ftp thru ipfilter nat

[Barry Loper]

Hi,
I looked thru the archives but couldn't find an answer to this issue.
I have a small network of private ips behind a rh 7.1 "router".
I'm trying to use iptables to masq the internal net to the router's
 external ip.  Using the info in the netfilter-HOWTO, the only rule
I set up was ipfilter -t nat -A POSTROUTING -o eth0 -j MASQUERADE
eth0 is public and eth1 is private.  Now, the internet connection seems 
to be working just peachy until I try to ftp from one of the masq'ed
 machines. I get error 500 only client ip address allowed PORT command.
ftp'ing from the router works fine.  I'm no routing guru but, i think that 
what i want to do is pretty straight forward.  I had it working just fine 
with rh 6.1 and ipchains but, I want this to work with iptables.
Thanks for any help.