CSE 461: Introduction to Computer Communication Networks, Winter 2018
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    Project 3: HTTP Proxy
Out: Monday, February 12th
Due: Monday, March 5th, 11:59 PM
Turnin: Online
Teams: Teams of 3

Project Overview.

In this project, you will write an HTTP proxy that passes requests and data between multiple web clients and web servers, concurrently. The HTTP proxy is capable of both relaying HTTP requests and HTTP CONNECT tunneling. You'll point a browser at your proxy, so that it sends all page requests to your code instead of directly to the page's origin server.

  1. For non-CONNECT HTTP requests, you'll slightly edit the HTTP request header and send it and any payload the request might carry to the origin server, and then slightly edit the HTTP response header and send it and any response payload back to the browser.
  2. For CONNECT HTTP requests, you'll establish a TCP connection to the server named in the request, send an HTTP success response to the browser, and then simply pass through any data sent by the browser or the remote server to the other end of the communication.
  3. Your proxy should be capable of handling the traffic caused by real user browsing. A small portion of that traffic is generated by the user's typing URLs or clicking on links. Much of the traffic is caused by the contents of the pages the user has asked for - both elements embedded in those pages (e.g., images) and Javascript loaded with it can result in many additional HTTP transactions.


Hypertext Transfer Protocol (HTTP)

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:

  • An initial line (a request or response line, as defined below)
  • Zero or more header lines
  • A blank line (CRLF)
  • An optional message body.

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):

  • A client creates a connection to the server.
  • 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)
  • 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).
  • 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 Linux prompt, type:

telnet www.washington.edu 80

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

Connected to www.washington.edu (
Escape character is '^]'.

type the following:

GET http://www.washington.edu/ HTTP/1.0

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

HTTP/1.1 200 OK
Date: Mon, 19 Oct 2015 22:30:09 GMT
Server: Apache/2.2.24 (Unix) mod_ssl/2.2.24 OpenSSL/1.0.1e-fips PHP/5.6.11 mod_pubcookie/3.3.4a mod_uwa/3.2.1
Last-Modified: Mon, 19 Oct 2015 22:26:09 GMT
ETag: "6e7597-a941-5227ca1c6c240"
Accept-Ranges: bytes
Content-Length: 43329
Connection: close
Content-Type: text/html

 UW Homepage 
(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 washington 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 Proxy

HTTP is the protocol used to transfer information between browsers and web servers. HTTP is transmitted using TCP as the transport protocol.

An HTTP proxy is a program that can accept and reply to requests that would normally be directed to some web server. Proxies are an example of the use of "interposition" - placing something between two things that communicate using a well-defined interface - as shown in the figure below. Interposition is a generally useful technique. When possible, it allows new functionality to be injected into existing code with little or no modification to that code. For example, an HTTP proxy might be used for monitoring or debugging (by capturing a log of browser requests and server responses), to improve performance by maintaining a cache of web pages, or to enforce some policy about which sites can be accessed.

Assignment Details.

Non-CONNECT HTTP requests.

The requirements for our proxy are very modest: it merely prints out (at least the initial portion of) the first line of each HTTP request it receives from the browser, then fetches the requested page from the origin web server and returns it to the browser. This means that, for the most part, you don't have to know anything about HTTP; you simply read what the browser sends, print out (only) the first line, and pass that and all subsequent lines on to the web server. On the other side, you read everything the web server sends and pass it back to the browser. You keep forwarding data in this way, in each direction, until you detect that the source has closed the connection.

While that's the basic operation, there are two details that require a bit of processing of the HTTP stream. To make what follows more concrete, here's an example of what Firefox sent when I requested the page www.my.example.page.com. (I obtained this by running nc -l 46103 to set it listening for TCP connections on port 46103, and then configuring Firefox to use a proxy located at localhost:46103.)

$ nc -l 46103
GET http://www.my.example.page.com/ HTTP/1.1
Host: www.my.example.page.com
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:26.0) Gecko/20100101 Firefox/26.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
Determining the web server's address

When the browser sends an HTTP request to your proxy, you need to forward it on to the origin web server. You determine the web server by recognizing the Host line in the HTTP header. In the example above, the host is www.my.example.page.com. You should be insensitive to the case of the keyword Host, and you should be tolerant of white space anywhere it might plausibly appear. In general, the host name may be given as hostname:port or ip:port. If no port is specified, you should look for one in the URI given on the request line (the first line of the header). If there is no port there either, you should use 80 if the transport on the request line is either missing or is (case-insensitive) 'http://' and 443 if the transport is 'https://'.

The HTTP specification says that lines of the header are terminated by CRLF:

       CR             = <US-ASCII CR, carriage return (13)>
       LF             = <US-ASCII LF, linefeed (10)>

You should be lenient in interpreting this, though. For instance, you might see headers where the lines are terminated by a single LF.

HTTP does not require any particular ordering for the lines of the header, except that the request line (which is always of the general form shown in the example above) must be first.

The HTTP 1.1 specification requires that a Host line be provided in an HTTP request (but not in a reply). Your code does not have to work with HTTP 1.0, which doesn't require these lines. (But, I'd guess you'd have a hard time finding a browser that wanted to speak HTTP 1.0 in any case.)

Turning off keep-alive

The HTTP Connection: keep-alive line can be used to indicate that the browser (or server) wants to keep the TCP connection open even after the current HTTP request has been fully satisfied. This is a performance optimization: if the browser issues additional requests to the same server within a short time, the overhead of closing the current TCP connection and opening a new one is avoided.

Supporting keep-alive greatly complicates the proxy, because it needs to do enough HTTP parsing to understand where one HTTP request ends and the subsequent one begins (and similarly for responses coming from the server). HTTP doesn't have a simple framing mechanism for marking these boundaries. To avoid that, you should filter the request and response streams, removing any Connection: keep-alive, inserting a Connection: close, and converting any Proxy-connection: keep-alive to Proxy-connection: close. That should cause the browser and web server to close the TCP connection after each request. Each HTTP request now starts with the creation of a new TCP connection and ends with TCP close, making things simpler for the proxy.

The Tranformed Request Header

The final change we make is to lower the HTTP version of the request to HTTP 1.0. This is probably unnecessary, but the more discouragement to using persistent connections we can provide the better.

With that change, the header sent on to www.my.example.page.co is this:

$ nc -l 46103
GET http://www.my.example.page.com/ HTTP/1.0
Host: www.my.example.page.com
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:26.0) Gecko/20100101 Firefox/26.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: close


The HTTP request method CONNECT is used to establish a two-hop TCP connection between the client and some server. HTTP is used only to convey the CONNECT request between the client and the proxy, and to convey a success/failure response from the proxy back to the client. When the proxy receives the request, it determines the destination server (using the technique described above) and tries to open a TCP connection to it. If it succeeds, it returns an HTTP 200 OK response to the client. If the proxy fails to resolve the server address or connect to the server, it sends an HTTP 502 Bad Gateway response to the client and closes the connection.

At this point, nothing has yet been sent to the server, all that's happened is that a TCP connection has been established with it (in the success case). None of the HTTP request headers are ever sent to the server. Instead, the proxy simply forwards to the server any bytes it receives after the request header on its connection with the client, and forwards to the client any bytes it receives on its connection with the server. The client may send anything at all it wants on that TCP connection - it could be HTTP messages, or it could be something else completely. HTTPS uses this technique to allow TLS to negotiate a session key between the client and the server. The proxy is simply a conduit for a binary data exchange, and the client and server exchange the same messages over the tunnel as they would over a direct TCP connection with each other.

Solution Restrictions
As usual, our goal isn't to have dozens of HTTP proxy implementations, but rather to provide you with a reasonably specific development experience. For that reason, your implementation must conform to these restrictions:
  • We'd like you to build your proxy directly on TCP sockets. The language (or libraries available for the language) you use may offer you much higher level functionality - some form of HTTP server is often available, for instance - but you should not use it.
  • Your code may buffer entire HTTP headers, in either direction, before sending any portion of the (edited) header on to its destination, but you must not try to buffer the entire request or response. This means your code must stream at least the payload portion of the request and response - send it on as you receive it, rather than accumulate it until you have it all.
  • You are free to use any implementation approach you'd like (e.g., threads or event-loop). However, your implementation must be sufficiently concurrent that the handling of any single client request cannot substantially delay the handling of other, concurrent requests. (Additionally, it would be nice if your proxy didn't completely collapse if the face of a temporary, very high request rate, but that isn't required.)


We show here some sample output. The output is basically a trace of the HTTP request methods and URIs issued by the browser when fetching some page. It is very likely that two requests for the same page will result in different request streams. For one thing, the order of the requests is somewhat random. For another, the components of the page, and so the things fetched, can vary from one page fetch to another. On the other hand, some things must appear in each request trace, for instance, the request for the page itself. The result of this is that it's hard to say exactly what part of these traces your output must include.

The output follows a format that your code also must follow: each HTTP request line output must be preceded by '>>> ' (and your code should print that only for such output, except in the odd case that you're printing some data and the data includes it). Note the trailing space after the '>>>' characters, before the HTTP request line starts. You must print at least the HTTP method and URI given on the request line, but you can also print the entire request line (which additionally includes the HTTP version) if that's easier. The sample output prints only the method and URI.

Finally, you may print anything you want before the '>>> ' tag, and you may print any additional lines you want so long as they don't contain the '>>> ' tag. For example, you may want to print error messages, or even debugging information.

simple.txt Sample Output
simple.html Sample Output
Note that this is moving binary data.
Sample Output
CSE Winter Quarter 2015 Time Schedule Sample Output
Note that you get an unexpected error response, at least when using Firefox. That's expected.
Sample Output
www.whitehouse.gov Sample Output
Output is truncated to the first approximately 50 seconds.
Sample Output
http://www.google.com Sample Output

Configuring Firefox
To use the proxy we must configure the browser to send all its requests to the proxy, instead of directly to the web servers. In Firefox you do this using Preferences, then the Advanced icon, then the Network tab, then the Settings button for "Connection." Configure the proxy manually, giving the host and port your proxy is running on. You should, eventually, allow all types of traffic to pass through your proxy (although we care only about HTTP and HTTPS (SSL)), so you should check the "Use this proxy server for all protocols" check box. It might be easier during initial test to leave it unchecked, though, in which case your proxy will see only http:// requests.


run Script

To help us test your code, provide a run script that will build and invoke your proxy. The script should take a single argument: the port number the proxy's server socket should bind to, for example:

$ ./run 1234

Execution is terminated by end-of-file on stdin or by ctrl-C.


When you're ready to turn in your assignment, do the following:
  1. The files you submit should be placed in a directory named project3. There should be no other files in that directory.
  2. Create a README.txt file that contains the names and UW netid of the member(s) of your team.
  3. The readme must also contain all the instructions necessary to compile and run your code.
  4. Put the README.txt file, your HTTP proxy solution source code, and your run script in the project3 directory.
  5. Archive all the materials (project3 folder and everything in it) in a single .tar file named partner1netid_partner2netid.tar.
  6. Submit the partner1netid_partner2netid.tar file to the course dropbox.

Computer Science & Engineering
University of Washington
Box 352350
Seattle, WA  98195-2350
(206) 543-1695 voice, (206) 543-2969 fax
[comments to kheimerl at cs.washington.edu]