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NAME

nat-traverse - Use of UDP to traverse NAT gateways

SYNOPSIS

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

VERSION

This document describes nat-traverse v0.4.

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.

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.

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.
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!
"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.
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 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 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 -vlp 20000"
      user@right $ nat-traverse 10002:natgw-of-left:10001 \
            --cmd="nc -vlp 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. If you want reliable streams, use PPP on top of nat-traverse, as described above.

LIMITATIONS

Only IPv4 is supported, nat-traverse won't work with IPv6 addresses. Even though it would be relatively trivial to add IPv6 support, I refrained from doing that, as there's no need to use NAT with IPv6 (the address space IPv6 provides is sufficient).

If you do need IPv6 support, drop me a note and I'll patch nat-traverse.

        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 TastendrÃ¼cke an
        die Gegenseite weiterleiten, als auch beliebige Programme mit
        Verbindungen zur Gegenseite starten kann. Damit ist ein einfaches
        VPN schnell aufgebaut.

AUTHOR

You may want to visit nat-traverse's Freshmeat project page, <http://freshmeat.net/projects/nat-traverse/>, for new releases.

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.