[Gllug] networking problem

James Courtier-Dutton james.dutton at gmail.com
Fri Jul 15 09:18:45 UTC 2011

On 15 July 2011 08:46, t.clarke <tim at seacon.co.uk> wrote:
> Hi,
> we have a small problem at work which I am hoping might be solved with a small
> Linux box made up from an old PC  (coyote floppy distribution or similar would
> be good!) :-
> The problem is we have a device which is very old, unsupported now by the
> manufacturer and no manuals available it seems,  which 'talks' to the host
> on another network via a gateway of programmed-in (we assume) as
> its default gateway.  It looks very much like we are not going to be able to
> change this routing, and the gateway of will be disappearing shortly
> (to be replaced by another with address;  don't ask me why - not my
> doing!).   I believe the original decision was made before it was discovered
> that re-programming the  device with a new gateway was going to be problematical
> So, assuming re-programming the gateway on the device IS impossible, can we
> cheat and shove a linux box on the LAN with an IP of and a default
> gateway of, so that the device will send its packets to the linux
> box which will then forward (over the same eth0 interface) to, for
> ultimate delivery to the far end?   It would seem this will work in theory, but
> am I missing something??
> Tim

You don't give us enough information, but I will assume the subnet
mask is
First I would explore whether or not you can give the new GW device 2
IP addresses. Both and
This will save you setting up a new box.
It would help if you gave the make and model of the old box and the
new GW device.

In answer to your question. Adding a Linux box will work.
Lets call the old box with GW is Box A.
The new linux box with IP address is Box B
The new router is IP address is Box C.

What happens is this (Skipping the detailed ARP and MAC address stuff):
Box A will send its first packet to Box B.
Box B will forward it onto box C.
Box B will also send an ICMP redirect to Box A.
If Box A understands ICMP redirects, the next packet Box A sends to
the same destination IP address will be sent directly to Box C.

The nice result of this is that Box B will therefore see very little
traffic. Only the first IP packet of a new destination IP address.

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