Debian router with iptables problem

[ISPM <ispmarin@gmail.com>]

Hello all. I've been struggling to put together an alpha 164sx like a
router to my lab using debian hardened. I have a internal network that
have to acess the external world, and should be routed to the exterior
by the alpha. The alpha should route then to the gateway of my
building, a machine that i don't have acess. The debian have to
network cards, one in the internal net and the other in the external
(gateway) network. This is something like that:


                                                   |-----(lab computers)
(net)--(gateway)-------(alpha router)--|
                                                   |-----(https server)
 
The alpha should work like a firewall and a router. I've been trying
to assemble by myself using iptables. The alpha cannot have X, so
programs like firebuilder or firestarter can't be used.
The debian is a sarge instalation with the 2.4.26-1-generic kernel
from the netistaller, with most packages downgraded to stable and
hardened using harden. There is plenty scripts in the internet, but
none elucidated two things: how to use nat to route internal traffic
to external world and vice versa, so the internal network can use the
net and some services (ssh), and how to make the route to the gateway
works (this is the hardest part for me). Just some help would be
appreciated! I don't want to bother you all to give me the scripts.

Very very thanks!
-----------------------------------------------------------
Ivan S. P. Marin
Laboratório de Física Computacional
Computacional Physics Laboratory
lfc.ifsc.usp.br
Instituto de Física de São Carlos - USP
----------------------------------------------------------