NIS client will not work through firewall, help please

I have an Ubuntu lucid 10.4 running as NIS client, the machine was
working fine as a NIS client until i move the machine to a DMZ
firewall.

I tried to get NIS client working through the firewall by I fixing
ypbind port to a static port by modifying the file “/etc/default/nis”
YPBINDARGS="-p 835”

The above fixed ypbind port on 835, I then opened port 835 (ypbind)
and port 111 portmapper on the firewall.

rpcinfo -p
program vers proto port
100000 2 tcp 111 portmapper
100000 2 udp 111 portmapper
100007 2 udp 835 ypbind
100007 1 udp 835 ypbind
100007 2 tcp 835 ypbind
100007 1 tcp 835 ypbind

However when i try to start ypbind daemon, ypbind fail to bind to
ypserver

“ypcat passwd” show the following error message

YPBINDPROC_DOMAIN: Domain not bound
No such map passwd.byname. Reason: Can't bind to server which serves
this domain

Because I could not ping the NIS server i modified “/etc/default/nis”
to

YPBINDARGS="-no-dbus -p 835 -no-ping"

But this did not fix the problem, NIS client is still not binding to
ypserver through the firewall.

What other port do i need to open on the firewall?

What am i doing rough? Can you help please?