trying to get oddball setup to work

["Andrew St. Jean" <ast.000@sympatico.ca>]

I'm hoping someone can tell me if what I'm trying to do is possible and 
if so, how.

Here's a picture of my network topology:

machine x-------internet---------machine y-----private LAN----host a/b/c

Machine x has one interface with a public static IP.
Machine y has two interfaces, one with a public dynamic IP and the other 
with a private static IP.

Both machines x and y have iptables installed and running.
I have an ipsec tunnel (using FreeS/Wan) working between machines x and 
y. With this tunnel I can ping any of the hosts on my private LAN from 
machine x. I can also mount a shared partition from, say, host b onto 
machine x. I include this just to show that the ipsec tunnel is working.

What I want to do is use NAT to forward certain ports on machine x to 
machines on my private LAN. Right now, if I open a port in iptables on 
machine x, I can connect to machine x from the internet on that port. If 
I try to forward the port to host c, packets seem to disappear on 
machine x. I can see the packets arrive at machine x using tcpdump but 
nothing goes out again.

Thanks for any help provided.

Andrew