具体原因：使用yum搜索某些rpm包，找不到包是因为CentOS是RedHat企业版编译过来的，去掉了所有关于版权问题的东西。安装EPEL后可以很好的解决这个问题。EPEL(Extra Packages for Enterprise Linux )即企业版Linux的扩展包，提供了很多可共Centos使用的组件，安装完这个以后基本常用的rpm都可以找到。

Method #1从企业版Linux库配置包中安装扩展包。（推荐 ）

[root@localhost~]#sudoyuminstallepel-release
Loadedplugins:fastestmirror,langpacks
Loadingmirrorspeedsfromcachedhostfile
ResolvingDependencies
-->Runningtransactioncheck
--->Packageepel-release.noarch0:7-5willbeinstalled
-->FinishedDependencyResolution

DependenciesResolved

================================================================================
PackageArchVersionRepositorySize
================================================================================
Installing:
epel-releasenoarch7-5extras14k

TransactionSummary
================================================================================
Install1Package

Totaldownloadsize:14k
Installedsize:24k
Isthisok[y/d/N]:y
Downloadingpackages:
epel-release-7-5.noarch.rpm|14kB00:01
Runningtransactioncheck
Runningtransactiontest
Transactiontestsucceeded
Runningtransaction
Installing:epel-release-7-5.noarch1/1
Verifying:epel-release-7-5.noarch1/1

Installed:
epel-release.noarch0:7-5

Complete!

Method #2 从dl.fedoraproject.org安装扩展包。

1.使用wget命令下载epel-release：

[root@localhost~]#cd/tmp
[root@localhost~]#wgethttps://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpmls*.rpm

或者通过wiki查找到对应版本的epel后，再下载：

[root@localhost~]#wgethttp://dl.fedoraproject.org/pub/epel/7/x86_64/epel-release-7-5.noarch.rpm 或 [root@localhost~]#wgethttp://mirrors.ustc.edu.cn/fedora/epel/7/x86_64/epel-release-7-5.noarch.rpm//中国科学技术大学镜像

2.安装：

[root@localhost~]#sudoyuminstallepel-release-7.noarch.rpm

注意：

1. epel的版本要与centos的版本相对应，否则安装失败，即centos7版本要对应epel7版本。

2.rpm安装完后，rpm -ql epel*，确提示未安装，此时要导入rpm的签名信息

#rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6 （root身份执行）