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Date: Tue, 15 Jan 2008 09:59:22 +0000
From: Tom Judge <tom@tomjudge.com>
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To: "Bruce M. Simpson" <bms@FreeBSD.org>
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Cc: freebsd-net@freebsd.org
Subject: Re: Programming interface MAC filter without enabling PROMISC on
 an interface from user space.
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Bruce M. Simpson wrote:
> Tom Judge wrote:
<SNIP>
>> Personally I can't see why this approach would be a problem,  but I am 
>> not a expert.  The address is defined in IEEE Std 802.1D-2004 as to 
>> not be forwarded by bridges (which I interpret as it being link local 
>> in a sense as switches/bridges are not allowed to forward the frame), 
>> so I can't see it being a problem registered on multiple interfaces.
> 
> SIOCADDMULTI memberships are specific to the interface you request them 
> on. I can't speak for the bridging code -- I don't think it does any 
> special handling of multicast frames, however I'm not sure if it's smart 
> enough not to forward this group. Like IN_LOCALGROUP() it might need its 
> own 'don't forward this' clause.
> 


Just for the record it seems that if_bridge replaces the destination MAC 
of a Ethernet multicast packet with its own MAC therefore making sure 
that the packets are not forwarded.  Andrew can you confirm this 
assumption?  (Based on sys/net/if_bridge.c lines 2011-2018 on RELENG_6_2)

Tom