transparent squid proxy

As an experiment, I wanted set up a "black box" that has two interfaces 
(eth0 and eth1) that would act as a transparent squid proxy. I wanted to 
be able to plug this device in line between the network and the router 
and *not* to have to configure it at all.

My first attempts have resulted in a partial success:
1) I loaded up centos 4.3 server, installed iptables and ebtables.
2) created a transparent bridge between eth0 (internal network) and eth1 
(plugged into the router)
3) plugged in the box - all traffic was passed over the bridge

Next:

4) Added in squid as a transparent proxy, added an iptable rule
5) all traffic except http was passed over the bridge, and http was 
passed to squid which then downloaded what it needed.

However:

This scenario required the bridge to have an ip address on the network, 
and a default gateway. As I mentioned, I did not want to have configure 
the box at all (and therefore cannot have an ip address on the bridge).

 From what I can gather, squid is receiving the client request, but I 
presume what is happening is that squid has no idea on where to go to 
get the information.

Is it possible to add an ebtable / iptable rule to say "when coming from 
the local machine on port x forward / use device eth1 ?

Thanks.

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