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Date:	Mon, 8 Jan 2007 10:23:27 -0800
From:	Brendan Cully <>
To:	Ben Greear <>
Subject: Re: Question on advanced routing and/or virtual routers.

On Monday, 08 January 2007 at 09:08, Ben Greear wrote:
> Hello!
> I am curious if a single machine can be made to look like several 
> routers.  Please consider the following
> configuration:
> Linux Router-A has 4 ethernet interfaces.  PC-A is connected to eth0 and 
> has IP
> eth0 on Router-A has IP
> eth1 has IP, and eth1 is connected directly to eth2
> eth2 has IP
> eth3 is connected 'upstream' and has IP
> I would like for PC-A to be able to ping and as 
> normal.
> The part I'm not sure how to make work is that I want to be able to ping 
> and
> have the packet route out of eth1 and into eth2 (PC-A -> eth0 -> eth1 -> 
> eth2), and have the return packet follow the
> eth2 -> eth1 -> eth0 -> PC-A path.  A trace-route from PC-A should show 
> each of these hops (or, at least eth0 and eth2.)
> The eventual goal is to have arbitrary numbers of 'routers' in a single 
> Linux machine for emulation
> purposes.
> I was thinking that I might could accomplish this using multiple routing 
> tables and perhaps
> specific subnet routes for each each virtual router, specifying which 
> interface the packets should
> leave in order to find the next hop.
> Has anyone tried something similar to this or have ideas for how to best 
> proceed?

I started something like this a while ago (posted at
<> with a couple of replies
by Thomas Graf, but I can't seem to find it in the archives) but then
dropped the ball. It seems to work fairly well with a one-line kernel
patch to allow route lookup before the local address check. Oh, and I
didn't get traceroute working quite right either - I think there was
some trick to finding the source address for the generated reply.

I've got some info and code here:
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