CentOS 7 官方配置的镜像源已经停止访问，若想正常使用yum的话，需要更改镜像源

1、备份原镜像源配置

sudo cp -v /etc/yum.repos.d/CentOS-Base.repo{,-backup}

2、编辑配置文件

vi /etc/yum.repos.d/CentOS-Base.repo

按i进入编辑模式

覆盖替换如下内容：

[base]
name=CentOS-7.9.2009 - Base
baseurl=https://vault.centos.org/7.9.2009/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=1
metadata_expire=never

[updates]
name=CentOS-7.9.2009 - Updates
baseurl=https://vault.centos.org/7.9.2009/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=1
metadata_expire=never

[extras]
name=CentOS-7.9.2009 - Extras
baseurl=https://vault.centos.org/7.9.2009/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=1
metadata_expire=never

[centosplus]
name=CentOS-7.9.2009 - CentOSPlus
baseurl=https://vault.centos.org/7.9.2009/centosplus/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=0
metadata_expire=never

按esc退出编辑模式

输入wq!保存并退出

3、清除yum缓存

sudo yum clean all

4、重建yum镜像源

sudo yum makecache