1、问题现象
由于CentOS停止维护，mirrorlist.centos.org网站也关闭不可访问。导致CentOS默认配置的yum.repos.d源也不可用，所以执行yum命令会报“Could not resolve host: mirrorlist.centos.org”错误。具体如下：
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; 未知的错误"
One of the configured repositories failed (未知),
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. Run the command with the repository temporarily disabled
yum --disablerepo=<repoid> ...4. Disable the repository permanently, 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>
or
subscription-manager repos --disable=<repoid>5. 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
二、解决办法
通过一下命令，使用新文件替换CentOS原有的CentOS-Base.repo文件，。
#先备份原来的CentOS-Base.repo文件
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo_old#从阿里云下载新文件，替代CentOS-Base.repo
curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo#重建yum元数据缓存（大约需1分钟）
yum makecache
以上完成后，yum命令即可正常使用。