One of my Linux server running Redhat version 3 is dead. I already build the new server but I need to get few of the license files from the old server. Currently, I booted the old server using the CDROM. But when I go to /mnt/sysimage/var, I don't see any of files? I also went to /mnt/sysimage/usr and I don't see any thing.
Booting from cdrom won't necessarily mount the disks for you, or mount them in the right places if it does. A rescue cdrom may make few to no assumptions on what belongs where and rely on you to decide that instead by mounting things manually. Even the device names may not be identical.
I believe you're looking for the license files in /etc of the old server, which would be on the root partition of the old server. So yeah, use df. It's probably /mnt/sysimage/ and look for the etc dir under it