Configuring KVM Guest as FIrewall/Router

Since my motherboard does not support Vt-d, I'm having trouble setting up my KVM Guest as the router/firewall (Untangle). In Hyper-V, this was easy: set WAN ethernet X to not be shared with the host, and set the mac address.

I want traffic to be WAN -> Untangle Guest -> LAN

What do I need to do to get Untangle to get a DHCP lease from my modem?

I've tried (and several other combinations - although I haven't tried setting the address static):

#WAN Interface
auto eth2
iface eth2 inet dhcp

auto br2
iface br2 inet manual
brige_ports eth2
bridge_fd 9
bridge_hello 2
bridge_maxage 12
bridge_stp off

I spent hours googling and trying different settings and restarting my modem, but to no avail. My guest was never able to get a lease from my modem. Any ideas?

As an aside, I got it working this way (which I'm not content with):

WAN->Host Machine on eth2->IP Masqueraded/forwarded to Eth1 (which is setup as a dhcp server on that port)->Untangle on br1->Lan on br0.

Problem is that my host is not behind Untangle and I'm pretty sure this is not a very "proper" setup.

Any helpful pointers?