WHAT IS A VPN?

VPN is an acronym, it stands for:

Virtual

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

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

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

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 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.

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 almost trivial.

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.

MAXIMIZE SECURITY

   ./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).

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 = 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 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):

   #!/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 (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 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 first.example.net:/etc/. --exclude hostkey
   rsync -avzessh /etc/gvpe 133.55.82.9:/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.

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, e.g.:

   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:

   [Unit]
   Description=gvpe
   After=network.target
   Before=remote-fs.target
   [Service]
   ExecStart=/path/to/gvpe -D node
   KillMode=process
   Restart=always
   [Install]
   WantedBy=multi-user.target