Completely faking connection to server in script

Is your feature request related to a problem? Please describe. I haven’t found a good way to completely intercept connections (and all requests) to a server from a script.

Describe the solution you’d like

Perhaps there should be a FakeLayer or some other mechanism which would simulate the procedure of connecting to an upstream server, without actually doing so.

Describe alternatives you’ve considered

Here is the best solution I found so far:

from mitmproxy import http
from mitmproxy.proxy.protocol import TlsLayer

# The server to which we wish to intercept connections.
# When the hostname doesn't exist, this becomes a hard requirement.
_redirect_source = 'nonexisting.example.net'

# The server we will be redirecting the connection to.
# Can be any server which listens on the same ports as _redirect_source.
_redirect_target = 'www.google.com'


# Change the server address at connection time.
def serverconnect(cc):
    if cc.address[0] == _redirect_source:
        cc._my_real_address = cc.address  # Save original address
        cc.address = (_redirect_target, cc.address[1])
        return True


# For TLS, we must also patch the SNI hostname.
# Otherwise, certificate validation will fail.
def next_layer(next_layer):
    if isinstance(next_layer, TlsLayer):
        if next_layer.server_conn.address[0] == _redirect_source:
            next_layer._custom_server_sni = _redirect_target


def request(flow: http.HTTPFlow) -> None:
    # Now, undo the hostname patch in serverconnect,
    # so that mitmproxy's UI shows the right URL.
    try:
        flow.server_conn.address = flow.server_conn._my_real_address
        flow.request.host = flow.server_conn.address[0]
        flow.request.port = flow.server_conn.address[1]
    except AttributeError:
        pass
    # Now send our fake response
    flow.response = ...

Additional context

The above workaround is not perfect as it still does require a target server to redirect the connection to. (Using a local server poses difficulties as it would have to have a valid SSL certificate for the target redirect domain name.)

[mhils]

Yes, setting connection_strategy to lazy is exactly what should be done here. Thanks for the heads-up, @yufengzjj! 🍰

[petromoldovan]

@yufengzjj thanks! i will try that!