gateway with one interface

hi,

I have a small LAN with about 3 windows machiens, 1 debian server (etch), and adsl router with 4 ports.
Currently, I am using the router as a gateway (192.168.0.1), and the debian server is 192.168.0.254. The client machiens are configured to use 192.168.0.1 as a gateway.

What i need to do is to use the linux server 192.168.0.254 as a gateway for all the client machines, thereby i can monitor traffic and gives me more control. But the problem is the debian machine has only one interface and it is not directly connected to internet. It is also using the router as the gateway to internet (just like the other client machines).

So my question is, can this be done? if so any pointers is appreciated. I have checked routing/iptables info on the web, but in all the cases, the linux machine is directly connected to the net and has two interfaces.

thanks,
meekaaku

ps:
I could do squid as an http proxy, but i need to do for all types of traffic.

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