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首先，我们弄清楚什么是epel源：
epel（Extra Packages for Enterprise Linux）是一个软件源，里面包含了许多基本源中不存在的软件。
它由Fedora Project 推出。
地址：http://fedoraproject.org/wiki/EPEL
Extra Packages for Enterprise Linux (or EPEL) is a Fedora Special Interest Group that creates, maintains, and manages a high quality set of additional packages for Enterprise Linux, including, but not limited to, Red Hat Enterprise Linux (RHEL), CentOS and Scientific Linux (SL), Oracle Linux (OL).包括，但不限于RHEL/Centos/scientific linux/Oracle linux .
EPEL packages are usually based on their Fedora counterparts and will never conflict with or replace packages in the base Enterprise Linux distributions. EPEL uses much of the same infrastructure as Fedora, including buildsystem, bugzilla instance, updates manager, mirror manager and more.EPEL包基于Fedora副本并且不会与基础的企业版linux包冲突或者是替换
How can I use these extra packages？
EPEL has an 'epel-release' package that includes gpg keys for package signing and repository information. Installing this package for your Enterprise Linux version should allow you to use normal tools such as yum to install packages and their dependencies. By default the stable EPEL repo is enabled, there is also a 'epel-testing' repository that contains packages that are not yet deemed stable.
epel-release包包含了安装包签名的gpg keys和仓库信息。安装这个包应该允许你使用比如像yum这样的常规工具来安装包和他们的依赖。默认的epel repo是激活的，同时也有一个叫 'epel-testing'的仓库不视为稳定的。
注意点：
对于centos用户：
You can install EPEL by running yum install epel-release. The package is included in the CentOS Extras repository, enabled by default.
对于RHN(red hat network)用户：
You need to also enable the 'optional' repository to use EPEL packages as they depend on packages in that repository. This can be done by enabling the RHEL optional subchannel for RHN-Classic. For certificate-based subscriptions see Red Hat Subscription Management Guide. For EPEL 7, in addition to the 'optional' repository (rhel-7-server-optional-rpms), you also need to enable the 'extras' repository (rhel-7-server-extras-rpms).
红帽提供３个网络模块功能：
更新：方便地对您的系统进行软件更新。
管理：增强管理能力并提高您的 Linux 实施的可扩展性。
Provisioning：系统快照、集成的 kickstarts、配置管理以及更多功能。
How to subscribe a system to a child channel using Red Hat Network (RHN) Classic ?
https://access.redhat.com/solutions/11312
查看系统安装了哪些repo使用如下命令：
yum repolist
2. 安装过程：
下载安装了epel-release后，安装其他包报错：
Error: Cannot retrieve repository metadata (repomd.xml) for repository: epel. Please verify its path and try again
然后，我注释掉了这行：
#mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-7&arch=$basearch
修改了repo配置文件后，需要执行以下步骤：
[root@localhost yum.repos.d]# yum clean all
然后检查：
[root@localhost yum.repos.d]# yum list
之后又报错：
Error: xz compression not available
经过一番查询：是由于我的系统版本是6.8,但是安装了epel-release-7-8.noarch，
因此，我需要先卸载安装包，然后清理缓存：
yum remove epel-release
rm -rf /var/cache/yum/x86_64/6Server/epel/
