TCP three-way-handshake. by responding with a SYN/ACK TCP packet. The machine running Nmap then tears down the nascent connection by responding with a RST rather than sending an ACK packet which would complete the three-way-handshake and establish a full connection. The RST packet is sent by the kernel of the machine running Nmap in response to the unexpected SYN/ACK, not by Nmap itself. Nmap does not care whether the port is open or closed. Either the RST or SYN/ACK response discussed previously tell Nmap that the host is available and responsive. On Unix boxes, only the privileged user root. is generally able to send and receive raw TCP packets.. For unprivileged users, a workaround is automatically employed. whereby the connect system call is initiated against each target port. This has the effect of sending a SYN packet to the target host, in an attempt to establish a connection. If connect returns with a quick success or an ECONNREFUSED failure, the underlying TCP stack must have received a SYN/ACK or RST and the host is marked available. If the connection attempt is left hanging until a timeout is reached, the host is marked as down. This workaround is also used for IPv6 connections, as raw IPv6 packet building support is not yet available in Nmap.. -PA port list (TCP ACK Ping) . The TCP ACK ping is quite similar to the just-discussed SYN ping. The difference, as you could likely guess, is that the TCP ACK flag is set instead of the SYN flag. Such an ACK packet purports to be acknowledging data over an established TCP connection, but no such connection exists. So remote hosts should always respond with a RST packet, disclosing their existence in the process. The -PA option uses the same default port as the SYN probe (80) and can also take a list of destination ports in the same format. If an unprivileged user tries this, or an IPv6 target is specified, the connect workaround discussed previously is used. This workaround is imperfect because connect is actually sending a SYN packet rather than an ACK. The reason for offering both SYN and ACK ping probes is to maximize the chances of bypassing firewalls. Many administrators configure routers and other simple firewalls to block incoming SYN packets except for those destined for public services like the company web site or mail server. This prevents other incoming connections to the organization, while allowing users to make unobstructed outgoing connections to the Internet. This non-stateful approach takes up few resources on the firewall/router and is widely supported by hardware and software filters. The Linux Netfilter/iptables. firewall software offers the --syn convenience option to implement this stateless approach. When stateless firewall rules such as this are in place, SYN ping probes (-PS) are likely to be blocked when sent to closed target ports. In such cases, the ACK probe shines as it cuts right through these rules. Another common type of firewall uses stateful rules that drop unexpected packets. This feature was initially found mostly on high-end firewalls, though it has become much more common over the years. The Linux Netfilter/iptables system supports this through the --state option, which categorizes packets based on connection state. A SYN probe is more likely to work against such a system, as unexpected ACK packets are generally recognized as bogus and dropped. A solution to this quandary is to send both SYN and ACK probes by specifying -PS and -PA. -PU port list (UDP Ping) . Another host discovery option is the UDP ping, which sends a UDP packet to the given ports. For most ports, the packet will be empty, though for a few a protocol-specific payload will be sent that is more likely to get a response.. See the file payload.cc. for exactly which ports have payloads. The --data-length. option sends a fixed-length random payload for all ports. The port list takes the same format as with the previously discussed -PS and -PA options. If no ports are specified, the default is 40125. This default can be configured at compile-time by changing DEFAULT_UDP_PROBE_PORT_SPEC. in nmap.h.. A highly uncommon port is used by default because sending to open ports is often undesirable for this particular scan type. Upon hitting a closed port on the target machine, the UDP probe should elicit an ICMP port unreachable packet in return. This signifies to Nmap that the machine is up and available. Many other types of ICMP errors, such as host/network unreachables or TTL exceeded are indicative of a down or unreachable host. A lack of response is also interpreted this way. If an open port is reached, most services simply ignore the empty packet and fail to return any response. This is why the default probe port is 40125, which is highly unlikely to be in use. A few services, such as the Character Generator (chargen) protocol, will respond to an empty UDP packet, and thus disclose to Nmap that the machine is available. The primary advantage of this scan type is that it bypasses firewalls and filters that only screen TCP. For example, I once owned a Linksys BEFW11S4 wireless broadband router. The external interface of this device filtered all TCP ports by default, but UDP probes would still elicit port unreachable messages and thus give away the device. -PY port list (SCTP INIT Ping) . This option sends an SCTP packet containing a minimal INIT chunk. The default destination port is 80 (configurable at compile time by changing DEFAULT_SCTP_PROBE_PORT_SPEC in nmap.h). Alternate ports can be specified as a parameter. The syntax is the same as for the -p except that port type specifiers like S: are not allowed. Examples are -PY22 and -PY22,80,179,5060. Note that there can be no space between -PY and the port list. If multiple probes are specified they will be sent in parallel. The INIT chunk suggests to the remote system that you are attempting to establish an association. Normally the destination port will be closed, and an ABORT chunk will be sent back. If the port happens to be open, the target will take the second step of an SCTP four-way-handshake. by responding with an INIT-ACK chunk. If the machine running Nmap has a functional SCTP stack, then it tears down the nascent association by responding with an ABORT chunk rather than sending a COOKIE-ECHO chunk which would be the next step in the four-way-handshake. The ABORT packet is sent by the kernel of the machine running Nmap in response to the unexpected INIT-ACK, not by Nmap itself. Nmap does not care whether the port is open or closed. Either the ABORT or INIT-ACK response discussed previously tell Nmap that the host is available and responsive. On Unix boxes, only the privileged user root. is generally able to send and receive raw SCTP packets.. Using SCTP INIT Pings is currently not possible for unprivileged users.. The same limitation applies to IPv6, which is currently not supported for SCTP INIT Ping.. -PE; -PP; -PM (ICMP Ping Types) . In addition to the unusual TCP, UDP and SCTP host discovery types discussed previously, Nmap can send the standard packets sent by the ubiquitous ping program. Nmap sends an ICMP type 8 (echo request) packet to the target IP addresses, expecting a type 0 (echo reply) in return from available hosts.. Unfortunately for network explorers, many hosts and firewalls now block these packets, rather than responding as required by RFC 1122[2]. For this reason, ICMP-only scans are rarely reliable enough against unknown targets over the Internet. But for system administrators monitoring an internal network, they can be a practical and efficient approach. Use the -PE option to enable this echo request behavior. While echo request is the standard ICMP ping query, Nmap does not stop there. The ICMP standards (RFC 792[3]. and RFC 950[4]. “a host SHOULD NOT implement these messages”. Timestamp and address mask queries can be sent with the -PP and -PM options, respectively. A timestamp reply (ICMP code 14) or address mask reply (code 18) discloses that the host is available. These two queries can be valuable when administrators specifically block echo request packets while forgetting that other ICMP queries can be used for the same purpose. -PO protocol list (IP Protocol Ping) . The newest host discovery option is the IP protocol ping, which sends IP packets with the specified protocol number set in their IP header. The protocol list takes the same format as do port lists in the previously discussed TCP, UDP and SCTP host discovery options. If no protocols are specified, the default is to send multiple IP packets for ICMP (protocol 1), IGMP (protocol 2), and IP-in-IP (protocol 4). The default protocols can be configured at compile-time by changing DEFAULT_PROTO_PROBE_PORT_SPEC. in nmap.h. Note that for the ICMP, IGMP, TCP (protocol 6), UDP (protocol 17) and SCTP (protocol 132), the packets are sent with the proper protocol headers. while other protocols are sent with no additional data beyond the IP header (unless the --data-length. option is specified). This host discovery method looks for either responses using the same protocol as a probe, or ICMP protocol unreachable messages which signify that the given protocol isn´t supported on the destination host. Either type of response signifies that the target host is alive. -PR (ARP Ping) . One of the most common Nmap usage scenarios is to scan an ethernet LAN. On most LANs, especially those using private address ranges specified by RFC 1918[5], the vast majority of IP addresses are unused at any given time. When Nmap tries to send a raw IP packet such as an ICMP echo request, the operating system must determine the destination hardware (ARP) address corresponding to the target IP so that it can properly address the ethernet frame. This is often slow and problematic, since operating systems weren´t written with the expectation that they would need to do millions of ARP requests against unavailable hosts in a short time period. ARP scan puts Nmap and its optimized algorithms in charge of ARP requests. And if it gets a response back, Nmap doesn´t even need to worry about the IP-based ping packets since it already knows the host is up. This makes ARP scan much faster and more reliable than IP-based scans. So it is done by default when scanning ethernet hosts that Nmap detects are on a local ethernet network. Even if different ping types (such as -PE or -PS) are specified, Nmap uses ARP instead for any of the targets which are on the same LAN. If you absolutely don´t want to do an ARP scan, specify --send-ip. --traceroute (Trace path to host) . Traceroutes are performed post-scan using information from the scan results to determine the port and protocol most likely to reach the target. It works with all scan types except connect scans (-sT) and idle scans (-sI). All traces use Nmap´s dynamic timing model and are performed in parallel. Traceroute works by sending packets with a low TTL (time-to-live) in an attempt to elicit ICMP Time Exceeded messages from intermediate hops between the scanner and the target host. Standard traceroute implementations start with a TTL of 1 and increment the TTL until the destination host is reached. Nmap´s traceroute starts with a high TTL and then decrements the TTL until it reaches zero. Doing it backwards lets Nmap employ clever caching algorithms to speed up traces over multiple hosts. On average Nmap sends 5–10 fewer packets per host, depending on network conditions. If a single subnet is being scanned (i.e. 192.168.0.0/24) Nmap may only have to send a single packet to most hosts. -n (No DNS resolution) . Tells Nmap to never do reverse DNS resolution on the active IP addresses it finds. Since DNS can be slow even with Nmap´s built-in parallel stub resolver, this option can slash scanning times. -R (DNS resolution for all targets) . Tells Nmap to always do reverse DNS resolution on the target IP addresses. Normally reverse DNS is only performed against responsive (online) hosts. --system-dns (Use system DNS resolver) . By default, Nmap resolves IP addresses by sending queries directly to the name servers configured on your host and then listening for responses. Many requests (often dozens) are performed in parallel to improve performance. Specify this option to use your system resolver instead (one IP at a time via the getnameinfo call). This is slower and rarely useful unless you find a bug in the Nmap parallel resolver (please let us know if you do). The system resolver is always used for IPv6 scans. --dns-servers server1[,server2[,...]] (Servers to use for reverse DNS queries) . By default, Nmap determines your DNS servers (for rDNS resolution) from your resolv.conf file (Unix) or the Registry (Win32). Alternatively, you may use this option to specify alternate servers. This option is not honored if you are using --system-dns or an IPv6 scan. Using multiple DNS servers is often faster, especially if you choose authoritative servers for your target IP space. This option can also improve stealth, as your requests can be bounced off just about any recursive DNS server on the Internet. This option also comes in handy when scanning private networks. Sometimes only a few name servers provide proper rDNS information, and you may not even know where they are. You can scan the network for port 53 (perhaps with version detection), then try Nmap list scans (-sL) specifying each name server one at a time with --dns-servers until you find one which works. PORT SCANNING BASICS While Nmap has grown in functionality over the years, it began as an efficient port scanner, and that remains its core function. The simple command nmap target scans more than 1660 TCP ports on the host target. While many port scanners have traditionally lumped all ports into the open or closed states, Nmap is much more granular. It divides ports into six states: open, closed, filtered, unfiltered, open|filtered, or closed|filtered. These states are not intrinsic properties of the port itself, but describe how Nmap sees them. For example, an Nmap scan from the same network as the target may show port 135/tcp as open, while a scan at the same time with the same options from across the Internet might show that port as filtered. The six port states recognized by Nmap An application is actively accepting TCP connections, UDP datagrams or SCTP associations on this port. Finding these is often the primary goal of port scanning. Security-minded people know that each open port is an avenue for attack. Attackers and pen-testers want to exploit the open ports, while administrators try to close or protect them with firewalls without thwarting legitimate users. Open ports are also interesting for non-security scans because they show services available for use on the network. A closed port is accessible (it receives and responds to Nmap probe packets), but there is no application listening on it. They can be helpful in showing that a host is up on an IP address (host discovery, or ping scanning), and as part of OS detection. Because closed ports are reachable, it may be worth scanning later in case some open up. Administrators may want to consider blocking such ports with a firewall. Then they would appear in the filtered state, discussed next. Nmap cannot determine whether the port is open because packet filtering prevents its probes from reaching the port. The filtering could be from a dedicated firewall device, router rules, or host-based firewall software. These ports frustrate attackers because they provide so little information. Sometimes they respond with ICMP error messages such as type 3 code 13 (destination unreachable: communication administratively prohibited), but filters that simply drop probes without responding are far more common. This forces Nmap to retry several times just in case the probe was dropped due to network congestion rather than filtering. This slows down the scan dramatically. The unfiltered state means that a port is accessible, but Nmap is unable to determine whether it is open or closed. Only the ACK scan, which is used to map firewall rulesets, classifies ports into this state. Scanning unfiltered ports with other scan types such as Window scan, SYN scan, or FIN scan, may help resolve whether the port is open. Nmap places ports in this state when it is unable to determine whether a port is open or filtered. This occurs for scan types in which open ports give no response. The lack of response could also mean that a packet filter dropped the probe or any response it elicited. So Nmap does not know for sure whether the port is open or being filtered. The UDP, IP protocol, FIN, NULL, and Xmas scans classify ports this way. This state is used when Nmap is unable to determine whether a port is closed or filtered. It is only used for the IP ID idle scan. PORT SCANNING TECHNIQUES As a novice performing automotive repair, I can struggle for hours trying to fit my rudimentary tools (hammer, duct tape, wrench, etc.) to the task at hand. When I fail miserably and tow my jalopy to a real mechanic, he invariably fishes around in a huge tool chest until pulling out the perfect gizmo which makes the job seem effortless. The art of port scanning is similar. Experts understand the dozens of scan techniques and choose the appropriate one (or combination) for a given task. Inexperienced users and script kiddies,. on the other hand, try to solve every problem with the default SYN scan. Since Nmap is free, the only barrier to port scanning mastery is knowledge. That certainly beats the automotive world, where it may take great skill to determine that you need a strut spring compressor, then you still have to pay thousands of dollars for it. Most of the scan types are only available to privileged users.. This is because they send and receive raw packets,. which requires root access on Unix systems. Using an administrator account on Windows is recommended, though Nmap sometimes works for unprivileged users on that platform when WinPcap has already been loaded into the OS. Requiring root privileges was a serious limitation when Nmap was released in 1997, as many users only had access to shared shell accounts. Now, the world is different. Computers are cheaper, far more people have always-on direct Internet access, and desktop Unix systems (including Linux and Mac OS X) are prevalent. A Windows version of Nmap is now available, allowing it to run on even more desktops. For all these reasons, users have less need to run Nmap from limited shared shell accounts. This is fortunate, as the privileged options make Nmap far more powerful and flexible. While Nmap attempts to produce accurate results, keep in mind that all of its insights are based on packets returned by the target machines (or firewalls in front of them). Such hosts may be untrustworthy and send responses intended to confuse or mislead Nmap. Much more common are non-RFC-compliant hosts that do not respond as they should to Nmap probes. FIN, NULL, and Xmas scans are particularly susceptible to this problem. Such issues are specific to certain scan types and so are discussed in the individual scan type entries. This section documents the dozen or so port scan techniques supported by Nmap. Only one method may be used at a time, except that UDP scan (-sU) and any one of the SCTP scan types (-sY, -sZ) may be combined with any one of the TCP scan types. As a memory aid, port scan type options are of the form -sC, where C is a prominent character in the scan name, usually the first. The one exception to this is the deprecated FTP bounce scan (-b). By default, Nmap performs a SYN Scan, though it substitutes a connect scan if the user does not have proper privileges to send raw packets (requires root access on Unix) or if IPv6 targets were specified. Of the scans listed in this section, unprivileged users can only execute connect and FTP bounce scans. -sS (TCP SYN scan) . SYN scan is the default and most popular scan option for good reasons. It can be performed quickly, scanning thousands of ports per second on a fast network not hampered by restrictive firewalls. SYN scan is relatively unobtrusive and stealthy, since it never completes TCP connections. It also works against any compliant TCP stack rather than depending on idiosyncrasies of specific platforms as Nmap´s FIN/NULL/Xmas, Maimon and idle scans do. It also allows clear, reliable differentiation between the open, closed, and filtered states. This technique is often referred to as half-open scanning, because you don´t open a full TCP connection. You send a SYN packet, as if you are going to open a real connection and then wait for a response. A SYN/ACK indicates the port is listening (open), while a RST (reset) is indicative of a non-listener. If no response is received after several retransmissions, the port is marked as filtered. The port is also marked filtered if an ICMP unreachable error (type 3, code 1, 2, 3, 9, 10, or 13) is received. -sT (TCP connect scan) . TCP connect scan is the default TCP scan type when SYN scan is not an option. This is the case when a user does not have raw packet privileges or is scanning IPv6 networks. Instead of writing raw packets as most other scan types do, Nmap asks the underlying operating system to establish a connection with the target machine and port by issuing the connect system call. This is the same high-level system call that web browsers, P2P clients, and most other network-enabled applications use to establish a connection. It is part of a programming interface known as the Berkeley Sockets API. Rather than read raw packet responses off the wire, Nmap uses this API to obtain status information on each connection attempt. When SYN scan is available, it is usually a better choice. Nmap has less control over the high level connect call than with raw packets, making it less efficient. The system call completes connections to open target ports rather than performing the half-open reset that SYN scan does. Not only does this take longer and require more packets to obtain the same information, but target machines are more likely to log the connection. A decent IDS will catch either, but most machines have no such alarm system. Many services on your average Unix system will add a note to syslog, and sometimes a cryptic error message, when Nmap connects and then closes the connection without sending data. Truly pathetic services crash when this happens, though that is uncommon. An administrator who sees a bunch of connection attempts in her logs from a single system should know that she has been connect scanned. -sU (UDP scans) . While most popular services on the Internet run over the TCP protocol, UDP[6] services are widely deployed. DNS, SNMP, and DHCP (registered ports 53, 161/162, and 67/68) are three of the most common. Because UDP scanning is generally slower and more difficult than TCP, some security auditors ignore these ports. This is a mistake, as exploitable UDP services are quite common and attackers certainly don´t ignore the whole protocol. Fortunately, Nmap can help inventory UDP ports. UDP scan is activated with the -sU option. It can be combined with a TCP scan type such as SYN scan (-sS) to check both protocols during the same run. UDP scan works by sending a UDP packet to every targeted port. For some common ports such as 53 and 161, a protocol-specific payload is sent, but for most ports the packet is empty.. The --data-length option can be used to send a fixed-length random payload to every port. If an ICMP port unreachable error (type 3, code 3) is returned, the port is closed. Other ICMP unreachable errors (type 3, codes 1, 2, 9, 10, or 13) mark the port as filtered. Occasionally, a service will respond with a UDP packet, proving that it is open. If no response is received after retransmissions, the port is classified as open|filtered. This means that the port could be open, or perhaps packet filters are blocking the communication. Version detection (-sV) can be used to help differentiate the truly open ports from the filtered ones. A big challenge with UDP scanning is doing it quickly. Open and filtered ports rarely send any response, leaving Nmap to time out and then conduct retransmissions just in case the probe or response were lost. Closed ports are often an even bigger problem. They usually send back an ICMP port unreachable error. But unlike the RST packets sent by closed TCP ports in response to a SYN or connect scan, many hosts rate limit. ICMP port unreachable messages by default. Linux and Solaris are particularly strict about this. For example, the Linux 2.4.20 kernel limits destination unreachable messages to one per second (in net/ipv4/icmp.c). Nmap detects rate limiting and slows down accordingly to avoid flooding the network with useless packets that the target machine will drop. Unfortunately, a Linux-style limit of one packet per second makes a 65,536-port scan take more than 18 hours. Ideas for speeding your UDP scans up include scanning more hosts in parallel, doing a quick scan of just the popular ports first, scanning from behind the firewall, and using --host-timeout to skip slow hosts. -sY (SCTP INIT scan) . SCTP[7] is a relatively new alternative to the TCP and UDP protocols, combining most characteristics of TCP and UDP, and also adding new features like multi-homing and multi-streaming. It is mostly being used for SS7/SIGTRAN related services but has the potential to be used for other applications as well. SCTP INIT scan is the SCTP equivalent of a TCP SYN scan. It can be performed quickly, scanning thousands of ports per second on a fast network not hampered by restrictive firewalls. Like SYN scan, INIT scan is relatively unobtrusive and stealthy, since it never completes SCTP associations. It also allows clear, reliable differentiation between the open, closed, and filtered states. This technique is often referred to as half-open scanning, because you don´t open a full SCTP association. You send an INIT chunk, as if you are going to open a real association and then wait for a response. An INIT-ACK chunk indicates the port is listening (open), while an ABORT chunk is indicative of a non-listener. If no response is received after several retransmissions, the port is marked as filtered. The port is also marked filtered if an ICMP unreachable error (type 3, code 1, 2, 3, 9, 10, or 13) is received. -sN; -sF; -sX (TCP NULL, FIN, and Xmas scans) . These three scan types (even more are possible with the --scanflags option described in the next section) exploit a subtle loophole in the TCP RFC[8] to differentiate between open and closed ports. Page 65 of RFC 793 says that “if the [destination] port state is CLOSED .... an incoming segment not containing a RST causes a RST to be sent in response.” Then the next page discusses packets sent to open ports without the SYN, RST, or ACK bits set, stating that: “you are unlikely to get here, but if you do, drop the segment, and return.” When scanning systems compliant with this RFC text, any packet not containing SYN, RST, or ACK bits will result in a returned RST if the port is closed and no response at all if the port is open. As long as none of those three bits are included, any combination of the other three (FIN, PSH, and URG) are OK. Nmap exploits this with three scan types: Null scan (-sN) Does not set any bits (TCP flag header is 0) FIN scan (-sF) Sets just the TCP FIN bit. Xmas scan (-sX) Sets the FIN, PSH, and URG flags, lighting the packet up like a Christmas tree. These three scan types are exactly the same in behavior except for the TCP flags set in probe packets. If a RST packet is received, the port is considered closed, while no response means it is open|filtered. The port is marked filtered if an ICMP unreachable error (type 3, code 1, 2, 3, 9, 10, or 13) is received. The key advantage to these scan types is that they can sneak through certain non-stateful firewalls and packet filtering routers. Another advantage is that these scan types are a little more stealthy than even a SYN scan. Don´t count on this though—most modern IDS products can be configured to detect them. The big downside is that not all systems follow RFC 793 to the letter. A number of systems send RST responses to the probes regardless of whether the port is open or not. This causes all of the ports to be labeled closed. Major operating systems that do this are Microsoft Windows, many Cisco devices, BSDI, and IBM OS/400. This scan does work against most Unix-based systems though. Another downside of these scans is that they can´t distinguish open ports from certain filtered ones, leaving you with the response open|filtered. -sA (TCP ACK scan) . This scan is different than the others discussed so far in that it never determines open (or even open|filtered) ports. It is used to map out firewall rulesets, determining whether they are stateful or not and which ports are filtered. The ACK scan probe packet has only the ACK flag set (unless you use --scanflags). When scanning unfiltered systems, open and closed ports will both return a RST packet. Nmap then labels them as unfiltered, meaning that they are reachable by the ACK packet, but whether they are open or closed is undetermined. Ports that don´t respond, or send certain ICMP error messages back (type 3, code 1, 2, 3, 9, 10, or 13), are labeled filtered. -sW (TCP Window scan) . Window scan is exactly the same as ACK scan except that it exploits an implementation detail of certain systems to differentiate open ports from closed ones, rather than always printing unfiltered when a RST is returned. It does this by examining the TCP Window field of the RST packets returned. On some systems, open ports use a positive window size (even for RST packets) while closed ones have a zero window. So instead of always listing a port as unfiltered when it receives a RST back, Window scan lists the port as open or closed if the TCP Window value in that reset is positive or zero, respectively. This scan relies on an implementation detail of a minority of systems out on the Internet, so you can´t always trust it. Systems that don´t support it will usually return all ports closed. Of course, it is possible that the machine really has no open ports. If most scanned ports are closed but a few common port numbers (such as 22, 25, 53) are filtered, the system is most likely susceptible. Occasionally, systems will even show the exact opposite behavior. If your scan shows 1000 open ports and three closed or filtered ports, then those three may very well be the truly open ones. -sM (TCP Maimon scan) . The Maimon scan is named after its discoverer, Uriel Maimon.. He described the technique in Phrack Magazine issue #49 (November 1996).. Nmap, which included this technique, was released two issues later. This technique is exactly the same as NULL, FIN, and Xmas scans, except that the probe is FIN/ACK. According to RFC 793[8] (TCP), a RST packet should be generated in response to such a probe whether the port is open or closed. However, Uriel noticed that many BSD-derived systems simply drop the packet if the port is open. --scanflags (Custom TCP scan) . Truly advanced Nmap users need not limit themselves to the canned scan types offered. The --scanflags option allows you to design your own scan by specifying arbitrary TCP flags.. Let your creative juices flow, while evading intrusion detection systems. whose vendors simply paged through the Nmap man page adding specific rules! The --scanflags argument can be a numerical flag value such as 9 (PSH and FIN), but using symbolic names is easier. Just mash together any combination of URG, ACK, PSH, RST, SYN, and FIN. For example, --scanflags URGACKPSHRSTSYNFIN sets everything, though it´s not very useful for scanning. The order these are specified in is irrelevant. In addition to specifying the desired flags, you can specify a TCP scan type (such as -sA or -sF). That base type tells Nmap how to interpret responses. For example, a SYN scan considers no-response to indicate a filtered port, while a FIN scan treats the same as open|filtered. Nmap will behave the same way it does for the base scan type, except that it will use the TCP flags you specify instead. If you don´t specify a base type, SYN scan is used. -sZ (SCTP COOKIE ECHO scan) . SCTP COOKIE ECHO scan is a more advanced SCTP scan. It takes advantage of the fact that SCTP implementations should silently drop packets containing COOKIE ECHO chunks on open ports, but send an ABORT if the port is closed. The advantage of this scan type is that it is not as obvious a port scan than an INIT scan. Also, there may be non-stateful firewall rulesets blocking INIT chunks, but not COOKIE ECHO chunks. Don´t be fooled into thinking that this will make a port scan invisible; a good IDS will be able to detect SCTP COOKIE ECHO scans too. The downside is that SCTP COOKIE ECHO scans cannot differentiate between open and filtered ports, leaving you with the state open|filtered in both cases. -sI zombie host[:probeport] (idle scan) . This advanced scan method allows for a truly blind TCP port scan of the target (meaning no packets are sent to the target from your real IP address). Instead, a unique side-channel attack exploits predictable IP fragmentation ID sequence generation on the zombie host to glean information about the open ports on the target. IDS systems will display the scan as coming from the zombie machine you specify (which must be up and meet certain criteria). This fascinating scan type is too complex to fully describe in this reference guide, so I wrote and posted an informal paper with full details at http://nmap.org/book/idlescan.html. Besides being extraordinarily stealthy (due to its blind nature), this scan type permits mapping out IP-based trust relationships between machines. The port listing shows open ports from the perspective of the zombie host. So you can try scanning a target using various zombies that you think might be trusted. (via router/packet filter rules). You can add a colon followed by a port number to the zombie host if you wish to probe a particular port on the zombie for IP ID changes. Otherwise Nmap will use the port it uses by default for TCP pings (80). -sO (IP protocol scan) . IP protocol scan allows you to determine which IP protocols (TCP, ICMP, IGMP, etc.) are supported by target machines. This isn´t technically a port scan, since it cycles through IP protocol numbers rather than TCP or UDP port numbers. Yet it still uses the -p option to select scanned protocol numbers, reports its results within the normal port table format, and even uses the same underlying scan engine as the true port scanning methods. So it is close enough to a port scan that it belongs here. Besides being useful in its own right, protocol scan demonstrates the power of open-source software. While the fundamental idea is pretty simple, I had not thought to add it nor received any requests for such functionality. Then in the summer of 2000, Gerhard Rieger. conceived the idea, wrote an excellent patch implementing it, and sent it to the nmap-hackers mailing list.. I incorporated that patch into the Nmap tree and released a new version the next day. Few pieces of commercial software have users enthusiastic enough to design and contribute their own improvements! Protocol scan works in a similar fashion to UDP scan. Instead of iterating through the port number field of a UDP packet, it sends IP packet headers and iterates through the eight-bit IP protocol field. The headers are usually empty, containing no data and not even the proper header for the claimed protocol. The exceptions are TCP, UDP, ICMP, SCTP, and IGMP. A proper protocol header for those is included since some systems won´t send them otherwise and because Nmap already has functions to create them. Instead of watching for ICMP port unreachable messages, protocol scan is on the lookout for ICMP protocol unreachable messages. If Nmap receives any response in any protocol from the target host, Nmap marks that protocol as open. An ICMP protocol unreachable error (type 3, code 2) causes the protocol to be marked as closed Other ICMP unreachable errors (type 3, code 1, 3, 9, 10, or 13) cause the protocol to be marked filtered (though they prove that ICMP is open at the same time). If no response is received after retransmissions, the protocol is marked open|filtered -b FTP relay host (FTP bounce scan) . An interesting feature of the FTP protocol (RFC 959[9]) is support for so-called proxy FTP connections. This allows a user to connect to one FTP server, then ask that files be sent to a third-party server. Such a feature is ripe for abuse on many levels, so most servers have ceased supporting it. One of the abuses this feature allows is causing the FTP server to port scan other hosts. Simply ask the FTP server to send a file to each interesting port of a target host in turn. The error message will describe whether the port is open or not. This is a good way to bypass firewalls because organizational FTP servers are often placed where they have more access to other internal hosts than any old Internet host would. Nmap supports FTP bounce scan with the -b option. It takes an argument of the form username:password@server:port. Server is the name or IP address of a vulnerable FTP server. As with a normal URL, you may omit username:password, in which case anonymous login credentials (user: anonymous password:-wwwuser@) are used. The port number (and preceding colon) may be omitted as well, in which case the default FTP port (21) on server is used. This vulnerability was widespread in 1997 when Nmap was released, but has largely been fixed. Vulnerable servers are still around, so it is worth trying when all else fails. If bypassing a firewall is your goal, scan the target network for open port 21 (or even for any FTP services if you scan all ports with version detection), then try a bounce scan using each. Nmap will tell you whether the host is vulnerable or not. If you are just trying to cover your tracks, you don´t need to (and, in fact, shouldn´t) limit yourself to hosts on the target network. Before you go scanning random Internet addresses for vulnerable FTP servers, consider that sysadmins may not appreciate you abusing their servers in this way. PORT SPECIFICATION AND SCAN ORDER In addition to all of the scan methods discussed previously, Nmap offers options for specifying which ports are scanned and whether the scan order is randomized or sequential. By default, Nmap scans the most common 1,000 ports for each protocol. -p port ranges (Only scan specified ports) . This option specifies which ports you want to scan and overrides the default. Individual port numbers are OK, as are ranges separated by a hyphen (e.g. 1-1023). The beginning and/or end values of a range may be omitted, causing Nmap to use 1 and 65535, respectively. So you can specify -p- to scan ports from 1 through 65535. Scanning port zero. is allowed if you specify it explicitly. For IP protocol scanning (-sO), this option specifies the protocol numbers you wish to scan for (0–255). When scanning both TCP and UDP ports, you can specify a particular protocol by preceding the port numbers by T: or U:. The qualifier lasts until you specify another qualifier. For example, the argument -p U:53,111,137,T:21-25,80,139,8080 would scan UDP ports 53, 111,and 137, as well as the listed TCP ports. Note that to scan both UDP and TCP, you have to specify -sU and at least one TCP scan type (such as -sS, -sF, or -sT). If no protocol qualifier is given, the port numbers are added to all protocol lists. Ports can also be specified by name according to what the port is referred to in the nmap-services. You can even use the wildcards * and ? with the names. For example, to scan FTP and all ports whose names begin with “http”, use -p ftp,http*. Be careful about shell expansions and quote the argument to -p if unsure. Ranges of ports can be surrounded by square brackets to indicate ports inside that range that appear in nmap-services. For example, the following will scan all ports in nmap-services equal to or below 1024: -p [-1024]. Be careful with shell expansions and quote the argument to -p if unsure. -F (Fast (limited port) scan) . Specifies that you wish to scan fewer ports than the default. Normally Nmap scans the most common 1,000 ports for each scanned protocol. With -F, this is reduced to 100. Nmap needs an nmap-services file with frequency information in order to know which ports are the most common. If port frequency information isn´t available, perhaps because of the use of a custom nmap-services file, -F means to scan only ports that are named in the services file (normally Nmap scans all named ports plus ports 1–1024). -r (Don´t randomize ports) . By default, Nmap randomizes the scanned port order (except that certain commonly accessible ports are moved near the beginning for efficiency reasons). This randomization is normally desirable, but you can specify -r for sequential (sorted from lowest to highest) port scanning instead. --port-ratio <decimal number between 0 and 1> Scans all ports in nmap-services file with a ratio greater than the number specified as the argument. --top-ports <integer of 1 or greater> Scans the N highest-ratio ports found in nmap-services file. SERVICE AND VERSION DETECTION Point Nmap at a remote machine and it might tell you that ports 25/tcp, 80/tcp, and 53/udp are open. Using its nmap-services. database of about 2,200 well-known services,. Nmap would report that those ports probably correspond to a mail server (SMTP), web server (HTTP), and name server (DNS) respectively. This lookup is usually accurate—the vast majority of daemons listening on TCP port 25 are, in fact, mail servers. However, you should not bet your security on this! People can and do run services on strange ports.. Even if Nmap is right, and the hypothetical server above is running SMTP, HTTP, and DNS servers, that is not a lot of information. When doing vulnerability assessments (or even simple network inventories) of your companies or clients, you really want to know which mail and DNS servers and versions are running. Having an accurate version number helps dramatically in determining which exploits a server is vulnerable to. Version detection helps you obtain this information. After TCP and/or UDP ports are discovered using one of the other scan methods, version detection interrogates those ports to determine more about what is actually running. The nmap-service-probes. database contains probes for querying various services and match expressions to recognize and parse responses. Nmap tries to determine the service protocol (e.g. FTP, SSH, Telnet, HTTP), the application name (e.g. ISC BIND, Apache httpd, Solaris telnetd), the version number, hostname, device type (e.g. printer, router), the OS family (e.g. Windows, Linux) and sometimes miscellaneous details like whether an X server is open to connections, the SSH protocol version, or the KaZaA user name). Of course, most services don´t provide all of this information. If Nmap was compiled with OpenSSL support, it will connect to SSL servers to deduce the service listening behind that encryption layer.. When RPC services are discovered, the Nmap RPC grinder. (-sR). is automatically used to determine the RPC program and version numbers. Some UDP ports are left in the open|filtered state after a UDP port scan is unable to determine whether the port is open or filtered. Version detection will try to elicit a response from these ports (just as it does with open ports), and change the state to open if it succeeds. open|filtered TCP ports are treated the same way. Note that the Nmap -A option enables version detection among other things. A paper documenting the workings, usage, and customization of version detection is available at http://nmap.org/book/vscan.html. When Nmap receives responses from a service but cannot match them to its database, it prints out a special fingerprint and a URL for you to submit if to if you know for sure what is running on the port. Please take a couple minutes to make the submission so that your find can benefit everyone. Thanks to these submissions, Nmap has about 3,000 pattern matches for more than 350 protocols such as SMTP, FTP, HTTP, etc.. Version detection is enabled and controlled with the following options: -sV (Version detection) . Enables version detection, as discussed above. Alternatively, you can use -A, which enables version detection among other things. --allports (Don´t exclude any ports from version detection) . By default, Nmap version detection skips TCP port 9100 because some printers simply print anything sent to that port, leading to dozens of pages of HTTP GET requests, binary SSL session requests, etc. This behavior can be changed by modifying or removing the Exclude directive in nmap-service-probes, or you can specify --allports to scan all ports regardless of any Exclude directive. --version-intensity intensity (Set version scan intensity) . When performing a version scan (-sV), Nmap sends a series of probes, each of which is assigned a rarity value between one and nine. The lower-numbered probes are effective against a wide variety of common services, while the higher numbered ones are rarely useful. The intensity level specifies which probes should be applied. The higher the number, the more likely it is the service will be correctly identified. However, high intensity scans take longer. The intensity must be between 0 and 9. The default is 7. When a probe is registered to the target port via the nmap-service-probes ports directive, that probe is tried regardless of intensity level. This ensures that the DNS probes will always be attempted against any open port 53, the SSL probe will be done against 443, etc. --version-light (Enable light mode) . This is a convenience alias for --version-intensity 2. This light mode makes version scanning much faster, but it is slightly less likely to identify services. --version-all (Try every single probe) . An alias for --version-intensity 9, ensuring that every single probe is attempted against each port. --version-trace (Trace version scan activity) . This causes Nmap to print out extensive debugging info about what version scanning is doing. It is a subset of what you get with --packet-trace. -sR (RPC scan) . This method works in conjunction with the various port scan methods of Nmap. It takes all the TCP/UDP ports found open and floods them with SunRPC program NULL commands in an attempt to determine whether they are RPC ports, and if so, what program and version number they serve up. Thus you can effectively obtain the same info as rpcinfo -p even if the target´s portmapper is behind a firewall (or protected by TCP wrappers). Decoys do not currently work with RPC scan.. This is automatically enabled as part of version scan (-sV) if you request that. As version detection includes this and is much more comprehensive, -sR is rarely needed. OS DETECTION One of Nmap´s best-known features is remote OS detection using TCP/IP stack fingerprinting. Nmap sends a series of TCP and UDP packets to the remote host and examines practically every bit in the responses. After performing dozens of tests such as TCP ISN sampling, TCP options support and ordering, IP ID sampling, and the initial window size check, Nmap compares the results to its nmap-os-db. database of more than a thousand known OS fingerprints and prints out the OS details if there is a match. Each fingerprint includes a freeform textual description of the OS, and a classification which provides the vendor name (e.g. Sun), underlying OS (e.g. Solaris), OS generation (e.g. 10), and device type (general purpose, router, switch, game console, etc). If Nmap is unable to guess the OS of a machine, and conditions are good (e.g. at least one open port and one closed port were found), Nmap will provide a URL you can use to submit the fingerprint if you know (for sure) the OS running on the machine. By doing this you contribute to the pool of operating systems known to Nmap and thus it will be more accurate for everyone. OS detection enables some other tests which make use of information that is gathered during the process anyway. One of these is TCP Sequence Predictability Classification. This measures approximately how hard it is to establish a forged TCP connection against the remote host. It is useful for exploiting source-IP based trust relationships (rlogin, firewall filters, etc) or for hiding the source of an attack. This sort of spoofing is rarely performed any more, but many machines are still vulnerable to it. The actual difficulty number is based on statistical sampling and may fluctuate. It is generally better to use the English classification such as “worthy challenge” or “trivial joke”. This is only reported in normal output in verbose (-v) mode. When verbose mode is enabled along with -O, IP ID sequence generation is also reported. Most machines are in the “incremental” class, which means that they increment the ID field in the IP header for each packet they send. This makes them vulnerable to several advanced information gathering and spoofing attacks. Another bit of extra information enabled by OS detection is a guess at a target´s uptime. This uses the TCP timestamp option (RFC 1323[10]) to guess when a machine was last rebooted. The guess can be inaccurate due to the timestamp counter not being initialized to zero or the counter overflowing and wrapping around, so it is printed only in verbose mode. A paper documenting the workings, usage, and customization of OS detection is available at http://nmap.org/book/osdetect.html. OS detection is enabled and controlled with the following options: -O (Enable OS detection) . Enables OS detection, as discussed above. Alternatively, you can use -A to enable OS detection along with other things. --osscan-limit (Limit OS detection to promising targets) . OS detection is far more effective if at least one open and one closed TCP port are found. Set this option and Nmap will not even try OS detection against hosts that do not meet this criteria. This can save substantial time, particularly on -PN scans against many hosts. It only matters when OS detection is requested with -O or -A. --osscan-guess; --fuzzy (Guess OS detection results) . When Nmap is unable to detect a perfect OS match, it sometimes offers up near-matches as possibilities. The match has to be very close for Nmap to do this by default. Either of these (equivalent) options make Nmap guess more aggressively. Nmap will still tell you when an imperfect match is printed and display its confidence level (percentage) for each guess. --max-os-tries (Set the maximum number of OS detection tries against a target) . When Nmap performs OS detection against a target and fails to find a perfect match, it usually repeats the attempt. By default, Nmap tries five times if conditions are favorable for OS fingerprint submission, and twice when conditions aren´t so good. Specifying a lower --max-os-tries value (such as 1) speeds Nmap up, though you miss out on retries which could potentially identify the OS. Alternatively, a high value may be set to allow even more retries when conditions are favorable. This is rarely done, except to generate better fingerprints for submission and integration into the Nmap OS database. NMAP SCRIPTING ENGINE (NSE) The Nmap Scripting Engine (NSE) is one of Nmap´s most powerful and flexible features. It allows users to write (and share) simple scripts (using the Lua programming language[11], Tasks we had in mind when creating the system include network discovery, more sophisticated version detection, vulnerability detection. NSE can even be used for vulnerability exploitation. To reflect those different uses and to simplify the choice of which scripts to run, each script contains a field associating it with one or more categories. Currently defined categories are safe, intrusive, malware, version, discovery, vuln, auth, and default. These are all described at http://nmap.org/book/nse-usage.html#nse-categories. Scripts are not run in a sandbox and thus could accidentally or maliciously damage your system or invade your privacy. Never run scripts from third parties unless you trust the authors or have carefully audited the scripts yourself. The Nmap Scripting Engine is described in detail at http://nmap.org/book/nse.html and is controlled by the following options: -sC . Performs a script scan using the default set of scripts. It is equivalent to --script=default. Some of the scripts in this category are considered intrusive and should not be run against a target network without permission. --script filename|category|directory|expression|all[,...] . Runs a script scan using the comma-separated list of filenames, script categories, and directories. Each element in the list may also be a Boolean expression describing a more complex set of scripts. Each element is interpreted first as an expression, then as a category, and finally as a file or directory name. The special argument all makes every script in Nmap´s script database eligible to run. The all argument should be used with caution as NSE may contain dangerous scripts including exploits, brute force authentication crackers, and denial of service attacks. File and directory names may be relative or absolute. Absolute names are used directly. Relative paths are looked for in the following places until found: --datadir $NMAPDIR ~/.nmap (not searched on Windows) NMAPDATADIR the current directory A scripts subdirectory is also tried in each of these. When a directory name is given, Nmap loads every file in the directory whose name ends with .nse. All other files are ignored and directories are not searched recursively. When a filename is given, it does not have to have the .nse extension; it will be added automatically if necessary. Nmap scripts are stored in a scripts subdirectory of the Nmap data directory by default (see http://nmap.org/book/data-files.html). For efficiency, scripts are indexed in a database stored in scripts/script.db,. which lists the category or categories in which each script belongs. When referring to scripts from script.db by name, you can use a shell-style ‘*’ wildcard. nmap --script "http-*" Loads all scripts whose name starts with http-, such as http-auth.nse and http-open-proxy.nse. The argument to --script had to be in quotes to protect the wildcard from the shell. More complicated script selection can be done using the and, or, and not operators to build Boolean expressions. The operators have the same precedence[12] as in Lua: not is the highest, followed by and and then or. You can alter precedence by using parentheses. Because expressions contain space characters it is necessary to quote them. nmap --script "not intrusive" Loads every script except for those in the intrusive category. nmap --script "default or safe" This is functionally equivalent to nmap --script "default,safe". It loads all scripts that are in the default category or the safe category or both. nmap --script "default and safe" Loads those scripts that are in both the default and safe categories. nmap --script "(default or safe or intrusive) and not http-*" Loads scripts in the default, safe, or intrusive categories, except for those whose names start with http-. --script-args name1=value1,name2={name3=value3},name4={value4,value5} . Lets you provide arguments to NSE scripts. Arguments are a comma-separated list of name=value pairs. Names and values may be strings not containing whitespace or the characters ‘{’, ‘}’, ‘=’, or ‘,’. To include one of these characters in a string, enclose the string in single or double quotes. Within a quoted string, ‘/’ escapes a quote. A backslash is only used to escape quotation marks in this special case; in all other cases a backslash is interpreted literally. Values may also be tables enclosed in {}, just as in Lua. A table may contain simple string values or more name-value pairs, including nested tables. An example of script arguments: --script-args auth={user=foo,pass=´,{}=bar´},userdb=C:/Path/To/File. The online NSE Documentation Portal at http://nmap.org/nsedoc/ lists the arguments that each script accepts. --script-trace . This option does what --packet-trace does, just one ISO layer higher. If this option is specified all incoming and outgoing communication performed by a script is printed. The displayed information includes the communication protocol, the source, the target and the transmitted data. If more than 5% of all transmitted data is not printable, then the trace output is in a hex dump format. Specifying --packet-trace enables script tracing too. --script-updatedb . This option updates the script database found in scripts/script.db which is used by Nmap to determine the available default scripts and categories. It is only necessary to update the database if you have added or removed NSE scripts from the default scripts directory or if you have changed the categories of any script. This option is generally used by itself: nmap --script-updatedb. TIMING AND PERFORMANCE One of my highest Nmap development priorities has always been performance. A default scan (nmap hostname) of a host on my local network takes a fifth of a second. That is barely enough time to blink, but adds up when you are scanning hundreds or thousands of hosts. Moreover, certain scan options such as UDP scanning and version detection can increase scan times substantially. So can certain firewall configurations, particularly response rate limiting. While Nmap utilizes parallelism and many advanced algorithms to accelerate these scans, the user has ultimate control over how Nmap runs. Expert users carefully craft Nmap commands to obtain only the information they care about while meeting their time constraints. Techniques for improving scan times include omitting non-critical tests, and upgrading to the latest version of Nmap (performance enhancements are made frequently). Optimizing timing parameters can also make a substantial difference. Those options are listed below. Some options accept a time parameter. This is specified in milliseconds by default, though you can append ‘s’, ‘m’, or ‘h’ to the value to specify seconds, minutes, or hours. So the --host-timeout arguments 900000, 900s, and 15m all do the same thing. --min-hostgroup numhosts; --max-hostgroup numhosts (Adjust parallel scan group sizes) . Nmap has the ability to port scan or version scan multiple hosts in parallel. Nmap does this by dividing the target IP space into groups and then scanning one group at a time. In general, larger groups are more efficient. The downside is that host results can´t be provided until the whole group is finished. So if Nmap started out with a group size of 50, the user would not receive any reports (except for the updates offered in verbose mode) until the first 50 hosts are completed. By default, Nmap takes a compromise approach to this conflict. It starts out with a group size as low as five so the first results come quickly and then increases the groupsize to as high as 1024. The exact default numbers depend on the options given. For efficiency reasons, Nmap uses larger group sizes for UDP or few-port TCP scans. When a maximum group size is specified with --max-hostgroup, Nmap will never exceed that size. Specify a minimum size with --min-hostgroup and Nmap will try to keep group sizes above that level. Nmap may have to use smaller groups than you specify if there are not enough target hosts left on a given interface to fulfill the specified minimum. Both may be set to keep the group size within a specific range, though this is rarely desired. These options do not have an effect during the host discovery phase of a scan. This includes plain ping scans (-sP). Host discovery always works in large groups of hosts to improve speed and accuracy. The primary use of these options is to specify a large minimum group size so that the full scan runs more quickly. A common choice is 256 to scan a network in Class C sized chunks. For a scan with many ports, exceeding that number is unlikely to help much. For scans of just a few port numbers, host group sizes of 2048 or more may be helpful. --min-parallelism numprobes; --max-parallelism numprobes (Adjust probe parallelization) . These options control the total number of probes that may be outstanding for a host group. They are used for port scanning and host discovery. By default, Nmap calculates an ever-changing ideal parallelism based on network performance. If packets are being dropped, Nmap slows down and allows fewer outstanding probes. The ideal probe number slowly rises as the network proves itself worthy. These options place minimum or maximum bounds on that variable. By default, the ideal parallelism can drop to one if the network proves unreliable and rise to several hundred in perfect conditions. The most common usage is to set --min-parallelism to a number higher than one to speed up scans of poorly performing hosts or networks. This is a risky option to play with, as setting it too high may affect accuracy. Setting this also reduces Nmap´s ability to control parallelism dynamically based on network conditions. A value of ten might be reasonable, though I only adjust this value as a last resort. The --max-parallelism option is sometimes set to one to prevent Nmap from sending more than one probe at a time to hosts. The --scan-delay option, discussed later, is another way to do this. --min-rtt-timeout time, --max-rtt-timeout time, --initial-rtt-timeout time (Adjust probe timeouts) . Nmap maintains a running timeout value for determining how long it will wait for a probe response before giving up or retransmitting the probe. This is calculated based on the response times of previous probes. If the network latency shows itself to be significant and variable, this timeout can grow to several seconds. It also starts at a conservative (high) level and may stay that way for a while when Nmap scans unresponsive hosts. Specifying a lower --max-rtt-timeout and --initial-rtt-timeout than the defaults can cut scan times significantly. This is particularly true for pingless (-PN) scans, and those against heavily filtered networks. Don´t get too aggressive though. The scan can end up taking longer if you specify such a low value that many probes are timing out and retransmitting while the response is in transit. If all the hosts are on a local network, 100 milliseconds is a reasonable aggressive --max-rtt-timeout value. If routing is involved, ping a host on the network first with the ICMP ping utility, or with a custom packet crafter such as hping2. that is more likely to get through a firewall. Look at the maximum round trip time out of ten packets or so. You might want to double that for the --initial-rtt-timeout and triple or quadruple it for the --max-rtt-timeout. I generally do not set the maximum RTT below 100 ms, no matter what the ping times are. Nor do I exceed 1000 ms. --min-rtt-timeout is a rarely used option that could be useful when a network is so unreliable that even Nmap´s default is too aggressive. Since Nmap only reduces the timeout down to the minimum when the network seems to be reliable, this need is unusual and should be reported as a bug to the nmap-dev mailing list.. --max-retries numtries (Specify the maximum number of port scan probe retransmissions) . When Nmap receives no response to a port scan probe, it could mean the port is filtered. Or maybe the probe or response was simply lost on the network. It is also possible that the target host has rate limiting enabled that temporarily blocked the response. So Nmap tries again by retransmitting the initial probe. If Nmap detects poor network reliability, it may try many more times before giving up on a port. While this benefits accuracy, it also lengthen scan times. When performance is critical, scans may be sped up by limiting the number of retransmissions allowed. You can even specify --max-retries 0 to prevent any retransmissions, though that is only recommended for situations such as informal surveys where occasional missed ports and hosts are acceptable. The default (with no -T template) is to allow ten retransmissions. If a network seems reliable and the target hosts aren´t rate limiting, Nmap usually only does one retransmission. So most target scans aren´t even affected by dropping --max-retries to a low value such as three. Such values can substantially speed scans of slow (rate limited) hosts. You usually lose some information when Nmap gives up on ports early, though that may be preferable to letting the --host-timeout expire and losing all information about the target. --host-timeout time (Give up on slow target hosts) . Some hosts simply take a long time to scan. This may be due to poorly performing or unreliable networking hardware or software, packet rate limiting, or a restrictive firewall. The slowest few percent of the scanned hosts can eat up a majority of the scan time. Sometimes it is best to cut your losses and skip those hosts initially. Specify --host-timeout with the maximum amount of time you are willing to wait. For example, specify 30m to ensure that Nmap doesn´t waste more than half an hour on a single host. Note that Nmap may be scanning other hosts at the same time during that half an hour, so it isn´t a complete loss. A host that times out is skipped. No port table, OS detection, or version detection results are printed for that host. --scan-delay time; --max-scan-delay time (Adjust delay between probes) . This option causes Nmap to wait at least the given amount of time between each probe it sends to a given host. This is particularly useful in the case of rate limiting.. Solaris machines (among many others) will usually respond to UDP scan probe packets with only one ICMP message per second. Any more than that sent by Nmap will be wasteful. A --scan-delay of 1s will keep Nmap at that slow rate. Nmap tries to detect rate limiting and adjust the scan delay accordingly, but it doesn´t hurt to specify it explicitly if you already know what rate works best. When Nmap adjusts the scan delay upward to cope with rate limiting, the scan slows down dramatically. The --max-scan-delay option specifies the largest delay that Nmap will allow. A low --max-scan-delay can speed up Nmap, but it is risky. Setting this value too low can lead to wasteful packet retransmissions and possible missed ports when the target implements strict rate limiting. Another use of --scan-delay is to evade threshold based intrusion detection and prevention systems (IDS/IPS).. --min-rate number; --max-rate number (Directly control the scanning rate) . Nmap´s dynamic timing does a good job of finding an appropriate speed at which to scan. Sometimes, however, you may happen to know an appropriate scanning rate for a network, or you may have to guarantee that a scan will be finished by a certain time. Or perhaps you must keep Nmap from scanning too quickly. The --min-rate and --max-rate options are designed for these situations. When the --min-rate option is given Nmap will do its best to send packets as fast as or faster than the given rate. The argument is a positive real number representing a packet rate in packets per second. For example, specifying --min-rate 300 means that Nmap will try to keep the sending rate at or above 300 packets per second. Specifying a minimum rate does not keep Nmap from going faster if conditions warrant. Likewise, --max-rate limits a scan´s sending rate to a given maximum. Use --max-rate 100, for example, to limit sending to 100 packets per second on a fast network. Use --max-rate 0.1 for a slow scan of one packet every ten seconds. Use --min-rate and --max-rate together to keep the rate inside a certain range. These two options are global, affecting an entire scan, not individual hosts. They only affect port scans and host discovery scans. Other features like OS detection implement their own timing. There are two conditions when the actual scanning rate may fall below the requested minimum. The first is if the minimum is faster than the fastest rate at which Nmap can send, which is dependent on hardware. In this case Nmap will simply send packets as fast as possible, but be aware that such high rates are likely to cause a loss of accuracy. The second case is when Nmap has nothing to send, for example at the end of a scan when the last probes have been sent and Nmap is waiting for them to time out or be responded to. It´s normal to see the scanning rate drop at the end of a scan or in between hostgroups. The sending rate may temporarily exceed the maximum to make up for unpredictable delays, but on average the rate will stay at or below the maximum. Specifying a minimum rate should be done with care. Scanning faster than a network can support may lead to a loss of accuracy. In some cases, using a faster rate can make a scan take longer than it would with a slower rate. This is because Nmap´s adaptive retransmission algorithms will detect the network congestion caused by an excessive scanning rate and increase the number of retransmissions in order to improve accuracy. So even though packets are sent at a higher rate, more packets are sent overall. Cap the number of retransmissions with the --max-retries option if you need to set an upper limit on total scan time. --defeat-rst-ratelimit . Many hosts have long used rate limiting. to reduce the number of ICMP error messages (such as port-unreachable errors) they send. Some systems now apply similar rate limits to the RST (reset) packets they generate. This can slow Nmap down dramatically as it adjusts its timing to reflect those rate limits. You can tell Nmap to ignore those rate limits (for port scans such as SYN scan which don´t treat non-responsive ports as open) by specifying --defeat-rst-ratelimit. Using this option can reduce accuracy, as some ports will appear non-responsive because Nmap didn´t wait long enough for a rate-limited RST response. With a SYN scan, the non-response results in the port being labeled filtered rather than the closed state we see when RST packets are received. This option is useful when you only care about open ports, and distinguishing between closed and filtered ports isn´t worth the extra time. -T paranoid|sneaky|polite|normal|aggressive|insane (Set a timing template) . While the fine-grained timing controls discussed in the previous section are powerful and effective, some people find them confusing. Moreover, choosing the appropriate values can sometimes take more time than the scan you are trying to optimize. So Nmap offers a simpler approach, with six timing templates. You can specify them with the -T option and their number (0–5) or their name. The template names are paranoid (0), sneaky (1), polite (2), normal (3), aggressive (4), and insane (5). The first two are for IDS evasion. Polite mode slows down the scan to use less bandwidth and target machine resources. Normal mode is the default and so -T3 does nothing. Aggressive mode speeds scans up by making the assumption that you are on a reasonably fast and reliable network. Finally insane mode. assumes that you are on an extraordinarily fast network or are willing to sacrifice some accuracy for speed. These templates allow the user to specify how aggressive they wish to be, while leaving Nmap to pick the exact timing values. The templates also make some minor speed adjustments for which fine-grained control options do not currently exist. For example, -T4. prohibits the dynamic scan delay from exceeding 10 ms for TCP ports and -T5 caps that value at 5 ms. Templates can be used in combination with fine-grained controls, and the fine-grained controls will you specify will take precedence over the timing template default for that parameter. I recommend using -T4 when scanning reasonably modern and reliable networks. Keep that option even when you add fine-grained controls so that you benefit from those extra minor optimizations that it enables. If you are on a decent broadband or ethernet connection, I would recommend always using -T4. Some people love -T5 though it is too aggressive for my taste. People sometimes specify -T2 because they think it is less likely to crash hosts or because they consider themselves to be polite in general. They often don´t realize just how slow -T polite. really is. Their scan may take ten times longer than a default scan. Machine crashes and bandwidth problems are rare with the default timing options (-T3) and so I normally recommend that for cautious scanners. Omitting version detection is far more effective than playing with timing values at reducing these problems. While -T0. and -T1. may be useful for avoiding IDS alerts, they will take an extraordinarily long time to scan thousands of machines or ports. For such a long scan, you may prefer to set the exact timing values you need rather than rely on the canned -T0 and -T1 values. The main effects of T0 are serializing the scan so only one port is scanned at a time, and waiting five minutes between sending each probe. T1 and T2 are similar but they only wait 15 seconds and 0.4 seconds, respectively, between probes. T3 is Nmap´s default behavior, which includes parallelization.. -T4 does the equivalent of --max-rtt-timeout 1250 --initial-rtt-timeout 500 --max-retries 6 and sets the maximum TCP scan delay to 10 milliseconds. T5 does the equivalent of --max-rtt-timeout 300 --min-rtt-timeout 50 --initial-rtt-timeout 250 --max-retries 2 --host-timeout 15m as well as setting the maximum TCP scan delay to 5 ms. FIREWALL/IDS EVASION AND SPOOFING Many Internet pioneers envisioned a global open network with a universal IP address space allowing virtual connections between any two nodes. This allows hosts to act as true peers, serving and retrieving information from each other. People could access all of their home systems from work, changing the climate control settings or unlocking the doors for early guests. This vision of universal connectivity has been stifled by address space shortages and security concerns. In the early 1990s, organizations began deploying firewalls for the express purpose of reducing connectivity. Huge networks were cordoned off from the unfiltered Internet by application proxies, network address translation, and packet filters. The unrestricted flow of information gave way to tight regulation of approved communication channels and the content that passes over them. Network obstructions such as firewalls can make mapping a network exceedingly difficult. It will not get any easier, as stifling casual reconnaissance is often a key goal of implementing the devices. Nevertheless, Nmap offers many features to help understand these complex networks, and to verify that filters are working as intended. It even supports mechanisms for bypassing poorly implemented defenses. One of the best methods of understanding your network security posture is to try to defeat it. Place yourself in the mind-set of an attacker, and deploy techniques from this section against your networks. Launch an FTP bounce scan, idle scan, fragmentation attack, or try to tunnel through one of your own proxies. In addition to restricting network activity, companies are increasingly monitoring traffic with intrusion detection systems (IDS). All of the major IDSs ship with rules designed to detect Nmap scans because scans are sometimes a precursor to attacks. Many of these products have recently morphed into intrusion prevention systems (IPS). that actively block traffic deemed malicious. Unfortunately for network administrators and IDS vendors, reliably detecting bad intentions by analyzing packet data is a tough problem. Attackers with patience, skill, and the help of certain Nmap options can usually pass by IDSs undetected. Meanwhile, administrators must cope with large numbers of false positive results where innocent activity is misdiagnosed and alerted on or blocked. Occasionally people suggest that Nmap should not offer features for evading firewall rules or sneaking past IDSs. They argue that these features are just as likely to be misused by attackers as used by administrators to enhance security. The problem with this logic is that these methods would still be used by attackers, who would just find other tools or patch the functionality into Nmap. Meanwhile, administrators would find it that much harder to do their jobs. Deploying only modern, patched FTP servers is a far more powerful defense than trying to prevent the distribution of tools implementing the FTP bounce attack. There is no magic bullet (or Nmap option) for detecting and subverting firewalls and IDS systems. It takes skill and experience. A tutorial is beyond the scope of this reference guide, which only lists the relevant options and describes what they do. -f (fragment packets); --mtu (using the specified MTU) . The -f option causes the requested scan (including ping scans) to use tiny fragmented IP packets. The idea is to split up the TCP header over several packets to make it harder for packet filters, intrusion detection systems, and other annoyances to detect what you are doing. Be careful with this! Some programs have trouble handling these tiny packets. The old-school sniffer named Sniffit segmentation faulted immediately upon receiving the first fragment. Specify this option once, and Nmap splits the packets into eight bytes or less after the IP header. So a 20-byte TCP header would be split into three packets. Two with eight bytes of the TCP header, and one with the final four. Of course each fragment also has an IP header. Specify -f again to use 16 bytes per fragment (reducing the number of fragments).. Or you can specify your own offset size with the --mtu option. Don´t also specify -f if you use --mtu. The offset must be a multiple of eight. While fragmented packets won´t get by packet filters and firewalls that queue all IP fragments, such as the CONFIG_IP_ALWAYS_DEFRAG option in the Linux kernel, some networks can´t afford the performance hit this causes and thus leave it disabled. Others can´t enable this because fragments may take different routes into their networks. Some source systems defragment outgoing packets in the kernel. Linux with the iptables. connection tracking module is one such example. Do a scan while a sniffer such as Wireshark. is running to ensure that sent packets are fragmented. If your host OS is causing problems, try the --send-eth. option to bypass the IP layer and send raw ethernet frames. Fragmentation is only supported for Nmap´s raw packet features, which includes TCP and UDP port scans (except connect scan and FTP bounce scan) and OS detection. Features such as version detection and the Nmap Scripting Engine generally don´t support fragmentation because they rely on your host´s TCP stack to communicate with target services. -D decoy1[,decoy2][,ME][,...] (Cloak a scan with decoys) . Causes a decoy scan to be performed, which makes it appear to the remote host that the host(s) you specify as decoys are scanning the target network too. Thus their IDS might report 5–10 port scans from unique IP addresses, but they won´t know which IP was scanning them and which were innocent decoys. While this can be defeated through router path tracing, response-dropping, and other active mechanisms, it is generally an effective technique for hiding your IP address. Separate each decoy host with commas, and you can optionally use ME. as one of the decoys to represent the position for your real IP address. If you put ME in the sixth position or later, some common port scan detectors (such as Solar Designer´s. excellent Scanlogd). are unlikely to show your IP address at all. If you don´t use ME, Nmap will put you in a random position. You can also use RND. to generate a random, non-reserved IP address, or RND:number to generate number addresses. Note that the hosts you use as decoys should be up or you might accidentally SYN flood your targets. Also it will be pretty easy to determine which host is scanning if only one is actually up on the network. You might want to use IP addresses instead of names (so the decoy networks don´t see you in their nameserver logs). Decoys are used both in the initial ping scan (using ICMP, SYN, ACK, or whatever) and during the actual port scanning phase. Decoys are also used during remote OS detection (-O). Decoys do not work with version detection or TCP connect scan. When a scan delay is in effect, the delay is enforced between each batch of spoofed probes, not between each individual probe. Because decoys are sent as a batch all at once, they may temporarily violate congestion control limits. It is worth noting that using too many decoys may slow your scan and potentially even make it less accurate. Also, some ISPs will filter out your spoofed packets, but many do not restrict spoofed IP packets at all. -S IP_Address (Spoof source address) . In some circumstances, Nmap may not be able to determine your source address (Nmap will tell you if this is the case). In this situation, use -S with the IP address of the interface you wish to send packets through. Another possible use of this flag is to spoof the scan to make the targets think that someone else is scanning them. Imagine a company being repeatedly port scanned by a competitor! The -e option and -PN are generally required for this sort of usage. Note that you usually won´t receive reply packets back (they will be addressed to the IP you are spoofing), so Nmap won´t produce useful reports. -e interface (Use specified interface) . Tells Nmap what interface to send and receive packets on. Nmap should be able to detect this automatically, but it will tell you if it cannot. --source-port portnumber; -g portnumber (Spoof source port number) . One surprisingly common misconfiguration is to trust traffic based only on the source port number. It is easy to understand how this comes about. An administrator will set up a shiny new firewall, only to be flooded with complains from ungrateful users whose applications stopped working. In particular, DNS may be broken because the UDP DNS replies from external servers can no longer enter the network. FTP is another common example. In active FTP transfers, the remote server tries to establish a connection back to the client to transfer the requested file. Secure solutions to these problems exist, often in the form of application-level proxies or protocol-parsing firewall modules. Unfortunately there are also easier, insecure solutions. Noting that DNS replies come from port 53 and active FTP from port 20, many administrators have fallen into the trap of simply allowing incoming traffic from those ports. They often assume that no attacker would notice and exploit such firewall holes. In other cases, administrators consider this a short-term stop-gap measure until they can implement a more secure solution. Then they forget the security upgrade. Overworked network administrators are not the only ones to fall into this trap. Numerous products have shipped with these insecure rules. Even Microsoft has been guilty. The IPsec filters that shipped with Windows 2000 and Windows XP contain an implicit rule that allows all TCP or UDP traffic from port 88 (Kerberos). In another well-known case, versions of the Zone Alarm personal firewall up to 2.1.25 allowed any incoming UDP packets with the source port 53 (DNS) or 67 (DHCP). Nmap offers the -g and --source-port options (they are equivalent) to exploit these weaknesses. Simply provide a port number and Nmap will send packets from that port where possible. Nmap must use different port numbers for certain OS detection tests to work properly, and DNS requests ignore the --source-port flag because Nmap relies on system libraries to handle those. Most TCP scans, including SYN scan, support the option completely, as does UDP scan. --data-length number (Append random data to sent packets) . Normally Nmap sends minimalist packets containing only a header. So its TCP packets are generally 40 bytes and ICMP echo requests are just 28. Some UDP ports. and IP protocols. get a custom payload by default. This option tells Nmap to append the given number of random bytes to most of the packets it sends, and not to use any protocol-specific payloads. (Use --data-length 0 for no random or protocol-specific payloads.. OS detection (-O) packets are not affected. because accuracy there requires probe consistency, but most pinging and portscan packets support this. It slows things down a little, but can make a scan slightly less conspicuous. --ip-options S|R [route]|L [route]|T|U ... ; --ip-options hex string (Send packets with specified ip options) . The IP protocol[13] offers several options which may be placed in packet headers. Unlike the ubiquitous TCP options, IP options are rarely seen due to practicality and security concerns. In fact, many Internet routers block the most dangerous options such as source routing. Yet options can still be useful in some cases for determining and manipulating the network route to target machines. For example, you may be able to use the record route option to determine a path to a target even when more traditional traceroute-style approaches fail. Or if your packets are being dropped by a certain firewall, you may be able to specify a different route with the strict or loose source routing options. The most powerful way to specify IP options is to simply pass in values as the argument to --ip-options. Precede each hex number with /x then the two digits. You may repeat certain characters by following them with an asterisk and then the number of times you wish them to repeat. For example, /x01/x07/x04/x00*36/x01 is a hex string containing 36 NUL bytes. Nmap also offers a shortcut mechanism for specifying options. Simply pass the letter R, T, or U to request record-route,. record-timestamp,. or both options together, respectively. Loose or strict source routing. may be specified with an L or S followed by a space and then a space-separated list of IP addresses. If you wish to see the options in packets sent and received, specify --packet-trace. For more information and examples of using IP options with Nmap, see http://seclists.org/nmap-dev/2006/q3/0052.html. --ttl value (Set IP time-to-live field) . Sets the IPv4 time-to-live field in sent packets to the given value. --randomize-hosts (Randomize target host order) . Tells Nmap to shuffle each group of up to 16384 hosts before it scans them. This can make the scans less obvious to various network monitoring systems, especially when you combine it with slow timing options. If you want to randomize over larger group sizes, increase PING_GROUP_SZ. in nmap.h. and recompile. An alternative solution is to generate the target IP list with a list scan (-sL -n -oN filename), randomize it with a Perl script, then provide the whole list to Nmap with -iL.. --spoof-mac MAC address, prefix, or vendor name (Spoof MAC address) . Asks Nmap to use the given MAC address for all of the raw ethernet frames it sends. This option implies --send-eth. to ensure that Nmap actually sends ethernet-level packets. The MAC given can take several formats. If it is simply the number 0, Nmap chooses a completely random MAC address for the session. If the given string is an even number of hex digits (with the pairs optionally separated by a colon), Nmap will use those as the MAC. If fewer than 12 hex digits are provided, Nmap fills in the remainder of the six bytes with random values. If the argument isn´t a zero or hex string, Nmap looks through nmap-mac-prefixes to find a vendor name containing the given string (it is case insensitive). If a match is found, Nmap uses the vendor´s OUI (three-byte prefix). and fills out the remaining three bytes randomly. Valid --spoof-mac argument examples are Apple, 0, 01:02:03:04:05:06, deadbeefcafe, 0020F2, and Cisco. This option only affects raw packet scans such as SYN scan or OS detection, not connection-oriented features such as version detection or the Nmap Scripting Engine. --badsum (Send packets with bogus TCP/UDP checksums) . Asks Nmap to use an invalid TCP, UDP or SCTP checksum for packets sent to target hosts. Since virtually all host IP stacks properly drop these packets, any responses received are likely coming from a firewall or IDS that didn´t bother to verify the checksum. For more details on this technique, see http://nmap.org/p60-12.html --adler32 (Use deprecated Adler32 instead of CRC32C for SCTP checksums) . Asks Nmap to use the deprecated Adler32 algorithm for calculating the SCTP checksum. If --adler32 is not given, CRC-32C (Castagnoli) is used. RFC 2960[14] originally defined Adler32 as checksum algorithm for SCTP; RFC 4960[7] later redefined the SCTP checksums to use CRC-32C. Current SCTP implementations should be using CRC-32C, but in order to elicit responses from old, legacy SCTP implementations, it may be preferrable to use Adler32. OUTPUT Any security tool is only as useful as the output it generates. Complex tests and algorithms are of little value if they aren´t presented in an organized and comprehensible fashion. Given the number of ways Nmap is used by people and other software, no single format can please everyone. So Nmap offers several formats, including the interactive mode for humans to read directly and XML for easy parsing by software. In addition to offering different output formats, Nmap provides options for controlling the verbosity of output as well as debugging messages. Output types may be sent to standard output or to named files, which Nmap can append to or clobber. Output files may also be used to resume aborted scans. Nmap makes output available in five different formats. The default is called interactive output,. and it is sent to standard output (stdout).. There is also normal output,. which is similar to interactive except that it displays less runtime information and warnings since it is expected to be analyzed after the scan completes rather than interactively. XML output. is one of the most important output types, as it can be converted to HTML, easily parsed by programs such as Nmap graphical user interfaces, or imported into databases. The two remaining output types are the simple grepable output. which includes most information for a target host on a single line, and sCRiPt KiDDi3 0utPUt. for users who consider themselves |<-r4d. While interactive output is the default and has no associated command-line options, the other four format options use the same syntax. They take one argument, which is the filename that results should be stored in. Multiple formats may be specified, but each format may only be specified once. For example, you may wish to save normal output for your own review while saving XML of the same scan for programmatic analysis. You might do this with the options -oX myscan.xml -oN myscan.nmap. While this chapter uses the simple names like myscan.xml for brevity, more descriptive names are generally recommended. The names chosen are a matter of personal preference, though I use long ones that incorporate the scan date and a word or two describing the scan, placed in a directory named after the company I´m scanning. While these options save results to files, Nmap still prints interactive output to stdout as usual. For example, the command nmap -oX myscan.xml target prints XML to myscan.xml and fills standard output with the same interactive results it would have printed if -oX wasn´t specified at all. You can change this by passing a hyphen character as the argument to one of the format types. This causes Nmap to deactivate interactive output, and instead print results in the format you specified to the standard output stream. So the command nmap -oX - target will send only XML output to stdout.. Serious errors may still be printed to the normal error stream, stderr.. Unlike some Nmap arguments, the space between the logfile option flag (such as -oX) and the filename or hyphen is mandatory. If you omit the flags and give arguments such as -oG- or -oXscan.xml, a backwards compatibility feature of Nmap will cause the creation of normal format output files named G- and Xscan.xml respectively. All of these arguments support strftime-like. conversions in the filename. %H, %M, %S, %m, %d, %y, and %Y are all exactly the same as in strftime. %T is the same as %H%M%S, %R is the same as %H%M, and %D is the same as %m%d%y. A % followed by any other character just yields that character (%% gives you a percent symbol). So -oX ´scan-%T-%D.xml´ will use an XML file in the form of scan-144840-121307.xml. Nmap also offers options to control scan verbosity and to append to output files rather than clobbering them. All of these options are described below. Nmap Output Formats -oN filespec (normal output) . Requests that normal output be directed to the given filename. As discussed above, this differs slightly from interactive output. -oX filespec (XML output) . Requests that XML output be directed to the given filename. Nmap includes a document type definition (DTD) which allows XML parsers to validate Nmap XML output. While it is primarily intended for programmatic use, it can also help humans interpret Nmap XML output. The DTD defines the legal elements of the format, and often enumerates the attributes and values they can take on. The latest version is always available from http://nmap.org/data/nmap.dtd. XML offers a stable format that is easily parsed by software. Free XML parsers are available for all major computer languages, including C/C++, Perl, Python, and Java. People have even written bindings for most of these languages to handle Nmap output and execution specifically. Examples are Nmap::Scanner[15] and Nmap::Parser[16] in Perl CPAN. In almost all cases that a non-trivial application interfaces with Nmap, XML is the preferred format. The XML output references an XSL stylesheet which can be used to format the results as HTML. The easiest way to use this is simply to load the XML output in a web browser such as Firefox or IE. By default, this will only work on the machine you ran Nmap on (or a similarly configured one) due to the hard-coded nmap.xsl filesystem path. Use the --webxml or --stylesheet options to create portable XML files that render as HTML on any web-connected machine. -oS filespec (ScRipT KIdd|3 oUTpuT) . Script kiddie output is like interactive output, except that it is post-processed to better suit the l33t HaXXorZ who previously looked down on Nmap due to its consistent capitalization and spelling. Humor impaired people should note that this option is making fun of the script kiddies before flaming me for supposedly “helping them”. -oG filespec (grepable output) . This output format is covered last because it is deprecated. The XML output format is far more powerful, and is nearly as convenient for experienced users. XML is a standard for which dozens of excellent parsers are available, while grepable output is my own simple hack. XML is extensible to support new Nmap features as they are released, while I often must omit those features from grepable output for lack of a place to put them. Nevertheless, grepable output is still quite popular. It is a simple format that lists each host on one line and can be trivially searched and parsed with standard Unix tools such as grep, awk, cut, sed, diff, and Perl. Even I usually use it for one-off tests done at the command line. Finding all the hosts with the SSH port open or that are running Solaris takes only a simple grep to identify the hosts, piped to an awk or cut command to print the desired fields. Grepable output consists of comments (lines starting with a pound (#)). and target lines. A target line includes a combination of six labeled fields, separated by tabs and followed with a colon. The fields are Host, Ports, Protocols, Ignored State, OS, Seq Index, IP ID, and Status. The most important of these fields is generally Ports, which gives details on each interesting port. It is a comma separated list of port entries. Each port entry represents one interesting port, and takes the form of seven slash (/) separated subfields. Those subfields are: Port number, State, Protocol, Owner, Service, SunRPC info, and Version info. As with XML output, this man page does not allow for documenting the entire format. A more detailed look at the Nmap grepable output format is available from http://nmap.org/book/output-formats-grepable-output.html. -oA basename (Output to all formats) . As a convenience, you may specify -oA basename to store scan results in normal, XML, and grepable formats at once. They are stored in basename.nmap, basename.xml, and basename.gnmap, respectively. As with most programs, you can prefix the filenames with a directory path, such as ~/nmaplogs/foocorp/ on Unix or c:/hacking/sco on Windows. Verbosity and debugging options -v (Increase verbosity level) . Increases the verbosity level, causing Nmap to print more information about the scan in progress. Open ports are shown as they are found and completion time estimates are provided when Nmap thinks a scan will take more than a few minutes. Use it twice or more for even greater verbosity. Most changes only affect interactive output, and some also affect normal and script kiddie output. The other output types are meant to be processed by machines, so Nmap can give substantial detail by default in those formats without fatiguing a human user. However, there are a few changes in other modes where output size can be reduced substantially by omitting some detail. For example, a comment line in the grepable output that provides a list of all ports scanned is only printed in verbose mode because it can be quite long. -d [level] (Increase or set debugging level) . When even verbose mode doesn´t provide sufficient data for you, debugging is available to flood you with much more! As with the verbosity option (-v), debugging is enabled with a command-line flag (-d) and the debug level can be increased by specifying it multiple times.. Alternatively, you can set a debug level by giving an argument to -d. For example, -d9 sets level nine. That is the highest effective level and will produce thousands of lines unless you run a very simple scan with very few ports and targets. Debugging output is useful when a bug is suspected in Nmap, or if you are simply confused as to what Nmap is doing and why. As this feature is mostly intended for developers, debug lines aren´t always self-explanatory. You may get something like: Timeout vals: srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==> srtt: 14987 rttvar: 14987 to: 100000. If you don´t understand a line, your only recourses are to ignore it, look it up in the source code, or request help from the development list (nmap-dev).. Some lines are self explanatory, but the messages become more obscure as the debug level is increased. --reason (Host and port state reasons) . Shows the reason each port is set to a specific state and the reason each host is up or down. This option displays the type of the packet that determined a port or hosts state. For example, A RST packet from a closed port or an echo reply from an alive host. The information Nmap can provide is determined by the type of scan or ping. The SYN scan and SYN ping (-sS and -PS) are very detailed, but the TCP connect scan (-sT) is limited by the implementation of the connect system call. This feature is automatically enabled by the debug option (-d). and the results are stored in XML log files even if this option is not specified. --stats-every time (Print periodic timing stats) . Periodically prints a timing status message after each interval of time. The time is a specification of the kind described in the section called “TIMING AND PERFORMANCE”; so for example, use --stats-every 10s to get a status update every 10 seconds. Updates are printed to interactive output (the screen) and XML output. --packet-trace (Trace packets and data sent and received) . Causes Nmap to print a summary of every packet sent or received. This is often used for debugging, but is also a valuable way for new users to understand exactly what Nmap is doing under the covers. To avoid printing thousands of lines, you may want to specify a limited number of ports to scan, such as -p20-30. If you only care about the goings on of the version detection subsystem, use --version-trace instead. If you only care about script tracing, specify --script-trace. With --packet-trace, you get all of the above. --open (Show only open (or possibly open) ports) . Sometimes you only care about ports you can actually connect to (open ones), and don´t want results cluttered with closed, filtered, and closed|filtered ports. Output customization is normally done after the scan using tools such as grep, awk, and Perl, but this feature was added due to overwhelming requests. Specify --open to only see open, open|filtered, and unfiltered ports. These three ports are treated just as they normally are, which means that open|filtered and unfiltered may be condensed into counts if there are an overwhelming number of them. --iflist (List interfaces and routes) . Prints the interface list and system routes as detected by Nmap. This is useful for debugging routing problems or device mischaracterization (such as Nmap treating a PPP connection as ethernet). --log-errors (Log errors/warnings to normal mode output file) . Warnings and errors printed by Nmap usually go only to the screen (interactive output), leaving any normal-format output files (usually specified with -oN) uncluttered. When you do want to see those messages in the normal output file you specified, add this option. It is useful when you aren´t watching the interactive output or when you want to record errors while debugging a problem. The error and warning messages will still appear in interactive mode too. This won´t work for most errors related to bad command-line arguments because Nmap may not have initialized its output files yet. In addition, some Nmap error and warning messages use a different system which does not yet support this option. An alternative to --log-errors is redirecting interactive output (including the standard error stream) to a file. Most Unix shells make this approach easy, though it can be difficult on Windows. Miscellaneous output options --append-output (Append to rather than clobber output files) . When you specify a filename to an output format flag such as -oX or -oN, that file is overwritten by default. If you prefer to keep the existing content of the file and append the new results, specify the --append-output option. All output filenames specified in that Nmap execution will then be appended to rather than clobbered. This doesn´t work well for XML (-oX) scan data as the resultant file generally won´t parse properly until you fix it up by hand. --resume filename (Resume aborted scan) . Some extensive Nmap runs take a very long time—on the order of days. Such scans don´t always run to completion. Restrictions may prevent Nmap from being run during working hours, the network could go down, the machine Nmap is running on might suffer a planned or unplanned reboot, or Nmap itself could crash. The administrator running Nmap could cancel it for any other reason as well, by pressing ctrl-C. Restarting the whole scan from the beginning may be undesirable. Fortunately, if normal (-oN) or grepable (-oG) logs were kept, the user can ask Nmap to resume scanning with the target it was working on when execution ceased. Simply specify the --resume option and pass the normal/grepable output file as its argument. No other arguments are permitted, as Nmap parses the output file to use the same ones specified previously. Simply call Nmap as nmap --resume logfilename. Nmap will append new results to the data files specified in the previous execution. Resumption does not support the XML output format because combining the two runs into one valid XML file would be difficult. --stylesheet path or URL (Set XSL stylesheet to transform XML output) . Nmap ships with an XSL stylesheet named nmap.xsl for viewing or translating XML output to HTML. The XML output includes an xml-stylesheet directive which points to nmap.xml where it was initially installed by Nmap (or in the current working directory on Windows). Simply load Nmap´s XML output in a modern web browser and it should retrieve nmap.xsl from the filesystem and use it to render results. If you wish to use a different stylesheet, specify it as the argument to --stylesheet. You must pass the full pathname or URL. One common invocation is --stylesheet http://nmap.org/data/nmap.xsl. This tells a browser to load the latest version of the stylesheet from Nmap.Org. The --webxml option does the same thing with less typing and memorization. Loading the XSL from Nmap.Org makes it easier to view results on a machine that doesn´t have Nmap (and thus nmap.xsl) installed. So the URL is often more useful, but the local filesystem location of nmap.xsl is used by default for privacy reasons. --webxml (Load stylesheet from Nmap.Org) . This convenience option is simply an alias for --stylesheet http://nmap.org/data/nmap.xsl. --no-stylesheet (Omit XSL stylesheet declaration from XML) . Specify this option to prevent Nmap from associating any XSL stylesheet with its XML output. The xml-stylesheet directive is omitted. MISCELLANEOUS OPTIONS This section describes some important (and not-so-important) options that don´t really fit anywhere else. -6 (Enable IPv6 scanning) . Since 2002, Nmap has offered IPv6 support for its most popular features. In particular, ping scanning (TCP-only), connect scanning, and version detection all support IPv6. The command syntax is the same as usual except that you also add the -6 option. Of course, you must use IPv6 syntax if you specify an address rather than a hostname. An address might look like 3ffe:7501:4819:2000:210:f3ff:fe03:14d0, so hostnames are recommended. The output looks the same as usual, with the IPv6 address on the “interesting ports” line being the only IPv6 give away. While IPv6 hasn´t exactly taken the world by storm, it gets significant use in some (usually Asian) countries and most modern operating systems support it. To use Nmap with IPv6, both the source and target of your scan must be configured for IPv6. If your ISP (like most of them) does not allocate IPv6 addresses to you, free tunnel brokers are widely available and work fine with Nmap. I use the free IPv6 tunnel broker. service at http://www.tunnelbroker.net. Other tunnel brokers are listed at Wikipedia[17]. 6to4 tunnels are another popular, free approach. -A (Aggressive scan options) . This option enables additional advanced and aggressive options. I haven´t decided exactly which it stands for yet. Presently this enables OS detection (-O), version scanning (-sV), script scanning (-sC) and traceroute (--traceroute). More features may be added in the future. The point is to enable a comprehensive set of scan options without people having to remember a large set of flags. However, because script scanning with the default set is considered intrusive, you should not use -A against target networks without permission. This option only enables features, and not timing options (such as -T4) or verbosity options (-v) that you might want as well. --datadir directoryname (Specify custom Nmap data file location) . Nmap obtains some special data at runtime in files named nmap-service-probes, nmap-services, nmap-protocols, nmap-rpc, nmap-mac-prefixes, and nmap-os-db. If the location of any of these files has been specified (using the --servicedb or --versiondb options), that location is used for that file. After that, Nmap searches these files in the directory specified with the --datadir option (if any). Any files not found there, are searched for in the directory specified by the NMAPDIR environmental variable. ~/.nmap. for real and effective UIDs (POSIX systems only) or location of the Nmap executable (Win32 only), and then a compiled-in location such as /usr/local/share/nmap or /usr/share/nmap . As a last resort, Nmap will look in the current directory. --servicedb services file (Specify custom services file) . Asks Nmap to use the specified services file rather than the nmap-services data file that comes with Nmap. Using this option also causes a fast scan (-F) to be used. See the description for --datadir for more information on Nmap´s data files. --versiondb service probes file (Specify custom service probes file) . Asks Nmap to use the specified service probes file rather than the nmap-service-probes data file that comes with Nmap. See the description for --datadir for more information on Nmap´s data files. --send-eth (Use raw ethernet sending) . Asks Nmap to send packets at the raw ethernet (data link) layer rather than the higher IP (network) layer. By default, Nmap chooses the one which is generally best for the platform it is running on. Raw sockets (IP layer). are generally most efficient for Unix machines, while ethernet frames are required for Windows operation since Microsoft disabled raw socket support. Nmap still uses raw IP packets on Unix despite this option when there is no other choice (such as non-ethernet connections). --send-ip (Send at raw IP level) . Asks Nmap to send packets via raw IP sockets rather than sending lower level ethernet frames. It is the complement to the --send-eth option discussed previously. --privileged (Assume that the user is fully privileged) . Tells Nmap to simply assume that it is privileged enough to perform raw socket sends, packet sniffing, and similar operations that usually require root privileges. on Unix systems. By default Nmap quits if such operations are requested but geteuid is not zero. --privileged is useful with Linux kernel capabilities and similar systems that may be configured to allow unprivileged users to perform raw-packet scans. Be sure to provide this option flag before any flags for options that require privileges (SYN scan, OS detection, etc.). The NMAP_PRIVILEGED. environmental variable may be set as an equivalent alternative to --privileged. --unprivileged (Assume that the user lacks raw socket privileges) . This option is the opposite of --privileged. It tells Nmap to treat the user as lacking network raw socket and sniffing privileges. This is useful for testing, debugging, or when the raw network functionality of your operating system is somehow broken. The NMAP_UNPRIVILEGED. environmental variable may be set as an equivalent alternative to --unprivileged. --release-memory (Release memory before quitting) . This option is only useful for memory-leak debugging. It causes Nmap to release allocated memory just before it quits so that actual memory leaks are easier to spot. Normally Nmap skips this as the OS does this anyway upon process termination. --interactive (Start in interactive mode) . Starts Nmap in interactive mode, which offers an interactive Nmap prompt allowing easy launching of multiple scans (either synchronously or in the background). This is useful for people who scan from multi-user systems as they often want to test their security without letting everyone else on the system know exactly which systems they are scanning. Use --interactive to activate this mode and then type h for help. This option is rarely used because proper shells are usually more familiar and feature-complete. This option includes a bang (!) operator for executing shell commands, which is one of many reasons not to install Nmap setuid root.. -V; --version (Print version number) . Prints the Nmap version number and exits. -h; --help (Print help summary page) . Prints a short help screen with the most common command flags. Running Nmap without any arguments does the same thing. RUNTIME INTERACTION During the execution of Nmap, all key presses are captured. This allows you to interact with the program without aborting and restarting it. Certain special keys will change options, while any other keys will print out a status message telling you about the scan. The convention is that lowercase letters increase the amount of printing, and uppercase letters decrease the printing. You may also press ‘?’ for help. v / V Increase / decrease the verbosity level d / D Increase / decrease the debugging Level p / P Turn on / off packet tracing ? Print a runtime interaction help screen Anything else Print out a status message like this: Stats: 0:00:08 elapsed; 111 hosts completed (5 up), 5 undergoing Service Scan Service scan Timing: About 28.00% done; ETC: 16:18 (0:00:15 remaining) EXAMPLES Here are some Nmap usage examples, from the simple and routine to a little more complex and esoteric. Some actual IP addresses and domain names are used to make things more concrete. In their place you should substitute addresses/names from your own network.. While I don´t think port scanning other networks is or should be illegal, some network administrators don´t appreciate unsolicited scanning of their networks and may complain. Getting permission first is the best approach. For testing purposes, you have permission to scan the host scanme.nmap.org. This permission only includes scanning via Nmap and not testing exploits or denial of service attacks. To conserve bandwidth, please do not initiate more than a dozen scans against that host per day. If this free scanning target service is abused, it will be taken down and Nmap will report Failed to resolve given hostname/IP: scanme.nmap.org. These permissions also apply to the hosts scanme2.nmap.org, scanme3.nmap.org, and so on, though those hosts do not currently exist. nmap -v scanme.nmap.org This option scans all reserved TCP ports on the machine scanme.nmap.org . The -v option enables verbose mode. nmap -sS -O scanme.nmap.org/24 Launches a stealth SYN scan against each machine that is up out of the 256 IPs on “class C” sized network where Scanme resides. It also tries to determine what operating system is running on each host that is up and running. This requires root privileges because of the SYN scan and OS detection. nmap -sV -p 22,53,110,143,4564 198.116.0-255.1-127 Launches host enumeration and a TCP scan at the first half of each of the 255 possible eight-bit subnets in the 198.116 class B address space. This tests whether the systems run SSH, DNS, POP3, or IMAP on their standard ports, or anything on port 4564. For any of these ports found open, version detection is used to determine what application is running. nmap -v -iR 100000 -PN -p 80 Asks Nmap to choose 100,000 hosts at random and scan them for web servers (port 80). Host enumeration is disabled with -PN since first sending a couple probes to determine whether a host is up is wasteful when you are only probing one port on each target host anyway. nmap -PN -p80 -oX logs/pb-port80scan.xml -oG logs/pb-port80scan.gnmap 216.163.128.20/20 This scans 4096 IPs for any web servers (without pinging them) and saves the output in grepable and XML formats. NMAP BOOK While this reference guide details all material Nmap options, it can´t fully demonstrate how to apply those features to quickly solve real-world tasks. For that, we released Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning. Topics include subverting firewalls and intrusion detection systems, optimizing Nmap performance, and automating common networking tasks with the Nmap Scripting Engine. Hints and instructions are provided for common Nmap tasks such as taking network inventory, penetration testing, detecting rogue wireless access points, and quashing network worm outbreaks. Examples and diagrams show actual communication on the wire. More than half of the book is available free online. See http://nmap.org/book for more information. BUGS Like its author, Nmap isn´t perfect. But you can help make it better by sending bug reports or even writing patches. If Nmap doesn´t behave the way you expect, first upgrade to the latest version available from http://nmap.org. If the problem persists, do some research to determine whether it has already been discovered and addressed. Try searching for the error message on our search page at http://insecure.org/search.html or at Google. Also try browsing the nmap-dev archives at http://seclists.org/.. Read this full manual page as well. If nothing comes of this, mail a bug report to nmap-dev@insecure.org. Please include everything you have learned about the problem, as well as what version of Nmap you are running and what operating system version it is running on. Problem reports and Nmap usage questions sent to nmap-dev@insecure.org are far more likely to be answered than those sent to Fyodor directly. If you subscribe to the nmap-dev list before posting, your message will bypass moderation and get through more quickly. Subscribe at http://cgi.insecure.org/mailman/listinfo/nmap-dev. Code patches to fix bugs are even better than bug reports. Basic instructions for creating patch files with your changes are available at http://nmap.org/data/HACKING. Patches may be sent to nmap-dev (recommended) or to Fyodor directly. AUTHOR Gordon “Fyodor” Lyon fyodor@insecure.org (http://insecure.org) Hundreds of people have made valuable contributions to Nmap over the years. These are detailed in the CHANGELOG. file which is distributed with Nmap and also available from http://nmap.org/changelog.html. LEGAL NOTICES Nmap Copyright and Licensing The Nmap Security Scanner is (C) 1996–2009 Insecure.Com LLC. Nmap is also a registered trademark of Insecure.Com LLC. This program is free software; you may redistribute and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; Version 2 with the clarifications and exceptions described below. This guarantees your right to use, modify, and redistribute this software under certain conditions. If you wish to embed Nmap technology into proprietary software, we sell alternative licenses (contact sales@insecure.com). Dozens of software vendors already license Nmap technology such as host discovery, port scanning, OS detection, and version detection. Note that the GPL places important restrictions on “derived works”, yet it does not provide a detailed definition of that term. To avoid misunderstandings, we consider an application to constitute a “derivative work” for the purpose of this license if it does any of the following: · Integrates source code from Nmap · Reads or includes Nmap copyrighted data files, such as nmap-os-db or nmap-service-probes. · Executes Nmap and parses the results (as opposed to typical shell or execution-menu apps, which simply display raw Nmap output and so are not derivative works.) · Integrates/includes/aggregates Nmap into a proprietary executable installer, such as those produced by InstallShield. · Links to a library or executes a program that does any of the above. The term “Nmap” should be taken to also include any portions or derived works of Nmap. This list is not exclusive, but is meant to clarify our interpretation of derived works with some common examples. Our interpretation applies only to Nmap—we don´t speak for other people´s GPL works. If you have any questions about the GPL licensing restrictions on using Nmap in non-GPL works, we would be happy to help. As mentioned above, we also offer alternative license to integrate Nmap into proprietary applications and appliances. These contracts have been sold to many security vendors, and generally include a perpetual license as well as providing for priority support and updates as well as helping to fund the continued development of Nmap technology. Please email sales@insecure.com for further information. As a special exception to the GPL terms, Insecure.Com LLC grants permission to link the code of this program with any version of the OpenSSL library which is distributed under a license identical to that listed in the included COPYING.OpenSSL file, and distribute linked combinations including the two.. You must obey the GNU GPL in all respects for all of the code used other than OpenSSL. If you modify this file, you may extend this exception to your version of the file, but you are not obligated to do so. If you received these files with a written license agreement or contract stating terms other than the terms above, then that alternative license agreement takes precedence over these comments. Creative Commons License for this Nmap Guide This Nmap Reference Guide is (C) 2005–2009 Insecure.Com LLC. It is hereby placed under version 3.0 of the Creative Commons Attribution License[18]. This allows you redistribute and modify the work as you desire, as long as you credit the original source. Alternatively, you may choose to treat this document as falling under the same license as Nmap itself (discussed previously). Source Code Availability and Community Contributions Source is provided to this software because we believe users have a right to know exactly what a program is going to do before they run it. This also allows you to audit the software for security holes (none have been found so far). Source code also allows you to port Nmap to new platforms, fix bugs, and add new features. You are highly encouraged to send your changes to nmap-dev@insecure.org for possible incorporation into the main distribution. By sending these changes to Fyodor or one of the Insecure.Org development mailing lists, it is assumed that you are offering the Nmap Project (Insecure.Com LLC) the unlimited, non-exclusive right to reuse, modify, and relicense the code. Nmap will always be available Open Source,. but this is important because the inability to relicense code has caused devastating problems for other Free Software projects (such as KDE and NASM). We also occasionally relicense the code to third parties as discussed above. If you wish to specify special license conditions of your contributions, just say so when you send them. No Warranty. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License v2.0 for more details at http://www.gnu.org/licenses/gpl-2.0.html, or in the COPYING file included with Nmap. It should also be noted that Nmap has occasionally been known to crash poorly written applications, TCP/IP stacks, and even operating systems.. While this is extremely rare, it is important to keep in mind. Nmap should never be run against mission critical systems unless you are prepared to suffer downtime. We acknowledge here that Nmap may crash your systems or networks and we disclaim all liability for any damage or problems Nmap could cause. Inappropriate Usage Because of the slight risk of crashes and because a few black hats like to use Nmap for reconnaissance prior to attacking systems, there are administrators who become upset and may complain when their system is scanned. Thus, it is often advisable to request permission before doing even a light scan of a network. Nmap should never be installed with special privileges (e.g. suid root) for security reasons.. Third-Party Software This product includes software developed by the Apache Software Foundation[19]. A modified version of the Libpcap portable packet capture library[20]. is distributed along with Nmap. The Windows version of Nmap utilized the Libpcap-derived WinPcap library[21]. instead. Regular expression support is provided by the PCRE library[22],. which is open-source software, written by Philip Hazel.. Certain raw networking functions use the Libdnet[23]. networking library, which was written by Dug Song.. A modified version is distributed with Nmap. Nmap can optionally link with the OpenSSL cryptography toolkit[24]. for SSL version detection support. The Nmap Scripting Engine uses an embedded version of the Lua programming language[25].. All of the third-party software described in this paragraph is freely redistributable under BSD-style software licenses. United States Export Control. Nmap only uses encryption when compiled with the optional OpenSSL support and linked with OpenSSL. When compiled without OpenSSL support, Insecure.Com LLC believes that Nmap is not subject to U.S. Export Administration Regulations (EAR)[26] export control. As such, there is no applicable ECCN (explort control classification number) and exportation does not require any special license, permit, or other governmental authorization. When compiled with OpenSSL support or distributed as source code, Insecure.Com LLC believes that Nmap falls under U.S. ECCN 5D002[27] (“Information Security Software”). We distribute Nmap under the TSU exception for publicly available encryption software defined in EAR 740.13(e)[28]. NOTES 1. Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning http://nmap.org/book/ 2. RFC 1122 http://www.rfc-editor.org/rfc/rfc1122.txt 3. RFC 792 http://www.rfc-editor.org/rfc/rfc792.txt 4. RFC 950 http://www.rfc-editor.org/rfc/rfc950.txt 5. RFC 1918 http://www.rfc-editor.org/rfc/rfc1918.txt 6. UDP http://www.rfc-editor.org/rfc/rfc768.txt 7. SCTP http://www.rfc-editor.org/rfc/rfc4960.txt 8. TCP RFC http://www.rfc-editor.org/rfc/rfc793.txt 9. RFC 959 http://www.rfc-editor.org/rfc/rfc959.txt 10. RFC 1323 http://www.rfc-editor.org/rfc/rfc1323.txt 11. Lua programming language http://lua.org 12. precedence http://www.lua.org/manual/5.1/manual.html#2.5.3 13. IP protocol http://www.rfc-editor.org/rfc/rfc791.txt 14. RFC 2960 http://www.rfc-editor.org/rfc/rfc2960.txt 15. Nmap::Scanner http://sourceforge.net/projects/nmap-scanner/ 16. Nmap::Parser http://nmapparser.wordpress.com/ 17. listed at Wikipedia http://en.wikipedia.org/wiki/List_of_IPv6_tunnel_brokers 18. Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0/ 19. Apache Software Foundation http://www.apache.org 20. Libpcap portable packet capture library http://www.tcpdump.org 21. WinPcap library http://www.winpcap.org 22. PCRE library http://www.pcre.org 23. Libdnet http://libdnet.sourceforge.net 24. OpenSSL cryptography toolkit http://www.openssl.org 25. Lua programming language http://www.lua.org 26. Export Administration Regulations (EAR) http://www.access.gpo.gov/bis/ear/ear_data.html 27. 5D002 http://www.access.gpo.gov/bis/ear/pdf/ccl5-pt2.pdf 28. EAR 740.13(e) http://www.access.gpo.gov/bis/ear/pdf/740.pdf Nmap 01/26/2010 NMAP(1) (责任编辑：IT) |