备份原有的源：
sudo mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
下载国内源：
sudo wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
清理并生成新缓存
sudo yum clean all sudo yum makecache fast
检查更改：
sudo yum repolist
