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From: Robert Watson <rwatson@FreeBSD.org>
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To: "George V. Neville-Neil" <gnn@FreeBSD.org>
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Cc: cvs-src@FreeBSD.org, src-committers@FreeBSD.org, cvs-all@FreeBSD.org
Subject: Re: cvs commit: src/sys/netinet ip_output.c
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On Fri, 29 Aug 2008, George V. Neville-Neil wrote:

> gnn         2008-08-29 20:42:58 UTC
>
>  FreeBSD src repository
>
>  Modified files:
>    sys/netinet          ip_output.c
>  Log:
>  SVN rev 182463 on 2008-08-29 20:42:58Z by gnn
>
>  Fix a bug whereby multicast packets that are looped back locally
>  wind up with the incorrect checksum on the wire when transmitted via
>  devices that do checksum offloading.

It wouldn't surprise me if there are other related but obscure issues here 
that this change fixes -- the output path for IP and below generally believes 
pretty firmly that it owns and can write to the mbuf parts going down the 
stack, such as prepending headers encapsulation headers, rewriting packets in 
NAT, etc.  Most will be very edge case because they probably work poorly or 
not at all with multicast anyway, but...

Robert N M Watson
Computer Laboratory
University of Cambridge