COS-461 Assignments: HTTP Proxy

Assignment 2: HTTP Proxy



In this assignment, you will implement a web proxy that passes requests and data between multiple web clients and web servers, concurrently. This will give you a chance to get to know one of the most popular application protocols on the Internet -- the Hypertext Transfer Protocol (HTTP) -- and give you an introduction to the Berkeley sockets API. When you're done with the assignment, you should be able to configure your web browser to use your personal proxy server as a web proxy.

Introduction: The Hypertext Transfer Protocol

The Hypertext Transfer Protocol (HTTP) is the protocol used for communication on this web: it defines how your web browser requests resources from a web server and how the server responds. For simplicity, in this assignment, we will be dealing only with version 1.0 of the HTTP protocol, defined in detail in RFC 1945. You may refer to that RFC while completing this assignment, but our instructions should be self-contained.

HTTP communications happen in the form of transactions; a transaction consists of a client sending a request to a server and then reading the response. Request and response messages share a common basic format:

The initial line and header lines are each followed by a "carriage-return line-feed" (\r\n) signifying the end-of-line.

For most common HTTP transactions, the protocol boils down to a relatively simple series of steps (important sections of RFC 1945 are in parenthesis):

  1. A client creates a connection to the server.
  2. The client issues a request by sending a line of text to the server. This request line consists of a HTTP method (most often GET, but POST, PUT, and others are possible), a request URI (like a URL), and the protocol version that the client wants to use (HTTP/1.0). The request line is followed by one or more header lines. The message body of the initial request is typically empty. (5.1-5.2, 8.1-8.3, 10, D.1)
  3. The server sends a response message, with its initial line consisting of a status line, indicating if the request was successful. The status line consists of the HTTP version (HTTP/1.0), a response status code (a numerical value that indicates whether or not the request was completed successfully), and a reason phrase, an English-language message providing description of the status code. Just as with the the request message, there can be as many or as few header fields in the response as the server wants to return. Following the CRLF field separator, the message body contains the data requested by the client in the event of a successful request. (6.1-6.2, 9.1-9.5, 10)
  4. Once the server has returned the response to the client, it closes the connection.

It's fairly easy to see this process in action without using a web browser. From a Unix prompt, type:

telnet 80

This opens a TCP connection to the server at listening on port 80 (the default HTTP port). You should see something like this:

Connected to
Escape character is '^]'.

type the following:


and hit enter twice. You should see something like the following:

HTTP/1.0 200 OK
Date: Tue, 16 Feb 2010 19:21:24 GMT
(More HTTP headers...)
Content-Type: text/html; charset=utf-8

(More HTML follows)

There may be some additional pieces of header information as well- setting cookies, instructions to the browser or proxy on caching behavior, etc. What you are seeing is exactly what your web browser sees when it goes to the Yahoo home page: the HTTP status line, the header fields, and finally the HTTP message body- consisting of the HTML that your browser interprets to create a web page. You may notice here that the server responds with HTTP 1.1 even though you requested 1.0. Some web servers refuse to serve HTTP 1.0 content.

HTTP Proxies

Ordinarily, HTTP is a client-server protocol. The client (usually your web browser) communicates directly with the server (the web server software). However, in some circumstances it may be useful to introduce an intermediate entity called a proxy. Conceptually, the proxy sits between the client and the server. In the simplest case, instead of sending requests directly to the server the client sends all its requests to the proxy. The proxy then opens a connection to the server, and passes on the client's request. The proxy receives the reply from the server, and then sends that reply back to the client. Notice that the proxy is essentially acting like both a HTTP client (to the remote server) and a HTTP server (to the initial client).

Why use a proxy? There are a few possible reasons:


Assignment Details

The Basics

Your task is to build a web proxy capable of accepting HTTP requests, forwarding requests to remote (origin) servers, and returning response data to a client. The proxy MUST handle concurrent requests by forking a process for each new client request using the fork() system call. You will only be responsible for implementing the GET method. All other request methods received by the proxy should elicit a "Not Implemented" (501) error (see RFC 1945 section 9.5 - Server Error).

This assignment can be completed in either C or C++. It should compile and run (using g++) without errors or warnings from the FC 010 cluster, producing a binary called proxy that takes as its first argument a port to listen from. Don't use a hard-coded port number.

You shouldn't assume that your server will be running on a particular IP address, or that clients will be coming from a pre-determined IP.


When your proxy starts, the first thing that it will need to do is establish a socket connection that it can use to listen for incoming connections. Your proxy should listen on the port specified from the command line and wait for incoming client connections. Each new client request is accepted, and a new process is spawned using fork() to handle the request. There should be a reasonable limit on the number of processes that your proxy can create (e.g., 100). Once a client has connected, the proxy should read data from the client and then check for a properly-formatted HTTP request -- but don't worry, we have provided you with libraries that parse the HTTP request lines and headers. Specifically, you will use our libraries to ensure that the proxy receives a request that contains a valid request line:

All other headers just need to be properly formatted:
In this assignment, client requests to the proxy must be in their absolute URI form (see RFC 1945, Section 5.1.2) -- as your browser will send if properly configured to explicitly use a proxy (as opposed to a transparent on-path proxies that some ISPs deploy, unbeknownst to their users). An invalid request from the client should be answered with an appropriate error code, i.e. "Bad Request" (400) or "Not Implemented" (501) for valid HTTP methods other than GET. Similarly, if headers are not properly formatted for parsing, your client should also generate a type-400 message.

Parsing Library

We have provided a parsing library to do string parsing on the header of the request. This library is in proxy_parse.[c|h] in the skeleton code. The library can parse the request into a structure called ParsedRequest which has fields for things like the host name (domain name) and the port. It also parses the custom headers into a set of ParsedHeader structs which each contain a key and value corresponding to the header. You can lookup headers by the key and modify them. The library can also recompile the headers into a string given the information in the structs.

More details as well as sample usage is available in proxy_parse.h, as well as example code on how to use the library. This library can also be used to verify that the headers are in the correct format since the parsing functions return error codes if this is not the case.

Parsing the URL

Once the proxy sees a valid HTTP request, it will need to parse the requested URL. The proxy needs at least three pieces of information: the requested host and port, and the requested path. See the URL (7) manual page for more info. You will need to parse the absolute URL specified in the request line using the given You can use the parsing library to help you. If the hostname indicated in the absolute URL does not have a port specified, you should use the default HTTP port 80.

Getting Data from the Remote Server

Once the proxy has parsed the URL, it can make a connection to the requested host (using the appropriate remote port, or the default of 80 if none is specified) and send the HTTP request for the appropriate resource. The proxy should always send the request in the relative URL + Host header format regardless of how the request was received from the client:

Accept from client:

Send to remote server:
GET / HTTP/1.0
Connection: close
(Additional client specified headers, if any...)

Note that we always send HTTP/1.0 flags and a Connection: close header to the server, so that it will close the connection after its response is fully transmitted, as opposed to keeping open a persistent connection (as we learned in Lecture 2). So while you should pass the client headers you receive on to the server, you should make sure you replace any Connection header received from the client with one specifying close, as shown. To add new headers or modify existing ones, use the HTTP Request Parsing Library we provide.

Returning Data to the Client

After the response from the remote server is received, the proxy should send the response message as-is to the client via the appropriate socket. To be strict, the proxy would be required to ensure a Connection: close is present in the server's response to let the client decide if it should close it's end of the connection after receiving the response. However, checking this is not required in this assignment for the following reasons. First, a well-behaving server would respond with a Connection: close anyway given that we ensure that we sent the server a close token. Second, we configure Firefox to always send a Connection: close by setting keepalive to false. Finally, we wanted to simplify the assignment so you wouldn't have to parse the server response.

Testing Your Proxy

Run your client with the following command:

./proxy <port>, where port is the port number that the proxy should listen on. As a basic test of functionality, try requesting a page using telnet:

telnet localhost <port>
Connected to localhost.localdomain (
Escape character is '^]'.

If your proxy is working correctly, the headers and HTML of the Google homepage should be displayed on your terminal screen. Notice here that we request the absolute URL ( instead of just the relative URL (/). A good sanity check of proxy behavior would be to compare the HTTP response (headers and body) obtained via your proxy with the response from a direct telnet connection to the remote server. Additionally, try requesting a page using telnet concurrently from two different shells.

For a slightly more complex test, you can configure your web browser to use your proxy server as its web proxy. See the section below for details.

Configuring a Web Browser to Use a Proxy


Version 3.x:

  1. Select Tools->Options (or Edit->Preferences) from the menu.
  2. Click on the 'Advanced' icon in the Options dialog.
  3. Select the 'Network' tab, and click on 'Settings' in the 'Connections' area.
  4. Select 'Manual Proxy Configuration' from the options available. In the boxes, enter the hostname and port where proxy program is running.

Earlier Versions:

  1. Upgrade your browser. You're vulnerable to security threats.

To stop using the proxy server, select 'No Proxy' in the connection settings dialog.

Configuring Firefox to use HTTP/1.0

Because Firefox defaults to using HTTP/1.1 and your proxy speaks HTTP/1.0, there are a couple of minor changes that need to be made to Firefox's configuration. Fortunately, Firefox is smart enough to know when it is connecting through a proxy, and has a few special configuration keys that can be used to tweak the browser's behavior.

  1. Type 'about:config' in the title bar.
  2. In the search/filter bar, type 'network.http.proxy'
  3. You should see three keys: network.http.proxy.keepalive, network.http.proxy.pipelining, and network.http.proxy.version.
  4. Set keepalive to false. Set version to 1.0. Make sure that pipelining is set to false.

Internet Explorer

Take a look at this page for complete instructions on enabling a proxy for various versions of Internet Explorer.

You should also do the following to make Internet Explorer work in a HTTP 1.0 compatible mode with your proxy:

  1. Under Internet Options, select the 'Advanced' tab.
  2. Scroll down to HTTP 1.1 Settings. Uncheck 'Use HTTP 1.1 through proxy connections'.

Socket Programming

In order to build your proxy you will need to learn and become comfortable programming sockets. The Berkeley sockets library is the standard method of creating network systems on Unix. There are a number of functions that you will need to use for this assignment:

You can find the details of these functions in the Unix man pages (most of them are in section 2) and in the Stevens Unix Network Programming book, particularly chapters 3 and 4. Other sections you may want to browse include the client-server example system in chapter 5 (you will need to write both client and server code for this assignment) and the name and address conversion functions in chapter 9.

Multi-Process Programming

In addition to the Berkeley sockets library, there are some functions you will need to use for creating and managing multiple processes: fork, waitpid.

You can find the details of these functions in the Unix man pages:



You should submit your completed proxy by the date posted on the course website to CS Dropbox. You will need to submit a tarball file containing the following:

Your tarball should be named cos461_ass1_USERNAME.tgz where USERNAME is your username. The sample Makefile in the skeleton zip file we provide will make this tarball for you with the make tar command.

Your proxy will be graded out of ten points, with the following criteria:

  1. When running make on your assignment, it should compile without errors or warnings on the FC 010 cluster machines and produce a binary named proxy. The first command line argument should be the port that the proxy will listen from.
  2. Your proxy should run silently- any status messages or diagnostic output should be off by default.
  3. You can complete the assignment in either C or C++.
  4. Your proxy should work with both Firefox and Internet Explorer.
  5. We'll first check that your proxy works correctly with a small number of major web pages, using the same scripts that we've given you to test your proxy. If your proxy passes all of these 'public' tests, you will get 6 of the possible points.
  6. We'll then check a number of additional URLs and transactions that you will not know in advance. If your proxy passes all of these tests, you get 2 additional points. These tests will check the overall robustness of your proxy, and how you handle certain edge cases. This may include sending your proxy incorrectly formed HTTP requests, large transfers, etc.
  7. Well written (good abstraction, error checking, readability) and well commented code will get 2 additional points, for a total of 10.
  8. The first student to submit a proxy that scores a perfect 10 will get an extra credit point!

A Note on Network Programming

Writing code that will interact with other programs on the Internet is a little different than just writing something for your own use. The general guideline often given for network programs is: be lenient about what you accept, but strict about what you send, also known as Postel's Law. That is, even if a client doesn't do exactly the right thing, you should make a best effort to process their request if it is possible to easily figure out their intent. On the other hand, you should ensure that anything that you send out conforms to the published protocols as closely as possible.

Check the FAQ for more specific guidelines.

Last updated: Wed Mar 23 08:10:45 -0400 2011