nat-traverse - NAT gateway traversal utility
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.
This document describes nat-traverse v0.7.
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.
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.
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.
Sets the number of initial garbage packets to send. The default, 10,
should work with most firewalls.
Sets the maximum number of seconds to wait for an acknowledgement by
Quits nat-traverse after the tunnel has been established
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-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
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
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.
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"
root@right # nat-traverse \
--cmd="pppd nodetach notty noauth"
"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
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
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
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
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:
# Command to execute on left resp. right:
root@left # until \
nat-traverse --quit-after-connect 60001:right:60001 \
sleep 5 \
root@right # until \
nat-traverse --quit-after-connect 60001:left:60001 \
sleep 5 \
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.)
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.
RFC 1631 at https://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 https://www.ietf.org/rfc/rfc3022.txt
Traditional IP Network Address Translator (Traditional NAT). P.
Srisuresh, K. Egevang. January 2001. (Obsoletes RFC1631) (Status:
RFC 1661 at https://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:
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
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.
Copyright (C) 2005, 2012, 2017 Ingo Blechschmidt,
The source code repository is hosted at
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.
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