Taken from
http://www.ccm.ece.vt.edu/~lscharf/samd/?topic=Linux&title=YP%2FNIS%2C+NFS%2C+RPC%2C+and+lokkit :

YP/NIS, NFS, RPC, and lokkit
Keywords:
Date Created: 2002-04-05
Author: Luke Scharf luke@vt.edu

Platform:

* RedHat 7.x

Problem:

* ypbind can find the ypserver, but "ypcat passwd" times out with an
RPC error. "rpcinfo -p" returns a correct-looking list of
portmapped services.
* NFS won't mount an exported directory; RPC times out.

Solution:
You can disable RedHat's built-in firewall by running /usr/sbin/lokkit.
You can get some measure of security back by using /etc/hosts.allow and
/etc/hosts.deny.

Explanation:
The RedHat firewall configurator (/usr/sbin/lokkit) sets up iptables so
that udp traffic is only accepted on specific ports and from specific
locations (like your DNS servers). The portmapper uses semi-arbitrary
ports (that's why the ports have to be mapped), so it's a lot more work
to set up machine-by-machine security.