Cannot connect to socket on '' ip address. Python

Question

He, I have a server python script that listens on ip address '' and port 1337 and now I am trying to connect on the client side to that socket. But for some reason I cannot do that. If i will change the binding address to "127.0.0.1" it will work just fine, but I have been told to use this ip address ''.

Can someone please explain me why this is heppening and what is this ip address mean?.

This is my server:

import socket
from threading import Thread

HOST = '' 
PORT = 1337

def main():
    try:
        s.bind((HOST, PORT))
    except socket.error as msg:
        print('Bind failed. Error Code : ' + str(msg[0]) + ' Message ' + msg[1])
        sys.exit()

    print('Socket bind complete')

    s.listen(10)
    print('Socket now listening')

    while 1:
        conn, addr = s.accept()
        print('Connected with ' + addr[0] + ':' + str(addr[1]))

        new_client = Thread(target=clientthread, args=(conn,))
        new_client.start()

    s.close()

if __name__ == "__main__":
    main()

This is my client:

import socket

HOST = ''
PORT = 1337

def main():
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect((HOST, PORT))
    print("Worked!")
    s.close()

if __name__ == "__main__":
    main

And this is the weeor that I am getting:

Traceback (most recent call last):
  File "C:\Users\magshimim\Downloads\responder_client.py", line 4, in <module>
    s.connect((HOST, PORT))
OSError: [WinError 10049] The requested address is not valid in its context

You can bind on '' on server side (which means to bind on all network adapters while binding e. g. on 127.0.0.1 means to only receive connections from same device) but on client side you need a valid IP address like 127.0.0.1. — Michael Butscher
– Michael Butscher, Commented Dec 18, 2022 at 12:27

JayPeerachai · Accepted Answer · 2022-12-18 12:32:33Z

2

The IP address '' (i.e., wildcard) in the server code represents the any address. This means that the server will listen on all available network interfaces.

'' is equivalent to 0.0.0.0 and is used only in the server, not a client. For a client, it requires knowing the actual IP address of the server in order to connect to it.

Consequently, using the IP address 127.0.0.1 in the client code is already correct.

answered Dec 18, 2022 at 12:32

JayPeerachai

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TessellatingHeckler · Accepted Answer · 2022-12-18 12:29:37Z

The documentation for Python's socket module says:

A pair (host, port) is used for the AF_INET address family, where host is a string representing either a hostname in internet domain notation like 'daring.cwi.nl' or an IPv4 address like '100.50.200.5', and port is an integer.

For IPv4 addresses, two special forms are accepted instead of a host address: '' represents INADDR_ANY, which is used to bind to all interfaces

As Michael Butscher comments, the server can bind to all interfaces for listening, that might include multiple internet connections, or a local network interface, or virtual machine networking, etc.

A client cannot connect to all interfaces - imagine trying to open a single connection from your computer to "all websites", it can't work, you would need many connections, one for each. You need to give an address to connect to, and 127.0.0.1 is a special address always meaning "loopback to this local computer".

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