I have multiple redhat 5 servers that are behind a firewall. I used the mirror script from here http://www.webmo.net/support/yum_repository.html and downloaded ftp://mirror.switch.ch/pool/3/mirror/centos/5.2/ updates and os to my internal repository. Changed my repo files on one of my servers to point to these directories. When I run yum check-update, it connects to the repository

[root@lxtpmb01 yum.repos.d]# yum check-update
Loading "rhnplugin" plugin
Loading "installonlyn" plugin
This system is not registered with RHN.
RHN support will be disabled.
Setting up repositories
rhel5-swrepo 100% |=========================| 951 B 00:00
Reading repository metadata in from local files
primary.xml.gz 100% |=========================| 434 kB 00:00
################################################## 959/959
primary.xml.gz 100% |=========================| 434 kB 00:00
################################################## 959/959


But then nothing. It didn't find any files that need to be updated.

Is this because the OS is a redhat release and is looking for redhat files and the updates are centos? Or I did something else wrong?

Centos only supports the most current point release. Since Centos 5.3 is out, there will be no further updates to 5.2. There actually have not been any Centos5.2 updates since RHEL5.3 was released(almost 2months?). You can copy (change your source address 5.2 to 5.3) the new files and try again.

The 5.2 mirror that I used has new files in the updates directory. They are certainly newer than the redhat server that I'm trying to update, which was last updated over a year ago. What I don't understand is why my yum check-update didn't recognize those newer files as files that it should use for updates.

I had to clear the yum cache on the server I was trying to update by rm'ing everything from /var/cache/yum. Once I did that and ran yum check-update it found all the new files on my repository.