I’ve a couple of fedora boxes that were coming up with this error when rkhunter was run.
Rootkit Hunter 1.2.8 is running
Determining OS… Unknown
Warning: This operating system is not fully supported!
Seems you just need to add a suitable line into /var/rkhunter/db/os.dat so rkhunter can find your md5 hashes again and properly verify them. e.g.
705:Fedora Core release 5 (Bordeaux) (i386):/usr/bin/md5sum:/bin:
or
706:Fedora Core release 6 (Zod) (i386):/usr/bin/md5sum:/bin:
References
http://www.webhostingtalk.com/showthread.php?t=497209