# yum update
Loaded plugins: fastestmirror
Determining fastest mirrors
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
Eg. $releasever is not a valid release or hasnt been released yet/
removing mirrorlist with no valid mirrors: /var/cache/yum/addons/mirrorlist.txt
Error: Cannot find a valid baseurl for repo: addons
I tried to manually install the package:
# wget http://centos.patan.com.ar/5.4/os/x86_64/CentOS/centos-release-5-4.el5.centos.1.x86_64.rpm
# wget http://centos.patan.com.ar/5.4/os/x86_64/CentOS/centos-release-notes-5.4-4.x86_64.rpm
# rpm -Uvh centos-release-5-4.el5.centos.1.x86_64.rpm centos-release-notes-5.4-4.x86_64.rpm --nodeps
and got it fixed.