本文章介绍的yum源支持的版本是CentOS 6系列，理论上支持RHEL6系列。
更新方法如下：

先进入yum源配置目录

cd /etc/yum.repos.d

备份系统自带的yum源

mv CentOS-Base.repo CentOS-Base.repo.save

下载其他更快的yum源

中科大的yum源：
wget http://centos.ustc.edu.cn/CentOS-Base.repo

163的yum源：
wget http://mirrors.163.com/.help/CentOS-Base-163.repo

sohu的yum源
wget http://mirrors.sohu.com/help/CentOS-Base-sohu.repo

更新完yum源后，建议更新一下，使操作立即生效

yum makecache

[yasi]:
在执行最后一步，即yum makecache的时候，遇到错误：
Loaded plugins: fastestmirror
Repository base is listed more than once in the configuration
Repository updates is listed more than once in the configuration
Repository extras is listed more than once in the configuration
Repository centosplus is listed more than once in the configuration
Repository contrib is listed more than once in the configuration
Loading mirror speeds from cached hostfile
 * addons: mirrors.163.com
 * base: mirror.metrocast.net
 * extras: mirror.beyondhosting.net
 * fasttrack: centos.vipernetworksystems.com
 * updates: centos.vipernetworksystems.com
http://mirrors.163.com/centos/6/addons/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 404"
Trying other mirror.
http://mirror.centos.org/centos/6/addons/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 404"
Trying other mirror.
Error: Cannot retrieve repository metadata (repomd.xml) for repository: addons. Please verify its path and try again
原因是，http://mirrors.163.com/centos/6/addons/x86_64/repodata/repomd.xml和 http://mirror.centos.org/centos/6/addons/x86_64/repodata/repomd.xml 确实不存在，可能被更新掉了。解决办法是，找到可用的URL替换之，或者把相应的.repo文件中的项整个注释掉。这里我采用后者。

例如，在CentOS-Base-163.repo中，注释掉整个addon项
#[addons]
#name=CentOS-$releasever - Addons - 163.com
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=addons
#baseurl=http://mirrors.163.com/centos/$releasever/addons/$basearch/
#gpgcheck=1
#pgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5

在CentOS-Base-sohu.repo中注释掉下面的内容
#[addons]
#name=CentOS-$releasever - Addons - sohu.com
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=addons
#baseurl=http://mirrors.sohu.com/centos/$releasever/addons/$basearch/
#gpgcheck=1
#gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5CentOS-Base-sohu.repo

然后，再执行yum makecache。之后，yum就可以找到我要安装的包了，比如gdb, subversion, gcc-c++ 等。

注意：

有时用yum list xxx找不到某个包，可用尝试用yum search xxx。
如，yum list vim 就报告 Error: No matching Packages to list。而 yum search vim 则返回：
vim-X11.x86_64 : The VIM version of the vi editor for the X Window System
vim-common.x86_64 : The common files needed by any version of the VIM editor
vim-enhanced.x86_64 : A version of the VIM editor which includes recent enhancements

vim-minimal.x86_64 : A minimal version of the VIM editor