Determining fastest mirrors
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; Unknown error"
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.备份原有镜像源：
sudo mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.bak
2.下载新的镜像源配置文件（例如阿里云）：
sudo curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
3.清理并重建缓存：
sudo yum clean all
sudo yum makecache
sudo yum install net-tools