Connection refused when setting port to zero and inside a docker container

I am trying to develop a test email server that should run in a docker container. The docker container is run with --network=host -P; though for unit tests I would like to use --network=none.

I am deliberately specifying that port 0 should be used (let the OS pick an available port). I then have a function to get the port that was actually used. The server code is

#! /usr/bin/env python3

import asyncio #pylint: disable=unused-import
from aiosmtpd.controller import Controller

class DemoEmailHandler:
    async def handle_DATA(self, server, session, envelope):
        message = {
            'peer' : session.peer,
            'mailfrom' : envelope.mail_from,
            'to': envelope.rcpt_tos,
            'data' : envelope.content}
        print(message)
        return '250 Message accepted for delivery'


class DemoEmailServer:
    def __init__(self):
        self._handler = DemoEmailHandler()
        self._server = Controller(
            handler=self._handler,
            hostname="0.0.0.0",
            port=0
        )
        self._server.start()
        self._port = self._server.server.sockets[0].getsockname()[1]

    def stop(self):
        """Tell the server to stop"""
        self._server.stop()

    def getPort(self) -> int:
        """Return the port the SMTP (email) server is actually on"""
        return self._port

SERVER = DemoEmailServer()
print(SERVER.getPort())
SERVER.stop()

I keep getting connection refused

Traceback (most recent call last):
  File "./demo_email_server.py", line 36, in <module>
    SERVER = DemoEmailServer()
  File "./demo_email_server.py", line 25, in __init__
    self._server.start()
  File "/usr/local/lib/python3.8/dist-packages/aiosmtpd-1.4.2-py3.8.egg/aiosmtpd/controller.py", line 223, in start
    self._trigger_server()
  File "/usr/local/lib/python3.8/dist-packages/aiosmtpd-1.4.2-py3.8.egg/aiosmtpd/controller.py", line 313, in _trigger_server
    s = stk.enter_context(create_connection((hostname, self.port), 1.0))
  File "/usr/lib/python3.8/socket.py", line 808, in create_connection
    raise err
  File "/usr/lib/python3.8/socket.py", line 796, in create_connection
    sock.connect(sa)
ConnectionRefusedError: [Errno 111] Connection refused

when running inside a docker container. If I run on the host then the server starts up fine.

(My host and container are both Ubuntu 20.04, python 3.8.5, the container has aiosmtpd 1.4.2 installed from source).

[waynew]

Okay, so… I know it’s been a very long time since I’ve touched this, but - yeah, the fix is pretty straightforward, I think.

First off, yeah I was able to reproduce this.

Second off, I think the correct thing for us to do here is in the _create_server function where we do self.loop.create_server we could change it to:

server = self.loop.create_server(...)
if not self.port:
    self.port = server.sockets[0].getsockname()[1]
return server

By making that change locally, it all worked 👍 We’ll want some test cases for that obviously, but it shouldn’t be too hard to do!

[KGHVerilator]

Hopefully this will work out; thank you for look into it.