linux-kernel - UDP broadcast packets not looped back

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Message-ID: <46C0CDFF.6080409@free.fr>
Date:	Mon, 13 Aug 2007 23:32:47 +0200
From:	Benoit PAPILLAULT <benoit.papillault@...e.fr>
To:	linux-kernel@...r.kernel.org
Subject: UDP broadcast packets not looped back

Hi there,

I wrote a small C program using the BSD socket API to send a UDP 
broadcast packet to all interfaces of 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, I found out that I just received the packet 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" (805 bytes)