问题一：无法正常使用yum命令。问题描述：终端执行yum-yupdate，报以下错误提示：Couldnotretrievemirrorlisthttp://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=stock32errorwas14:curl#6-"Couldnotresolvehost:mirrorlist.centos.org;Unknown