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.
SEE ALSO¶
- <RFC 1631 at http://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 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 TastendrA~Xcke an die Gegenseite
weiterleiten, als auch beliebige Programme mit Verbindungen zur Gegenseite
starten kann. Damit ist ein einfaches VPN schnell aufgebaut.
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.