Port forward for connections without shell access

[fullc0ntr0l]

What's the equivalent of ssh -N argument when doing local port forwarding with no shell access user?

ssh -N -L 8080:xxxxx:80 user@ssh-server

I'm trying to run the first example from here but it closes my connection

DEBUG:asyncssh:[conn=0] Trying password auth
INFO:asyncssh:[conn=0] Auth for user user succeeded
INFO:asyncssh:[conn=0] Creating local TCP forwarder from dynamic port to xxxxx, port 80
DEBUG:asyncssh:[conn=0] Assigning dynamic port 44119
INFO:asyncssh:[conn=0] Closing connection
INFO:asyncssh:[conn=0] Sending disconnect: Disconnected by application (11)
INFO:asyncssh:[conn=0] Connection closed

[fullc0ntr0l]

Actually that happens on user with shell access too. Here's the code I'm trying to execute

import asyncio
import asyncssh
import sys
import logging

logging.basicConfig(level="DEBUG")
asyncssh.set_debug_level(2)


async def run_client() -> None:
    async with asyncssh.connect(
        HOST,
        username=USERNAME,
        password=PASSWORD,
    ) as conn:
        listener = await conn.forward_local_port(
            "",
            0,
            REMOTE_ADDR,
            REMOTE_PORT,
        )
        await listener.wait_closed()


try:
    asyncio.get_event_loop().run_until_complete(run_client())
except (OSError, asyncssh.Error) as exc:
    sys.exit("SSH connection failed: " + str(exc))

[fullc0ntr0l]

That's weird, so listener.wait_closed is closing my connection whatever I'm using, local port or unix socket, and conn.wait_closed is keeping it alive. What version of python have you tested that? I'm on 3.12.0

[ronf]

Interesting. I'm running the AsyncSSH unit tests on Python 3.12 without a problem, but I am able to reproduce the problem of listener.wait_closed() not blocking like it should. but it working with conn.wait_closed(). I'll take a closer look. Both versions work fine for me on Python 3.11.

[fullc0ntr0l]

Could be related to this issue and fixed in next python version

gh-79033: Another attempt at fixing asyncio.Server.wait_closed(). It now blocks until both conditions are true: the server is closed, and there are no more active connections. (This means that in some cases where in 3.12.0 this function would incorrectly have returned immediately, it will now block; in particular, when there are no active connections but the server hasn’t been closed yet.)

[ronf]

Yes - it looks like it. I tracked it down so far to a change in Server.wait_closed() from:

        if self._sockets is None or self._waiters is None:

to:

        if self._waiters is None or self._active_count == 0:

If I add a sleep before calling wait_closed() to give time to successfully open an active socket on the listener, things work as they did before. However, with an active count of 0, it never tries to wait.

[ronf]

The following change in Python 3.12 seems to work (included in python/cpython#111336):

--- /tmp/base_events.py.orig	2023-11-04 09:16:12
+++ base_events.py	2023-11-04 09:16:37
@@ -377,7 +377,7 @@
             self._serving_forever_fut = None

     async def wait_closed(self):
-        if self._waiters is None or self._active_count == 0:
+        if self._waiters is None:
             return
         waiter = self._loop.create_future()
         self._waiters.append(waiter)