Re: [LARTC] Proxy ARP considered harmful. :)

From: Ard van Breemen <ard@telegraafnet.nl>
To: lartc@vger.kernel.org
Subject: Re: [LARTC] Proxy ARP considered harmful. :)
Date: Sun, 24 Feb 2002 20:06:24 +0000	[thread overview]
Message-ID: <marc-lartc-101458126300975@msgid-missing> (raw)
In-Reply-To: <marc-lartc-101443755702421@msgid-missing>

On Fri, Feb 22, 2002 at 11:11:44PM -0500, Adrian Chung wrote:
> I've got a DSL modem that bridges ethernet between three boxes here,
> and everything else on the same subnet at the ISP side.
> 
> With the three boxes plugged into a hub, with the DSL modem, I can't
> do aggregate bandwidth shaping, because there's no way for one both to
> know in relation to the other three how much bandwidth it's using.
> 
> I decided to use proxy-arp, and put two of the boxes behind a 2.4 box
> doing shaping:
> 
>    .225 \
>            -- .224 -- DSL Modem -- ISP (.252)
>    .226 /
> 
> And, following the HOWTO, proceeded to turn proxy_arp on for the left
> and right interface on .224 which both had IP address .224.
> 
> I set the routes up so that .225/6 went to the left, and .128/25 went
> to the right.
> 
> Everything seemed to work fine.
> 
> Except that my box started to answer ARP requests from and for
> everything on the ISP's .128/25 subnet.  So it caused lots of havoc.
Ehhh, so you did not set up a default gateway at .225 and .226.
That's your problem.
> The HOWTO assumes that you have a router of some sort between the
> proxy ARP box and the ISP, so that ARP requests never traverse the
> router.
Yep and no. It assumes you have a default gateway, which usually is a
local router, but it can also be the router of the ISP. So your problem
is .225, and .226 not arping for the ISP address.  There is nothing
wrong with the remainder of your setup.

> In my case, since it's a bridge, everything goes.
Yes, and in the normal situation you would have .252 as a default gateway.

TROUBLE SHOOTING FAQ:
If you see a large amount of arps, and a large growing neighbour table,
you definetely have forgotten the default gateway.
-- 
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