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=head1 NAME |
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GNU-VPE - Overview of the GNU Virtual Private Ethernet suite. |
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=head1 DESCRIPTION |
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GVPE is a suite designed to provide a virtual private network for multiple |
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nodes over an untrusted network. This document first gives an introduction |
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to VPNs in general and then describes the specific implementation of GVPE. |
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=head2 WHAT IS A VPN? |
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VPN is an acronym, it stands for: |
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=over 4 |
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=item Virtual |
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Virtual means that no physical network is created (of course), but a |
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network is I<emulated> by creating multiple tunnels between the member |
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nodes by encapsulating and sending data over another transport network. |
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Usually the emulated network is a normal IP or Ethernet, and the transport |
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network is the Internet. However, using a VPN system like GVPE to connect |
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nodes over other untrusted networks such as Wireless LAN is not uncommon. |
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=item Private |
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Private means that non-participating nodes cannot decode ("sniff)" nor |
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inject ("spoof") packets. This means that nodes can be connected over |
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untrusted networks such as the public Internet without fear of being |
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eavesdropped while at the same time being able to trust data sent by other |
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nodes. |
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In the case of GVPE, even participating nodes cannot sniff packets |
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send to other nodes or spoof packets as if sent from other nodes, so |
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communications between any two nodes is private to those two nodes. |
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=item Network |
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Network means that more than two parties can participate in the network, |
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so for instance it's possible to connect multiple branches of a company |
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into a single network. Many so-called "VPN" solutions only create |
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point-to-point tunnels, which in turn can be used to build larger |
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networks. |
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GVPE provides a true multi-point network in which any number of nodes (at |
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least a few dozen in practise, the theoretical limit is 4095 nodes) can |
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participate. |
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=back |
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=head2 GVPE DESIGN GOALS |
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=over 4 |
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=item SIMPLE DESIGN |
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Cipher, HMAC algorithms and other key parameters must be selected |
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at compile time - this makes it possible to only link in algorithms |
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you actually need. It also makes the crypto part of the source very |
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transparent and easy to inspect, and last not least this makes it possible |
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to hardcode the layout of all packets into the binary. GVPE goes a step |
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further and internally reserves blocks of the same length for all packets, |
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which virtually removes all possibilities of buffer overflows, as there is |
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only a single type of buffer and it's always of fixed length. |
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=item EASY TO SETUP |
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A few lines of config (the config file is shared unmodified between all |
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hosts) and a single run of C<gvpectrl> to generate the keys suffices to |
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make it work. |
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=item MAC-BASED SECURITY |
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Since every host has it's own private key, other hosts cannot spoof |
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traffic from this host. That makes it possible to filter packet by MAC |
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address, e.g. to ensure that packets from a specific IP address come, in |
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fact, from a specific host that is associated with that IP and not from |
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another host. |
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=back |
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=head1 PROGRAMS |
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Gvpe comes with two programs: one daemon (C<gvpe>) and one control program |
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(C<gvpectrl>). |
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=over 4 |
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=item gvpectrl |
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This program is used to generate the keys, check and give an overview of of the |
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configuration and to control the daemon (restarting etc.). |
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=item gvpe |
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This is the daemon used to establish and maintain connections to the other |
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network nodes. It should be run on the gateway of each VPN subnet. |
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=back |
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=head1 COMPILETIME CONFIGURATION |
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Please have a look at the C<gvpe.osdep(5)> manpage for platform-specific |
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information. |
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Gvpe hardcodes most encryption parameters. While this reduces flexibility, |
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it makes the program much simpler and helps making buffer overflows |
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impossible under most circumstances. |
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Here are a few recipes for compiling your gvpe, showing the extremes |
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(fast, small, insecure OR slow, large, more secure), between which you |
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should choose: |
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=head2 AS LOW PACKET OVERHEAD AS POSSIBLE |
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./configure --enable-hmac-length=4 --enable-rand-length=0 |
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Minimize the header overhead of VPN packets (the above will result in |
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only 4 bytes of overhead over the raw ethernet frame). This is a insecure |
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configuration because a HMAC length of 4 makes collision attacks based on |
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the birthday paradox pretty easy. |
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=head2 MINIMIZE CPU TIME REQUIRED |
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./configure --enable-cipher=bf --enable-digest=md4 |
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Use the fastest cipher and digest algorithms currently available in |
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gvpe. MD4 has been broken and is quite insecure, though, so using another |
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digest algorithm is recommended. |
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=head2 MAXIMIZE SECURITY |
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./configure --enable-hmac-length=16 --enable-rand-length=8 --enable-digest=sha1 |
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This uses a 16 byte HMAC checksum to authenticate packets (I guess 8-12 |
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would also be pretty secure ;) and will additionally prefix each packet |
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with 8 bytes of random data. In the long run, people should move to |
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SHA-256 and beyond). |
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In general, remember that AES-128 seems to be as secure but faster than |
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AES-192 or AES-256, more randomness helps against sniffing and a longer |
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HMAC helps against spoofing. MD4 is a fast digest, SHA1, RIPEMD160, SHA256 |
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are consecutively better, and Blowfish is a fast cipher (and also quite |
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secure). |
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=head1 HOW TO SET UP A SIMPLE VPN |
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In this section I will describe how to get a simple VPN consisting of |
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three hosts up and running. |
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=head2 STEP 1: configuration |
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First you have to create a daemon configuration file and put it into the |
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configuration directory. This is usually C</etc/gvpe>, depending on how you |
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configured gvpe, and can be overwritten using the C<-c> command line switch. |
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Put the following lines into C</etc/gvpe/gvpe.conf>: |
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udp-port = 50000 # the external port to listen on (configure your firewall) |
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mtu = 1400 # minimum MTU of all outgoing interfaces on all hosts |
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ifname = vpn0 # the local network device name |
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node = first # just a nickname |
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hostname = first.example.net # the DNS name or IP address of the host |
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node = second |
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hostname = 133.55.82.9 |
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node = third |
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hostname = third.example.net |
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The only other file necessary is the C<if-up> script that initializes the |
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virtual ethernet interface on the local host. Put the following lines into |
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C</etc/gvpe/if-up> and make it executable (C<chmod 755 /etc/gvpe/if-up>): |
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#!/bin/sh |
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ip link set $IFNAME address $MAC mtu $MTU up |
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[ $NODENAME = first ] && ip addr add 10.0.1.1 dev $IFNAME |
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[ $NODENAME = second ] && ip addr add 10.0.2.1 dev $IFNAME |
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[ $NODENAME = third ] && ip addr add 10.0.3.1 dev $IFNAME |
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ip route add 10.0.0.0/16 dev $IFNAME |
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This script will give each node a different IP address in the C<10.0/16> |
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network. The internal network (if gvpe runs on a router) should then be |
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set to a subset of that network, e.g. C<10.0.1.0/24> on node C<first>, |
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C<10.0.2.0/24> on node C<second>, and so on. |
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By enabling routing on the gateway host that runs C<gvpe> all nodes will |
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be able to reach the other nodes. You can, of course, also use proxy ARP |
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or other means of pseudo-bridging, or (best) full routing - the choice is |
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yours. |
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=head2 STEP 2: create the RSA key pairs for all hosts |
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Run the following command to generate all key pairs for all nodes (that |
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might take a while): |
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gvpectrl -c /etc/gvpe -g |
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This command will put the public keys into C<< |
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/etc/gvpe/pubkeys/I<nodename> >> and the private keys into C<< |
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/etc/gvpe/hostkeys/I<nodename> >>. |
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=head2 STEP 3: distribute the config files to all nodes |
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Now distribute the config files and private keys to the other nodes. This |
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should be done in two steps, since only the private keys meant for a node |
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should be distributed (so each node has only it's own private key). |
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The example uses rsync-over-ssh |
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First all the config files without the hostkeys should be distributed: |
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rsync -avzessh /etc/gvpe first.example.net:/etc/. --exclude hostkeys |
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rsync -avzessh /etc/gvpe 133.55.82.9:/etc/. --exclude hostkeys |
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rsync -avzessh /etc/gvpe third.example.net:/etc/. --exclude hostkeys |
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Then the hostkeys should be copied: |
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rsync -avzessh /etc/gvpe/hostkeys/first first.example.net:/etc/hostkey |
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rsync -avzessh /etc/gvpe/hostkeys/second 133.55.82.9:/etc/hostkey |
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rsync -avzessh /etc/gvpe/hostkeys/third third.example.net:/etc/hostkey |
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You should now check the configuration by issuing the command C<gvpectrl -c |
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/etc/gvpe -s> on each node and verify it's output. |
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=head2 STEP 4: starting gvpe |
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You should then start gvpe on each node by issuing a command like: |
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gvpe -D -l info first # first is the nodename |
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This will make the gvpe daemon stay in foreground. You should then see |
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"connection established" messages. If you don't see them check your |
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firewall and routing (use tcpdump ;). |
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If this works you should check your networking setup by pinging various |
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endpoints. |
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To make gvpe run more permanently you can either run it as a daemon (by |
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starting it without the C<-D> switch), or, much better, from your inittab |
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or equivalent. I use a line like this on all my systems: |
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t1:2345:respawn:/opt/gvpe/sbin/gvpe -D -L first >/dev/null 2>&1 |
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=head2 STEP 5: enjoy |
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... and play around. Sending a -HUP (C<gvpectrl -kHUP>) to the daemon |
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will make it try to connect to all other nodes again. If you run it from |
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inittab, as is recommended, C<gvpectrl -k> (or simply C<killall gvpe>) will |
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kill the daemon, start it again, making it read it's configuration files |
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again. |
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=head1 SEE ALSO |
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gvpe.osdep(5) for OS-dependent information, gvpe.conf(5), gvpectrl(8), |
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and for a description of the transports, protocol, and routing algorithm, |
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gvpe.protocol(7). |
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The GVPE mailing list, at L<http://lists.schmorp.de/>, or |
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C<gvpe@lists.schmorp.de>. |
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=head1 AUTHOR |
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Marc Lehmann <gvpe@schmorp.de> |
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=head1 COPYRIGHTS AND LICENSES |
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GVPE itself is distributed under the GENERAL PUBLIC LICENSE (see the file |
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COPYING that should be part of your distribution). |
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In some configurations it uses modified versions of the tinc vpn suite, |
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which is also available under the GENERAL PUBLIC LICENSE. |
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