Linux-centos8官方源已下线，建议切换centos-vault源

问题描述
使用yum安装时出现如下错误：

Errors during downloading metadata for repository 'AppStream':
  - Status code: 404 for http://mirrors.cloud.aliyuncs.com/centos/8/AppStream/x86_64/os/repodata/repomd.xml (IP: 100.100.2.148)
Error: Failed to download metadata for repo 'AppStream': Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried

问题分析
阿里云解释：centos8官方源已下线，建议切换centos-vault源

解决方法
1.删除AppStream源
rm -f /etc/yum.repos.d/CentOS-AppStream.repo
2.取消并备份旧yum源（可直接删除）
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
3.下载vault源
curl -o /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-vault-8.5.2111.repo
4.清除yum缓存
yum clean all
5.生成新缓存
 yum makecache
附加
如果出现 Couldn't resolve host 'mirrors.cloud.aliyuncs.com' ，非阿里云服务器无法访问其内网域名，需要作替换，eg：

sed -i -e '/mirrors.cloud.aliyuncs.com/d' -e '/mirrors.aliyuncs.com/d' /etc/yum.repos.d/CentOS-Base.repo

方法二：

$ sudo sed -i -e "s|mirrorlist=|#mirrorlist=|g" /etc/yum.repos.d/CentOS-*
$ sudo sed -i -e "s|#baseurl=http://mirror.centos.org|baseurl=http://vault.centos.org|g" /etc/yum.repos.d/CentOS-*