If you know the port number of a network service, you can find the service name using the getservbyport() socket class function from the socket library. You can optionally give the protocol name when calling this function.

Let us define a find_service_name() function, where the getservbyport() socket class function will be called with a few ports, for example, 80, 25. We can use Python's for-in loop construct.

Listing 1.4 shows finding_service_name as follows:

#!/usr/bin/env python
# Python Network Programming Cookbook -- Chapter -  1
# This program is optimized for Python 2.7.
# It may run on any other version with/without modifications.

import socket

def find_service_name():
    protocolname = 'tcp'
    for port in [80, 25]:
        print "Port: %s => service name: %s" %(port, socket.getservbyport(port, protocolname))
    print "Port: %s => service name: %s" %(53, socket.getservbyport(53, 'udp'))
    
if __name__ == '__main__':
    find_service_name()

If you run this script, you will see the following output:

$ python 1_4_finding_service_name.py 

Port: 80 => service name: http
Port: 25 => service name: smtp
Port: 53 => service name: domain

In this recipe, for-in statement is used to iterate over a sequence of variables. So for each iteration we use one IP address to convert them in their packed and unpacked format.