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NAME
nat-traverse - Use of UDP to traverse NAT gateways
SYNOPSIS
user@left $ nat-traverse 40000:natgw-of-right:40001
user@right $ nat-traverse 40001:natgw-of-left:40000
VERSION
This document describes nat-traverse v0.4.
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.
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 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* 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 -vlp 20000"
user@right $ nat-traverse 10002:natgw-of-left:10001 \
--cmd="nc -vlp 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. If you want reliable streams, use
PPP on top of nat-traverse, as described above.
LIMITATIONS
Only IPv4 is supported, nat-traverse won't work with IPv6 addresses.
Even though it would be relatively trivial to add IPv6 support, I
refrained from doing that, as there's no need to use NAT with IPv6 (the
address space IPv6 provides is sufficient).
If you do need IPv6 support, drop me a note and I'll patch nat-traverse.
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 Tastendrücke 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 Ingo Blechschmidt, <iblech@web.de>.
You may want to visit nat-traverse's Freshmeat project page,
<http://freshmeat.net/projects/nat-traverse/>, for new releases.
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.
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