提示的具体消息如下
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 
解决方案
vi CentOS-Base.repo
修改配置文件
base下方的baseurl替换成国内镜像源,将mirrorlist注释掉;
baseurl=http://mirrors.aliyun.com/centos/$releasever/extras/$basearch/
 #mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras
保存之后,记得要执行下面2条命令
      sudo yum clean all
        sudo yum makecache
然后就可以使用yum install 了