==== 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.