2.4.22 multicast address problem, pcap workaround

From: Dominic Robinson (d.robinson_at_videonetworks.com)
Date: 09/19/03

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    Date:	Fri, 19 Sep 2003 09:11:54 +0100
To: linux-kernel@vger.kernel.org

    Please cc to d.robinson@videonetworks.com

    My source machine sends multicast packets on all the following
    addresses: 230.1.1.1, 230.1.1.2 through to 230.1.1.64

    My target machine (on local network, or with cross-over cable)
    joins all these multicast groups, and receives packets on all
    of these addresses EXCEPT 230.1.1.18 and 230.1.1.32.

    However, if I run tcpdump at the same time as my program, I
    find my program suddenly starts receiving on 230.1.1.[18,32]
    If I kill tcpdump, my program stops receiving. It's the
    "pcap_open_live()" function in tcpdump which causes this,
    and as a workaround I just call it direct in my receiver.

    Could this be a bug in the multicast kernel code?

    NOTES:
    (1) 230.1.1.1 to 230.1.64.1 and 230.1.1.1 to 230.64.1.1
        has the same problem.
    (2) 230.1.1.1 to 230.1.1.63 works
    (3) 230.1.1.1 to 230.1.1.128 fails with slightly different
        behaviour
    (4) It makes no difference if I run a single program
        listening on all addresses, or 64 individual programs
        each listening on a single address.

    Thanks and regards,
    Dominic
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