This is an old revision of the document!
If you upgraded to Centos 8 and are now finding that dnf no longer updates and gives an error that it can't find mirrors:
Error: Failed to download metadata for repo 'appstream': Cannot prepare internal mirrorlist: No URLs in mirrorlist
Then this is because Redhat decided to withdraw Centos 8 and instead not provide a free to use stable linux distribution anymore. Most of use who use(d) RedHat also had at least one CentOS distribution to run as a test system but this can now no longer be done and you will need to license additional servers for this purpose, OR, move to another distribution as many have done.
To get your Centos 8 back updating and migrate to Centos 8 stream (which is considered unstable) you will need to do the following:
Login as root
Execute:
sed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-Linux-*
sed -i 's|#baseurl=http://mirror.centos.org|baseurl=http://vault.centos.org|g' /etc/yum.repos.d/CentOS-Linux-*
dnf install centos-release-stream -y
dnf swap centos-{linux,stream}-repos -y
dnf distro-sync -y
You will probably need to reboot after this has all finished.