netdev - Re: Multicast IP packet routed between 2 ports nic on the same host

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Date:	Mon, 6 Jun 2011 12:17:47 -0700
From:	David Stevens <dlstevens@...ibm.com>
To:	BONNEAU Guy <gbonneau@...anda.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	netdev-owner@...r.kernel.org
Subject: Re: Multicast IP packet routed between 2 ports nic on the same host

netdev-owner@...r.kernel.org wrote on 06/06/2011 06:40:26 AM:

> From: BONNEAU Guy <gbonneau@...anda.com>

> I open a second console and I use mreceive to join the same 
> multicast group 239.255.200.200:8000 to receive multicast data from 
> subnet 172.30.8.xx using the console command : ./mreceive -g 239.
> 255.200.200 -p 8000 -i 172.30.8.31 to the eth1 adapter of my 
> workstation. The application starts to receive multicast data and 
> advertises the data received. This is also the expected behaviour.
> 
> Now this is where the problem begins. As soon as the multicast data 
> begin to be received on the eth1 adapter the first console begins to
> advertise multicast data received on eth0 adapter. I am well aware 
> that the Linux kernel implements a multicast level 2 routing 
> capability. Thus at first glance this seems to be the expected 
> behaviour. However... I have forwarding disabled as well as 
> mc_forwarding disabled and rp_filter is enabled for both adapters. 
> Thus I don't expect the kernel to forward the multicast data from 
> eth1 to eth0. 

        Routing is between multiple machines. You're receiving the
packets on the sockets because they have a binding that matches.
If you only want to receive multicast packets from a particular
interface, then you need to use "SO_BINDTODEVICE" to restrict to
that interface.
        Group membership is per-interface, but socket bindings match
against any packets delivered to the entire machine, if you haven't
otherwise restricted it.

                                                                +-DLS

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