备份

mkdir /etc/yum.repos.d/backup
mv /etc/yum.repos.d/*.* /etc/yum.repos.d/backup

使用阿里源 ( cent os 7 )

wget -O /etc/yum.repos.d/CentOS7-Aliyun.repo http://mirrors.aliyun.com/repo/Centos-7.repo
wget -O /etc/yum.repos.d/epel-7-Aliyun.repo http://mirrors.aliyun.com/repo/epel-7.repo
yum clean all
yum makecache
#查看源列表
yum repolist all