Which port is being used by my socket

Question

Suppose I create a UDP socket in python, and then send a message using:

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

sock.sendto(MESSAGE, (DEST_IP, DEST_PORT))

How do I find out which source port my message was send from?

(I do not want to bind my socket to any specific port. But how do I find out which source port was used to send the message?)

Answer 1

The first getsockname() doesn't appear to work, but the second does. So perhaps it doesn't get valid values until after the first transmission:

#!/usr/local/cpython-3.3/bin/python

import socket

MESSAGE = b"Hello"
DEST_IP = '127.0.0.1'
DEST_PORT = 12345

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

print(sock.getsockname())
sock.sendto(MESSAGE, (DEST_IP, DEST_PORT))
print(sock.getsockname())

HTH

Help on method getsockname:

getsockname(...) method of socket._socketobject instance
    getsockname() -> address info

    Return the address of the local endpoint.  For IP sockets, the address
    info is a pair (hostaddr, port).

Answer 2

If you don't bind a UDP socket, an automatic bind occurs when you first send or receive. So getsockname() after that will tell you the allocated port number.