Homework 2: Simple Network Programming
======================================

Due Tuesday, Sep 9th, 12:40pm (at class start).

----

Section 0: Cleanup
------------------

Deal with my comments on the first homework: fix problems, etc.

Section 1: More Iterators and Generators
----------------------------------------

Implement both a re-entrant non-generator iterator, PrimeIter, *and* a
generator, prime_generator, that calculate prime numbers using
`Eratosthenes' Sieve
<http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes>`__. Your code
should pass the following tests: ::

  import homework2_primes, types
  
  prime5 = [2, 3, 5, 7, 11]
  
  primer = homework2_primes.PrimeIter()
  x = [ (i, j, k) for (i, j, k) in zip(primer, primer, range(5)) ]
  assert prime5 == [ i for (i, j, k) in x ]
  assert prime5 == [ j for (i, j, k) in x ]
  
  primer = homework2_primes.prime_generator()
  assert isinstance(primer, types.GeneratorType)
  x = [ i for (i, k) in zip(primer, range(5)) ]
  assert prime5 == x

Here is a potentially useful utility function that you should feel
free to use in your own code: ::

  def divides(primes, n):
    """
    'primes' is a list of prime numbers, 'n' is a positive integer.
    'divides' returns True if any number in 'primes' divides 'n'.
    """
    for trial in primes:
       if n % trial == 0: return True
    return False

Section 2: Write a spambot
--------------------------

Write a command-line program that connects to a specified server on
the SMTP port (port 25) and sends an e-mail from you to a specified
address; the e-mail should contain a specified word.  The program
should be named 'send-email' and be in the 'homework2/' subdirectory
of your svn archive.  So, for example, the following command ::

  python homework2/send-email teckla.idyll.org titus@cse491.msu.idyll.org blarny

would send an e-mail containing the word 'blarny' to
'titus@cse491.msu.idyll.org' using the mail server 'teckla.idyll.org'.

You don't need to do any error trapping or handling.

**Very important:** Actually sending spam is a violation of the `MSU
network policy <http://www.msu.edu/aup/>`__, and it is also very
unfriendly. Stick to using the server 'teckla.idyll.org' and your own
username @ cse491.msu.idyll.org, or else you may find your network
access cut off or worse.  Plus you could get me in trouble, which may
sound like fun but won't be - for you.  Promise.

Section 3: Write a simple TCP echo server
-----------------------------------------

Create a Python echo server that, when run, binds to the specified
port and echoes whatever data is sent to it back to the sender, up
until a simple '\\n.\\n' is received, at which point the connection is
terminated.  This echo server should take two arguments on the command
line: the host IP address and the port to which to bind.  For example,
::

  python homework2/echo-server localhost 4199

**Please do not use any Python network libraries other than 'socket' for
this assignment.**  You can (and should) process connections
synchronously; there's no need to handle multiple connections in
parallel.  (Guess what a future homework is? ;)

The echo server should echo *exactly* what it receives, with no
interpretation or parsing other than the end period.

Hint: you can use the spambot from section 2 to test your echo-server
without writing a new Python program; just set the spambot program to
print whatever it receives to the console, so you can verify that the
echo server is working properly.