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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

Using UDP for e.g. File Transfers

If considering extending this example for e.g. file transfers, keep in mind that UDP is not reliable. So you'll have to handle packets getting lost and packets arriving out of order. In effect, to get something reliable you'll need to implement something similar to TCP on top of UDP, and you might want to consider using TCP instead.

That being said, sometimes you need to use UDP, e.g. for UDP hole punching. In that case, consider TFTP for python or UDT for python


The official example of multicast can be found at /usr/share/doc/python2.3/examples/Demo/sockets/ (at least on Debian Sarge, after apt-get install python-examples). It worked on my machine, but I have yet to try it running on different machines. -- -- 2006-08-09 03:20:30

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 2005-04-14 13:19:00)

(I've replaced it with one that works. -- Asgeir S. Nilsen 2005-05-09 19:25:00)

(I've corrected the mreq according to the comment below -- Sebastian Setzer 2006-01-25 14:28:00)

(I added an "=" to the "4sl" struct packing. Both the old version and the new version work on my 32-bit machine, but the Python documentation for the struct module suggests that "l" would be 64 bits on an LP64 or LPI64 platform without it, so I thought it would be prudent to add. -- Kragen Sitaker 2010-04-28 07:03:00)

   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 # wrong: mreq = struct.pack("sl", socket.inet_aton(""), socket.INADDR_ANY)
   8 mreq = struct.pack("=4sl", socket.inet_aton(""), socket.INADDR_ANY)
  10 sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)
  12 while True:
  13   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)

Note the above example is missing a bind() call. This will work OK if there is a multicast route setup in the IP routing table {i.e. os.system('route add eth0')} But without a bind() nor route, the kernel will not determine which network interface to send the data on, and will return an error. There may be other ways for the "socket to network interface" mapping to be defined, but I forget what they are. (Chris David)

The above multicasting examples do not work on my computer, but I was able to fix them using code from Try these examples:

   1 # UDP multicast examples, Hugo Vincent, 2005-05-14.
   2 import socket
   4 def send(data, port=50000, addr=''):
   5         """send(data[, port[, addr]]) - multicasts a UDP datagram."""
   6         # Create the socket
   7         s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
   8         # Make the socket multicast-aware, and set TTL.
   9         s.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 20) # Change TTL (=20) to suit
  10         # Send the data
  11         s.sendto(data, (addr, port))
  13 def recv(port=50000, addr="", buf_size=1024):
  14         """recv([port[, addr[,buf_size]]]) - waits for a datagram and returns the data."""
  16         # Create the socket
  17         s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
  19         # Set some options to make it multicast-friendly
  20         s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
  21         try:
  22                 s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)
  23         except AttributeError:
  24                 pass # Some systems don't support SO_REUSEPORT
  25         s.setsockopt(socket.SOL_IP, socket.IP_MULTICAST_TTL, 20)
  26         s.setsockopt(socket.SOL_IP, socket.IP_MULTICAST_LOOP, 1)
  28         # Bind to the port
  29         s.bind(('', port))
  31         # Set some more multicast options
  32         intf = socket.gethostbyname(socket.gethostname())
  33         s.setsockopt(socket.SOL_IP, socket.IP_MULTICAST_IF, socket.inet_aton(intf))
  34         s.setsockopt(socket.SOL_IP, socket.IP_ADD_MEMBERSHIP, socket.inet_aton(addr) + socket.inet_aton(intf))
  36         # Receive the data, then unregister multicast receive membership, then close the port
  37         data, sender_addr = s.recvfrom(buf_size)
  38         s.setsockopt(socket.SOL_IP, socket.IP_DROP_MEMBERSHIP, socket.inet_aton(addr) + socket.inet_aton(''))
  39         s.close()
  40         return data

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 2005-01-19 19:54:19

You could do something like this:

   1 class McastSocket(socket.socket):
   2   def __init__(self, local_port, reuse=False):
   3     socket.socket.__init__(self, socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
   4     if(reuse):
   5       self.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
   6       if hasattr(socket, "SO_REUSEPORT"):
   7         self.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)
   8     self.bind(('', local_port))
   9   def mcast_add(self, addr, iface):
  10     sock.setsockopt(
  11         socket.IPPROTO_IP,
  12         socket.IP_ADD_MEMBERSHIP,
  13         socket.inet_aton(mcast_addr) + socket.inet_aton(mcast_iface))

Then to listen to multicast events locally:

   1 sock = McastSocket(local_port=12345, reuse=1)
   2 sock.mcast_add('', '')

The perl IO::Socket::Multicast class doesn't look much different from this.

-- PaulBrannan

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.


...Which is exactly what the pack statement is expected to do, according to the manual:

=UPDATE:= Definitive UDP multicasting example code can be found in PyZeroConf at

How can it be definitive, if you have to download a whole project in order to read simply how to do multicasting in Python?

* How to Write New Components for Kamaelia has example code for doing multicast send and receipt at the top.

Perhaps this is what you are looking for:

-- cswank

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