FAQ
I'm using MSW XP Pro with Python 2.4 to develop but production will be Linux
with Python 2.3. (could upgrade to 2.4 if absolutely necessary) I can also
switch to Linux for development if necessary.

I am writing some python to replace proprietary software that talks to a
timeclock via UDP.

The timeclock extracts the sending port from the UDP header and uses that
for all response messages.

I cannot find out how to set the sending port in the header. Windows XP
appears to set an arbitrary port. I've been using ethereal to analyze
network traffic and it seems that if I can set the sending port, I should be
OK.

I have been googling various combinations of "python udp ..." for the last
two hours and have not found anything that addresses how to set the sending
port. I'm guessing that this may be in setsockopt but don't see any
parameters that "click".

Any help would be greatly appreciated.

Fred Sells
fred at adventistcare dotttttt org


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  • Keirr at Dec 20, 2005 at 9:38 am
    Fred,

    It is quite possible I've misunderstood the problem :-) but have
    you tried anything like

    import socket
    tc_local_port = 9999
    tc_remote_port = 9999
    outgoing_if = "172.16.1.2" # say
    remote_tc_host = "172.16.1.3" # say
    # udp is the default for DGRAM
    tc_sock = socket(socket.AF_INET, socket.SOCK_DGRAM)
    tc_sock.bind((outgoing_if, tc_local_port))
    tc_sock.connect((remote_tc_host, tc_remote_port))

    If you send data with tc_sock, it should have a source port of
    tc_local_port.
    So, to set the source port, call bind. You should do that before
    calling connect, since
    calling connect on an unbound socket has the side effect of
    assigning an ephemeral port to the socket.
    Hope that was of some help to you.

    All the best,

    Keir.
  • Keirr at Dec 20, 2005 at 9:46 am
    A few trivial corrections, to my own post :-(
    tc_sock = socket(socket...
    should be
    tc_sock = socket.socket(socket...
    of course

    and, (while I'm here) when I stated that calling connect on an
    unbound socket caused
    a ephemeral port to be assigned, I should have written "calling connect
    on an unbound _UDP_ socket, etc. "

    Cheers,

    Keir.
  • Steve Horsley at Dec 22, 2005 at 8:19 pm

    Sells, Fred wrote:
    I'm using MSW XP Pro with Python 2.4 to develop but production will be Linux
    with Python 2.3. (could upgrade to 2.4 if absolutely necessary) I can also
    switch to Linux for development if necessary.

    I am writing some python to replace proprietary software that talks to a
    timeclock via UDP.

    The timeclock extracts the sending port from the UDP header and uses that
    for all response messages.

    I cannot find out how to set the sending port in the header. Windows XP
    appears to set an arbitrary port. I've been using ethereal to analyze
    network traffic and it seems that if I can set the sending port, I should be
    OK.

    I have been googling various combinations of "python udp ..." for the last
    two hours and have not found anything that addresses how to set the sending
    port. I'm guessing that this may be in setsockopt but don't see any
    parameters that "click".

    Try binding the address ('',0) which means any address, any port
    on this box. Then get the bound address with getsockname(). Below
    is a copy of an interactive try-out...


    import socket
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    s.bind(('',0))
    s
    <socket._socketobject object at 0xb7d9ee0c>
    s.getsockname()
    ('0.0.0.0', 32775)
    >>>

    So in this case, port 32775 was chosen to bind to.

    Steve

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postedDec 20, '05 at 4:33a
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