
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()
