netdev - Re: IGMP joins come from the wrong SA/interface

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Message-ID: <20140204220809.GB7526@sesse.net>
Date:	Tue, 4 Feb 2014 23:08:09 +0100
From:	"Steinar H. Gunderson" <sgunderson@...foot.com>
To:	netdev@...r.kernel.org
Subject: Re: IGMP joins come from the wrong SA/interface

On Thu, Jan 30, 2014 at 11:44:11PM +0100, Hannes Frederic Sowa wrote:
> The routing lookup is done at IP_ADD_MEMBERSHIP time. I really wonder why you
> have routed the 239.0.0.0/8 range to eth0.11. It seems to me that the kernel
> does what you told it to do. ;)
> 
> multicast flag on ip route is just used for multicast forwarding and does not
> matter for local multicast. Also if we find unicast route first (more
> specific) kernel does not do backtracking if destination is in multicast
> scope.

Hah, you're right. The issue was a combination of:

 1. mediatomb's initscript on Debian at some point started to add a bogus
    239.0.0.0/8 route (and I didn't notice this because I earlier tested with
    addresses outside this range).
 2. I didn't properly understand that the multicast flag on the route did not
    matter (although it really should!).
 3. rp_filter ate the data packets when they actually arrived. (I don't know
    why I never had this problem before, but I certainly didn't.)

So in various debugging rounds, I managed to fix #1 and #2 to various
degrees, but then #3 would come and make it appear like nothing actually
happened. I didn't see this before tracking it all the way up to the upstream
routers and observing that they actually _did_ send out packets...

/* Steinar */
-- 
Homepage: http://www.sesse.net/
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