Python connected datagram socket

Question

I'm building a simple client-server multiplayer game and I want to have connected UDP sockets. However, when I call the listen() method it produces Operation not supported exception.

try:
    server = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    server.bind((host, port))
    server.listen(5)
except socket.error, (value, message):
    print "E: " + message  # E: Operation not supported

Is there a way to have connected datagram sockets?

Answer 1

UDP protocol is connectionless and thus you really cannot create connections between 2 sockets in the same manner as with the TCP client and server; thus you also cannot do the listen system call on an UDP socket, as it concerns only the TCP server sockets. Instead you use socket.recvfrom to receive datagrams from any address:

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind((host, port))
data, addr = sock.recvfrom(65536)  # this will fit the maximum datagram

You can respond to the client (if they still have a socket open), by destructuring the addr which is a host, port tuple for socket.AF_INET, socket.SOCK_DGRAM.

client_host, client_port = addr

You can send data back there with sendto

sock.sendto(data, (client_host, client_port))

If you want to use a well-known port, you will bind the socket; if not, the system will assign a port for you.

---

It is possible to do a connect system call with datagram sockets on many socket implementations; this serves as a sort of filter, that drops the packets coming from unwanted sources, and for setting the default outgoing address for sock.send (it is still OK to use sock.sendto on such socket to another address, though the response might be dropped because of the "connection"). This is very useable on a client or between 2 nodes if they agree to use 2 well known ports with each other.

However if you do connect on server, it cannot serve any other requests on this socket. Also, listen with its queues only relates to SOCK_STREAM sockets.

Thus in case of many clients to 1 server, you could have the server listen to socket 12345, and when a client contacts the server, the server could respond from server:12345 with a message that contains another port number that the client should use with the server.