epel是centos的衍生发行版，用来弥补centos更新比较滞后的一些扩展源，是由fedora社区打造，安装后就可以yum安装以前需要编译安装的软件，比如nginx，iftop，openvpn等等。
需要下载系统对应的epel包 http://dl.fedoraproject.org/pub/epel/
PS：如果有些需要安装的软件用yum不能安装，对于新装的系统，可以先考虑有没有安装epel扩展源。
EPEL提供了上万个软件包
安装位置/etc/yum.repos.d/epel.repo文件
wget http://dl.fedoraproject.org/pub/epel/7/x86_64/e/epel-release-7-5.noarch.rpm
rpm -ivh epel-release-7-5.noarch.rpm
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EPEL安装源：（这种方法更简单，不需要大规模yum -y update）
CentOS/RHEL 5 ：
rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-5.noarch.rpm
CentOS/RHEL 6 ：
rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm
CentOS/RHEL 7 ：
rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
Ok 安装完毕。
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命令安装：
yum search epel-release
yum -y install epel-release
即可。
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手动添加epel源配置
vim /etc/yum.repos.d/epel.repo
[epel]
name=Extra Packages for Enterprise Linux 7 - $basearch
#baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch
mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-7&arch=$basearch
failovermethod=priority
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
[epel-debuginfo]
name=Extra Packages for Enterprise Linux 7 - $basearch - Debug
#baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch/debug
mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-debug-7&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=1
[epel-source]
name=Extra Packages for Enterprise Linux 7 - $basearch - Source
#baseurl=http://download.fedoraproject.org/pub/epel/7/SRPMS
mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-source-7&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=1
然后执行：
yum clean all
yum makecache
即可
其实yum -y install epel-release和rpm -ivh xxx都是在/etc/yum.repos.d/路径下自动生成epel.repo和epel-testing.repo文件。但是默认情况下只有epel-testing.repo文件是生效的，如果想使用测试版，vim epel-testing.repo，把enabled=0改成enabled=1即可。
如果不想使用epel的yum源，把enabled=1改成enabled=0，或者直接卸载rpm -e epel-release.
