SSH forward and reverse tunneling

Abdul Wahab Junaid

3 years ago

SSH (Secure Shell) tunneling is a technique that allows you to securely forward network traffic from one networked device to another. There are two main types of SSH tunneling: forward tunneling and reverse tunneling.

SSH Forward Tunneling:
- Description:
  - Forward tunneling is the more common type of SSH tunneling. It allows you to establish a secure connection from a local machine to a remote machine, forwarding specific ports or services through the encrypted SSH connection.
- Use Cases:
  - Remote Access:
    - Accessing a service (e.g., a web server) on a remote machine securely.
  - Bypassing Firewalls:
    - Accessing services behind firewalls or network restrictions.
  - Secure Data Transfer:
    - Transmitting data securely over an untrusted network.
- Example:
  - Suppose you have a web server running on a remote machine, and you want to access it securely from your local machine. You can use the following command to create a forward tunnel:
```
ssh -L local_port:remote_server:remote_port user@remote_server
```
    This forwards traffic from local_port on your local machine to remote_port on the remote server.
SSH Reverse Tunneling:
- Description:
  - Reverse tunneling is used to create a secure connection from a remote machine to a local machine, allowing the remote machine to access services on the local machine.
- Use Cases:
  - Remote Support:
    - Allowing remote support personnel to access services on a local machine securely.
  - Exposing Local Services:
    - Making services on a local machine accessible from the internet securely.
- Example:
  - Suppose you have a service on your local machine that you want someone on a remote machine to access securely. You can use the following command to create a reverse tunnel:
```
ssh -R remote_port:local_machine:local_port user@local_machine
```
    This forwards traffic from remote_port on the remote machine to local_port on your local machine.

In both forward and reverse tunneling, the SSH protocol is used to create a secure, encrypted connection between the two machines, providing a secure way to transmit data over potentially insecure networks. These techniques are valuable for various scenarios, including remote access, bypassing network restrictions, and securing data transfer.

Python

#!/usr/bin/env python

"""
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 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] [:]",
        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()

This Python script demonstrates how to perform reverse port forwarding over SSH using the Paramiko library. Reverse port forwarding allows a remote machine to access services on the local machine through an SSH tunnel.

Explanation:

Importing Libraries:
- The script imports necessary modules, including socket, select, sys, threading, and paramiko.
Constants:
- The script defines some constants, such as the default SSH port (22) and a default port for the reverse tunnel (4000).
Global Variable:
- g_verbose is a global variable used to control verbose output. If set to True, it prints informative messages.
Handler Function:
- The handler function is responsible for managing the data transfer between the SSH channel and the local/remote sockets.
Reverse Forward Tunnel Function:
- The reverse_forward_tunnel function sets up the reverse forwarding tunnel. It requests port forwarding from the remote server and continuously accepts incoming connections to the specified port.
Verbose Function:
- The verbose function prints messages if the global variable g_verbose is set to True.
Help Message:
- The HELP string provides a brief description of the script’s purpose.
Helper Functions:
- get_host_port: Parses a string in the format “hostname:port” into host and port components.
- parse_options: Parses command-line options using the OptionParser class.
Main Function:
- The main function is the entry point of the script. It parses command-line options, establishes an SSH connection to the specified server, and initiates the reverse forwarding tunnel.
Script Execution:
- The script checks if it’s being run directly (if __name__ == "__main__":) and calls the main function.

This script provides a command-line interface for setting up reverse port forwarding over SSH, allowing a remote machine to access local services securely. Users can specify the SSH server, remote host and port, and other parameters through command-line options. The Paramiko library is used for SSH-related functionalities.