[root@localhost ~]# yum -y install epel-release && yum -y update
CentOS Linux 8 - AppStream 60 B/s | 38 B 00:00
错误：为 repo 'appstream' 下载元数据失败 : Cannot prepare internal mirrorlist: No URLs in mirrorlist
解决方法：
运行一下命令，就能解决：
sed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-Linux-*
sed -i 's|#baseurl=http://mirror.centos.org|baseurl=http://vault.centos.org|g' /etc/yum.repos.d/CentOS-Linux-*
之后，不会收到更新，需要重新安装包，运行下面安装包：
sudo dnf install centos-release-stream -y
sudo dnf swap centos-{linux,stream}-repos -y
sudo dnf distro-sync -y
