[Cialug] Routing between subnets with iptables

[Don Ellis]

We have a network set up with two subnets behind a NAT. We are able to
get out of the network from either subnet.

What we want to do is be able to connect between a system on one
subnet and a system on the other subnet.

Does anyone have examples of doing this using iptables?

systemA in subnet0: 10.0.1.200
systemB in subnet1: 10.0.3.200

I have tried making changes in the iptables configuration, but I don't
have enough experience in iptables to know where to use -i, -o, -d,
and -s (I think those are the values to be adjusted here). Also, I'm
making assumptions about what table and chain the rules need to be
added to, and when to append (-A) and when to insert (-I).

Except for my first adjustment to something someone else had done
wrong, my shooting in the dark has had no results.

TIA,

--Don Ellis