centos 更换yum源

centos源更换为，163或者阿里的。

1、首先备份
mv /etc/yum.repos.d/Centos-Base.repo /etc/yum.repos.d/Centos-Base.repo.backup

2、下载repo文件到目录下
cd /etc/yum.repos.d
wget http://mirrors.163.com/.help/CentOS7-Base-163.repo
3、yum clean all //清除缓存
yum makecache //生成新的缓存

163和aliyun镜像：
http://mirrors.163.com
http://mirrors.163.com/.help/centos.html
https://mirrors.aliyun.com/
https://mirrors.aliyun.com/help/centos