Name

nat-traverse – Use of UDP to traverse NAT gateways

Synopsis

To create a simple text-only tunnel, use the commands

user@left  $ nat-traverse 40000:natgw-of-right:40001
user@right $ nat-traverse 40001:natgw-of-left:40000

where 40000 is an unused UDP port on left and 40001 is an unused UDP port on right.

Description

Screenshot of nat-traverse in action Screenshot of nat-traverse in action

nat-traverse establishes connections between nodes which are behind NAT gateways, i.e. hosts which do not have public IP addresses. Additionally, you can setup a small VPN by using pppd on top of nat-traverse. nat-traverse does not need an external server on the Internet, and it isn't necessary to reconfigure the involved NAT gateways, either. nat-traverse works out-of-the-box.

Network diagram Example network architecture nat-traversal can work with

See below for how this is achieved.

In other words: nat-traverse is a bit like Harm, but doesn't have Harm's limitation that one peer has to have a public IP address.

Limitation: nat-traverse does not work with gateways which change the port numbers. This is a fundamental problem of nat-traverse's design, as the changed port numbers are (in general) not predictable.

Download

nat-traverse is a Perl 5 program using only core modules, so neither compilation nor installation of Perl modules is necessary. You do need a copy of Perl (>= 5.6.1), though.

Simply grab nat-traverse-0.6.pl, rename it to nat-traverse, and flag it executable by using chmod. Alternatively, you can download nat-traverse packed in a bz2-compressed tarball.

Note to Debian users: The great grml project provides a Debian packet of nat-traverse (but not the newest version).

Options

local_port:peer:remote_port (required)

Sets the local port to use and the remote address to connect to.

Note that you have to give the IP address or hostname of the NAT gateway of the host you want to connect to, as the target host doesn't have a public IP address.

--cmd="pppd..."

Runs the specified command after establishing the connection.

The command will be run with its STDIN and STDOUT bound to the socket, i.e. everything the command writes to STDOUT will be forwarded to the peer.

If no command is specified, nat-traverse will relay input from STDIN to the peer and vice versa, i.e. nat-traverse degrades to netcat.

--window=10

Sets the number of initial garbage packets to send. The default, 10, should work with most firewalls.

--timeout=10

Sets the maximum number of seconds to wait for an acknowledgement by the peer.

--quit-after-connect

Quits nat-traverse after the tunnel has been established successfully.

nat-traverse returns a non-0 statuscode to indicate that it wasn't able to establish a tunnel.

--quit-after-connect is useful if you want another program to use the tunnel. For example, you could configure OpenVPN to use the the same ports as nat-traverse -- thus OpenVPN would be able to cross NAT gateways.

--version, --help

Technique

nat-traverse establishes connections between hosts behind NAT gateways without need for reconfiguration of the involved NAT gateways.

  1. Firstly, nat-traverse on host left sends garbage UDP packets to the NAT gateway of right. These packets are, of course, discarded by the firewall.

  2. Then right's nat-traverse sends garbage UDP packets to the NAT gateway of left. These packets are not discarded, as left's NAT gateway thinks these packets are replies to the packets sent in step 1!

  3. left's nat-traverse continues to send garbage packets to right's NAT gateway. These packets are now not dropped either, as the NAT gateway thinks the packets are replies to the packets sent in step 2.

  4. Finally, both hosts send an acknowledgement packet to signal readiness. When these packets are received, the connection is established and nat-traverse can either relay STDIN/STDOUT to the socket or execute a program.

Examples

Setup of a small VPN with PPP

It's easy to setup a small VPN (Virtual Private Network) by using the Point-to-Point Protocol Daemon, pppd:

  root@left # nat-traverse \
      --cmd="pppd updetach noauth passive notty \
             ipparam vpn 10.0.0.1:10.0.0.2"
      40000:natgw-of-right:40001
  root@right # nat-traverse \
      --cmd="pppd nodetach notty noauth"
      40001:natgw-of-left:40000
pppd running on top of nat-traverse pppd running on top of nat-traverse

pppd creates a new interface, typically ppp0. Using this interface, you can ping 10.0.0.1 or 10.0.0.2. As you can see, pppd upgrades the data-only tunnel nat-traverse provides to a full IP tunnel. Thus you can establish reliable TCP connections over the tunnel, even though nat-traverse uses UDP! Furthermore, you could even add IPv6 addresses to ppp0 by running ip -6 addr add...!

Note though that although this VPN is arguably a private network, it is not secured in any way. You may want to use SSH to encrypt the connection.

Port Forwarding with netcat

You can use netcat to forward one of your local UDP or TCP ports to an arbitrary UDP or TCP port of the remote host, similar to ssh -L or ssh -R:

  user@left  $ nat-traverse 10001:natgw-of-right:10002 \
      --cmd="nc -vl 20000"
  user@right $ nat-traverse 10002:natgw-of-left:10001 \
      --cmd="nc -v localhost 22"

As soon as the tunnel is established (using UDP ports 10001 and 10002), left's TCP port 20000 is forwarded to right's SSH Daemon (TCP port 22):

  user@some-other-host $ ssh -p 20000 user@left
  # Will connect to right's SSH daemon!

But do note that you lose the reliability of TCP in this example, as the actual data is transported via UDP; so this is only a toy example. If you want reliable streams, use PPP on top of nat-traverse, as described above.

Setup of a VPN with OpenVPN

You can use OpenVPN over nat-traverse if you want to have a secure VPN.

Using OpenVPN over nat-traverse requires only one change to OpenVPN's configuration file, presuming that you don't want to use OpenVPN's multi-client mode: You have to adjust the port and lport options accordingly, for example:

  # Options to add to left's and right's OpenVPN config:
  port  60001
  lport 60001

  # Command to execute on left resp. right:
  root@left  # until \
                 nat-traverse --quit-after-connect 60001:right:60001 \
               do \
                 sleep 5 \
               done; \
               openvpn [...]
  root@right # until \
                 nat-traverse --quit-after-connect 60001:left:60001 \
               do \
                 sleep 5 \
               done; \
               openvpn [...]

The until loop ensures that OpenVPN will not be started before nat-traverse was able to establish the connection. Michael Kugele (michael (at) kugele.net) also reported a way to still be able to use OpenVPN's multi-client mode with nat-traverse: As all instances of nat-traverse have to use unique ports (because a connection is identified by the source/destination port combination), you've to use redirection rules to redirect the ports used by nat-traverse to the port the OpenVPN daemon listens on:

  iptables -t nat -A PREROUTING -p udp \
    --dport $LPORT -j DNAT --to $HOST:$PORT
  iptables -t nat -A PREROUTING -p udp \
    --dport $PORT -j REDIRECT --to-port $LPORT

($LPORT specifies the source port nat-traverse uses on the server side, and $HOST:$PORT is the address of the OpenVPN server.)

Limitations

Only IPv4 is supported, nat-traverse won't work with IPv6 addresses. Drop me a note if you do need IPv6 support.

nat-traverse does not work with gateways which change the port numbers. This is a fundamental problem of nat-traverse's design, as the changed port numbers are (in general) not predictable.

See also

RFC 1631

The IP Network Address Translator (NAT). K. Egevang, P. Francis. May 1994. (Obsoleted by RFC3022) (Status: INFORMATIONAL)

RFC 3022

Traditional IP Network Address Translator (Traditional NAT). P. Srisuresh, K. Egevang. January 2001. (Obsoletes RFC1631) (Status: INFORMATIONAL)

RFC 1661

The Point-to-Point Protocol (PPP). W. Simpson, Ed.. July 1994. (Obsoletes RFC1548) (Updated by RFC2153) (Also STD0051) (Status: STANDARD)

pppd

Website of Paul's PPP Package (open source implementation of the Point-to-Point Protocol (PPP) on Linux and Solaris)

German talk about nat-traverse

Dieser Vortrag zeigt, wie man einen Tunnel zwischen zwei Computern, die beide hinter NAT-Gateways sitzen, hinbekommt. Dazu wird ein neues Programm vorgestellt, welches sowohl einfache Tastendr├╝cke an die Gegenseite weiterleiten, als auch beliebige Programme mit Verbindungen zur Gegenseite starten kann. Damit ist ein einfaches VPN schnell aufgebaut.

You may want to visit nat-traverse's Freecode project page.

Changelog

v0.6, 2012-12-24

Fixed the port forwarding example in the documentation, thanks to Marcin Zaj─ůczkowski.

v0.5, 2012-02-12

Adjusted the length of garbage and acknowledgements packets to make nat-traverse work under Windows and Android. Thanks to Jacobo de Pedro for finding this bug!

v0.4, 2005-08-23

New option --quit-after-connect to quit nat-traverse after establishing the tunnel.

v0.3, 2005-06-29

Made nat-traverse work with Perl 5.6.1 (previously Perl 5.8.0 was required).

v0.2, 2005-06-26

Fixed a race condition which caused the two nat-traverse instances to sometimes miss each other.

v0.1, 2005-06-25

Initial release.

Author

Copyright (C) 2005, 2012 Ingo Blechschmidt, <iblech@web.de>.

License

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program 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 General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.