[CentOS] NFS and firewall issues

Hi,

I have a small home LAN, where one machine (192.168.1.1) connects to the
internet via dialup (no DSL here) and shares the connection with IP
masquerading. To achieve this, I tried out various recipes found on the
internet, and finally put this in /etc/rc.d/rc.local:

--8<------ rc.local ------------
#!/bin/sh
#
# This script will be executed *after* all the other init scripts.
# You can put your own initialization stuff in here if you don't
# want to do the full Sys V style init stuff.

touch /var/lock/subsys/local

# Setup IP Masquerading
iptables -F; iptables -t nat -F; iptables -t mangle -F
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE

# Secure it
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -m state --state NEW -i ! ppp0 -j ACCEPT
iptables -P INPUT DROP
iptables -A FORWARD -i ppp0 -o ppp0 -j REJECT
--8<---------------------------

I don't understand much about iptables. Documentation about it is on my TODO
list (I'll keep it for after my death:oD).

Now I have a problem. I fiddled a bit with NFS, and I had a firewall-related
problem with it. In short, when iptables (the firewall) runs, I can't do an NFS
mount. But as soon as I stop it (service iptables stop), NFS mounts are
possible.

Now what can I do to 1) go on sharing my ppp connection 2) be able to do NFS
mounts while 3) still having a reasonable secure system?

Cheers,

Niki Kovacs