netdev - Re: Multicast socket behaviour?

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Message-ID: <20080813111950.GA2034@hmsreliant.think-freely.org>
Date:	Wed, 13 Aug 2008 07:19:50 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Daniel Ng <daniel_ng11@...os.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Multicast socket behaviour?

On Wed, Aug 13, 2008 at 08:21:58AM +0000, Daniel Ng wrote:
> Hi,
> 
> The below C code registers the socket with the multicast group 'HELLO_GROUP':
> 
>     mreq.imr_multiaddr.s_addr=inet_addr(HELLO_GROUP);
>     mreq.imr_address.s_addr=htonl(INADDR_ANY);
> 
>     mreq.imr_ifindex = if_nametoindex("ppp1");
> 
>     if (setsockopt(fd,IPPROTO_IP,IP_ADD_MEMBERSHIP,&mreq,sizeof(mreq)) < 0) {
>         perror("setsockopt");
>         exit(1);
>     }
> 
> I understand that if 'INADDR_ANY' is used, it is up to the kernel to decide 
> what action to take.
> 
> >From my experiments, it seems that the interface corresponding to the 
> highest '224.0.0.0' entry in the routing table is used.
> 
> Would someone please explain why this is so? 
> 
> How difficult would it be to have the kernel join the HELLO_GROUP on *all* 
> available multicast-capable interfaces? Why isn't this currently implemented?
> 
> I'm using 2.6.14.
> 
> Cheers,
> Daniel
> 

I had this same confusion on netdev a few weeks ago:
http://marc.info/?l=linux-netdev&m=121580189427089&w=2

Its working as it should, allbeit somewhat counter-intuitively.  Although there
may be a bug in your kernel, you're several releases back.

Neil


-- 
/****************************************************
 * Neil Horman <nhorman@...driver.com>
 * Software Engineer, Red Hat
 ****************************************************/
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