From mboxrd@z Thu Jan 1 00:00:00 1970 From: Patrick McHardy Subject: Re: multicasting netlink messages to groups > 31 from userspace Date: Wed, 04 Jul 2007 16:30:40 +0200 Message-ID: <468BAF10.6080208@trash.net> References: <1183492286.4021.21.camel@johannes.berg> <468BAADC.4030203@trash.net> <1183558702.3812.33.camel@johannes.berg> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Cc: netdev , jamal , Thomas Graf To: Johannes Berg Return-path: Received: from stinky.trash.net ([213.144.137.162]:38773 "EHLO stinky.trash.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758557AbXGDObr (ORCPT ); Wed, 4 Jul 2007 10:31:47 -0400 In-Reply-To: <1183558702.3812.33.camel@johannes.berg> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Johannes Berg wrote: > On Wed, 2007-07-04 at 16:12 +0200, Patrick McHardy wrote: > >>Johannes Berg wrote: >> >>>Hey, >>> >>>Looking through the code that uses NL_NONROOT_SEND I just realised that >>>it's impossible to send multicast messages from userspace to multicast >>>groups with IDs higher than 31. That's not really good given that >>>everywhere else we handle multicast groups up to 2^32-1 :/ >> >> >>Why do you want to send to a multicast group from userspace? > > > Why not, what's wrong with that? The kernel doesn't have any multicast listeners (yet). > Actually, I think I mentioned this earlier, I was thinking about doing > wireless configuration as a group where both the kernel and possibly a > userspace process listen on that multicast group and processes that want > to configure a device just send to that group. Then the kernel ignores > the message if a userspace process is handling the specific device > completely. For example changing the BSSID: if the kernel is doing MLME > then it changes the BSSID, but if a userspace process is doing it then > the kernel doesn't do anything since BSSID changing is a pure MLME > function, but for consistency it'd be nice if both could be done the > same way, hence a multicast group. > > This was actually suggested by Herbert since it's easy to find out if > that multicast group has a listener and not so easy if a special generic > netlink socket in userspace is (still) open. I wonder if thats really a good idea to use multicast for device configuration. Unicast transmissions from userspace to kernel are reliable when you don't use MSG_DONTWAIT. For multicasts doing the same would mean blocking on each receiver when the receive queue is full.