Error: Failed to download metadata for repo ‘AppStream‘

　　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