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==== NAME ==== |
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GNU-VPE - Overview of the GNU Virtual Private Ethernet suite. |
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==== DESCRIPTION ==== |
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GVPE is a suite designed to provide a virtual private network for |
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multiple nodes over an untrusted network. This document first gives an |
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introduction to VPNs in general and then describes the specific |
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implementation of GVPE. |
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== WHAT IS A VPN? == |
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VPN is an acronym, it stands for: |
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: Virtual means that no physical network is created (of course), but a |
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network is *emulated* by creating multiple tunnels between the |
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member nodes by encapsulating and sending data over another |
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transport network. |
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Usually the emulated network is a normal IP or Ethernet, and the |
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transport network is the Internet. However, using a VPN system like |
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GVPE to connect nodes over other untrusted networks such as Wireless |
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LAN is not uncommon. |
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: Private means that non-participating nodes cannot decode ("sniff)" |
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nor inject ("spoof") packets. This means that nodes can be connected |
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over untrusted networks such as the public Internet without fear of |
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being eavesdropped while at the same time being able to trust data |
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sent by other nodes. |
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In the case of GVPE, even participating nodes cannot sniff packets |
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send to other nodes or spoof packets as if sent from other nodes, so |
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communications between any two nodes is private to those two nodes. |
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: Network means that more than two parties can participate in the |
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network, so for instance it's possible to connect multiple branches |
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of a company into a single network. Many so-called "vpn" solutions |
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only create point-to-point tunnels, which in turn can be used to |
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build larger networks. |
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GVPE provides a true multi-point network in wich any number of nodes |
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(at least a few dozen in practise, the theoretical limit is 4095 |
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nodes) can participate. |
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== GVPE DESIGN GOALS == |
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: SIMPLE DESIGN |
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Cipher, HMAC algorithms and other key parameters must be selected at |
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compile time - this makes it possible to only link in algorithms you |
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actually need. It also makes the crypto part of the source very |
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transparent and easy to inspect, and last not least this makes it |
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possible to hardcode the layout of all packets into the binary. GVPE |
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goes a step further and internally reserves blocks of the same |
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length for all packets, which virtually removes all possibilities of |
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buffer overflows, as there is only a single type of buffer and it's |
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always of fixed length. |
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: EASY TO SETUP |
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A few lines of config (the config file is shared unmodified between |
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all hosts) and a single run of ``gvpectrl'' to generate the keys |
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suffices to make it work. |
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: MAC-BASED SECURITY |
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Since every host has it's own private key, other hosts cannot spoof |
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traffic from this host. That makes it possible to filter packet by |
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MAC address, e.g. to ensure that packets from a specific IP address |
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come, in fact, from a specific host that is associated with that IP |
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and not from another host. |
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==== PROGRAMS ==== |
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Vpe comes with two programs: one daemon (``gvpe'') and one control |
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program (``gvpectrl''). |
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: gvpectrl |
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Is used to generate the keys, check and give an overview of of the |
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configuration and contorl the daemon (restarting etc.). |
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: gvpe |
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Is the daemon used to establish and maintain connections to the |
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other network members. It should be run on the gateway machine. |
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==== COMPILETIME CONFIGURATION ==== |
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Please have a look at the ``gvpe.osdep(5)'' manpage for |
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platform-specific information. |
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Here are a few recipes for compiling your gvpe, showing the extremes |
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(fast, small, insecure OR slow, large, more secure), between you should |
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choose: |
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== AS LOW PACKET OVERHEAD AS POSSIBLE == |
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./configure --enable-hmac-length=4 --enable-rand-length=0 |
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Minimize the header overhead of VPN packets (the above will result in |
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only 4 bytes of overhead over the raw ethernet frame). This is a |
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insecure configuration because a HMAC length of 4 makes collision |
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attacks based on the birthday paradox easy, though. |
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== MINIMIZE CPU TIME REQUIRED == |
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./configure --enable-cipher=bf --enable-digest=md4 |
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Use the fastest cipher and digest algorithms currently available in |
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gvpe. MD4 has been broken and is quite insecure, though. |
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== MAXIMIZE SECURITY == |
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./configure --enable-hmac-length=16 --enable-rand-length=8 --enable-digest=sha1 |
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This uses a 16 byte HMAC checksum to authenticate packets (I guess 8-12 |
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would also be pretty secure ;) and will additionally prefix each packet |
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with 8 bytes of random data. In the long run, people should move to |
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SHA-224 and beyond, but support in openssl is missing as of writing this |
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document. |
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In general, remember that AES-128 seems to be more secure and faster |
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than AES-192 or AES-256, more randomness helps against sniffing and a |
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longer HMAC helps against spoofing. MD4 is a fast digest, SHA1 or |
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RIPEMD160 are better, and Blowfish is a fast cipher (and also quite |
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secure). |
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==== HOW TO SET UP A SIMPLE VPN ==== |
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In this section I will describe how to get a simple VPN consisting of |
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three hosts up and running. |
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== STEP 1: configuration == |
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First you have to create a daemon configuation file and put it into the |
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configuration directory. This is usually ``/etc/gvpe'', depending on how |
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you configured gvpe, and can be overwritten using the ``-c'' commandline |
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switch. |
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Put the following lines into ``/etc/gvpe/gvpe.conf'': |
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udp-port = 50000 # the external port to listen on (configure your firewall) |
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mtu = 1400 # minimum MTU of all outgoing interfaces on all hosts |
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ifname = vpn0 # the local network device name |
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node = first # just a nickname |
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hostname = first.example.net # the DNS name or IP address of the host |
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node = second |
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hostname = 133.55.82.9 |
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node = third |
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hostname = third.example.net |
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The only other file neccessary if the ``if-up'' script that initializes |
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the local ethernet interface. Put the following lines into |
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``/etc/gvpe/if-up'' and make it execute (``chmod 755 /etc/gvpe/if-up''): |
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#!/bin/sh |
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ip link set $IFNAME address $MAC mtu $MTU up |
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[ $NODENAME = first ] && ip addr add 10.0.1.1 dev $IFNAME |
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[ $NODENAME = second ] && ip addr add 10.0.2.1 dev $IFNAME |
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[ $NODENAME = third ] && ip addr add 10.0.3.1 dev $IFNAME |
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ip route add 10.0.0.0/16 dev $IFNAME |
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This script will give each node a different IP address in the |
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``10.0/16'' network. The internal network (e.g. the ``eth0'' interface) |
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should then be set to a subset of that network, e.g. ``10.0.1.0/24'' on |
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node ``first'', ``10.0.2.0/24'' on node ``second'', and so on. |
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By enabling routing on the gateway host that runs ``gvpe'' all nodes |
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will be able to reach the other nodes. You can, of course, also use |
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proxy arp or other means of pseudo-bridging (or even real briding), or |
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(best) full routing - the choice is yours. |
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== STEP 2: create the RSA key pairs for all hosts == |
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Run the following command to generate all key pairs (that might take a |
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while): |
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gvpectrl -c /etc/gvpe -g |
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This command will put the public keys into |
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``/etc/gvpe/pubkeys/*nodename*'' and the private keys into |
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``/etc/gvpe/hostkeys/*nodename*''. |
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== STEP 3: distribute the config files to all nodes == |
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Now distribute the config files to the other nodes. This should be done |
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in two steps, since the private keys should not be distributed. The |
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example uses rsync-over-ssh |
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First all the config files without the hostkeys should be distributed: |
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rsync -avzessh /etc/gvpe first.example.net:/etc/. --exclude hostkeys |
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rsync -avzessh /etc/gvpe 133.55.82.9:/etc/. --exclude hostkeys |
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rsync -avzessh /etc/gvpe third.example.net:/etc/. --exclude hostkeys |
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Then the hostkeys should be copied: |
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rsync -avzessh /etc/gvpe/hostkeys/first first.example.net:/etc/hostkey |
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rsync -avzessh /etc/gvpe/hostkeys/second 133.55.82.9:/etc/hostkey |
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rsync -avzessh /etc/gvpe/hostkeys/third third.example.net:/etc/hostkey |
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You should now check the configration by issuing the command ``gvpectrl |
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-c /etc/gvpe -s'' on each node and verify it's output. |
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== STEP 4: starting gvpe == |
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You should then start gvpe on each node by issuing a command like: |
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gvpe -D -linfo first # first is the nodename |
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This will make the gvpe stay in foreground. You should then see |
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"connection established" messages. If you don't see them check your |
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firewall and routing (use tcpdump ;). |
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If this works you should check your networking setup by pinging various |
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endpoints. |
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To make gvpe run more permanently you can either run it as a daemon (by |
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starting it without the ``-D'' switch), or, much better, from your |
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inittab. I use a line like this on my systems: |
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t1:2345:respawn:/opt/gvpe/sbin/gvpe -D -L first >/dev/null 2>&1 |
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== STEP 5: enjoy == |
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... and play around. Sending a -HUP (``gvpectrl -kHUP'') to the daemon |
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will make it try to connect to all other nodes again. If you run it from |
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inittab, as is recommended, ``gvpectrl -k'' (or simply ``killall gvpe'') |
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will kill the daemon, start it again, making it read it's configuration |
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files again. |
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==== SEE ALSO ==== |
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gvpe.osdep(5) for OS-depedendent information, gvpe.conf(5), gvpectrl(8), |
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and for a description of the transports, protocol, and routing |
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algorithm, gvpe.protocol(7). |
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The GVPE mailinglist, at <http://lists.schmorp.de/>, or |
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``gvpe@lists.schmorp.de''. |
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==== AUTHOR ==== |
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Marc Lehmann <gvpe@schmorp.de> |
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==== COPYRIGHTS AND LICENSES ==== |
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GVPE itself is distributed under the GENERAL PUBLIC LICENSE (see the |
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file COPYING that should be part of your distribution). |
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In some configurations it uses modified versions of the tinc vpn suite, |
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which is also available under the GENERAL PUBLIC LICENSE. |
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