Doing a yum update, I received this error and the update quit:

Errors during downloading metadata for repository 'appstream':
  - Curl error (6): Couldn't resolve host name for http://mirrorlist.centos.org/?release=8-stream&arch=x86_64&repo=AppStream&infra=stock [Could not resolve host: mirrorlist.centos.org]
Error: Failed to download metadata for repo 'appstream': Cannot prepare internal mirrorlist: Curl error (6): Couldn't resolve host name for http://mirrorlist.centos.org/?release=8-stream&arch=x86_64&repo=AppStream&infra=stock [Could not resolve host: mirrorlist.centos.org]

Because CentOS 8 reached end-of-life, the yum repositories at “mirrorlist.centos.org” stopped working and need to be updated to new CentOS 8 Stream repos.

You can batch edit all the files in /etc/yum.repos.d/ with these commands:

sed -i s/mirror.centos.org/vault.centos.org/g /etc/yum.repos.d/*.repo
sed -i s/^#.*baseurl=http/baseurl=http/g /etc/yum.repos.d/*.repo
sed -i s/^mirrorlist=http/#mirrorlist=http/g /etc/yum.repos.d/*.repo

The content of the repo files will change like this:

[baseos]
name=CentOS Stream $releasever - BaseOS
#mirrorlist=http://mirrorlist.centos.org/?release=$stream&arch=$basearch&repo=BaseOS&infra=$infra
baseurl=http://vault.centos.org/$contentdir/$stream/BaseOS/$basearch/os/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial