1、首先备份系统自带yum源配置文件/etc/yum.repos.d/CentOS-Base.repo
    [root@localhost ~]# mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup

    2、下载ailiyun的yum源配置文件到/etc/yum.repos.d/
    CentOS7
    [root@localhost ~]# wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
    CentOS6
    [root@localhost ~]# wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo
    CentOS5
    [root@localhost ~]# wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-5.repo

    3、运行yum makecache生成缓存
    [root@localhost ~]# yum makecache