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.

.. code-block:: bash

    $ 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:

.. code-block:: bat

    netstat -an

* Lets write a program to talk between computers.

.. code-block:: python
    :linenos:
    :caption: 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


.. code-block:: python
    :linenos:
    :caption: 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
====================================

.. code-block:: python
    :linenos:
    :caption: 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)

.. code-block:: python
    :linenos:
    :caption: 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 .