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From: Robert Watson <rwatson@FreeBSD.org>
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To: Derek Tattersall <dlt@mebtel.net>
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Cc: freebsd-net@freebsd.org
Subject: Re: Multicast source address in recvfrom()
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On Sun, 1 Feb 2009, Derek Tattersall wrote:

> In order to become familiar with multicast implementation using FreeBSD, I 
> found via Google a pair of test programs which multicast sent a simple text 
> message and received the text message.  I added some code to report the 
> source address, because none of the references that I looked at specified 
> the source IP address in the frame.
>
> I ran the sender on A -current system, AMD64 vintage last week.  The 
> receiver was on a -current system I386 vintage last week.  TCPDUMP shows the 
> source IP address in the frame as (correctly) 192.168.0.15.  The receiver 
> reports the source IP address as 200.231.191.191.  I have also run the same 
> test with an OpenBSD 4.4 Release I386 system as the receiver.  The openBSD 
> system reports the sender as 192.168.0.15.  A Fedora 10 system reported the 
> source IP address as 0.0.0.0.
>
> Googling the RFCs and other information and referring to Comer's and 
> Stevens' books on TCPIP I can't determine what should be reported.  Does 
> anybody have clue for me?

Hi Derek:

It might depend on how you're querying the source address.  Could you post a 
code excerpt?

Robert N M Watson
Computer Laboratory
University of Cambridge