MWE for https://stackoverflow.com/questions/77661426/paramiko-request-port-forward-with-and-without-handler

paramiko_multiple_port_forwards_on_one_transport.py

# This script does what it's supposed to: it opens multiple tunnels with different ports on the SSH host and handles them 
# concurrently and it can react to the port that is being accessed by the client.

import threading
import time
import paramiko
import select
import queue

user = input('username')
host = input('hostname')

client = paramiko.SSHClient()
client.load_system_host_keys()
client.set_missing_host_key_policy(paramiko.WarningPolicy())

client.connect(
    host,
    22,
    username=user,
    look_for_keys=True
)


request_queue = queue.Queue()


def handler():
    while True:
        request = request_queue.get()
        if request:
            channel, _, target_address = request
            print("processing request from target address" + str(target_address))
            r, *others = select.select([channel], [], [])
            while r == [channel]:
                print('receiving.')
                r[0].recv(1024)
                r, *others = select.select([channel], [], [], 1)
            channel.send(f'Hello world {target_address[1]}.'.encode())
            channel.close()


def dispatcher(channel, source_address, target_address):
    global tunnel_channel
    print("Queueing request.")
    request_queue.put((channel, source_address, target_address))


def obsolete_dispatcher():
    raise NotImplementedError("This dispatcher will never be called!")


transport = client.get_transport()
# It's a little unexpected that the second call to request_port_forward overrides the handler.
transport.request_port_forward('', 30000, obsolete_dispatcher)
transport.request_port_forward('', 30001, dispatcher)

for _ in range(3):
    threading.Thread(target=handler).start()


while True:
    time.sleep(1)

paramiko_port_forward_using_accept.py

# Stripped down to an MWE from https://github.com/paramiko/paramiko/blob/main/demos/rforward.py
#
# This script transfers the first few bytes from the first request to the ssh client and exits; not useful,
# but works as expected.
#
# Problem with this: the port number is lost, so this can't be extended to multiple tunnels on different ports that 
# are handled differently depending on the port that is being accessed

import paramiko
import select

user = input('username')
host = input('hostname')

client = paramiko.SSHClient()
client.load_system_host_keys()
client.set_missing_host_key_policy(paramiko.WarningPolicy())

client.connect(
    host,
    22,
    username=user,
    look_for_keys=True
)


def handler(channel, source_address, target_address):
    print("Selecting...")
    selects = select.select([channel],[],[])
    print("Selected: " + str(selects))


transport = client.get_transport()
transport.request_port_forward('', 30000)
channel = transport.accept(1000)
handler(channel, None, None)

# Output:
# Selecting...
# Selected: ([<paramiko.Channel 0 (open) window=2097152 in-buffer=142 -> <paramiko.Transport at 0x262df700 (cipher aes128-ctr, 128 bits) (active; 1 open channel(s))>>], [], [])

paramiko_port_forward_with_handler.py

# This script waits for the first request and then just hangs, trying to read the first few bytes from the request.
# Expected: it should transfer the first few bytes from every request to the ssh client (and then close the connection).
#
# The problem seems to be a threading issue in paramiko; confusing because the example script shows that connections can 
# indeed be processed in threads.

import time
import paramiko
import select

user = input('username')
host = input('hostname')

client = paramiko.SSHClient()
client.load_system_host_keys()
client.set_missing_host_key_policy(paramiko.WarningPolicy())

client.connect(
    host,
    22,
    username=user,
    look_for_keys=True
)


def handler(channel, source_address, target_address):
    print("Selecting...")
    # The following line never returns.
    selects = select.select([channel],[],[])
    print("Selected: " + str(selects))


transport = client.get_transport()
transport.request_port_forward('', 30000, handler)

while True:
    time.sleep(1)

# Output:
# Selecting...

Run either of these scripts, enter an SSH host name and your user name. Then, on a different shell, try wget <hostname>:30000.
The result should be the same but isn't.

The SO question was resolved and
paramiko_multiple_port_forwards_on_one_transport.py demonstrates how we can handle multiple port forwards with just one SSH client.