Unable to connect to RHN network - ping works but nslookup fails

You should have proper entries in /etc/hosts and /etc/nsswitch.conf

Usually, you'll want its hosts line to look like(in /etc/nsswitch.conf):

hosts:      files dns

Antoher file to check is /etc/resolv.conf
check the permissions of all these files.

Read: http://superuser.com/questions/704785/ping-cant-resolve-hostname-but-nslookup-can