Can any one please tell me how to configure iptables to accept portmap services. I want to allow only nfs and nis services but the problem is as they are dependent on portmap, is is tough to predict the port no for me. Thanks in advance.

RPC services use random ports specified by portmap so under the default configurations, setting open ports with iptables is difficult.
You will need to force a number of rpc services to use fixed ports.

If your using RedHat to do this, go through the /etc/init.d/ files related to nfs to find out how you can fix the relevant ports for rpc.statd, rpc.nfsd, rpc.mountd, rpc.lockd and rpc.rquotad. A good place to start would be the portmap file.

search for lines starting as follows:

daemon rpc.statd

and add the option to fix the port:

daemon rpc.statd -p 3334

You can then easily configure iptables to keep these ports open. be sure to keep the portmap 111 port open.