CentOS8yum安装遇到Failed to download metadata for repo 'AppStream'解决

CentOS上使用yum安装软件，总是报错：

[root@localhost ~]# yum update
CentOS-8 - AppStream 70 B/s | 38 B 00:00
Error: Failed to download metadata for repo 'AppStream': Cannot prepare internal mirrorlist: No URLs in mirrorlist

或者 yum install epel-release 也是报同样的错误

原因是CentOS8的官方yum源已经停止维护了，需要切换到 vault.centos.org

按如下操作即可：

cd /etc/yum.repos.d/

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

yum clean all && yum makecache

后面继续使用yum安装软件就可以了