备份源文件：CentOS-AppStream.repo
    内容：

    1. [aliyun-AppStream]
    3. name=CentOS-$releasever - AppStream
    4. baseurl=https://mirrors.aliyun.com/centos/$releasever/AppStream/$basearch/os/
    5. gpgcheck=1
    6. enabled=1
    7. gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

    备份源文件：CentOS-BaseOS.repo

    1. [aliyun-BaseOS]
    2. name=CentOS-$releasever - Base
    3. baseurl=https://mirrors.aliyun.com/centos/$releasever/BaseOS/$basearch/os/
    4. gpgcheck=1
    5. enabled=1
    6. gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

    备份源文件：CentOS-Extras.repo

    1. [aliyun-extras]
    2. name=CentOS-$releasever - Extras
    3. baseurl=https://mirrors.aliyun.com/centos/$releasever/extras/$basearch/os/
    4. gpgcheck=1
    5. enabled=1
    6. gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

    执行命令

    1. dnf -y install epel-release
    4. dnf clean all
    7. dnf makecache
    10. dnf repolist