From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sebastian Fett Subject: Re: ARP response with link local IP, why not broadcast Date: Wed, 22 Jul 2015 09:49:49 +0200 Message-ID: <55AF4B1D.4030208@gmx.de> References: <55AE5958.3020009@gmx.de> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Cc: netdev To: Sowmini Varadhan Return-path: Received: from mout.gmx.net ([212.227.15.15]:52007 "EHLO mout.gmx.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753138AbbGVHty (ORCPT ); Wed, 22 Jul 2015 03:49:54 -0400 In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: > On Tue, Jul 21, 2015 at 4:38 PM, Sebastian Fett wrote: >> Hello! >> >> According to RFC3927 every ARP packet (reply and request) should be sent as >> link layer broadcast as long as the sender IP is a link local address. (see >> chapter 2.5). > > Because broadcast replies are noisy and should be avoided. > if possible- it creates a broadcast flood that would wake up all receivers, > and is especially undesirable in today's world, where bcast would wake > up sleepy devices, or require other inefficient processes in a cloud env. > See also https://www.ietf.org/id/draft-nordmark-6man-dad-approaches-01.txt > >> That functionality would help me a lot with a use case I have with our >> application. > > what is your use case? > My problem ist a local network of audio devices. It is a valid possibility that two halfs of the setup are set up individually (Stage left and stage right). Both local networks will auto configure themselves via link local and will be stable. But there always can be two devices with the same IP in both networks. At one point those two networks will be connected. With the current behaviour the conflicting devices will never know of each other and the address conflict. The devices are controlled by a central PC using avahi/bonjour. It will know of all conflicting devices, but will only be able to talk to the one that happens to be in it's ARP cache. And renewing that cache will not change anything, because it will happen with unicast messages. I looked at a Dante Controller (an audio data streaming device). And here all ARP messages are answered with broadcasts. I think that behaviour is acceptable because it only happens in local networks. Waking up sleeping devices will not be a concern there. That brings me to another question. When I react to an ARP packet in a userspace program, can I keep that packet from reaching the kernel as well? I would like to avoid to completely handle ARP in userspace. >> >> But it is not implemented in the kernel that way. >> Does anyone know why? Sebastian