COMP3310 2021 - Assignment 2: An annoying web-proxy

Background: • This assignment is worth 15% of the final mark

• It is due by 23:59 Sunday 9 May AEST - note: CANBERRA TIME (gmt+10)

• Late submissions will not be accepted, except in special circumstances

o Extensions must be requested well before the due date, via the course convenor, 

with appropriate evidence.

This is a coding assignment, to enhance and check your network programming skills. The main focus 

is on native socket programming, and your ability to understand and implement the key elements of 

an application protocol from its RFC specification.

Assignment 2 outline

A web-proxy is a simple web-client and web-server wrapped in a single application. It receives 

requests from one or more clients (web-browsers) for particular content URLs, and forwards them 

on to the intended server, then returns the result to your web-browser - in some form. How is this 

useful? 

• It can cache content, so the second and later clients to make the same request get a more 

rapid response, and free up network capacity.

• It can filter content, to ensure that content coming back is ‘safe’, e.g. for children or your 

home, or for staff/their computers inside an organisation. • It can filter requests, to ensure that people don’t access things they shouldn’t, for whatever 

policy reasons one might have. • It can listen to requests/responses and learn things, i.e. snoop on the traffic. Getting people 

to use your proxy though is a different challenge... o And of course it can listen to and modify requests/responses, for fun or profit.

For this assignment, you need to write a web proxy in C or Java, without the use of any external 

web/http-related libraries (html-parsing support is ok). ENGN students with limited C/Java 

backgrounds should talk with their tutors as we have other options there, though the requirements 

will be the same and more closely considered. As most networking server code is written in C, with 

other languages a distant second, it is worth learning it.

Your code MUST open sockets in the standard socket() API way, as per the tutorial exercises. Your 

code MUST make appropriate and correctly-formed HTTP/1.0 (RFC1945) or HTTP/1.1 enhanced 

requests (to a web-server, as a client) and responses (to a web-browser, as a server) on its own, and 

capture/interpret the results on its own in both directions. You will be handcrafting HTTP packets, so 

you’ll need to understand the structures of requests/responses and HTTP headers.

Wireshark will be helpful for debugging purposes, compare it to a direct web-browser transaction. 

The most common trap is not getting your line-ending ‘\n\n’ right on requests, and this is rather OS 

and language-specific. Remember to be conservative in what you send and reasonably liberal in what 

you accept.

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What your successful and highly-rated proxy will need to do:

1. Act as a proxy against a website we name. You must allow that name to be specified either 

as a command line argument or read from a file. 

2. Rewrite (simple) html links that originally pointed to the website to now point to your 

proxy, so all subsequent requests also go via your proxy.

a. Sometimes links are not written in pure style, e.g. they are