Screenshot of nat-traverse in action
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 UDP port on right.
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. 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 below for how this is achieved.
In other words: nat-traverse is a bit like Harm, but doesn't have Harm's limitation that one peer has to have a public IP address.
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.
nat-traverse is a Perl 5 program using only core modules, so neither compilation nor installation of Perl modules is necessary. You do need a copy of Perl (>= 5.6.1).
Simply grab nat-traverse-0.7.pl, rename it to
nat-traverse, and flag it executable by using chmod.
Alternatively, you can download nat-traverse packed in a bz2-compressed tarball.
Example network architecture nat-traversal can work with
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=10Sets the number of initial garbage packets to send. The default, 10, should work with most firewalls.
--timeout=10Sets the maximum number of seconds to wait for an acknowledgement by the peer.
--quit-after-connectQuits 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 a 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, --helpnat-traverse establishes connections between hosts behind NAT gateways without need for reconfiguration of the involved NAT gateways.
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.
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!
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.
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.
It's easy to setup a small 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 nat-traverse 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.
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:
pppd running on top of nat-traverse
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.
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 port 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.)
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.
The IP Network Address Translator (NAT). K. Egevang, P. Francis. May 1994. (Obsoleted by RFC3022) (Status: INFORMATIONAL)
Traditional IP Network Address Translator (Traditional NAT). P. Srisuresh, K. Egevang. January 2001. (Obsoletes RFC1631) (Status: INFORMATIONAL)
The Point-to-Point Protocol (PPP). W. Simpson, Ed.. July 1994. (Obsoletes RFC1548) (Updated by RFC2153) (Also STD0051) (Status: STANDARD)
Website of Paul's PPP Package (open source implementation of the Point-to-Point Protocol (PPP) on Linux and Solaris)
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.
Fixed a minor syntactical issue which caused a warning on modern Perl and relicensed under GPL version 3 or later.
Fixed the port forwarding example in the documentation, thanks to Marcin Zajączkowski.
Adjusted the length of garbage and acknowledgements packets to make nat-traverse work under Windows and Android. Thanks to Jacobo de Pedro for finding this bug!
New option --quit-after-connect to quit nat-traverse after
establishing the tunnel.
Made nat-traverse work with Perl 5.6.1 (previously Perl 5.8.0 was required).
Fixed a race condition which caused the two nat-traverse instances to sometimes miss each other.
Initial release.
Copyright (C) 2005, 2012, 2017 Ingo Blechschmidt, <iblech@speicherleck.de>.
The source code repository is hosted at GitLab.
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.