netdev - UDP broadcast packets not looped back

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Message-ID: <46D9778F.9070403@free.fr>
Date:	Sat, 01 Sep 2007 16:30:39 +0200
From:	Benoit PAPILLAULT <benoit.papillault@...e.fr>
To:	netdev@...r.kernel.org
Subject: UDP broadcast packets not looped back

Hi there,

I've already sent this email to lkml (see 
http://lkml.org/lkml/2007/8/13/1144) and got no feedback so far. I was 
told that netdev might be a more appropriate list, so here I am!

I wrote a small C program using the BSD socket API to send a UDP
broadcast packet to all interfaces on my machine. To do so, i sendto()
it to 255.255.255.255 after using setsockopt(SO_BINDTODEVICE) on the
correct interface. It works perfectly.

However, in the same program, either using the same socket or another on
the same UDP port, I found out that I just received packets i'm sending 
as well. And do not want it. I perfectly understand that some program 
might be interested in broadcast packets they sent, but i don't.

I have seen that in the multicast world, every program can choose this
behaviour throught the setsockopt IP_MULTICAST_LOOP. I do not want to
use multicast since I don't want my packets to be routed (if there was
ever a multicast router on the link). But I admit I could use a TTL=1
packet in this case.

Anyway, I dig five minutes into the kernel source code I have at hand
and wrote a small patch to add IP_MULTICAST_LOOP to broadcast packets.
Since it might be interesting to other people, I just submit it here. It
has been testing on Ubuntu and kernel 2.6.23-rc1 from the wireless.git
repository. It should be very easy to adapt to other kernel version.

I have not seen this feature as being standard throught the OpenGroup
specification
(http://www.opengroup.org/onlinepubs/009695399/functions/setsockopt.html),
but it might be usefull anyway.

Comments welcome,
Benoit

View attachment "wireless-dev-udp-broadcast.diff" of type "text/x-patch" (806 bytes)