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       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 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 which any number of nodes (at least a few
           dozen in practise, the theoretical limit is 4095 nodes) can participate.

           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.

           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.

           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 (gvpectrl).

           This program is used to generate the keys, check and give an overview of of the
           configuration and to control the daemon (restarting etc.).

           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 information.

       Gvpe hardcodes most encryption parameters. While this reduces flexibility, it makes the
       program much simpler and helps making buffer overflows impossible under most

       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 trivial.

          ./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

   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 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 = # the DNS name or IP address of the host

          node = second
          hostname =

          node = third
          hostname =

       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 dev $IFNAME
          [ $NODENAME = second ] && ip addr add dev $IFNAME
          [ $NODENAME = third  ] && ip addr add dev $IFNAME
          ip route add 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. on node first, 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 pair for each node
       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 and nodekey.privkey. The former should be copied to
       /etc/gvpe/pubkey/nodename on the host where your config file is (you will have to create
       the pubkey directory first):

          scp nodekey confighost:/etc/gvpe/pubkey/nodename

       The private key nodekey.privkey should be moved to /etc/gvpe/hostkey:

          mkdir -p /etc/gvpe
          mv nodekey.privkey /etc/gvpe/hostkey

   STEP 3: distribute the config files to all nodes
       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 --exclude hostkey
          rsync -avzessh /etc/gvpe       --exclude hostkey
          rsync -avzessh /etc/gvpe --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.

   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 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 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 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 inittab,

          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



       gvpe.osdep(5) for OS-dependent information, gvpe.conf(5), gvpectrl(8), and for a
       description of the transports, protocol, and routing algorithm, gvpe.protocol(7).

       The GVPE mailing list, at <>, or


       Marc Lehmann <>


       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.