在CentOS下运行yum install flash-plugin或yum install mplayer的时候，提示库里没有找到这个软件？为什么会这样？因为CentOS是RHEL编译过来的，去掉了所有关于版权问题的东西。因此，在没有使用第三方软件库的情况下，很多软件无法用yum install来直接安装。

EPEL(Extra Packages for Enterprise Linux)软件库提供为RHEL系列(4.x、5.x、6.x等等)重新编译的Fedora组件。这个软件库在不替换系统组件方面下了功夫。在某些情况下它尝试直接针对CentOS兼容性的问题，但坚决否定软件库间的兼容性是目标之一。当它与其它第三方软件库混合使用时，问题可能会出现。故此，当你使用EPEL时，尤其是当你亦会运用其它件库的组件时，请确保你使用yum的Priorities插件。

RPMForge库现在已经拥有超过10000种的CentOS的软件包，被CentOS社区认为是最安全也是最稳定的一个第三方软件库。

下面讲解下如何使用EPEL与RPMForge、RPMFusion软件仓库：

一、安装yum-priorities插件。

这个插件是用来设置yum在调用软件源时的顺序的。因为官方提供的软件源，都是比较稳定和被推荐使用的。因此，官方源的顺序要高于第三方源的顺序。如何保证这个顺序，就需要安装yum-priorities这插件了。

[root@CentOS ~]# yum install yum-priorities

安装完后需要设置/etc/yum.repos.d/目录下的.repo相关文件（如CentOS-Base.repo），在这些文件中插入顺序指令：priority=N （N为1到99的正整数，数值越小越优先），一般的配置是这样的：
[base], [addons], [updates], [extras] … priority=1
[CentOSplus],[contrib] … priority=2
其他第三的软件源为：priority=N （推荐N>10）

如我的CentOS-Base.repo配置为（CentOS 6.0）：
# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the 
# remarked out baseurl= line instead.
#
#
[base]
name=CentOS-$releasever - Base
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
priority=1

#released updates 
[updates]
name=CentOS-$releasever - Updates
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
priority=1

#packages used/produced in the build but not released
[addons]
name=CentOS-$releasever – Addons
mirrorlist=http://mirrorlist.CentOS.org/?release=$releasever&arch=$basearch&repo=addons
#baseurl=http://mirror.CentOS.org/CentOS/$releasever/addons/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
priority=1

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras
#baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
priority=1

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
priority=2

#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=contrib
#baseurl=http://mirror.centos.org/centos/$releasever/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
priority=2

二、下载与安装相应EPEL的rpm文件包

1、下载与安装EPEL的rpm文件包

32位系统
[root@CentOS ~]# rpm -ivh http://download.fedora.redhat.com/pub/epel/6/i386/epel-release-6-5.noarch.rpm

64位系统
[root@CentOS ~]# rpm -ivh http://download.fedora.redhat.com/pub/epel/6/x86_64/epel-release-6-5.noarch.rpm

如果您不清楚您的系统内核，请使用uname -a命令来查看您的内核信息。

2、导入DAG的PGP Key

[root@CentOS ~]# rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-6

3、设置/etc/yum.repos.d/epel.repo文件中源的级别，就是添加priority=11这句。
如我的epel.repo文件内容：
[epel]
name=Extra Packages for Enterprise Linux 6 - $basearch
#baseurl=http://download.fedoraproject.org/pub/epel/6/$basearch
mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-6&arch=$basearch
failovermethod=priority
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-6
priority=11

[epel-debuginfo]
name=Extra Packages for Enterprise Linux 6 - $basearch - Debug
#baseurl=http://download.fedoraproject.org/pub/epel/6/$basearch/debug
mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-debug-6&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-6
gpgcheck=1

[epel-source]
name=Extra Packages for Enterprise Linux 6 - $basearch - Source
#baseurl=http://download.fedoraproject.org/pub/epel/6/SRPMS
mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-source-6&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-6
gpgcheck=1

三、下载与安装相应rpmforge的rpm文件包

1、下载rpmforge的rpm文件包

32位系统
[root@CentOS ~]# wget http://pkgs.repoforge.org/rpmforge-release/rpmforge-release-0.5.2-2.el6.rf.i686.rpm

64位系统
[root@CentOS ~]# wget rpmforge-release-0.5.2-2.el6.rf.x86_64.rpm

2、安装DAG的PGP Key

[root@CentOS ~]# rpm --import http://dag.wieers.com/rpm/packages/RPM-GPG-KEY.dag.txt

3、验证rpmforge的rpm文件包

[root@CentOS ~]# rpm -K rpmforge-release-0.5.2-2.el6.rf.*.rpm
如果报XXX not an rpm package的错，查看是否wget下载的包不完整，包的大小是12K

4、安装rpmforge的rpm文件包

[root@CentOS ~]# rpm -ivh rpmforge-release-0.5.2-2.el6.rf.*.rpm

5、设置/etc/yum.repos.d/rpmforge.repo文件中源的级别，就是添加priority=12这句，如我的rpmforge.repo文件内容：
### Name: RPMforge RPM Repository for RHEL 6 - dag
### URL: http://rpmforge.net/
[rpmforge]
name = RHEL $releasever - RPMforge.net - dag
baseurl = http://apt.sw.be/redhat/el6/en/$basearch/rpmforge
mirrorlist = http://apt.sw.be/redhat/el6/en/mirrors-rpmforge
#mirrorlist = file:///etc/yum.repos.d/mirrors-rpmforge
enabled = 1
protect = 0
gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY-rpmforge-dag
gpgcheck = 1
priority=12

[rpmforge-extras]
name = RHEL $releasever - RPMforge.net - extras
baseurl = http://apt.sw.be/redhat/el6/en/$basearch/extras
mirrorlist = http://apt.sw.be/redhat/el6/en/mirrors-rpmforge-extras
#mirrorlist = file:///etc/yum.repos.d/mirrors-rpmforge-extras
enabled = 0
protect = 0
gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY-rpmforge-dag
gpgcheck = 1

[rpmforge-testing]
name = RHEL $releasever - RPMforge.net - testing
baseurl = http://apt.sw.be/redhat/el6/en/$basearch/testing
mirrorlist = http://apt.sw.be/redhat/el6/en/mirrors-rpmforge-testing
#mirrorlist = file:///etc/yum.repos.d/mirrors-rpmforge-testing
enabled = 0
protect = 0
gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY-rpmforge-dag
gpgcheck = 1

四、下载与安装相应rpmfusion的rpm文件包

（CentOS官方说这个软件库里面的软件稳定性不如rpmforge，选用！rpmfusion官方站：http://rpmfusion.org）

[root@CentOS ~]# rpm -Uvh http://download1.rpmfusion.org/free/el/updates/testing/6/i386/rpmfusion-free-release-6-0.1.noarch.rpm

[root@CentOS ~]# rpm -Uvh http://download1.rpmfusion.org/nonfree/el/updates/testing/5/i386/rpmfusion-nonfree-release-5-0.1.noarch.rpm

五、测试

1、测试升级

[root@CentOS ~]# yum check-update

输出信息应该会有以下两行：

Loading “priorities” plugin
…
XX packages excluded due to repository priority protections

2、测试各插件是否生效

[root@CentOS ~]# yum check-update
如果出现类似以下输出，则表示配置成功。
Loaded plugins: fastestmirror, priorities, refresh-packagekit
Loading mirror speeds from cached hostfile
下面是一长串软件列表，最后一列形如：
epel    
rpmforge
base
说明各个软件包的来源。

备注：
1、如果yum使用过程中出现类似以下错误，多半是因为rpmfusion的server无法连接，可以不用管它，或者将/etc/yum.repo.d/目录下rpmfusion相关的的repo文件中的enable=1改为0，关掉rpmfusion相关的源列表。
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
 Eg. </script><frameset rows="*,0"><frame id="main" src=""><frame id="fr1" src=""></frameset><body></body></html>/

2、rpmfusion还会出现类似如下错误：
Error: Cannot retrieve repository metadata (repomd.xml) for repository: rpmfusion-foo. Please verify its path and try again
原因引自http://rpmfusion.org/FAQ#head-33eabca99c1c4fa289a1d757bda49721cd27871a：
Yum cannot connect to the RPM Fusion servers if you get above message. That could be a problem with your local network or a problem with the MirrorManager servers from RPM Fusion. If it's the latter use this comment to temporary work around the problem:
方法是去除所有baseurl和mirrorlist前面的#如下：
su -c "sed -i 's|^#baseurl|baseurl| ; s|^mirrorlist|#mirrorlist|' /etc/yum.repos.d/rpmfusion*free*repo"

To undo the change later use this command:
su -c "sed -i 's|^baseurl|#baseurl| ; s|^#mirrorlist|mirrorlist|' /etc/yum.repos.d/rpmfusion*free*repo"

 

还可以参考：

http://hi.baidu.com/nacoo2000/blog/item/afeeb4396748813fb9998f18.html

http://hi.baidu.com/nacoo2000/blog/item/792e3b87d057172566096e27.html