Centos 9 Could not resolve host: mirrors.centos.org问题

1. vim /etc/yum.repos.d/CentOS-Base.repo

[base]
name=CentOS-$releasever - Base
baseurl=https://vault.centos.org/7.9.2009/os/$basearch
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#released updates 
[updates]
name=CentOS-$releasever - Updates
baseurl=https://vault.centos.org/7.9.2009/updates/$basearch
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
baseurl=https://vault.centos.org/7.9.2009/extras/$basearch
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
baseurl=https://vault.centos.org/7.9.2009/centosplus/$basearch
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

2. yum clean all && yum makecache