netdev - Re: multicasting netlink messages to groups

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Message-Id: <1183558702.3812.33.camel@johannes.berg>
Date:	Wed, 04 Jul 2007 16:18:22 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	netdev <netdev@...r.kernel.org>, jamal <hadi@...erus.ca>,
	Thomas Graf <tgraf@...g.ch>
Subject: Re: multicasting netlink messages to groups > 31 from userspace

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?

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.

johannes

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