1.备份源

rename '.repo' '.repo.bak' /etc/yum.repos.d/CentOS*.repo

2.下载CentOS源（如果curl不存在或者不好用，手动下载源后上传到/etc/yum.repos.d/目录）：

curl -o /etc/yum.repos.d/Centos-vault-8.5.2111.repo https://mirrors.aliyun.com/repo/Centos-vault-8.5.2111.repo
curl -o /etc/yum.repos.d/epel-archive-8.repo https://mirrors.aliyun.com/repo/epel-archive-8.repo

3.清理&重建缓存

yum clean all && yum makecache