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.\" ========================================================================
.\"
.IX Title "NAT-TRAVERSE 1"
.TH NAT-TRAVERSE 1 "2017-10-28" "perl v5.24.2" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.SH "NAME"
nat\-traverse \- NAT gateway traversal utility
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
To create a simple text-only tunnel, use the commands
.PP
.Vb 2
\&  user@left  $ nat\-traverse 40000:natgw\-of\-right:40001
\&  user@right $ nat\-traverse 40001:natgw\-of\-left:40000
.Ve
.PP
where \f(CW40000\fR is an unused \s-1UDP\s0 port on \f(CW\*(C`left\*(C'\fR and \f(CW40001\fR is an unused port on
\&\f(CW\*(C`right\*(C'\fR. See \*(L"\s-1EXAMPLES\*(R"\s0 for more.
.SH "VERSION"
.IX Header "VERSION"
This document describes nat-traverse v0.7.
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
nat-traverse establishes connections between nodes which are behind \s-1NAT\s0
gateways, i.e. hosts which do \fInot\fR have public \s-1IP\s0 addresses. Additionally,
you can setup a small \s-1VPN\s0 by using pppd on top of nat-traverse (see
\&\*(L"\s-1EXAMPLES\*(R"\s0).  nat-traverse does \fInot\fR need an external server on the
Internet, and it isn't necessary to reconfigure the involved \s-1NAT\s0 gateways,
either. \fInat-traverse works out-of-the-box.\fR
.PP
See \*(L"\s-1TECHNIQUE\*(R"\s0 for how this is achieved.
.PP
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.
.SH "OPTIONS"
.IX Header "OPTIONS"
.ie n .IP """\fIlocal_port\fP:\fIpeer\fP:\fIremote_port\fP"" (required)" 4
.el .IP "\f(CW\f(CIlocal_port\f(CW:\f(CIpeer\f(CW:\f(CIremote_port\f(CW\fR (required)" 4
.IX Item "local_port:peer:remote_port (required)"
Sets the local port to use and the remote address to connect to.
.Sp
Note that you have to give the \s-1IP\s0 address or hostname of the \fI\s-1NAT\s0 gateway\fR of
the host you want to connect to, as the target host doesn't have a public \s-1IP\s0
address.
.ie n .IP """\-\-cmd=""\fIpppd...\fP""""" 4
.el .IP "\f(CW\-\-cmd=``\f(CIpppd...\f(CW''\fR" 4
.IX Item "--cmd=""pppd..."""
Runs the specified command after establishing the connection.
.Sp
The command will be run with its \s-1STDIN\s0 and \s-1STDOUT\s0 bound to the socket, i.e.
everything the command writes to \s-1STDOUT\s0 will be forwarded to the peer.
.Sp
If no command is specified, nat-traverse will relay input from \s-1STDIN\s0 to the peer
and vice versa, i.e. nat-traverse degrades to netcat.
.ie n .IP """\-\-window=\fI10\fP""" 4
.el .IP "\f(CW\-\-window=\f(CI10\f(CW\fR" 4
.IX Item "--window=10"
Sets the number of initial garbage packets to send. The default, 10, should
work with most firewalls.
.ie n .IP """\-\-timeout=\fI10\fP""" 4
.el .IP "\f(CW\-\-timeout=\f(CI10\f(CW\fR" 4
.IX Item "--timeout=10"
Sets the maximum number of seconds to wait for an acknowledgement by the peer.
.ie n .IP """\-\-quit\-after\-connect""" 4
.el .IP "\f(CW\-\-quit\-after\-connect\fR" 4
.IX Item "--quit-after-connect"
Quits nat-traverse after the tunnel has been established successfully.
.Sp
nat-traverse returns a non\-\f(CW0\fR statuscode to indicate that it wasn't able to
establish the tunnel.
.Sp
\&\f(CW\*(C`\-\-quit\-after\-connect\*(C'\fR 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 \s-1NAT\s0 gateways.
.ie n .IP """\-\-version"", ""\-\-help""" 4
.el .IP "\f(CW\-\-version\fR, \f(CW\-\-help\fR" 4
.IX Item "--version, --help"
.SH "TECHNIQUE"
.IX Header "TECHNIQUE"
nat-traverse establishes connections between hosts behind \s-1NAT\s0 gateways without need
for reconfiguration of the involved \s-1NAT\s0 gateways.
.IP "1." 4
Firstly, nat-traverse on host \f(CW\*(C`left\*(C'\fR sends garbage \s-1UDP\s0 packets to the \s-1NAT\s0 gateway
of \f(CW\*(C`right\*(C'\fR. These packets are, of course, discarded by the firewall.
.IP "2." 4
Then \f(CW\*(C`right\*(C'\fR's nat-traverse sends garbage \s-1UDP\s0 packets to the \s-1NAT\s0 gateway of
\&\f(CW\*(C`left\*(C'\fR. These packets are \fInot\fR discarded, as \f(CW\*(C`left\*(C'\fR's \s-1NAT\s0 gateway thinks
these packets are replies to the packets sent in step 1!
.IP "3." 4
\&\f(CW\*(C`left\*(C'\fR's nat-traverse continues to send garbage packets to \f(CW\*(C`right\*(C'\fR's \s-1NAT\s0 gateway.
These packets are now not dropped either, as the \s-1NAT\s0 gateway thinks the packets
are replies to the packets sent in step 2.
.IP "4." 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 \s-1STDIN/STDOUT\s0 to the socket or execute a program.
.SH "EXAMPLES"
.IX Header "EXAMPLES"
.SS "Setup of a small \s-1VPN\s0 with \s-1PPP\s0"
.IX Subsection "Setup of a small VPN with PPP"
It's easy to setup a \s-1VPN \s0(Virtual Private Network) by using the Point-to-Point
Protocol Daemon, \f(CW\*(C`pppd\*(C'\fR:
.PP
.Vb 7
\&  root@left # nat\-traverse \e
\&      \-\-cmd="pppd updetach noauth passive notty \e
\&             ipparam vpn 10.0.0.1:10.0.0.2"
\&      40000:natgw\-of\-right:40001
\&  root@right # nat\-traverse \e
\&      \-\-cmd="pppd nodetach notty noauth"
\&      40001:natgw\-of\-left:40000
.Ve
.PP
\&\f(CW\*(C`pppd\*(C'\fR creates a new interface, typically \f(CW\*(C`ppp0\*(C'\fR.  Using this interface, you
can ping \f(CW10.0.0.1\fR or \f(CW10.0.0.2\fR. As you can see, \f(CW\*(C`pppd\*(C'\fR upgrades the
data-only tunnel nat-traverse provides to a full \s-1IP\s0 tunnel. Thus you can
establish reliable \s-1TCP\s0 connections over the tunnel, even though the tunnel uses
\&\s-1UDP\s0!  Furthermore, you could even add IPv6 addresses to \f(CW\*(C`ppp0\*(C'\fR by running \f(CW\*(C`ip
\&\-6 addr add...\*(C'\fR!
.PP
Note though that although this \s-1VPN \s0\fIis\fR arguably a private network, it is \fInot\fR
secured in any way. You may want to use \s-1SSH\s0 to encrypt the connection.
.SS "Port Forwarding with netcat"
.IX Subsection "Port Forwarding with netcat"
You can use \f(CW\*(C`netcat\*(C'\fR to forward one of your local \s-1UDP\s0 or \s-1TCP\s0 ports to an
arbitrary \s-1UDP\s0 or \s-1TCP\s0 port of the remote host, similar to \f(CW\*(C`ssh \-L\*(C'\fR or \f(CW\*(C`ssh
\&\-R\*(C'\fR:
.PP
.Vb 4
\&  user@left  $ nat\-traverse 10001:natgw\-of\-right:10002 \e
\&        \-\-cmd="nc \-vl 20000"
\&  user@right $ nat\-traverse 10002:natgw\-of\-left:10001 \e
\&        \-\-cmd="nc \-v localhost 22"
.Ve
.PP
As soon as the tunnel is established (using \s-1UDP\s0 ports \f(CW10001\fR and \f(CW10002\fR),
\&\f(CW\*(C`left\*(C'\fR's \s-1TCP\s0 port \f(CW20000\fR is forwarded to \f(CW\*(C`right\*(C'\fR's \s-1SSH\s0 Daemon (\s-1TCP\s0 port
\&\f(CW22\fR):
.PP
.Vb 2
\&  user@some\-other\-host $ ssh \-p 20000 user@left
\&  # Will connect to right\*(Aqs SSH daemon!
.Ve
.PP
But do note that you lose the reliability of \s-1TCP\s0 in this example, as the actual
data is transported via \s-1UDP\s0; so this is only a toy example. If you want
reliable streams, use \s-1PPP\s0 on top of nat-traverse, as described above.
.SS "Setup of a \s-1VPN\s0 with OpenVPN"
.IX Subsection "Setup of a VPN with OpenVPN"
You can use OpenVPN <https://openvpn.net/> over nat-traverse if you want to
have a \fIsecure\fR \s-1VPN.\s0
.PP
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 \f(CW\*(C`code\*(C'\fR and \f(CW\*(C`lport\*(C'\fR options
accordingly, for example:
.PP
.Vb 3
\&  # Options to add to left\*(Aqs and right\*(Aqs OpenVPN config:
\&  port  60001
\&  lport 60001
\&
\&  # Command to execute on left resp. right:
\&  root@left  # until \e
\&                 nat\-traverse \-\-quit\-after\-connect 60001:right:60001 \e
\&               do \e
\&                 sleep 5 \e
\&               done; \e
\&               openvpn [...]
\&  root@right # until \e
\&                 nat\-traverse \-\-quit\-after\-connect 60001:left:60001 \e
\&               do \e
\&                 sleep 5 \e
\&               done; \e
\&               openvpn [...]<!\-\-
.Ve
.PP
The \f(CW\*(C`until\*(C'\fR loop ensures that OpenVPN will not be started before
nat-traverse was able to establish the connection. Michael Kugele
(\f(CW\*(C`michael (at) kugele.net\*(C'\fR) 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:
.PP
.Vb 4
\&  iptables \-t nat \-A PREROUTING \-p udp \e
\&    \-\-dport $LPORT \-j DNAT \-\-to $HOST:$PORT
\&  iptables \-t nat \-A PREROUTING \-p udp \e
\&    \-\-dport $PORT \-j REDIRECT \-\-to\-port $LPORT
.Ve
.PP
\&\f(CW$LPORT\fR specifies the source port nat-traverse uses on the server
side, and \f(CW\*(C`$HOST:$PORT\*(C'\fR is the address of the OpenVPN server.)
.SH "LIMITATIONS"
.IX Header "LIMITATIONS"
Only IPv4 is supported, nat-traverse won't work with IPv6 addresses. Drop me a
note if you do need IPv6 support.
.PP
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.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
.IP "\s-1RFC 1631\s0 at https://www.ietf.org/rfc/rfc1631.txt <https://www.ietf.org/rfc/rfc1631.txt>" 4
.IX Item "RFC 1631 at https://www.ietf.org/rfc/rfc1631.txt <https://www.ietf.org/rfc/rfc1631.txt>"
The \s-1IP\s0 Network Address Translator (\s-1NAT\s0). K. Egevang, P. Francis.  May 1994.
(Obsoleted by \s-1RFC3022\s0) (Status: \s-1INFORMATIONAL\s0)
.IP "\s-1RFC 3022\s0 at https://www.ietf.org/rfc/rfc3022.txt <https://www.ietf.org/rfc/rfc3022.txt>" 4
.IX Item "RFC 3022 at https://www.ietf.org/rfc/rfc3022.txt <https://www.ietf.org/rfc/rfc3022.txt>"
Traditional \s-1IP\s0 Network Address Translator (Traditional \s-1NAT\s0). P.  Srisuresh,
K. Egevang. January 2001.  (Obsoletes \s-1RFC1631\s0) (Status: \s-1INFORMATIONAL\s0)
.IP "\s-1RFC 1661\s0 at https://www.ietf.org/rfc/rfc1661.txt <https://www.ietf.org/rfc/rfc1661.txt>" 4
.IX Item "RFC 1661 at https://www.ietf.org/rfc/rfc1661.txt <https://www.ietf.org/rfc/rfc1661.txt>"
The Point-to-Point Protocol (\s-1PPP\s0). W. Simpson, Ed.. July 1994.  (Obsoletes
\&\s-1RFC1548\s0) (Updated by \s-1RFC2153\s0) (Also \s-1STD0051\s0) (Status: \s-1STANDARD\s0)
.IP "<https://ppp.samba.org/>" 4
.IX Item "<https://ppp.samba.org/>"
Website of Paul's \s-1PPP\s0 Package (open source implementation of the
Point-to-Point Protocol (\s-1PPP\s0) on Linux and Solaris)
.IP "German talk about nat-traverse at https://www.speicherleck.de/iblech/nat\-traverse/nat\-traverse\-talk.pdf <https://www.speicherleck.de/iblech/nat-traverse/nat-traverse-talk.pdf>" 4
.IX Item "German talk about nat-traverse at https://www.speicherleck.de/iblech/nat-traverse/nat-traverse-talk.pdf <https://www.speicherleck.de/iblech/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 Tastendru\*:cke an die Gegenseite
weiterleiten, als auch beliebige Programme mit Verbindungen zur Gegenseite
starten kann. Damit ist ein einfaches \s-1VPN\s0 schnell aufgebaut.
.SH "AUTHOR"
.IX Header "AUTHOR"
Copyright (C) 2005, 2012, 2017 Ingo Blechschmidt, <iblech@speicherleck.de>.
.PP
The source code repository is hosted at <https://gitlab.com/iblech/nat\-traverse>.
.SH "LICENSE"
.IX Header "LICENSE"
This program is free software; you can redistribute it and/or modify it under
the terms of the \s-1GNU\s0 General Public License as published by the Free Software
Foundation; either version 3 of the License, or (at your option) any later
version.
.PP
This program is distributed in the hope that it will be useful, but \s-1WITHOUT ANY
WARRANTY\s0; without even the implied warranty of \s-1MERCHANTABILITY\s0 or \s-1FITNESS FOR A
PARTICULAR PURPOSE.\s0 See the \s-1GNU\s0 General Public License for more details.
.PP
You should have received a copy of the \s-1GNU\s0 General Public License along with
this program; if not, write to the Free Software Foundation, Inc., 51 Franklin
Street, Fifth Floor, Boston, \s-1MA 02110\-1301, USA.\s0