背景：原来的yum源，无法下载yum包了。新增一个阿里云的（网易163的源失效了，无法使用）
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=stock error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"处理方式：
1、切换到目录：cd /etc/yum.repos.d
2、下载
wget -O Centos-7.repo https://mirrors.aliyun.com/repo/Centos-7.repo
或
curl -o Centos-7.repo https://mirrors.aliyun.com/repo/Centos-7.repo
3、生成缓存 yum makecache 
)
