Revision 18 as of 2005-05-10 02:31:45

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UDP Communication


See also SoapOverUdp, TcpCommunication


Here's simple code to post a note by UDP in Python:

   1 import socket
   3 UDP_IP=""
   4 UDP_PORT=5005
   5 MESSAGE="Hello, World!"
   7 print "UDP target IP:", UDP_IP
   8 print "UDP target port:", UDP_PORT
   9 print "message:", MESSAGE
  11 sock = socket.socket( socket.AF_INET, # Internet
  12                       socket.SOCK_DGRAM ) # UDP
  13 sock.sendto( MESSAGE, (UDP_IP, UDP_PORT) )


Here's simple code to receive UDP messages in Python:

   1 import socket
   3 UDP_IP=""
   4 UDP_PORT=5005
   6 sock = socket.socket( socket.AF_INET, # Internet
   7                       socket.SOCK_DGRAM ) # UDP
   8 sock.bind( (UDP_IP,UDP_PORT) )
  10 while True:
  11     data, addr = sock.recvfrom( 1024 ) # buffer size is 1024 bytes
  12     print "received message:", data


* It would seem easy to extend this to a simple means to open a file on the sender side, send datagrams to the receiver side, and write those packets to a file there - I just wonder about synchronisation issues regarding the buffer...Anyone smart care to put something down, say as a simple practical extension of what is already here? (And if you do it pls delete this message) *


I've been googling for some time now, and still have yet to find a working example of Python multicast listening.

(The example below has been updated to work -- Steven Spencer DateTime(2005-04-14T13:19:00Z))

(I've replaced it with one that works. -- Asgeir S. Nilsen DateTime(2005-05-09T19:25:00Z))

   1 import socket
   2 import struct
   4 sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
   5 sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
   6 sock.bind(('', 4242))
   7 mreq = struct.pack("sl", socket.inet_aton(""), socket.INADDR_ANY)
   8 sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)
  10 while True:
  11   print sock.recv(10240)

The mreq packing is based on [ some code that I found,] that does not work. On my computer, at least.

Sending to multicast groups is just fine; Here's some functional text:

   1 import socket
   3 sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
   4 sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 2)
   5 sock.sendto("robot", ("", 1000))

(You might want to reconsider the IP_MULTICAST_TTL setting here -- the recommended value for local-network multicasts is < 32, whilst a value > 32 indicates multicasts which should traverse onto the Internet -- Asgeir S. Nilsen)

At this point, I'm beginning to think: "Python multicast simply does not work."

It's too bad we don't have anything as simple as this:

   1 import UDP
   3 sock = UDP.MulticastListener("", 1000)  # Listen on port 1000
   4 print sock.recv(100)

   1 import UDP
   3 UDP.send("Hello, world!", "", 1000)

...or something like that.

-- LionKimbro DateTime(2005-01-19T19:54:19Z)

I was able to get the above example to work fine on a linux platform, with one small change. I had to put a "4sl" in the pack statement for creating mreq. It seems as if when I didn't have a 4, the pack statement was just using the first octet (somehow dropping the other octets), so I could only create the multi-cast "listener" on a ip. After some debugging, I put the 4 in front of the "s", which forced it to get all 4 octets from the inet_aton, and everything worked fine. Hope this helps.


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