更换yum源

cp /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.bak（备份可选）

wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo    alibaba的yum源

wget -P /etc/yum.repos.d/ http://mirrors.aliyun.com/repo/epel-7.repo

yum clean all

yum makecache