This Python script demonstrates how to set up a reverse SSH tunnel using the paramiko library. A reverse SSH tunnel allows you to forward a port on a remote SSH server back to a local machine, enabling access to services on the local machine from the remote server. This is similar to the -R option in OpenSSH.

Code

Imports and Constants

import getpass  # For securely reading passwords
import os  # For OS-related functions
import socket  # For creating and managing network sockets
import select  # For I/O multiplexing
import sys  # For system-specific parameters and functions
import threading  # For handling threading
from optparse import OptionParser  # For parsing command-line options

import paramiko  # For implementing SSH functionality

SSH_PORT = 22  # Default SSH port
DEFAULT_PORT = 4000  # Default port for remote forwarding

g_verbose = True  # Global variable to control verbose output

Helper Functions

handler(chan, host, port):

Handles data transfer between the SSH channel and the remote host.
Establishes a connection to the remote host and forwards data bidirectionally.

def handler(chan, host, port):
    sock = socket.socket()
    try:
        sock.connect((host, port))  # Connect to the remote host
    except Exception as e:
        verbose("Forwarding request to %s:%d failed: %r" % (host, port, e))
        return

    verbose(
        "Connected!  Tunnel open %r -> %r -> %r"
        % (chan.origin_addr, chan.getpeername(), (host, port))
    )
    while True:
        r, w, x = select.select([sock, chan], [], [])  # Wait for data
        if sock in r:
            data = sock.recv(1024)  # Receive data from the remote host
            if len(data) == 0:
                break
            chan.send(data)  # Send data to the SSH channel
        if chan in r:
            data = chan.recv(1024)  # Receive data from the SSH channel
            if len(data) == 0:
                break
            sock.send(data)  # Send data to the remote host
    chan.close()
    sock.close()
    verbose("Tunnel closed from %r" % (chan.origin_addr,))

reverse_forward_tunnel(server_port, remote_host, remote_port, transport):

Sets up a reverse port forwarding tunnel.
Listens for incoming connections on the specified server port and forwards them to the remote host.

def reverse_forward_tunnel(server_port, remote_host, remote_port, transport):
    transport.request_port_forward("", server_port)  # Request port forwarding
    while True:
        chan = transport.accept(1000)  # Accept incoming connections
        if chan is None:
            continue
        thr = threading.Thread(
            target=handler, args=(chan, remote_host, remote_port)
        )  # Start a new thread to handle the connection
        thr.setDaemon(True)
        thr.start()

verbose(s):

Prints verbose output if g_verbose is True.

def verbose(s):
    if g_verbose:
        print(s)

get_host_port(spec, default_port):

Parses a string in the format host:port into a host and port.

def get_host_port(spec, default_port):
    args = (spec.split(":", 1) + [default_port])[:2]
    args[1] = int(args[1])
    return args[0], args[1]

Command-Line Options

The script uses OptionParser to parse command-line arguments.

def parse_options():
    global g_verbose

    parser = OptionParser(
        usage="usage: %prog [options] <ssh-server>[:<server-port>]",
        version="%prog 1.0",
        description=HELP,
    )
    parser.add_option(
        "-q",
        "--quiet",
        action="store_false",
        dest="verbose",
        default=True,
        help="squelch all informational output",
    )
    parser.add_option(
        "-p",
        "--remote-port",
        action="store",
        type="int",
        dest="port",
        default=DEFAULT_PORT,
        help="port on server to forward (default: %d)" % DEFAULT_PORT,
    )
    parser.add_option(
        "-u",
        "--user",
        action="store",
        type="string",
        dest="user",
        default=getpass.getuser(),
        help="username for SSH authentication (default: %s)"
        % getpass.getuser(),
    )
    parser.add_option(
        "-K",
        "--key",
        action="store",
        type="string",
        dest="keyfile",
        default=None,
        help="private key file to use for SSH authentication",
    )
    parser.add_option(
        "",
        "--no-key",
        action="store_false",
        dest="look_for_keys",
        default=True,
        help="don't look for or use a private key file",
    )
    parser.add_option(
        "-P",
        "--password",
        action="store_true",
        dest="readpass",
        default=False,
        help="read password (for key or password auth) from stdin",
    )
    parser.add_option(
        "-r",
        "--remote",
        action="store",
        type="string",
        dest="remote",
        default=None,
        metavar="host:port",
        help="remote host and port to forward to",
    )
    options, args = parser.parse_args()

    if len(args) != 1:
        parser.error("Incorrect number of arguments.")
    if options.remote is None:
        parser.error("Remote address required (-r).")

    g_verbose = options.verbose
    server_host, server_port = get_host_port(args[0], SSH_PORT)
    remote_host, remote_port = get_host_port(options.remote, SSH_PORT)
    return options, (server_host, server_port), (remote_host, remote_port)

Main Function

The main() function sets up the SSH connection and starts the reverse tunnel.

def main():
    options, server, remote = parse_options()

    password = None
    if options.readpass:
        password = getpass.getpass("Enter SSH password: ")

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

    verbose("Connecting to ssh host %s:%d ..." % (server[0], server[1]))
    try:
        client.connect(
            server[0],
            server[1],
            username=options.user,
            key_filename=options.keyfile,
            look_for_keys=options.look_for_keys,
            password=password,
        )
    except Exception as e:
        print("*** Failed to connect to %s:%d: %r" % (server[0], server[1], e))
        sys.exit(1)

    verbose(
        "Now forwarding remote port %d to %s:%d ..."
        % (options.port, remote[0], remote[1])
    )

    try:
        reverse_forward_tunnel(
            options.port, remote[0], remote[1], client.get_transport()
        )
    except KeyboardInterrupt:
        print("C-c: Port forwarding stopped.")
        sys.exit(0)

How to Run the Code

Install Dependencies:
- Install the paramiko library using pip:
```
pip install paramiko
```

Run the Script:

Save the script to a file, e.g., reverse_ssh_tunnel.py.

Run the script with the required arguments:

python reverse_ssh_tunnel.py -p <remote_port> -r <remote_host:remote_port> <ssh_server>

Example:

python reverse_ssh_tunnel.py -p 4000 -r localhost:8080 user@ssh.example.com

Connect to the Tunnel:
- The script will forward traffic from ssh.example.com:4000 to localhost:8080.

Notes

This script is for educational purposes and should not be used in production environments without proper security measures.
Ensure the SSH server allows remote port forwarding.
Use strong passwords or key-based authentication for SSH connections.
The script handles keyboard interrupts (Ctrl+C) gracefully.

#!/usr/bin/env python

# Copyright (C) 2008  Robey Pointer <robeypointer@gmail.com>
#
# This file is part of paramiko.
#
# Paramiko is free software; you can redistribute it and/or modify it under the
# terms of the GNU Lesser General Public License as published by the Free
# Software Foundation; either version 2.1 of the License, or (at your option)
# any later version.
#
# Paramiko is distributed in the hope that it will be useful, but WITHOUT ANY
# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
# A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more
# details.
#
# You should have received a copy of the GNU Lesser General Public License
# along with Paramiko; if not, write to the Free Software Foundation, Inc.,
# 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA.

"""
Sample script showing how to do remote port forwarding over paramiko.

This script connects to the requested SSH server and sets up remote port
forwarding (the openssh -R option) from a remote port through a tunneled
connection to a destination reachable from the local machine.
"""

import getpass
import os
import socket
import select
import sys
import threading
from optparse import OptionParser

import paramiko

SSH_PORT = 22
DEFAULT_PORT = 4000

g_verbose = True


def handler(chan, host, port):
    sock = socket.socket()
    try:
        sock.connect((host, port))
    except Exception as e:
        verbose("Forwarding request to %s:%d failed: %r" % (host, port, e))
        return

    verbose(
        "Connected!  Tunnel open %r -> %r -> %r"
        % (chan.origin_addr, chan.getpeername(), (host, port))
    )
    while True:
        r, w, x = select.select([sock, chan], [], [])
        if sock in r:
            data = sock.recv(1024)
            if len(data) == 0:
                break
            chan.send(data)
        if chan in r:
            data = chan.recv(1024)
            if len(data) == 0:
                break
            sock.send(data)
    chan.close()
    sock.close()
    verbose("Tunnel closed from %r" % (chan.origin_addr,))


def reverse_forward_tunnel(server_port, remote_host, remote_port, transport):
    transport.request_port_forward("", server_port)
    while True:
        chan = transport.accept(1000)
        if chan is None:
            continue
        thr = threading.Thread(
            target=handler, args=(chan, remote_host, remote_port)
        )
        thr.setDaemon(True)
        thr.start()


def verbose(s):
    if g_verbose:
        print(s)


HELP = """\
Set up a reverse forwarding tunnel across an SSH server, using paramiko. A
port on the SSH server (given with -p) is forwarded across an SSH session
back to the local machine, and out to a remote site reachable from this
network. This is similar to the openssh -R option.
"""


def get_host_port(spec, default_port):
    "parse 'hostname:22' into a host and port, with the port optional"
    args = (spec.split(":", 1) + [default_port])[:2]
    args[1] = int(args[1])
    return args[0], args[1]


def parse_options():
    global g_verbose

    parser = OptionParser(
        usage="usage: %prog [options] <ssh-server>[:<server-port>]",
        version="%prog 1.0",
        description=HELP,
    )
    parser.add_option(
        "-q",
        "--quiet",
        action="store_false",
        dest="verbose",
        default=True,
        help="squelch all informational output",
    )
    parser.add_option(
        "-p",
        "--remote-port",
        action="store",
        type="int",
        dest="port",
        default=DEFAULT_PORT,
        help="port on server to forward (default: %d)" % DEFAULT_PORT,
    )
    parser.add_option(
        "-u",
        "--user",
        action="store",
        type="string",
        dest="user",
        default=getpass.getuser(),
        help="username for SSH authentication (default: %s)"
        % getpass.getuser(),
    )
    parser.add_option(
        "-K",
        "--key",
        action="store",
        type="string",
        dest="keyfile",
        default=None,
        help="private key file to use for SSH authentication",
    )
    parser.add_option(
        "",
        "--no-key",
        action="store_false",
        dest="look_for_keys",
        default=True,
        help="don't look for or use a private key file",
    )
    parser.add_option(
        "-P",
        "--password",
        action="store_true",
        dest="readpass",
        default=False,
        help="read password (for key or password auth) from stdin",
    )
    parser.add_option(
        "-r",
        "--remote",
        action="store",
        type="string",
        dest="remote",
        default=None,
        metavar="host:port",
        help="remote host and port to forward to",
    )
    options, args = parser.parse_args()

    if len(args) != 1:
        parser.error("Incorrect number of arguments.")
    if options.remote is None:
        parser.error("Remote address required (-r).")

    g_verbose = options.verbose
    server_host, server_port = get_host_port(args[0], SSH_PORT)
    remote_host, remote_port = get_host_port(options.remote, SSH_PORT)
    return options, (server_host, server_port), (remote_host, remote_port)


def main():
    options, server, remote = parse_options()

    password = None
    if options.readpass:
        password = getpass.getpass("Enter SSH password: ")

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

    verbose("Connecting to ssh host %s:%d ..." % (server[0], server[1]))
    try:
        client.connect(
            server[0],
            server[1],
            username=options.user,
            key_filename=options.keyfile,
            look_for_keys=options.look_for_keys,
            password=password,
        )
    except Exception as e:
        print("*** Failed to connect to %s:%d: %r" % (server[0], server[1], e))
        sys.exit(1)

    verbose(
        "Now forwarding remote port %d to %s:%d ..."
        % (options.port, remote[0], remote[1])
    )

    try:
        reverse_forward_tunnel(
            options.port, remote[0], remote[1], client.get_transport()
        )
    except KeyboardInterrupt:
        print("C-c: Port forwarding stopped.")
        sys.exit(0)


if __name__ == "__main__":
    main()

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