Python network programming(bind to external address)

Question

I am a newbie to python, and just few days back I started trying my hands on network programming(I am a newbie there too)

Now I found a neat client server program which was running quite simply on my computer, but when I replaced the local addresses, and told my friend to run the client script, it just wont respond.

My global I.P address : 120.59.XX.XXX

My Ipv4 address as returned by ipconfig : 192.168.1.2 (I am connected to internet through a router)

My gateway address : 192.168.1.1

Port used : 1060 (I tested this port locally and it wasn't in use)

#server.py

import socket
import sys

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

MAX = 65535
PORT = 1060

server.bind(('192.168.1.2', PORT))
print 'Listening at', server.getsockname()
while True:
    data, address = server.recvfrom(MAX)
    print 'The client at', address, 'says', repr(data)
    server.sendto('Your data was %d bytes' % len(data), address)

Client Code :

#client.py

import socket
import sys

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

PORT = 1060
MAX = 65536

client.sendto('Hello Server!', ('120.59.XX.XXX', PORT))
data, address = client.recvfrom(MAX)
print 'The server', address, 'says', repr(data)

I started server.py on my computer and told my friend to start client.py, I allowed incoming connections to python through firewall, also I added 1060 port to windows incoming connections list.

Still it is not responding, and I am unable to decipher why(I have a dynamic IP address, but for the current session it remains constant and hence should work, also 1060 is a well known port and shouldn't be a problem right?)

Ahmed El-Hassany · Accepted Answer · 2012-05-22 15:53:38Z

1

You need to add a port forwarding rule in your router! something like from port 1060 forward to 192.168.1.2 port 1060.

answered May 22, 2012 at 15:53

Ahmed El-Hassany

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4 Comments

Kartik Anand Over a year ago

yes this is what I first thought, but then how come utorrent is able to use ports without me forwarding them?and 1060 is well known used by SAP

Nikolai Fetissov Over a year ago

It goes out connecting to a server somewhere, i.e. the TCP connection is initiated from your PC.

Dr. Jan-Philip Gehrcke Over a year ago

NAT techniques often work kind of intelligent. If a connection is initiated by a machine within the local network, most NAT implementations map the response back to this machine. If a connection is initiated from the outside, it comes unexpectedly (from the "router"'s point of view). If there is no rule specified how to handle such an unexpected connection (like a port forward rule), the router rejects the connection.

Kartik Anand Over a year ago

so if I could trick the router, I mean send a random(or empty) message to any URL by UDP, get my random port number(which OS would assign), and then bind my server to that port number.will it work?

Nikolai Fetissov · Accepted Answer · 2012-05-22 15:56:43Z

1

You need a port-forward on the router, which would forward connections to router's external (global) address on port 1060 to your desktop IP 192.168.1.2 port 1060.

answered May 22, 2012 at 15:56

Nikolai Fetissov

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14 Comments

Kartik Anand Over a year ago

Ok but if I use a well known port, should it work then?because I used 80(I know I shouldn't) and it still didn't work.

Nikolai Fetissov Over a year ago

Well known ports like 80 and 25 are often blocked by ISPs on incoming. Use a random high number port like 42424.

Kartik Anand Over a year ago

I forwarded 1060 to 192.168.1.2 my local IP, and it still isn't working.

Nikolai Fetissov Over a year ago

The router might assume TCP when you do port-forward. See if there's an option there to enable UDP, which you are trying to do.

Nikolai Fetissov Over a year ago

Two other things to check - whether local firewall on windows pc allows udp:1060 in, and if you have any logging capability on that router, see if you are actually getting those packets from your friend (they might be filtered/dropped somewhere else).

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