我是从 Linux 迁移过来的 FreeBSD 新用户，Linux 中使用的是 netfilter 防火墙框架（LCTT 译注：netfilter 是由 Rusty Russell 提出的 Linux 2.4 内核防火墙框架）。那么在 FreeBSD 上，我该如何设置 PF 防火墙，来保护只有一个公共 IP 地址和端口的 web 服务器呢？

PF 是包过滤器packet filter的简称。它是为 OpenBSD 开发的，但是已经被移植到了 FreeBSD 以及其它操作系统上。PF 是一个包状态过滤引擎。在这篇教程中，我将向你展示如何在 FreeBSD 10.x 以及 11.x 中设置 PF 防火墙，从而来保护 web 服务器。

第一步：开启 PF 防火墙

你需要把下面这几行内容添加到文件 /etc/rc.conf 文件中：

1. # echo 'pf_enable="YES"' >> /etc/rc.conf
2. # echo 'pf_rules="/usr/local/etc/pf.conf"' >> /etc/rc.conf
3. # echo 'pflog_enable="YES"' >> /etc/rc.conf
4. # echo 'pflog_logfile="/var/log/pflog"' >> /etc/rc.conf

在这里：

1. pf_enable="YES" - 开启 PF 服务
2. pf_rules="/usr/local/etc/pf.conf" - 从文件 /usr/local/etc/pf.conf 中读取 PF 规则
3. pflog_enable="YES" - 为 PF 服务打开日志支持
4. pflog_logfile="/var/log/pflog" - 存储日志的文件，即日志存于文件 /var/log/pflog 中

第二步：在 /usr/local/etc/pf.conf 文件中创建防火墙规则

输入下面这个命令打开文件（超级用户模式下）：

1. # vi /usr/local/etc/pf.conf

在文件中添加下面这些 PF 规则集：

1. # vim: set ft=pf
2. # /usr/local/etc/pf.conf
3.  
4. ## 设置公共端口 ##
5. ext_if="vtnet0"
6.  
7. ## 设置服务器公共 IP 地址 ##
8. ext_if_ip="172.xxx.yyy.zzz"
9.  
10. ## 设置并删除下面这些公共端口上的 IP 范围 ##
11. martians = "{ 127.0.0.0/8, 192.168.0.0/16, 172.16.0.0/12, \
12. 10.0.0.0/8, 169.254.0.0/16, 192.0.2.0/24, \
13. 0.0.0.0/8, 240.0.0.0/4 }"
14.  
15. ## 设置 http(80)/https (443) 端口 ##
16. webports = "{http, https}"
17.  
18. ## 启用下面这些服务 ##
19. int_tcp_services = "{domain, ntp, smtp, www, https, ftp, ssh}"
20. int_udp_services = "{domain, ntp}"
21.  
22. ## 跳过回环端口 - 跳过端口上的所有 PF 处理 ##
23. set skip on lo
24.  
25. ## 设置 PF 应该统计的端口信息，如发送/接收字节数，通过/禁止的包的数目 ##
26. set loginterface $ext_if
27.  
28. ## 设置默认策略 ##
29. block return in log all
30. block out all
31.  
32. # 基于 IP 分片的错误处理来防御攻击
33. scrub in all
34.  
35. # 删除所有不可达路由地址
36. block drop in quick on $ext_if from $martians to any
37. block drop out quick on $ext_if from any to $martians
38.  
39. ## 禁止欺骗包
40. antispoof quick for $ext_if
41.  
42. # 打开 SSH 端口，SSH 服务仅从 VPN IP 139.xx.yy.zz 监听 22 号端口
43. # 出于安全原因，我不允许/接收 SSH 流量
44. pass in quick on $ext_if inet proto tcp from 139.xxx.yyy.zzz to $ext_if_ip port = ssh flags S/SA keep state label "USER_RULE: Allow SSH from 139.xxx.yyy.zzz"
45. ## 使用下面这些规则来为所有来自任何 IP 地址的用户开启 SSH 服务 #
46. ## pass in inet proto tcp to $ext_if port ssh
47. ### [ OR ] ###
48. ## pass in inet proto tcp to $ext_if port 22
49.  
50. # Allow Ping-Pong stuff. Be a good sysadmin
51. pass inet proto icmp icmp-type echoreq
52.  
53. # All access to our Nginx/Apache/Lighttpd Webserver ports
54. pass proto tcp from any to $ext_if port $webports
55.  
56. # 允许重要的发送流量
57. pass out quick on $ext_if proto tcp to any port $int_tcp_services
58. pass out quick on $ext_if proto udp to any port $int_udp_services
59.  
60. # 在下面添加自定义规则

保存并关闭文件。欢迎来参考我的规则集。如果要检查语法错误，可以运行：

1. # service pf check

或

1. /etc/rc.d/pf check

或

1. # pfctl -n -f /usr/local/etc/pf.conf

第三步：开始运行 PF 防火墙

命令如下。请小心，如果是基于 SSH 的会话，你可能会和服务器断开连接。

开启 PF 防火墙：

1. # service pf start

停用 PF 防火墙：

1. # service pf stop

检查语法错误：

1. # service pf check

重启服务：

1. # service pf restart

查看 PF 状态：

1. # service pf status

示例输出：

1. Status: Enabled for 0 days 00:02:18 Debug: Urgent
2.  
3. Interface Stats for vtnet0 IPv4 IPv6
4. Bytes In 19463 0
5. Bytes Out 18541 0
6. Packets In
7. Passed 244 0
8. Blocked 3 0
9. Packets Out
10. Passed 136 0
11. Blocked 12 0
12.  
13. State Table Total Rate
14. current entries 1
15. searches 395 2.9/s
16. inserts 4 0.0/s
17. removals 3 0.0/s
18. Counters
19. match 19 0.1/s
20. bad-offset 0 0.0/s
21. fragment 0 0.0/s
22. short 0 0.0/s
23. normalize 0 0.0/s
24. memory 0 0.0/s
25. bad-timestamp 0 0.0/s
26. congestion 0 0.0/s
27. ip-option 0 0.0/s
28. proto-cksum 0 0.0/s
29. state-mismatch 0 0.0/s
30. state-insert 0 0.0/s
31. state-limit 0 0.0/s
32. src-limit 0 0.0/s
33. synproxy 0 0.0/s
34. map-failed 0 0.0/s

开启/关闭/重启 pflog 服务的命令

输入下面这些命令：

1. # service pflog start
2. # service pflog stop
3. # service pflog restart

第四步：pfctl 命令的简单介绍

你需要使用 pfctl 命令来查看 PF 规则集和参数配置，包括来自包过滤器packet filter的状态信息。让我们来看一下所有常见命令：

显示 PF 规则信息

1. # pfctl -s rules

示例输出：

1. block return in log all
2. block drop out all
3. block drop in quick on ! vtnet0 inet from 172.xxx.yyy.zzz/24 to any
4. block drop in quick inet from 172.xxx.yyy.zzz/24 to any
5. pass in quick on vtnet0 inet proto tcp from 139.aaa.ccc.ddd to 172.xxx.yyy.zzz/24 port = ssh flags S/SA keep state label "USER_RULE: Allow SSH from 139.aaa.ccc.ddd"
6. pass inet proto icmp all icmp-type echoreq keep state
7. pass out quick on vtnet0 proto tcp from any to any port = domain flags S/SA keep state
8. pass out quick on vtnet0 proto tcp from any to any port = ntp flags S/SA keep state
9. pass out quick on vtnet0 proto tcp from any to any port = smtp flags S/SA keep state
10. pass out quick on vtnet0 proto tcp from any to any port = http flags S/SA keep state
11. pass out quick on vtnet0 proto tcp from any to any port = https flags S/SA keep state
12. pass out quick on vtnet0 proto tcp from any to any port = ftp flags S/SA keep state
13. pass out quick on vtnet0 proto tcp from any to any port = ssh flags S/SA keep state
14. pass out quick on vtnet0 proto udp from any to any port = domain keep state
15. pass out quick on vtnet0 proto udp from any to any port = ntp keep state

显示每条规则的详细内容

1. # pfctl -v -s rules

在每条规则的详细输出中添加规则编号：

1. # pfctl -vvsr show

显示状态信息

1. # pfctl -s state
2. # pfctl -s state | more
3. # pfctl -s state | grep 'something'

如何在命令行中禁止 PF 服务

1. # pfctl -d

如何在命令行中启用 PF 服务

1. # pfctl -e

如何在命令行中刷新 PF 规则/NAT/路由表

1. # pfctl -F all

示例输出：

1. rules cleared
2. nat cleared
3. 0 tables deleted.
4. 2 states cleared
5. source tracking entries cleared
6. pf: statistics cleared
7. pf: interface flags reset

如何在命令行中仅刷新 PF 规则

1. # pfctl -F rules

如何在命令行中仅刷新队列

1. # pfctl -F queue

如何在命令行中刷新统计信息（它不是任何规则的一部分）

1. # pfctl -F info

如何在命令行中清除所有计数器

1. # pfctl -z clear

第五步：查看 PF 日志

PF 日志是二进制格式的。使用下面这一命令来查看：

1. # tcpdump -n -e -ttt -r /var/log/pflog

示例输出：

1. Aug 29 15:41:11.757829 rule 0/(match) block in on vio0: 86.47.225.151.55806 > 45.FOO.BAR.IP.23: S 757158343:757158343(0) win 52206 [tos 0x28]
2. Aug 29 15:41:44.193309 rule 0/(match) block in on vio0: 5.196.83.88.25461 > 45.FOO.BAR.IP.26941: S 2224505792:2224505792(0) ack 4252565505 win 17520 (DF) [tos 0x24]
3. Aug 29 15:41:54.628027 rule 0/(match) block in on vio0: 45.55.13.94.50217 > 45.FOO.BAR.IP.465: S 3941123632:3941123632(0) win 65535
4. Aug 29 15:42:11.126427 rule 0/(match) block in on vio0: 87.250.224.127.59862 > 45.FOO.BAR.IP.80: S 248176545:248176545(0) win 28200 <mss 1410,sackOK,timestamp 1044055305 0,nop,wscale 8> (DF)
5. Aug 29 15:43:04.953537 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.7475: S 1164335542:1164335542(0) win 1024
6. Aug 29 15:43:05.122156 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.7475: R 1164335543:1164335543(0) win 1200
7. Aug 29 15:43:37.302410 rule 0/(match) block in on vio0: 94.130.12.27.18080 > 45.FOO.BAR.IP.64857: S 683904905:683904905(0) ack 4000841729 win 16384 <mss 1460>
8. Aug 29 15:44:46.574863 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.7677: S 3451987887:3451987887(0) win 1024
9. Aug 29 15:44:46.819754 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.7677: R 3451987888:3451987888(0) win 1200
10. Aug 29 15:45:21.194752 rule 0/(match) block in on vio0: 185.40.4.130.55910 > 45.FOO.BAR.IP.80: S 3106068642:3106068642(0) win 1024
11. Aug 29 15:45:32.999219 rule 0/(match) block in on vio0: 185.40.4.130.55910 > 45.FOO.BAR.IP.808: S 322591763:322591763(0) win 1024
12. Aug 29 15:46:30.157884 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.6511: S 2412580953:2412580953(0) win 1024 [tos 0x28]
13. Aug 29 15:46:30.252023 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.6511: R 2412580954:2412580954(0) win 1200 [tos 0x28]
14. Aug 29 15:49:44.337015 rule 0/(match) block in on vio0: 189.219.226.213.22640 > 45.FOO.BAR.IP.23: S 14807:14807(0) win 14600 [tos 0x28]
15. Aug 29 15:49:55.161572 rule 0/(match) block in on vio0: 5.196.83.88.25461 > 45.FOO.BAR.IP.40321: S 1297217585:1297217585(0) ack 1051525121 win 17520 (DF) [tos 0x24]
16. Aug 29 15:49:59.735391 rule 0/(match) block in on vio0: 36.7.147.209.2545 > 45.FOO.BAR.IP.3389: SWE 3577047469:3577047469(0) win 8192 <mss 1460,nop,wscale 8,nop,nop,sackOK> (DF) [tos 0x2 (E)]
17. Aug 29 15:50:00.703229 rule 0/(match) block in on vio0: 36.7.147.209.2546 > 45.FOO.BAR.IP.3389: SWE 1539382950:1539382950(0) win 8192 <mss 1460,nop,wscale 8,nop,nop,sackOK> (DF) [tos 0x2 (E)]
18. Aug 29 15:51:33.880334 rule 0/(match) block in on vio0: 45.55.22.21.53510 > 45.FOO.BAR.IP.2362: udp 14
19. Aug 29 15:51:34.006656 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.6491: S 151489102:151489102(0) win 1024 [tos 0x28]
20. Aug 29 15:51:34.274654 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.6491: R 151489103:151489103(0) win 1200 [tos 0x28]
21. Aug 29 15:51:36.393019 rule 0/(match) block in on vio0: 60.191.38.78.4249 > 45.FOO.BAR.IP.8000: S 3746478095:3746478095(0) win 29200 (DF)
22. Aug 29 15:51:57.213051 rule 0/(match) block in on vio0: 24.137.245.138.7343 > 45.FOO.BAR.IP.5358: S 14134:14134(0) win 14600
23. Aug 29 15:52:37.852219 rule 0/(match) block in on vio0: 122.226.185.125.51128 > 45.FOO.BAR.IP.23: S 1715745381:1715745381(0) win 5840 <mss 1420,sackOK,timestamp 13511417 0,nop,wscale 2> (DF)
24. Aug 29 15:53:31.309325 rule 0/(match) block in on vio0: 189.218.148.69.377 > 45.FOO.BAR.IP5358: S 65340:65340(0) win 14600 [tos 0x28]
25. Aug 29 15:53:31.809570 rule 0/(match) block in on vio0: 13.93.104.140.53184 > 45.FOO.BAR.IP.1433: S 39854048:39854048(0) win 1024
26. Aug 29 15:53:32.138231 rule 0/(match) block in on vio0: 13.93.104.140.53184 > 45.FOO.BAR.IP.1433: R 39854049:39854049(0) win 1200
27. Aug 29 15:53:41.459088 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.6028: S 168338703:168338703(0) win 1024
28. Aug 29 15:53:41.789732 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.6028: R 168338704:168338704(0) win 1200
29. Aug 29 15:54:34.993594 rule 0/(match) block in on vio0: 212.47.234.50.5102 > 45.FOO.BAR.IP.5060: udp 408 (DF) [tos 0x28]
30. Aug 29 15:54:57.987449 rule 0/(match) block in on vio0: 51.15.69.145.5100 > 45.FOO.BAR.IP.5060: udp 406 (DF) [tos 0x28]
31. Aug 29 15:55:07.001743 rule 0/(match) block in on vio0: 190.83.174.214.58863 > 45.FOO.BAR.IP.23: S 757158343:757158343(0) win 27420
32. Aug 29 15:55:51.269549 rule 0/(match) block in on vio0: 142.217.201.69.26112 > 45.FOO.BAR.IP.22: S 757158343:757158343(0) win 22840 <mss 1460>
33. Aug 29 15:58:41.346028 rule 0/(match) block in on vio0: 169.1.29.111.29765 > 45.FOO.BAR.IP.23: S 757158343:757158343(0) win 28509
34. Aug 29 15:59:11.575927 rule 0/(match) block in on vio0: 187.160.235.162.32427 > 45.FOO.BAR.IP.5358: S 22445:22445(0) win 14600 [tos 0x28]
35. Aug 29 15:59:37.826598 rule 0/(match) block in on vio0: 94.74.81.97.54656 > 45.FOO.BAR.IP.3128: S 2720157526:2720157526(0) win 1024 [tos 0x28]stateful
36. Aug 29 15:59:37.991171 rule 0/(match) block in on vio0: 94.74.81.97.54656 > 45.FOO.BAR.IP.3128: R 2720157527:2720157527(0) win 1200 [tos 0x28]
37. Aug 29 16:01:36.990050 rule 0/(match) block in on vio0: 182.18.8.28.23299 > 45.FOO.BAR.IP.445: S 1510146048:1510146048(0) win 16384

如果要查看实时日志，可以运行：

1. # tcpdump -n -e -ttt -i pflog0

如果你想了解更多信息，可以访问 PF FAQ 和 FreeBSD HANDBOOK 以及下面这些 man 页面：

1. # man tcpdump
2. # man pfctl
3. # man pf

关于作者

我是 nixCraft 的创立者，一个经验丰富的系统管理员，同时也是一位 Linux 操作系统/Unix shell 脚本培训师。我在不同的行业与全球客户工作过，包括 IT、教育、国防和空间研究、以及非营利组织。你可以在 Twitter、Facebook 或 Google+ 上面关注我。