=head1 NAME GNU-VPE - Overview of the GNU Virtual Private Ethernet suite. =head1 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. =head2 WHAT IS A VPN? VPN is an acronym, it stands for: =over 4 =item Virtual Virtual means that no physical network is created (of course), but a network is I 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. =item 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. =item 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. =back =head2 GVPE DESIGN GOALS =over 4 =item 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. =item 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. =item 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. =back =head1 PROGRAMS Gvpe comes with two programs: one daemon (C) and one control program (C). =over 4 =item 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.). =item gvpe 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. =back =head1 COMPILETIME CONFIGURATION Please have a look at the C 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 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: =head2 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. =head2 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. =head2 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). =head1 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. =head2 STEP 1: configuration First you have to create a daemon configuration file and put it into the configuration directory. This is usually C, depending on how you configured gvpe, and can be overwritten using the C<-c> command line switch. Put the following lines into C: 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 C script that initializes the virtual ethernet interface on the local host. Put the following lines into C and make it executable (C): #!/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 C<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. C<10.0.1.0/24> on node C, C<10.0.2.0/24> on node C, and so on. By enabling routing on the gateway host that runs C 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. =head2 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, F and F. The former should be copied to F<< /etc/gvpe/pubkey/I >> on the host where your config file is (you will have to create the F directory first): scp nodekey confighost:/etc/gvpe/pubkey/nodename The private key F should be moved to F: mkdir -p /etc/gvpe mv nodekey.privkey /etc/gvpe/hostkey =head2 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 C on each node and verify it's output. =head2 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 C<-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 =head2 STEP 5: enjoy ... and play around. Sending a -HUP (C) to the daemon will make it try to connect to all other nodes again. If you run it from inittab, as is recommended, C (or simply C) will kill the daemon, start it again, making it read it's configuration files again. To run the GVPE daemon permanently, you can either add it to your SysV F, 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 ike 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 =head1 SEE ALSO 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 L, or C. =head1 AUTHOR Marc Lehmann =head1 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.