VirtualBox: Set Network Adaptor for exclusive use of Guest VM

In a previous question I asked how to configure m0n0wall in a VM. Now I would like to set a network addaptor in my machine to be used for the guest VM only, that is, not allow any traffic from the host machine to pass through. I have so far been unable to find a way of doing this. I am using VBox 4.0.0.

I have two physical network cards, one which I would like the host to use and one for the guest.

I've tried disabling the two boxes in red (which then disabled the boxes in green) and left the "VirtualBox Bridged Networking Driver" ticked, but this hasn't worked (or maybe I was expecting this to work more easily than I had hoped).

The idea is to have the VM running as a gateway on the host machine.

I just tested this running an Ubuntu 11.10 Guest on a Windows 7 host with VirtualBox 4.1.16.

I set the guest VM's Virtualbox configuration I set the networking mode to "Bridged", and attached to my host's LAN connection.

On the host, I disabled all but "VirtualBox Bridged Networking Driver" in the connection properties list.

I was subsequently able to access the internet from my guest OS, but not from the host.

My intended use was as a virtual appliance to bypass my university network's requirement for Windows machines to install proprietary tracking software. The university only performs OS Fingerprinting before authentication on the network, so it is possible to boot to Linux, spoof a MAC address, authenticate, reboot into windows, spoof the same MAC address and connect to the network without authentication or tracking software.

I have not yet tested this technique with a virtual OS configured in the manner described above, but I will update my response with complete testing when I do.

Another option would be to get a USB network adapter and have VirtualBox capture that USB device and present it to the guest operating system. That way on the guest will be the only one to be able to access that device directly.