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.
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
- 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
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
- 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
- EASY TO SETUP
- A few lines of config (the config file is shared unmodified
between all hosts) and generating an RSA key-pair on each node 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.
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 (restarting
- This is the daemon used to establish and maintain
connections to the other network nodes. It should be run on the gateway of
each VPN subnet.
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 should choose:
./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 almost
./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=12 --enable-digest=ripemd610
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 12
bytes of random data.
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).
In this section I will describe how to get a simple VPN consisting of three
hosts up and running.
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 line 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 = 22.214.171.124
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 /etc/gvpe/if-up):
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.
Next you have to generate the RSA keys for the nodes. While you can set up GVPE
so you can generate all keys on a single host and centrally distribute all
keys, it is safer to generate the key for each node on the node, so that the
secret/private key does not have to be copied over the network.
To do so, run the following command to generate a key pair:
gvpectrl -c /etc/gvpe -g nodekey
This will create two files, nodekey
former should be copied to /etc/gvpe/pubkey/nodename
the host where your config file is (you will have to create the pubkey
scp nodekey confighost:/etc/gvpe/pubkey/nodename
The private key nodekey.privkey
should be moved to
mkdir -p /etc/gvpe
mv nodekey.privkey /etc/gvpe/hostkey
Now distribute the config files and public keys to the other nodes.
The example uses rsync-over-ssh to copy the config file and all the public keys:
rsync -avzessh /etc/gvpe first.example.net:/etc/. --exclude hostkey
rsync -avzessh /etc/gvpe 126.96.36.199:/etc/. --exclude hostkey
rsync -avzessh /etc/gvpe third.example.net:/etc/. --exclude hostkey
You should now check the configuration by issuing the command gvpectrl -c
/etc/gvpe -s on each node and verify it's output.
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
... 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 gvpectrl
-k (or simply killall gvpe) will kill the daemon, start it again, making it
read it's configuration files again.
To run the GVPE daemon permanently from your SysV init, you can add it to your
t1:2345:respawn:/bin/sh -c "exec nice -n-20 /path/to/gvpe -D node >/var/log/gvpe.log 2>&1"
For systems using systemd, you can use a unit file similar to this one:
ExecStart=/path/to/gvpe -D node
(5) for OS-dependent information, gvpe.conf
(8), and for a description of the transports, protocol, and
routing algorithm, gvpe.protocol
The GVPE mailing list, at <http://lists.schmorp.de/>, or
Marc Lehmann <email@example.com>
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.