Homework - Python Proxy Server

For a programming exercise (from Computer Networking: A Top-Down Approach (6th Edition) by Kurose and Ross), we're trying to develop a simple proxy server in python.

We were given the following code, wherever it says #Fill in start. ... #Fill in end. that is where we need to write code. My specific question and attempts will be below this original snippet.

We need to start the python server with: python proxyserver.py [server_ip] then navigate to localhost:8888/google.com and it should work when we're finished.

from socket import *
import sys
if len(sys.argv) <= 1:
  print 'Usage : "python ProxyServer.py server_ip"\n[server_ip : It is the IP Address Of Proxy Server'
  sys.exit(2)

# Create a server socket, bind it to a port and start listening 
tcpSerSock = socket(AF_INET, SOCK_STREAM)
# Fill in start.
# Fill in end.

while 1:
  # Strat receiving data from the client
  print 'Ready to serve...'
  tcpCliSock, addr = tcpSerSock.accept()
  print 'Received a connection from:', addr 
  message = # Fill in start. # Fill in end. print message

  # Extract the filename from the given message print message.split()[1]
  filename = message.split()[1].partition("/")[2] print filename
  fileExist = "false"
  filetouse = "/" + filename
  print filetouse
  try:
    # Check wether the file exist in the cache
    f = open(filetouse[1:], "r")
    outputdata = f.readlines()
    fileExist = "true"

    # ProxyServer finds a cache hit and generates a response message     
    tcpCliSock.send("HTTP/1.0 200 OK\r\n") 
    tcpCliSock.send("Content-Type:text/html\r\n")
    # Fill in start.
    # Fill in end.
      print 'Read from cache'
  # Error handling for file not found in cache
except IOError:
  if fileExist == "false":
    # Create a socket on the proxyserver
    c = # Fill in start. # Fill in end. 
    hostn = filename.replace("www.","",1)
    print hostn
      try:
        # Connect to the socket to port 80
        # Fill in start.
        # Fill in end.

        # Create a temporary file on this socket and ask port 80 for the file requested by the client
        fileobj = c.makefile('r', 0)
        fileobj.write("GET "+"http://" + filename + "HTTP/1.0\n\n")

        # Read the response into buffer
        # Fill in start.
        # Fill in end.

        # Create a new file in the cache for the requested file. 
        # Also send the response in the buffer to client socket and the corresponding file in the cache
        tmpFile = open("./" + filename,"wb")
        # Fill in start.
        # Fill in end.
      except:
        print "Illegal request"
    else:
      # HTTP response message for file not found 
      # Fill in start.
      # Fill in end.
  # Close the client and the server sockets
    tcpCliSock.close()
# Fill in start.
# Fill in end.

Where it says:

# Create a socket on the proxyserver
c = # Fill in start. # Fill in end.

I tried:

c = socket(AF_INET, SOCK_STREAM)

This seems to be how you create a socket, then for connecting to port 80 of the host, I have:

c.connect((hostn, 80))

Here, hostn is correctly google.com according to some local print statements I have. The next section for me to fill in says to #Read response into buffer but I don't really understand what that means. I presume it has something to do with the fileobj that is created just above.

Thanks in advance, please let me know if I missed anything I should be adding.

UPDATE

My current code can be found here to see what I've been trying:

https://github.com/ardavis/Computer-Networks/blob/master/Lab%203/ProxyServer.py

from socket import * import sys if len(sys.argv) <= 1: print 'Usage: "python ProxyServer.py server_ip"\n[server_ip : It is the IP Address of the Proxy Server' sys.exit(2) # Create a server socket, bind it to a port and start listening tcpSerPort = 8888 tcpSerSock = socket(AF_INET, SOCK_STREAM) # Prepare a server socket tcpSerSock.bind(('', tcpSerPort)) tcpSerSock.listen(5) while True: # Start receiving data from the client print 'Ready to serve...' tcpCliSock, addr = tcpSerSock.accept() print 'Received a connection from: ', addr message = tcpCliSock.recv(1024) # Extract the filename from the given message print message.split()[1] filename = message.split()[1].partition("/")[2] fileExist = "false" filetouse = "/" + filename try: # Check whether the file exists in the cache f = open(filetouse[1:], "r") outputdata = f.readlines() fileExist = "true" print 'File Exists!' # ProxyServer finds a cache hit and generates a response message tcpCliSock.send("HTTP/1.0 200 OK\r\n") tcpCliSock.send("Content-Type:text/html\r\n") # Send the content of the requested file to the client for i in range(0, len(outputdata)): tcpCliSock.send(outputdata[i]) print 'Read from cache' # Error handling for file not found in cache except IOError: print 'File Exist: ', fileExist if fileExist == "false": # Create a socket on the proxyserver print 'Creating socket on proxyserver' c = socket(AF_INET, SOCK_STREAM) hostn = filename.replace("www.", "", 1) print 'Host Name: ', hostn try: # Connect to the socket to port 80 c.connect((hostn, 80)) print 'Socket connected to port 80 of the host' # Create a temporary file on this socket and ask port 80 # for the file requested by the client fileobj = c.makefile('r', 0) fileobj.write("GET " + "http://" + filename + " HTTP/1.0\n\n") # Read the response into buffer buff = fileobj.readlines() # Create a new file in the cache for the requested file. # Also send the response in the buffer to client socket # and the corresponding file in the cache tmpFile = open("./" + filename, "wb") for i in range(0, len(buff)): tmpFile.write(buff[i]) tcpCliSock.send(buff[i]) except: print 'Illegal request' else: # HTTP response message for file not found # Do stuff here print 'File Not Found...Stupid Andy' a = 2 # Close the socket and the server sockets tcpCliSock.close() # Do stuff here

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