在没有光盘的情况，需要安装软件包，就要用到共网的yum源来安装了。
RedHat linux 默认是安装了yum软件的，但是由于激活认证的原因让redhat无法直接进行yum安装一些软件，如果我们需要在redhat下直接yum安装软件，我们只用把yum的源修改成CentOS的就好了，然后把源里面的变量全部修改成实际的值，这样就能使用yum直接安装我们需要的软件了。
使用说明
1、到http://mirrors.163.com的 centos帮助文档 中下载CentOS6-Base-163.repo文件，存放到/etc/yum.repo.d中
Centos 5 wget http://mirrors.163.com/.help/CentOS5-Base-163.repo
Centos 6 wget http://mirrors.163.com/.help/CentOS6-Base-163.repo
操作如下：
2、首先备份/etc/yum.repos.d/CentOS-Base.repo
3、编辑CentOS6-Base-163.repo文件，将其中的$releasever更改为centos的版本号，如果是RedHat 6.2 X86_64为例“baseurl=http://mirrors.163.com/centos/6.2/os/x86_64/”，本文档是以RedHat 6.2 32位为例所写！
[base]
name=CentOS-6.2 - Base - 163.com
baseurl=http://mirrors.163.com/centos/6.2/os/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6.2
#released updates
[updates]
name=CentOS-6.2 - Updates - 163.com
baseurl=http://mirrors.163.com/centos/6.2/updates/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6.2
#additional packages that may be useful
[extras]
name=CentOS-6.2 - Extras - 163.com
baseurl=http://mirrors.163.com/centos/6.2/extras/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6.2
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-6 .2- Plus - 163.com
baseurl=http://mirrors.163.com/centos/6.2/centosplus/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus
gpgcheck=1
enabled=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6.2
#contrib - packages by Centos Users
[contrib]
name=CentOS-6.2 - Contrib - 163.com
baseurl=http://mirrors.163.com/centos/6.2/contrib/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=contrib
gpgcheck=1
enabled=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6.2
编辑完后保存，运行：
yum clean all 清除原有缓存
yum makecache 获取yum列表