UDP broadcast does not work to loopback device without network connection?

Discussion in 'Java' started by franklin.bowen@gmail.com, Mar 14, 2006.

  1. Guest

    While developing some Java tools that use UDP broadcasting, I found
    that my laptop will not broadcast packets back to the loopback device
    *IF* I do not have a network connection (IP address assigned to one of
    my interfaces) even if i turn on broadcasting and share ports. Does
    anyone know if this is expected (or maybe undefined?) behaviour in
    networking in general, a Java specific issue, a bug, or some other
    option I have not listed?

    Thanks!
    , Mar 14, 2006
    #1
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  2. Re: UDP broadcast does not work to loopback device without networkconnection?

    wrote:
    > While developing some Java tools that use UDP broadcasting, I found
    > that my laptop will not broadcast packets back to the loopback device
    > *IF* I do not have a network connection (IP address assigned to one of
    > my interfaces) even if i turn on broadcasting and share ports. Does
    > anyone know if this is expected (or maybe undefined?) behaviour in
    > networking in general, a Java specific issue, a bug, or some other
    > option I have not listed?


    I assume this is a Windows 2000 or XP system? If so, this is a very common
    problem, and you can easily find the fix if you search for "disable windows
    media sense" on your favorite search site. Media sense was another 'helpful'
    Microsoft idea....

    --
    Phil Frisbie, Jr.
    Hawk Software
    http://www.hawksoft.com
    Phil Frisbie, Jr., Mar 14, 2006
    #2
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  3. Guest

    Yes, the system is running XP. Thanks for the information. I got
    "around" the problem by installing an ethernet PC card (USB would
    probably work just as well) and plugging an ethernet cable between the
    two NICs.

    They auto-configured to different subnets (169.254.87.x and
    169.254.104.x) but I just set the last two octets in my IPv4 broadcast
    address to 0xFF and I can now continue my development work. Yay!

    Thanks again for the useful reply! I appreciate it.
    , Mar 15, 2006
    #3
  4. Guest

    wrote:
    > Yes, the system is running XP. Thanks for the information. I got
    > "around" the problem by installing an ethernet PC card (USB would
    > probably work just as well) and plugging an ethernet cable between the
    > two NICs.
    >
    > They auto-configured to different subnets (169.254.87.x and
    > 169.254.104.x) but I just set the last two octets in my IPv4 broadcast
    > address to 0xFF and I can now continue my development work. Yay!


    I bet you'll find they're not in different subnets afterall. The mask
    on those addresses should be a /16. You might want to put something
    into your code so that it automagically figures out the correct
    broadcast address to use.

    I've had to do similar nonsense with my PowerBook: For some reason the
    perl pcap libraries refuse to read from a capture file if my NIC
    doesn't have a link. It makes no sense, but I haven't got to the
    bottom of it yet. My workaround is to carry a PCMCIA ethernet card
    (for which I don't even have drivers) and string that NIC over to my
    onboard Ethernet.

    Frustratingly, an Ethernet loopback plug like this one doesn't seem to
    do the trick:

    http://docs.sun.com/app/docs/doc/816-1667-05/6m82opb3g?a=view

    Maybe loopback plugs cannot work in a transceiver that supports
    auto-crossover?

    /chris
    , Mar 15, 2006
    #4
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