Python Networking Basics

• When you use your web browser, you usually type some address, like the one in your URL bar, now.

• That URL isn’t the real address. It is a name that corresponds to the real address.

$ nslookup gnomon.securitystandard.org
Server:             10.168.123.11
Address:    10.168.123.11#53
Non-authoritative answer:
gnomon.securitystandard.org canonical name = hermes.securitystandard.org.
Name:       hermes.securitystandard.org
Address: 185.172.165.184

• The real address of gnomon is 185.172.165.184 !

• So, what are those numbers? We’re back to binary fun!

IP Addresses

• Each of these numbers separated by dots is called an “octet.”

• Here is how they look in binary. Let’s do the math and prove it.

185	172	165	184
10111001	10101100	10100101	10111000

• Quiz yourself: What is the biggest number that fits in an octet?

Talking between computers

• To listen for a connection on your computer from another computer, you must hold socket.

• A socket is an IP address and a port number. To see which ports are in use on your computer, try this command:

netstat -an

• Lets write a program to talk between computers.

Talk server

#!/bin/env python
import socket
# We establish a variable called server that opens a socket.
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# We will listen on port 9087 on 0.0.0.0, which means *every IP*
# Notice that we pass something in parentheses
listentuple = ('0.0.0.0', 9087)
server.bind(listentuple) # Own the IP and port!
server.listen(50) # Start listening
print("Starting server.")
data = ''

while True:
    conn, addr = server.accept() # See who is talking to us
    print('Connected with ' + addr[0] + ':' + str(addr[1]))
    while "bye" not in data:
        data = conn.recv(1024)
        conn.sendall('You said ' + data + '\n')
        print("I got " + data)
    print("Stopping server.")
    server.shutdown(0)
    server.close()
    break

Talk client

import socket
def netcat(hostname, port, content):
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect((hostname, port))
    s.sendall(content)
    s.shutdown(socket.SHUT_WR)
    while 1:
        data = s.recv(1024)
        if data == "":
            break
        print("Received:", repr(data))
    print("Connection closed.")
    s.close()
# Use...
#   sendtohost('xxx.xxx.xxx.xxx', portnumber, "What I wanted to say.")

Downloading Things from the Internet

A function that checks if we fed Python an IP and a port to make a socket

import socket
def isipandport(socketstring):
    '''Tests for an ipv4 address and a realistic port number'''
    # If socketstring is empty, it is false, so we return False and quit.
    if not socketstring:
        return(False)
    # Let us make sure it contains a string or we transform it into one.
    socketstring = str(socketstring)
    # We will try to use it as an address.
    try:
        ip, port = socketstring.split(':')
        socket.inet_aton(ip)
        port = int(port)
    # If we fail, we will complain.
    except:
        print('malformed socket in ' + socketstring)
        return(False)
    # If the port number is less than 7 or greater than the maximum integer,
    #  We will whine about it, but accept it.
    if (port < 7) or (port > 65535):
        print('port out of range in ' + socketstring)
    return(True)

a set of commands to copy the web page for the Python programming language.

import urllib.request
f = urllib.request.urlopen('http://www.python.org/')
v1 = f.read()
FH = open('c:/temp/mycopy.html', 'w')
FH.write(v1)
FH.close()

• Now, try this:

  • Ask the user for an IP and port number. If the user gives you one, say ‘Thanks!’ If not, say ‘Wrong!’

  • Download the Python web page and display your local copy in a web browser by opening the URL file:///c:/temp/mycopy.html .