Provided by: gvpe_2.24-2_i386 bug


       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
           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

           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 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
           (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

       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 based on the birthday paradox pretty easy.


          ./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).


       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
       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

          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 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 --exclude hostkeys
          rsync -avzessh /etc/gvpe --exclude hostkeys
          rsync -avzessh /etc/gvpe --exclude hostkeys

       Then the hostkeys should be copied:

          rsync -avzessh /etc/gvpe/hostkeys/first
          rsync -avzessh /etc/gvpe/hostkeys/second
          rsync -avzessh /etc/gvpe/hostkeys/third

       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
       files again.


       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.