Provided by: gvpe_2.24-2_i386
GNU-VPE - Overview of the GNU Virtual Private Ethernet suite.
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
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
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 which any number of
nodes (at least a few dozen in practise, the theoretical limit is
4095 nodes) can participate.
GVPE DESIGN GOALS
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.
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.
Gvpe comes with two programs: one daemon (gvpe) and one control program
This program is used to generate the keys, check and give an
overview of of the configuration and to control the daemon
This is the daemon used to establish and maintain connections to
the other network nodes. It should be run on the gateway of each
Please have a look at the gvpe.osdep(5) manpage for platform-specific
Gvpe hardcodes most encryption parameters. While this reduces
flexibility, it makes the program much simpler and helps making buffer
overflows impossible under most circumstances.
Here are a few recipes for compiling your gvpe, showing the extremes
(fast, small, insecure OR slow, large, more secure), between which you
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 pretty easy.
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, so using
another digest algorithm is recommended.
./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-256 and beyond).
In general, remember that AES-128 seems to be as secure but 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, RIPEMD160,
SHA256 are consecutively 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 configuration 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 command
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 = 188.8.131.52
node = third
hostname = third.example.net
The only other file necessary is the if-up script that initializes the
virtual ethernet interface on the local host. Put the following lines
into /etc/gvpe/if-up and make it executable (chmod 755
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 (if gvpe runs on a router) 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 (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 for all nodes (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 and private keys to the other nodes.
This should be done in two steps, since only the private keys meant for
a node should be distributed (so each node has only it's own private
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 184.108.40.206:/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 220.127.116.11:/etc/hostkey
rsync -avzessh /etc/gvpe/hostkeys/third third.example.net:/etc/hostkey
You should now check the configuration 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 -l info first # first is the nodename
This will make the gvpe daemon 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
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
or equivalent. I use a line like this on all 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
gvpe.osdep(5) for OS-dependent information, gvpe.conf(5), gvpectrl(8),
and for a description of the transports, protocol, and routing
The GVPE mailing list, at <http://lists.schmorp.de/>, or
Marc Lehmann <firstname.lastname@example.org>
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.