修改CentOS的yum源，改为阿里云的镜像

1、备份

mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup

2、下载新的CentOS-Base.repo 到/etc/yum.repos.d/

CentOS 6

curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo

CentOS 7

curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo

CentOS 8

curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-8.repo

3、清除缓存，重新生成缓存

清除缓存

yum clean all

重新设置缓存

yum makecache