At the end of this document, you will find the skeleton code for a simple proxy server. You are to complete the skeleton code. The places where you need to fill in code are marked with #Fill in start and #Fill in end. Each place may require one or more lines of code. Note 1: The code uses the method socket.makefile that you can read more about it online. You can alternatively use .send and .recv methods. You should inspect the code to lean how each line works. Note 2: This is not the most elegant code for this Step. For example, the proxy throws away many useful headers in the client request. We give you bonus points if you come up with a better way of sending requests from proxy to server and response from proxy to client.

2. Running the Proxy Server: Run the proxy server program using your command prompt. Then request a web page from your browser. Direct the requests to the proxy server using your IP address and port number. For e.g. http://localhost:8888/www.google.com To use the proxy server with browser and proxy on separate computers, you will need the IP address on which your proxy server is running. In this case, while running the proxy, you will have to replace the localhost with the IP address of the computer where the proxy server is running. Also note the port number used. You will replace the port number used here 8888 with the port number you have used in your server code at which your proxy server is listening.

3. Configuring your Browser: You can also directly configure your web browser to use your proxy. This depends on your browser. In Internet Explorer, you can set the proxy in Tools > Internet Options > Connections tab > LAN Settings. In Mozilla, you can set the proxy in Tools > Options > Advanced tab > Network tab > Connection Settings. In both cases you need to give the address of the proxy and the port number that you gave when you ran the proxy server. You should be able to run the proxy and the browser on the same computer without any problem. With this approach, to get a web page using the proxy server, you simply provide the URL of the page you want. For e.g. http://www.google.com At the end of step 1, you have a simple proxy server. You should verify that you indeed get the web pages via the proxy server.

Step 2: Add Error Handling The skeleton code does no error handling. This can be a problem especially when the client requests an object which is not available, since the "404 Not found" response usually has no response body and the proxy assumes there is a body and tries to read it. Add error handling capability to proxy.

Step 3: Cache Verification and Replacement In practice, the proxy server must verify that the cached responses are still valid and that they are the correct responses to the client's requests. Add the simple caching functionality described above using conditional GET. Your implementation will need to be able to replace the objects in the cache with their up-to-date versions. For this you need to implement some internal data structure in the proxy to keep track of which objects are cached, their last modified dates, and where they are on the disk. You can keep this data structure in main memory; there is no need to make it persist across shutdowns.

What to Hand in: Please submit a zip file using the format . zip. Your zip file must include: complete proxy server code (called ProxyServer.py) screenshots at the client side verifying that you indeed get the web page via the proxy server. A README text file that explains the IP address and port number for your proxy. Also explain what data structure you used for caching.

A Skeleton Python Code for the Simple Proxy Server from socket import import ays 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 por t and start listening tcpSe r Sock=socket (AF_INET, SOCK-STREAM} Fill in star t . # Fill in end while 1: # star t receiving data from the client print 'Ready to serve topclisock, addrtcpSerSock.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. splito [1]) filenamemesage.spliti [1.partition"") [2] print (filename) filebxist"false' filetouse""+ filename print(filetousel try ..') t Check wether the file exist in the cache fopen(filetouse [l:,"r" outputdataf.readlines ( fileExist = "true" ProxyServer finds a cache hit and generates a response message tcpClisock.send"HTTP/1.0 200 OKn") tcpClisock.send("Content-Type: text/html 4 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( host n 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.ma kefile ('r', 0) 1nst ead of using end and recv, we can use makefile fileobj.vrite("GET "+"http://" + filename + HTTP/1.0 ") #If this line does not work, wr i te separate lines for "Get " and "Host:.*" # Read the response into buffer # Fill in star t . Fill in end. # Create a new file in the cache for the regue sted 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 me ssage for file not found # Fill in star # Fill in end. t . # Close the client and the server sockets tcpclisock.close 4 Fill in start. Fill in end. A Skeleton Python Code for the Simple Proxy Server from socket import import ays 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 por t and start listening tcpSe r Sock=socket (AF_INET, SOCK-STREAM} Fill in star t . # Fill in end while 1: # star t receiving data from the client print 'Ready to serve topclisock, addrtcpSerSock.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. splito [1]) filenamemesage.spliti [1.partition"") [2] print (filename) filebxist"false' filetouse""+ filename print(filetousel try ..') t Check wether the file exist in the cache fopen(filetouse [l:,"r" outputdataf.readlines ( fileExist = "true" ProxyServer finds a cache hit and generates a response message tcpClisock.send"HTTP/1.0 200 OKn") tcpClisock.send("Content-Type: text/html 4 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( host n 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.ma kefile ('r', 0) 1nst ead of using end and recv, we can use makefile fileobj.vrite("GET "+"http://" + filename + HTTP/1.0 ") #If this line does not work, wr i te separate lines for "Get " and "Host:.*" # Read the response into buffer # Fill in star t . Fill in end. # Create a new file in the cache for the regue sted 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 me ssage for file not found # Fill in star # Fill in end. t . # Close the client and the server sockets tcpclisock.close 4 Fill in start. Fill in end