在CentOS下運行yum install flash-plugin或yum install mplayer的時候，提示庫裡沒有找到這個軟件？為什麼會這樣？因為CentOS是RHEL編譯過來的，去掉了所有關於版權問題的東西。因此，在沒有使用第三方軟件庫的情況下，很多軟件無法用yum install來直接安裝。
EPEL(Extra Packages for Enterprise Linux)軟件庫提供為RHEL系列(4.x、5.x、6.x等等)重新編譯的Fedora組件。這個軟件庫在不替換系統組件方面下了功夫。在某些情況下它嘗試直接針對CentOS兼容性的問題，但堅決否定軟件庫間的兼容性是目標之一。當它與其它第三方軟件庫混合使用時，問題可能會出現。故此，當你使用EPEL時，尤其是當你亦會運用其它件庫的組件時，請確保你使用yum的Priorities插件。
RPMForge庫現在已經擁有超過10000種的CentOS的軟件包，被CentOS社區認為是最安全也是最穩定的一個第三方軟件庫。
RedHat Linux 本地yum源的配置 http://www.linuxidc.com/Linux/2013-08/88217.htm
RedHat 6.2 Linux修改yum源免費使用CentOS源 http://www.linuxidc.com/Linux/2013-07/87383.htm
配置EPEL YUM源 http://www.linuxidc.com/Linux/2012-10/71850.htm
Redhat 本地yum源配置 http://www.linuxidc.com/Linux/2012-11/75127.htm
yum的配置文件說明 http://www.linuxidc.com/Linux/2013-04/83298.htm
RedHat 6.1下安裝yum(圖文) http://www.linuxidc.com/Linux/2013-06/86535.htm
YUM 安裝及清理 http://www.linuxidc.com/Linux/2013-07/87163.htm
下面講解下如何使用EPEL與RPMForge、RPMFusion軟件倉庫：
一、安裝yum-priorities插件。
這個插件是用來設置yum在調用軟件源時的順序的。因為官方提供的軟件源，都是比較穩定和被推薦使用的。因此，官方源的順序要高於第三方源的順序。如何保證這個順序，就需要安裝yum-priorities這插件了。
[root@CentOS ~]# yum install yum-priorities
安裝完後需要設置/etc/yum.repos.d/目錄下的.repo相關文件（如CentOS-Base.repo），在這些文件中插入順序指令：priority=N （N為1到99的正整數，數值越小越優先），一般的配置是這樣的：
[base], [addons], [updates], [extras] … priority=1
[CentOSplus],[contrib] … priority=2
其他第三的軟件源為：priority=N （推薦N>10）
如我的CentOS-Base.repo配置為（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、下載與安裝相應EPEL的rpm文件包
1、下載與安裝EPEL的rpm文件包
32位系統
[root@CentOS ~]# rpm -ivh http://download.fedora.redhat.com/pub/epel/6/i386/epel-release-6-5.noarch.rpm
64位系統
[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統內核，請使用uname -a命令來查看您的內核信息。
2、導入DAG的PGP Key
[root@CentOS ~]# rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-6
3、設置/etc/yum.repos.d/epel.repo文件中源的級別，就是添加priority=11這句。
如我的epel.repo文件內容：
[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
更多詳情見請繼續閱讀下一頁的精彩內容： http://www.linuxidc.com/Linux/2014-08/106024p2.htm
