UDP tunneling through TCP

You may find yourself in a situation where you want to forward some UDP traffic through a pair of TCP sockets. I had this exact need when I worked in a very limited network environment that had a problematic DNS server. Domain name lookups succeeded only after 2 or 3 attempts making web browsing a really bad experience with a lot of refreshes necessary to get all page components right. I was able, though, to connect through SSH to my university’s shell account.

SSH is able to forward TCP traffic but not UDP traffic. DNS is transported over UDP (port 53). So, I had to write a utility that tunnels UDP traffic through TCP sockets.

The setup was the following:

  • My computer was setup with localhost as a DNS server.
  • The local part of the utility listened to UDP port 53 and forwarded data to TCP localhost:7001 (arbitrary port number).
  • My SSH client was setup to forward port 7001 to localhost:7002.
  • The remote part of the utility (which was running on my shell account) listened to TCP 7002 and when it received data, it forwarded them to a reliable DNS server.

The local part of the utility is the following:

#!/usr/bin/python

import logging
import socket
import sys

def get_domain(data):
  dominio = ''
  tipo = (ord(data[2]) >> 3) & 15   # Opcode bits
  if tipo == 0:                     # Standard query
    ini=12
    lon=ord(data[ini])
    while lon != 0:
      dominio+=data[ini+1:ini+lon+1]+'.'
      ini+=lon+1
      lon=ord(data[ini])
    return dominio
  return 'unknown (tipo=%d)' % (tipo)

_log = logging.getLogger('my_logger')
_log.setLevel(logging.INFO)
#_log.setLevel(logging.DEBUG)
_log.addHandler(logging.StreamHandler())

udp_port = int(sys.argv[1])
tcp_host = sys.argv[2]
tcp_port = int(sys.argv[3])
_log.info('Listening at UDP port %d and '\
          'forwarding to TCP %s:%d' % (udp_port, tcp_host, tcp_port))

tcp_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
tcp_socket.connect((tcp_host, tcp_port))

udp_socket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
udp_socket.bind(('', udp_port))
while 1:
  try:
    buffer, addr = udp_socket.recvfrom(4096)
    _log.debug('Relaying from %s: [%s]' % (str(addr), buffer))
    _log.info('DNS query from [%s] for [%s]' % (addr[0], get_domain(buffer)))
    tcp_socket.send(buffer)
    buffer = tcp_socket.recv(4096)
    _log.debug('Got response: [%s]' % buffer)
    udp_socket.sendto(buffer, addr)
  except socket.error, e:
    _log.info('Socket error: %s' % e)

The remote part of the utility is the following:

#!/usr/bin/python

import logging
import socket
import sys

_log = logging.getLogger('my_logger')
_log.setLevel(logging.INFO)
#_log.setLevel(logging.DEBUG)
_log.addHandler(logging.StreamHandler())

tcp_port = int(sys.argv[1])
udp_host = sys.argv[2]
udp_port = int(sys.argv[3])
_log.info('Listening at TCP port %d and '\
 'forwarding to UDP %s:%d' % (tcp_port, udp_host, udp_port))

udp_socket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

tcp_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
tcp_socket.bind(('', tcp_port))
tcp_socket.listen(1)
tcp_conn, tcp_addr = tcp_socket.accept()
_log.info('Got TCP connection from %s' % (str(tcp_addr)))

while 1:
 buffer = tcp_conn.recv(4096)
 _log.debug('Relaying from %s: [%s]' % (str(tcp_addr), buffer))
 udp_socket.sendto(buffer, (udp_host, udp_port))
 buffer, _ = udp_socket.recvfrom(4096)
 _log.debug('Got response: [%s]' % (buffer))
 tcp_conn.send(buffer)

Update 31/10/2009: Apparently this is not big news. Searching for UDP tunneling through TCP in a search engine gave me back as the first result the following page: http://zarb.org/~gc/html/udp-in-ssh-tunneling.html which perfectly describes how to accomplish the same task by using netcat or socat. The only drawback is that you need a FIFO to make it work, which means that you can’t use it out of the box on Windows operating system.

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