.. _overview: Overview ======== Mitmproxy has a powerful scripting API that allows you to control almost any aspect of traffic being proxied. In fact, much of mitmproxy's own core functionality is implemented using the exact same API exposed to scripters (see :src:`mitmproxy/addons`). A simple example ---------------- Scripting is event driven, with named handlers on the script object called at appropriate points of mitmproxy's operation. Here's a complete mitmproxy script that adds a new header to every HTTP response before it is returned to the client: .. literalinclude:: ../../examples/add_header.py :caption: :src:`examples/add_header.py` :language: python All events that deal with an HTTP request get an instance of `HTTPFlow `_, which we can use to manipulate the response itself. We can now run this script using mitmdump, and the new header will be added to all responses passing through the proxy: >>> mitmdump -s add_header.py Using classes ------------- In the example above, the script object is the ``add_header`` module itself. That is, the handlers are declared at the global level of the script. This is great for quick hacks, but soon becomes limiting as scripts become more sophisticated. When a script first starts up, the `start `_, event is called before anything else happens. You can replace the current script object by returning it from this handler. Here's how this looks when applied to the example above: .. literalinclude:: ../../examples/classes.py :caption: :src:`examples/classes.py` :language: python So here, we're using a module-level script to "boot up" into a class instance. From this point on, the module-level script is removed from the handler chain, and is replaced by the class instance. Handling arguments ------------------ Scripts can handle their own command-line arguments, just like any other Python program. Let's build on the example above to do something slightly more sophisticated - replace one value with another in all responses. Mitmproxy's `HTTPRequest `_ and `HTTPResponse `_ objects have a handy `replace `_ method that takes care of all the details for us. .. literalinclude:: ../../examples/arguments.py :caption: :src:`examples/arguments.py` :language: python We can now call this script on the command-line like this: >>> mitmdump -dd -s "./arguments.py html faketml" Whenever a handler is called, mitpmroxy rewrites the script environment so that it sees its own arguments as if it was invoked from the command-line. Logging and the context ----------------------- Scripts should not output straight to stderr or stdout. Instead, the `log `_ object on the ``ctx`` context module should be used, so that the mitmproxy host program can handle output appropriately. So, mitmdump can print colorised script output to the terminal, and mitmproxy console can place script output in the event buffer. Here's how this looks: .. literalinclude:: ../../examples/context_logging.py :caption: :src:`examples/context_logging.py` :language: python The ``ctx`` module also exposes the mitmproxy master object at ``ctx.master`` for advanced usage. Running scripts on saved flows ------------------------------ When a flow is loaded from disk, the sequence of events that the flow would have gone through on the wire is partially replayed. So, for instance, an HTTP flow loaded from disk will trigger `requestheaders `_, `request `_, `responseheaders `_ and `response `_ in order. We can use this behaviour to transform saved traffic using scripts. For example, we can invoke the replacer script from above on saved traffic as follows: >>> mitmdump -dd -s "./arguments.py html fakehtml" -r saved -w changed This command starts the ``arguments`` script, reads all the flows from ``saved`` transforming them in the process, then writes them all to ``changed``. The mitmproxy console tool provides interactive ways to run transforming scripts on flows - for instance, you can run a one-shot script on a single flow through the ``|`` (pipe) shortcut. Concurrency ----------- The mitmproxy script mechanism is single threaded, and the proxy blocks while script handlers execute. This hugely simplifies the most common case, where handlers are light-weight and the blocking doesn't have a performance impact. It's possible to implement a concurrent mechanism on top of the blocking framework, and mitmproxy includes a handy example of this that is fit for most purposes. You can use it as follows: .. literalinclude:: ../../examples/nonblocking.py :caption: :src:`examples/nonblocking.py` :language: python Developing scripts ------------------ Mitmproxy monitors scripts for modifications, and reloads them on change. When this happens, the script is shut down (the `done `_ event is called), and the new instance is started up as if the script had just been loaded (the `start `_ and `configure `_ events are called).