repo
什么是repo文件？
repo文件是Fedora中yum源（软件仓库）的配置文件，通常一个repo文件定义了一个或者多个软件仓库的细节内容，例如我们将从哪里下载需要安装或者升级的软件包，repo文件中的设置内容将被yum读取和应用！
YUM的工作原理并不复杂，每一个 RPM软件的头（header）里面都会纪录该软件的依赖关系，那么如果可以将该头的内容纪录下来并且进行分析，可以知道每个软件在安装之前需要额外安装 哪些基础软件。也就是说，在服务器上面先以分析工具将所有的RPM档案进行分析，然后将该分析纪录下来，只要在进行安装或升级时先查询该纪录的文件，就可 以知道所有相关联的软件。所以YUM的基本工作流程如下：
服务器端：在服务器上面存放了所有的RPM软件包，然后以相关的功能去分析每个RPM文件的依赖性关系，将这些数据记录成文件存放在服务器的某特定目录内。
客户端：如果需要安装某个软件时，先下载服务器上面记录的依赖性关系文件(可通过WWW或FTP方式)，通过对服务器端下载的纪录数据进行分析，然后取得所有相关的软件，一次全部下载下来进行安装。
CentOS5-Base.repo
# 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-$releasever - Base - 163.com
baseurl=http://vault.centos.org/5.11/os/$basearch/
gpgcheck=1
gpgkey=http://vault.centos.org/RPM-GPG-KEY-CentOS-5
#released updates
[updates]
name=CentOS-$releasever - Updates - 163.com
baseurl=http://vault.centos.org/5.11/updates/$basearch/
gpgcheck=1
gpgkey=http://vault.centos.org/RPM-GPG-KEY-CentOS-5
#packages used/produced in the build but not released
[addons]
name=CentOS-$releasever - Addons - 163.com
baseurl=http://vault.centos.org/5.11/addons/$basearch/
gpgcheck=1
gpgkey=http://vault.centos.org/RPM-GPG-KEY-CentOS-5
#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras - 163.com
baseurl=http://vault.centos.org/5.11/extras/$basearch/
gpgcheck=1
gpgkey=http://vault.centos.org/RPM-GPG-KEY-CentOS-5
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus - 163.com
baseurl=http://vault.centos.org/5.11/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://vault.centos.org/RPM-GPG-KEY-CentOS-5
#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib - 163.com
baseurl=http://vault.centos.org/5.11/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://vault.centos.org/RPM-GPG-KEY-CentOS-5
epel
什么是epel
如果既想获得 RHEL 的高质量、高性能、高可靠性，又需要方便易用(关键是免费)的软件包更新功能，那么 Fedora Project 推出的 EPEL(Extra Packages for Enterprise Linux)正好适合你。EPEL(http://fedoraproject.org/wiki/EPEL) 是由 Fedora 社区打造，为 RHEL 及衍生发行版如 CentOS、Scientific Linux 等提供高质量软件包的项目。
安装epel
rpm -ivh epel-release-latest-5.noarch.rpm
导入key：
rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-5
如果用比较新的软件，用epel-test.repo这个文件就行了
别忘了安装yum install yum-priorities
CentOS5入口：https://archives.fedoraproject.org/pub/archive/epel/
CentOS6入口：https://dl.fedoraproject.org/pub/epel/6/x86_64/
epel.repo
[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=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL
[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=0
[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=0
epel-testing.repo
[epel-testing]
name=Extra Packages for Enterprise Linux 5 - Testing - $basearch
#baseurl=http://download.fedoraproject.org/pub/epel/testing/5/$basearch
mirrorlist=http://mirrors.fedoraproject.org/mirrorlist?repo=testing-epel5&arch=$basearch
failovermethod=priority
enabled=0
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL
[epel-testing-debuginfo]
name=Extra Packages for Enterprise Linux 5 - Testing - $basearch - Debug
#baseurl=http://download.fedoraproject.org/pub/epel/testing/5/$basearch/debug
mirrorlist=http://mirrors.fedoraproject.org/mirrorlist?repo=testing-debug-epel5&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL
gpgcheck=1
[epel-testing-source]
name=Extra Packages for Enterprise Linux 5 - Testing - $basearch - Source
#baseurl=http://download.fedoraproject.org/pub/epel/testing/5/SRPMS
mirrorlist=http://mirrors.fedoraproject.org/mirrorlist?repo=testing-source-epel5&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL
gpgcheck=1
$ yum clean all
$ yum makecache
执行完上面两句命令就可以正常使用成功啦。
常用命令
1）指定目录安装
$ yum -c /etc/yum.conf --installroot=/usr/local --releasever=/ install httpd
2）只进行下载rpm包，不进行安装
$ yum install --downloadonly --downloaddir=/tmp cacti
转载链接：
1、yum的repo文件详解、以及epel简介、yum源的更换
https://www.cnblogs.com/nineep/p/6795692.html
RPM包下载链接：
https://download.csdn.net/download/weixin_38642130/10921500