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 port
    on "right". See "EXAMPLES" for more.

VERSION
    This document describes nat-traverse v0.5.

DESCRIPTION
    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 (see "EXAMPLES"). 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.*

    See "TECHNIQUE" for how this is achieved.

    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.

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 the 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 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" 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 the
    tunnel 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 "code" 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 at http://www.ietf.org/rfc/rfc1631.txt>
        The IP Network Address Translator (NAT). K. Egevang, P. Francis. May
        1994. (Obsoleted by RFC3022) (Status: INFORMATIONAL)

    <RFC 3022 at http://www.ietf.org/rfc/rfc3022.txt>
        Traditional IP Network Address Translator (Traditional NAT). P.
        Srisuresh, K. Egevang. January 2001. (Obsoletes RFC1631) (Status:
        INFORMATIONAL)

    <RFC 1661 at http://www.ietf.org/rfc/rfc1661.txt>
        The Point-to-Point Protocol (PPP). W. Simpson, Ed.. July 1994.
        (Obsoletes RFC1548) (Updated by RFC2153) (Also STD0051) (Status:
        STANDARD)

    <http://ppp.samba.org/>
        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 at
    http://linide.sourceforge.net/nat-traverse/nat-traverse-talk.pdf>
        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 Tastendrücke an
        die Gegenseite weiterleiten, als auch beliebige Programme mit
        Verbindungen zur Gegenseite starten kann. Damit ist ein einfaches
        VPN schnell aufgebaut.

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

    You may want to visit nat-traverse's Freecode project page,
    <http://freecode.com/projects/nat-traverse/>.

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.

