Ubuntu Security :: How To Make A Transparent Firewall

I'm involved in a project to students set up a network security training lab using vmware. I want to simulate (in a very rough way) scanning through a poorly configured router or firewall. The easiest way I can think of to simulate this is to use a linux vmware image with two virtual nic cards to act as a firewall with the attacker on the outside network and a domain controller, web server, and database server on the inside network.

I would like to start students off with a firewall script that exposes everything on their internal network to the attacker. Is there an easy way to (mis)configure iptables to do this?. The model I'm trying to replicate is something like this. Attackers were on a 10.10.x.x network, defenders were on a 192.168.x.x network. As an attacker I could nmap 192.168.x.x and see every machine and every service on the defenders side even if they moved a service to an unexpected location. how I can implement a similar configuration using a linux image as firewalls/routers in vmware?