linux-kernel - Re: [PATCH] net/ipv4/arp.c: Fix arp reply when sender ip 0 (was: Strange behavior in arp probe reply, bug or feature?)

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Message-ID: <20071119130610.GB6952@ms2.inr.ac.ru>
Date:	Mon, 19 Nov 2007 16:06:10 +0300
From:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
To:	Jonas Danielsson <the.sator@...il.com>
Cc:	linux-kernel@...r.kernel.org, davem@...emloft.net,
	jmorris@...ei.org, netdev@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] net/ipv4/arp.c: Fix arp reply when sender ip 0 (was: Strange behavior in arp probe reply, bug or feature?)

Hello!

> Is there a reason that the target hardware address isn't the target
> hardware address?

It is bound only to the fact that linux uses protocol address
of the machine, which responds. It would be highly confusing
(more than confusing :-)), if we used our protocol address and hardware
address of requestor.

But if you use zero protocol address as source, you really can use
any hw address.

> The dhcp clients I examined, and the implementation of the arpcheck
> that I use will compare the target hardware field of the arp-reply and
> match it against its own mac, to verify the reply. And this fails with
> the current implementation in the kernel.

1. Do not do this. Mainly, because you already know that this does not work
   with linux. :-) Logically, target hw address in arp reply is just
   a nonsensial redundancy, it should not be checked and even looked at.

2. What's about your suggestion, I thought about this and I am going to agree.

   Arguments, which convinced me are:

   - arping still works.
   - any piece of reasonable software should work.
   - if Windows understands DaD (is it really true? I cannot believe)
     and it is unhappy about our responce and does not block use
     of duplicate address only due to this, we _must_ accomodate ASAP.
   - if we do,we have to use 0 protocol address, no choice.

Alexey
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