The result there was to create the ability to have zones on a private network segment, behind a proxy/router zone.
What, however, if you want the zones on one of those private segments to communicate with zones on a different private segment?
Consider the following two proxy zones:
A: address 192.168.10.1, subnet 10.1.0.0/16
B: address 192.168.10.2, subnet 10.2.0.0/16
And we want the zones in the 10.1.0.0 and 10.2.0.0 subnets to talk to each other. The first step is to add routes, so that packets from system A destined for the 10.2.0.0 subnet are sent to host B. (And vice versa.)
A: route add net 10.2.0.0/16 192.168.10.2
B: route add net 10.1.0.0/16 192.168.10.1
This doesn't quite work. The packets are sent, but recall that the proxy zone is doing NAT on behalf of the zones behind it. So packets leaving 10.1.0.0 get NATted on the way out, get delivered successfully to the 10.2.0.0 destination but then the reply packet gets NATted on its way back, so it doesn't really work.
So, all that's needed is to not NAT the packets that are going to the other private subnet. Remember the original NAT rule in ipnat.conf on host A would have been:
map pnic0 10.1.0.0/16 -> 0/32 portmap tcp/udp auto
map pnic0 10.1.0.0/16 -> 0/32
and we don't want to NAT anything that is going to 10.2.0.0, which would be:
map pnic0 from 10.1.0.0/16 ! to 10.2.0.0/16 -> 0/32 portmap tcp/udp auto
map pnic0 from 10.1.0.0/16 ! to 10.2.0.0/16 -> 0/32
And that's all there is to it. You now have a very simple private software-defined network with the 10.1 and 10.2 subnets joined together.
If you think this looks like the approach underlying Project Calico, you would be right. In Calico, you build up the network by managing routes (many more as it's per-host rather than the per-subnet I have here), although Calico has a lot more manageability and smarts built in to it rather than manually adding routes to each host.
While simple, there are obvious problems associated with scaling such a solution.
While adding and deleting routes isn't so bad, listing all the subnets in ipnat.conf would be tedious to say the least. The solution here would be to use the ippool facility to group the subnets.
How do we deal with a dynamic environment? While the back-end zones would come and go all the time, I expect the proxy/router zone topology to be fairly stable, so configuration churn would be fairly low.
The mechanism described here isn't limited to a single host, it easily spans multiple hosts. (With the simplistic routing as I've described it here, those hosts would have to be on the same network, but that's not a fundamental limitation.) My scripts in Tribblix just save details of how the proxy/router zones on a host are configured locally, so I need to extend the logic to a network-wide configuration store. That, at least, is well-known territory.