3. First Attempt: Single NAT

On my first try I simply used a third intermediate network which was routed to network 1 and connected the NAT box in-between.

ASCII Art 2: Attempt 1

          Network 1
        192.168.150.0 (Corp)
              |
            Router
              |
           Network 2
        192.168.180.0 (Intermediate)
                   |
          NAT BOX  |
          eth0 192.168.180.180
          eth1 192.168.150.180
                   |
        Network 3  |
        192.168.150.0 (Untrusted Network)

Using a combination of Destination NAT mappings and a Source NAT remapping, this was a partial success. I could access all the stuff on Network 3 from any network that WASN'T network 1. Why ? Localhost routing policy on the NAT box.

The local routing policy on a normal linux system is usually:

• Directly Connected Interfaces
• Static Routes
• Default Routes

In that order. So since I was directly connected to 192.168.150.0 there was no way for the NAT box to talk to Network 1 since it was technically already directly connected to it, or at least it's doppleganger. I'm sure there's some way to overcome this using Alexey's ip util but I did not look into this further.