Receiving INADDR_BROADCAST packets

From: Bruce M Simpson (bms_at_spc.org)
Date: 08/22/03

  • Next message: Supote Leelasupphakorn: "How can I subnetting my network ?" 
    Date: Fri, 22 Aug 2003 12:16:36 +0100
To: freebsd-net@freebsd.org

    Hi all,

    Ok, the broadcast sending problem is solved, as far as I'm concerned.

    However, the reception problem isn't:
       14766 broadcast/multicast datagrams dropped due to no socket

    This despite:
    wi0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
            inet 1.234.56.78 netmask 0xff000000 broadcast 1.255.255.255
    udp4 0 0 1.234.56.78.654 *.*

    I can:
    a) patch udp_input() to pass INADDR_BROADCAST traffic to all sockets bound
       to the port and on the interface where the traffic was received.
       Possibly conditionalise this interface based on a socket-level option.
       e.g. 'Yes, I'm an interface-bound socket and I'd like undirected
       broadcasts on my interface thankyou very much'. This would be done in
       such a way which was 'opt-in' and didn't impact existing applications.
    b) preserve the existing 4.2BSD kludge of binding to INADDR_ANY and not
       knowing what interface an undirected broadcast came in on.
    c) same as b) but use IP_RECVIF to determine which interface the broadcast
       came in on, and change my network event dispatch code accordingly.
    d) use a SOCK_RAW socket.
    e) use BPF.

    b), d), e) are not solutions. They are cop-outs.
    Please don't suggest them. Really.

    I note that Apple are using IP_RECVIF in their mDNS code. The problem
    there is not the same (they're dealing with multicasts), but similar.

    I already cache the interface index in my internal RIB and track RTM_IFINFO,
    RTM_IFANNOUNCE events, so this isn't too big a hit (the fact that this
    ioctl provides an index, not a name).

    I'm going to go with solution c) for now, but has anyone considered a)?
    (Little choice really, we have a code freeze coming up, and I'd like my
    code to run unchanged on 4.9-RELEASE).

    BMS
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