问题

Loaded plugins: fastestmirror
Determining fastest mirrors
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=vag error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"One of the configured repositories failed (Unknown),and yum doesn't have enough cached data to continue. At this point the onlysafe 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 workingupstream. This is most often useful if you are using a newerdistribution release than is supported by the repository (and thepackages for the previous distribution release still work).3. Run the command with the repository temporarily disabledyum --disablerepo=<repoid> ...4. Disable the repository permanently, so yum won't use it by default. Yumwill then just ignore the repository until you permanently enable itagain or use --enablerepo for temporary usage:yum-config-manager --disable <repoid>orsubscription-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 muchslower). If it is a very temporary problem though, this is often a nicecompromise:yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=trueCannot find a valid baseurl for repo: base/7/x86_64

解决方法

检查dns

cat /etc/resolv.conf

1. ‌阿里公共DNS，地址为223.5.5.5和223.6.6.6。
2. ‌百度DNS，地址为180.76.76.76，支持Windows、Mac和Linux系统。
3. DNSPod Public DNS，隶属于‌腾讯云，地址为119.29.29.29。
4. 114 DNS，高速且稳定，地址为114.114.114.114和114.114.115.115。‌

ping百度确认网络畅通

ping www.baidu.com

备份当前的yum源

注意查看CentOS-Base.repo文件的路径，路径可能会有不同，根据自己的路径来

mv /etc/yum.repos.d/backup/CentOS-Base.repo /etc/yum.repos.d/backup/CentOS-Base.repo.backup

 下载新的CentOS-Base.repo到原目录

wget -O /etc/yum.repos.d/backup/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo

或者

curl -o /etc/yum.repos.d/backup/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo

更新yum缓存 

yum clean all
yum makecache