mitmproxy/examples/tls_passthrough.py

"""
This inline script allows conditional TLS Interception based
on a user-defined strategy.

Example:

    > mitmdump -s tls_passthrough.py

    1. curl --proxy http://localhost:8080 https://example.com --insecure
    // works - we'll also see the contents in mitmproxy

    2. curl --proxy http://localhost:8080 https://example.com --insecure
    // still works - we'll also see the contents in mitmproxy

    3. curl --proxy http://localhost:8080 https://example.com
    // fails with a certificate error, which we will also see in mitmproxy

    4. curl --proxy http://localhost:8080 https://example.com
    // works again, but mitmproxy does not intercept and we do *not* see the contents

Authors: Maximilian Hils, Matthew Tuusberg
"""
from __future__ import (absolute_import, print_function, division)
import collections
import random

from enum import Enum

from libmproxy.exceptions import TlsProtocolException
from libmproxy.protocol import TlsLayer, RawTCPLayer


class InterceptionResult(Enum):
    success = True
    failure = False
    skipped = None


class _TlsStrategy(object):
    """
    Abstract base class for interception strategies.
    """
    def __init__(self):
        # A server_address -> interception results mapping
        self.history = collections.defaultdict(lambda: collections.deque(maxlen=200))

    def should_intercept(self, server_address):
        """
        Returns:
            True, if we should attempt to intercept the connection.
            False, if we want to employ pass-through instead.
        """
        raise NotImplementedError()

    def record_success(self, server_address):
        self.history[server_address].append(InterceptionResult.success)

    def record_failure(self, server_address):
        self.history[server_address].append(InterceptionResult.failure)

    def record_skipped(self, server_address):
        self.history[server_address].append(InterceptionResult.skipped)


class ConservativeStrategy(_TlsStrategy):
    """
    Conservative Interception Strategy - only intercept if there haven't been any failed attempts
    in the history.
    """

    def should_intercept(self, server_address):
        if InterceptionResult.failure in self.history[server_address]:
            return False
        return True


class ProbabilisticStrategy(_TlsStrategy):
    """
    Fixed probability that we intercept a given connection.
    """
    def __init__(self, p):
        self.p = p
        super(ProbabilisticStrategy, self).__init__()

    def should_intercept(self, server_address):
        return random.uniform(0, 1) < self.p


class TlsFeedback(TlsLayer):
    """
    Monkey-patch _establish_tls_with_client to get feedback if TLS could be established
    successfully on the client connection (which may fail due to cert pinning).
    """

    def _establish_tls_with_client(self):
        server_address = self.server_conn.address
        tls_strategy = self.script_context.tls_strategy

        try:
            super(TlsFeedback, self)._establish_tls_with_client()
        except TlsProtocolException as e:
            tls_strategy.record_failure(server_address)
            raise e
        else:
            tls_strategy.record_success(server_address)


# inline script hooks below.


def start(context, argv):
    if len(argv) == 2:
        context.tls_strategy = ProbabilisticStrategy(float(argv[1]))
    else:
        context.tls_strategy = ConservativeStrategy()


def next_layer(context, next_layer):
    """
    This hook does the actual magic - if the next layer is planned to be a TLS layer,
    we check if we want to enter pass-through mode instead.
    """
    if isinstance(next_layer, TlsLayer) and next_layer._client_tls:
        server_address = next_layer.server_conn.address

        if context.tls_strategy.should_intercept(server_address):
            # We try to intercept.
            # Monkey-Patch the layer to get feedback from the TLSLayer if interception worked.
            next_layer.__class__ = TlsFeedback
            next_layer.script_context = context
        else:
            # We don't intercept - reply with a pass-through layer and add a "skipped" entry.
            context.log("TLS passthrough for %s" % repr(next_layer.server_conn.address), "info")
            next_layer_replacement = RawTCPLayer(next_layer.ctx, logging=False)
            next_layer.reply(next_layer_replacement)
            context.tls_strategy.record_skipped(server_address)