Ubuntu Manpage: nat-traverse - Use of UDP to traverse NAT gateways

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.

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.