修改CentOS的yum源，改為阿里雲的鏡像
1、備份
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
2、下載新的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、清除緩存，重新生成緩存
清除緩存
yum clean all
重新設置緩存
yum makecache