Ubuntu Manpage: nat-traverse - NAT gateway traversal utility

NAME

       nat-traverse - NAT gateway traversal utility

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 v0.7.

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 <https://openvpn.net/> 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, 2017 Ingo Blechschmidt, <iblech@speicherleck.de>.

       The source code repository is hosted at <https://gitlab.com/iblech/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 3 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.

perl v5.24.2                                       2017-10-28                                    NAT-TRAVERSE(1)