报错如下：
Loaded plugins: fastestmirror,langpacks
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=stock error was
12: Timeout on http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=stock: (28,'Resolving timed out after 30383 milliseconds')
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 using a newer
distribution release than is supported by the repository (and the
packages for the prevIoUs distribution release still work).
3. disable the repository,so yum won't use it by default. Yum will then
just ignore the repository until you permanently enable it again or use
--enablerepo for temporary usage:
yum-config-manager --disable <repoid>
4. Configure the failing repository to be skipped,if it is unavailable.
Note that yum will try to contact the repo. when it runs most commands,
so will have to try and fail each time (and thus. yum will be be much
slower). If it is a very temporary problem though,this is often a nice
compromise:
yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=true
Cannot find a valid baseurl for repo: base/7/x86_64
这种错误，是因为没有配置/etc/resolv.conf。
nameserver 8.8.8.8
保存退出即可
