先说结论，以阿里云为例，复制以下代码执行即可：
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
mv /etc/yum.repos.d/CentOS-AppStream.repo /etc/yum.repos.d/CentOS-AppStream.repo.backup
mv /etc/yum.repos.d/CentOS-Extras.repo /etc/yum.repos.d/CentOS-Extras.repo.backup
mv /etc/yum.repos.d/CentOS-PowerTools.repo /etc/yum.repos.d/CentOS-PowerTools.repo.backup
mv /etc/yum.repos.d/CentOS-centosplus.repo /etc/yum.repos.d/CentOS-centosplus.repo.backup
curl -o /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-8.repo
dnf makecache
在写这篇博客的时候，阿里云的源是这样用的：
# 备份
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
curl -o /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-8.rep
# 或者
# wget -O /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-8.repo
不过在dnf makecache的时候会报以下问题：
