I'm trying to configure a virtual machine (VirtualBox guest running Backtrack 4) with a bridged adapter over a VPN connection. The VPN is is hosted by the cybersecurity club at my university, and connects to a sandboxed LAN designed for penetration testing against various servers that the club has built.

My host (Windows 7 Ultimate) connects to the VPN fine and is assigned an IP through DHCP, but for some reason the VM can't do the same thing, and I'm not sure why. It's like OpenVPN is filtering out packets from the MAC address it doesn't recognize.

I want the virtual machine to bridge over the VPN connection, because our IT office has very strict policies about what you can and can't do on the network. I want to be able to run active attacks (ARP spoofing, nmap, Nessus scans) in the sandbox environment without risking the traffic accidentally going over the university network and getting my internet access revoked. Bridging over the VPN connection and running all attacks from inside the VM would solve that problem.

Any idea why the host can use this interface, but the VM can't?