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