我安装了全新的 Cent-OS 7，但是当我想更新或获取新软件包时，出现
此错误：
Loaded plugins: fastestmirror, langpacks
One of the configured repositories failed (Unknown),
and yum doesn't have enough cached data to continue. At this point the only
safe thing yum can do is fail. There are a few ways to work "fix" this:
1. Contact the upstream for the repository and get them to fix the problem.
2. Reconfigure the baseurl/etc. for the repository, to point to a working
upstream. This is most often useful if you are …