Yesterday I published version 0.4 of mitmproxy - an intercepting proxy for HTTP/S traffic. The tool already has pretty complete documentation, but I've decided to write a series of less formal tutorials to showcase its abilities. Below is the first, and simplest, of these - keep an eye on the blog for more in the coming days.

A 30-second client playback example

My local cafe is serviced by a rickety and unreliable wireless network, generously sponsored with ratepayers' money by our city council. After connecting, you are redirected to an SSL-protected page that prompts you for a username and password. Once you've entered your details, you are free to enjoy the intermittent dropouts, treacle-like speeds and incorrectly configured transparent proxy.

I tend to automate this kind of thing at the first opportunity, on the theory that time spent now will be more than made up in the long run. In this case, I might use Firebug to ferret out the form post parameters and target URL, then fire up an editor to write a little script using Python's urllib to simulate a submission. That's a lot of futzing about. With mitmproxy we can do the job in literally 30 seconds, without having to worry about any of the details. Here's how.

1. Run mitmdump to record our HTTP conversation to a file.

> mitmdump -w wireless-login

2. Point your browser at the mitmdump instance.

I use a tiny Firefox addon called Toggle Proxy to switch quickly to and from mitmproxy. I'm assuming you've already configured your browser with mitmproxy's SSL certificate authority.

3. Log in as usual.

And that's it! You now have a serialized version of the login process in the file wireless-login, and you can replay it at any time like this:

> mitmdump -c wireless-login

Embellishments

We're really done at this point, but there are a couple of embellishments we could make if we wanted. I use wicd to automatically join wireless networks I frequent, and it lets me specify a command to run after connecting. I used the client replay command above and voila! - totally hands-free wireless network startup.

We might also want to prune requests that download CSS, JS, images and so forth. These add only a few moments to the time it takes to replay, but they're not really needed and I somehow feel compelled trim them anyway. So, we fire up the mitmproxy console tool on our serialized conversation, like so:

> mitmproxy wireless-login

We can now go through and manually delete (using the d keyboard shortcut) everything we want to trim. When we're done, we use S to save the conversation back to the file.