How to make a Cisco Forward Packets/Bridge Networks

Hi Experts,

I want to turn my Cisco border router into a Bridge.

I have a Cisco 1700 with a WIC-1ADSL Card that is doing PPPoA.

When it connects to the internet using interface dialer0, an IP address is assigned by my ISP, and my router does NAT all communications.

Now what I want to do is pass through EVERY packet to a device on the ethernet network.

SO: I have this...

Internet <--> Cisco1720 <--> Internal network

I want:

Internet <--> Cisco1720 <--> Other Device (Linux) <--> internal Network

This means that all the NATing will be carried out on my linux box, which means that the IP Address will probably need to be on the Cisco and the Linux Box so this is where I come unstuck.

The reason I want to do this is because I want to be able to "inspect" (as in use wireshark) every single packet that arrives at my border, of course because of the NAT the cisco drops many packets and does not forward them into my network.

Is this possible or do I really need to look at some other hardware options?  Such as an ADSL Card for my Linux Box - unfortunately in NZ ADSL is not delivered over Ethernet - If it was PPPoE I would just throw the Cisco away...

Thanks!!!