Yum repository mirror for old CentOS' versions
Most CentOS' mirrors don't host full mirror of Yum repository for all versions of CentOS. If you are using older version of CentOS, you might one-day found the yum update is broken since the mirror no longer contains the files you needed.
In this case, you can modify the file /etc/yum.repos.d/CentOS-Base.repo and change all paths point to the following:
.. baseurl=http://vault.centos.org/... ..
Then do a yum update to reflect the changes.