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Exercise 3a: spy on requests and responses

What you need to know

We've seen that there is a web browser, which makes a request to a web server, and receives a response.

graph LR;
    Browser-- request -->Server;
    Server-- response -->Browser;
Loading

That request, and its response, travels across wires and through airwaves. Maybe it contains your credit card number, your secret crush, or your health details.

What if someone is listening in?

graph LR;
    Browser-- request -->Spy;
    Hacker --> Server
    Server-- response -->Spy;
    Hacker-->Browser;
Loading

Let's be such a hacker! We're going to use a tool called spy.py which can spy on the requests and responses as they go past.

Getting started

Make an HTML page within src/server/pages/crush.html with your secret crush, in HTML.

Running spy.py

In a previous exercise, you had both the browser and the server running at the same time. You may have had two terminals running. You will need a third for this

  • don't forget to cd if necessary in the new terminal.

So, while you still have the browser and server running, also start

python3 src/spy/spy.py 8001 8000

Spying on an HTTP request

Run the simple web browser. Navigate to http://localhost:8001/crush.html.

Note that it's 8001 not 8000!

You should see the HTTP request go past, and the response.

Here's an example request (yours will look slightly different):

GET /crush.html HTTP/1.1
Host: localhost:8000
User-Agent: python-requests/2.31.0
Accept-Encoding: gzip, deflate
Accept: */*
Connection: keep-alive

Here's an example response (again yours will be a bit different):

HTTP/1.0 200 OK
Server: SimpleHTTP/0.6 Python/3.12.2
Date: Fri, 16 Feb 2024 16:43:01 GMT
Content-type: text/html
Content-Length: 344
Last-Modified: Fri, 16 Feb 2024 16:13:49 GMT

<html>
<head>
<title>About my crush</title>
</head>
<body>
<p>My secret crush is Elsa from Frozen.</p>
</body>
</html>

This is the information flowing over the network between the browser and the server.

We're doing it on the same computer as the browser and server, but your information passes across dozens of computers on the way, and normally, any of them could spy.

Questions to answer

  • In the request, what HTTP version is being used?
  • What's the "content type" of the response from the server? (HTTP servers can supply images, video, and audio as well as HTML - you might have noticed them)

What do we do?

What's with 8000 and 8001?

These are "port numbers". Different applications on your computer might be "listening" for network connections on different "ports". Our server was listening on port 8000 - that's why it says 8000 within

http://localhost:8000/.

Our spy is listening on port 8001, and then it's passing on any connections to port 8000. So the data flows via the spy first and then onto the server.