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    SubjectRouting problem with udp, and a multihomed host in 2.4.20

    I have three subnets. A, B and C.

    I have a Linux 2.4.20 server, named bartok, with three interfaces, one
    on each subnet (so it can listen to broadcast requests everywhere).

    This server has a default route pointing at A.

    I have a router that routes between all these subnets, and others.

    I have a client that sits on subnet B.

    When my client tries to talk to bartok, it might try any of 3 IP
    addresses (Due to round-robining in the DNS).

    All IP addresses work find when establishing a TCP connection
    e.g. telnet or ssh.
    They don't for UDP.
    e.g. rpcinfo -u bartok mountd

    If I use the address on B, it works fine (as you would expect - same
    subnet).
    If I use the address on A it works fine (that is the default route
    interface).

    But if I use the address on C, it doesn't.

    What happens is:

    - request goes from client to router
    - request goes from router to bartok interface on C
    - bartok issues an ARP for client on C interface which is WRONG
    - nobody replies to the ARP because client is on B, not C.

    If I turn on proxy-arp on the router I can get the reply back, but I
    would rather not do that.

    So why does a reply to a UDP request arriving on subnet C from some
    other subnet try to ARP out on subnet C instead of being routed
    normally, while replies to TCP requests get routed properly and work
    fine?

    Is this a bug, or is there some configuration I can change?

    I have double checked the subnet masks and broadcast addresses and
    they work fine.
    We have rp_filter set to 0 on all interfaces.
    forwarding is set to 0, but setting it to 1 makes no difference.

    Thanks,
    NeilBrown
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