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 EL4 及 EL5 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。
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-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/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
priority=1#released updates
[updates]
name=CentOS-$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-5
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/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
priority=1#additional packages that may be useful
[extras]
name=CentOS-$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-5
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/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
priority=2#contrib – packages by CentOS Users
[contrib]
name=CentOS-$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-5
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/5/i386/epel-release-5-4.noarch.rpm
64位系统
[root@CentOS ~]# rpm -ivh http://download.fedora.redhat.com/pub/epel/5/x86_64/epel-release-5-4.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
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 5 – $basearch
#baseurl=http://download.fedoraproject.org/pub/epel/5/$basearch
mirrorlist=http://mirrors.fedoraproject.org/mirrorlist?repo=epel-5&arch=$basearch
failovermethod=priority
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL
priority=11[epel-debuginfo]
name=Extra Packages for Enterprise Linux 5 – $basearch – Debug
#baseurl=http://download.fedoraproject.org/pub/epel/5/$basearch/debug
mirrorlist=http://mirrors.fedoraproject.org/mirrorlist?repo=epel-debug-5&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL
gpgcheck=1[epel-source]
name=Extra Packages for Enterprise Linux 5 – $basearch – Source
#baseurl=http://download.fedoraproject.org/pub/epel/5/SRPMS
mirrorlist=http://mirrors.fedoraproject.org/mirrorlist?repo=epel-source-5&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL
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://packages.sw.be/rpmforge-release/rpmforge-release-0.5.1-1.el5.rf.i386.rpm
64位系统
[root@CentOS ~]# wget http://packages.sw.be/rpmforge-release/rpmforge-release-0.5.1-1.el5.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-0.5.1-1.el5.rf.*.rpm
4，安装rpmforge的rpm文件包
[root@CentOS ~]# rpm -i rpmforge-release-0.5.1-1.el5.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 Red Hat Enterprise 5 – dag
### URL: http://rpmforge.net/
[rpmforge]
name = Red Hat Enterprise $releasever – RPMforge.net – dag
baseurl = http://apt.sw.be/redhat/el5/en/$basearch/rpmforge
mirrorlist = http://apt.sw.be/redhat/el5/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
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/5/i386/rpmfusion-free-release-5-0.1.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，安装下mplayer播放器试试，如果安装成功了就没有问题了。
[root@CentOS ~]# yum install mplayer