netdev - Multicast socket behaviour?

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Message-ID:  <loom.20080813T082132-707@post.gmane.org>
Date:	Wed, 13 Aug 2008 08:21:58 +0000 (UTC)
From:	Daniel Ng <daniel_ng11@...os.com>
To:	netdev@...r.kernel.org
Subject:  Multicast socket behaviour?

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

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