Cannot retrieve repository metadata (repomd.xml) for repository: base

1. CentOS6.8 yum安装报错
   Cannot retrieve repository metadata (repomd.xml) for repository: base. Please verify its path and try again
2. 解决方法
   1. 备份现有repo仓库
      mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
   2. 使用阿里云仓库repo
      curl -o /etc/yum.repos.d/CentOS-Base.repo https://www.xmpan.com/Centos-6-Vault-Aliyun.repo
   3. 更新yum
      yum clean all
      yum makecache
3. 处理完yum 源问题，即可正常安装