• Philip Downey's avatar
    IGMP: Inhibit reports for local multicast groups · df2cf4a7
    Philip Downey authored
    The range of addresses between 224.0.0.0 and 224.0.0.255 inclusive, is
    reserved for the use of routing protocols and other low-level topology
    discovery or maintenance protocols, such as gateway discovery and
    group membership reporting.  Multicast routers should not forward any
    multicast datagram with destination addresses in this range,
    regardless of its TTL.
    
    Currently, IGMP reports are generated for this reserved range of
    addresses even though a router will ignore this information since it
    has no purpose.  However, the presence of reserved group addresses in
    an IGMP membership report uses up network bandwidth and can also
    obscure addresses of interest when inspecting membership reports using
    packet inspection or debug messages.
    
    Although the RFCs for the various version of IGMP (e.g.RFC 3376 for
    v3) do not specify that the reserved addresses be excluded from
    membership reports, it should do no harm in doing so.  In particular
    there should be no adverse effect in any IGMP snooping functionality
    since 224.0.0.x is specifically excluded as per RFC 4541 (IGMP and MLD
    Snooping Switches Considerations) section 2.1.2. Data Forwarding
    Rules:
    
        2) Packets with a destination IP (DIP) address in the 224.0.0.X
           range which are not IGMP must be forwarded on all ports.
    
    IGMP reports for local multicast groups can now be optionally
    inhibited by means of a system control variable (by setting the value
    to zero) e.g.:
        echo 0 > /proc/sys/net/ipv4/igmp_link_local_mcast_reports
    
    To retain backwards compatibility the previous behaviour is retained
    by default on system boot or reverted by setting the value back to
    non-zero e.g.:
        echo 1 >  /proc/sys/net/ipv4/igmp_link_local_mcast_reports
    Signed-off-by: default avatarPhilip Downey <pdowney@brocade.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    df2cf4a7
igmp.c 70.5 KB