1. 备份CentOS-Base.repo

   mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup

2. 下载新的CentOS-Base.repo 到/etc/yum.repos.d/

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

   说明：在执行上面之前，需要执行which wget确认系统中是否存在wget命令

或者

curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo

3. 执行yum makecache生成缓存