MITM proxy program to forward (tee) traffic between two TCP ports

forwarder.py

import trio  # python3 -m pip install --upgrade trio
from functools import partial


async def forward_from_a_to_b(a, b):
    async for chunk in a:
        print(f"=> {chunk}", flush=True)
        await b.send_all(chunk)


async def proxy(args, listen_stream):
    target_stream = await trio.open_tcp_stream(args.forward_host, args.forward_port)
    async with trio.open_nursery() as nursery:
        nursery.start_soon(forward_from_a_to_b, listen_stream, target_stream)
        nursery.start_soon(forward_from_a_to_b, target_stream, listen_stream)


async def main(args):
    handler = partial(proxy, args)
    await trio.serve_tcp(handler, args.listen_port)


if __name__ == '__main__':
    import argparse

    parser = argparse.ArgumentParser()
    parser.add_argument('listen_port', type=int, help="Listen port")
    parser.add_argument('forward_port', type=int, help="Port traffic is forwarded to")
    parser.add_argument('--forward_host', type=str, default="127.0.0.1", help="Host to forward traffic to")
    args = parser.parse_args()

    trio.run(main, args)