1)
mv /etc/yum.repos.d/CentOS-base.repo /etc/yum.repos.d/CentOS-base.repo.backup
2)
wget -O /etc/yum.repos.d/CentOS-base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
或者
curl -o /etc/yum.repos.d/CentOS-base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
3)
yum clean all
yum makecache
yum update -y
所有的yum源
yum repolist all
可用的yum源
yum repolist enabled
