centos8 yum 报错：
Error: Failed to download metadata for repo ‘appstream’: Cannot prepare internal mirrorlist: No URLs in mirrorlist
详细信息如下：
CentOS Linux 8 - AppStream 4.2 B/s | 38 B 00:09 Error: Failed to download metadata for repo 'appstream': Cannot prepare internal mirrorlist: No URLs in mirrorlist解决 step1
cd /etc/yum.repos.d/step2
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-*step3
yum update -y
