I start to wonder when I try to update the repo in my igos release 6. It always showed error. I suspect that the repository address in /etc/yum.repos.d/fedora-core.repo and other is expire. So I browsed and find out in fedora forum, yes it was already move to archive.

So change all the address in .repo file in baseurl to :
baseurl=http://archive.fedoraproject.org/pub/archive/fedora/…… (the end section should not be remove)

finally as root do this
#yum update

this thing happens with the ubuntu 6.10 to earliear.