This is the info manual for tinc, a Virtual Private Network daemon.

Copyright © 1998,199,2000 Ivo Timmermans <itimmermans@bigfoot.com>, Guus Sliepen <guus@sliepen.warande.net> and Wessel Dankers <wsl@nl.linux.org>.

$Id: tinc.texi,v 1.8.4.9 2000/11/30 23:39:55 zarq Exp $

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Introduction
************

tinc is a Virtual Private Network (VPN) daemon that uses tunneling and encryption to create a secure private network between hosts on the Internet.

Because the tunnel appears to the IP level network code as a normal network device, there is no need to adapt any existing software.

This tunneling allows VPN sites to share information with each other over the Internet without exposing any information to others.

This document is the manual for tinc.  Included are chapters on how to configure your computer to use tinc, as well as the configuration process of tinc itself.

Virtual Private Networks
========================

A Virtual Private Network or VPN is a network that can only be accessed by a few elected computers that participate.  This goal is achievable in more than just one way.

Private networks can consist of a single stand-alone ethernet LAN.  Or even two computers hooked up using a null-modem cable.  In these cases, it is obvious that the network is private, no one can access it from the outside.  But if your computers are linked to the internet, the network is not private anymore, unless one uses firewalls to block all private traffic.  But then, there is no way to send private data to trusted computers on the other end of the internet.

This problem can be solved by using virtual networks.  Virtual networks can live on top of other networks, but do not interfere with each other.  Mostly, virtual networks appear like a singe LAN, even though they can span the entire world.  But virtual networks can't be secured by using firewalls, because the traffic that flows through it has to go through the internet, where other people can look at it.

When one introduces encryption, we can form a true VPN.  Other people may see encrypted traffic, but if they don't know how to decipher it (they need to know the key for that), they cannot read the information that flows through the VPN.  This is what tinc was made for.

tinc uses normal IP datagrams to encapsulate data that goes over the VPN network link.  In this case it's also clear that the network is virtual, because no direct network link has to exist between to participants.

As is the case with either type of VPN, anybody could eavesdrop.  Or worse, alter data.  Hence it's probably advisable to encrypt the data that flows over the network.

tinc
====

I really don't quite remember what got us started, but it must have been Guus' idea.  He wrote a simple implementation (about 50 lines of C) that used the ethertap device that Linux knows of since somewhere about kernel 2.1.60.  It didn't work immediately and he improved it a bit.  At this stage, the project was still simply called vpnd.

Since then, a lot has changed---to say the least.

tinc now supports encryption, it consists of a single daemon (tincd) for both the receiving and sending end, it has become largely runtime-configurable---in short, it has become a full-fledged professional package.

A lot can---and will be---changed.  I have a few things that I'd like to see in the future releases of tinc.  Not everything will be available in the near future.  Our first objective is to make tinc work perfectly as it stands, and then add more advanced features.

Meanwhile, we're always open-minded towards new ideas.  And we're available too.

Supported platforms
===================

tinc works on Linux, FreeBSD and Solaris.  These are the three platforms that are supported by the universial TUN/TAP device driver, so if support for other operating systems is added to this driver, perhaps tinc will run on them as well.  Without this driver, tinc will most likely compile and run, but it will not be able to send or receive data packets.

For a more up to date list, please check the list on our website: http://tinc.nl.linux.org/platforms.html.

Linux
-----

tinc was first written for Linux running on an intel x86 processor, so this is the best supported platform.  The protocol however, and actually anything about tinc, has been rewritten to support random byte ordering and arbitrary word length.  So in theory it should run on other processors that Linux runs on.  Take care however, we haven't been able to really test it yet.  If you want to run tinc on another platform than x86, and want to tell us how it went, please do so.

tinc uses the ethertap device that is provided in the standard kernel since version 2.1.60, so anything above that (2.2.x, 2.3.x, and the 2.4.0-testx (which is current at the time of this writing) kernel versions) is able to support tinc.

FreeBSD
-------

tinc on FreeBSD relies on the universial TUN/TAP driver for its data acquisition from the kernel.  Therefore, tinc suports the same platforms as this driver.  These are: FreeBSD 3.x, 4.x, 5.x.

Solaris
-------

tinc on Solaris relies on the universial TUN/TAP driver for its data acquisition from the kernel.  Therefore, tinc suports the same platforms as this driver.  These are: Solaris, 2.1.x.

Installing tinc: preparations
*****************************

This chapter contains information on how to prepare your system to support tinc.

Configuring the kernel
======================

If you are running Linux, chances are good that your kernel already supports all the devices that tinc needs for proper operation.  For example, the standard kernel from Redhat Linux already has support for ethertap and netlink compiled in.  Debian users can use the modconf utility to select the modules.  If your Linux distribution supports this method of selecting devices, look out for something called `ethertap', and `netlink_dev'.  You need both these devices.

If you can install these devices in a similar manner, you may skip this section.

Configuring the Linux kernel
----------------------------

Since this particular implementation only runs on 2.1 or higher Linux kernels, you should grab one (2.2 is current at this time).  A 2.0 port is not really possible, unless someone tells me someone ported the ethertap and netlink devices back to 2.0.

If you are unfamiliar with the process of configuring and compiling a new kernel, you should read the Kernel HOWTO (http://howto.linuxberg.com/LDP/HOWTO/Kernel-HOWTO.html) first.  Do that now!

Here are the options you have to turn on when configuring a new kernel.

For kernel 2.2.x:

     Code maturity level options
     [*] Prompt for development and/or incomplete code/drivers
     Networking options
     [*] Kernel/User netlink socket
     <*> Netlink device emulation
     Network device support
     <*> Ethertap network tap

For kernel 2.3.x and 2.4.x:

     Code maturity level options
     [*] Prompt for development and/or incomplete code/drivers
     Networking options
     [*] Kernel/User netlink socket
     <*> Netlink device emulation
     Network device support
     <*> Universal TUN/TAP device driver support

Any other options not mentioned here are not relevant to tinc.  If you decide to build any of these as dynamic kernel modules, it's a good idea to add these lines to /etc/modules.conf.

     alias tap0 ethertap
     alias char-major-36 netlink_dev

If you have a 2.4 kernel, you can also choose to use the `Ethertap network tap' device.  This is marked obsolete, because the universal TUN/TAP driver is a newer implementation that is supposed to be used in favor of ethertap.  For tinc, it doesn't really matter which one you choose; based on the device file name, tinc will make the right choice about what protocol to use.

Finally, after having set up other options, build the kernel and boot it.  Unfortunately it's not possible to insert these modules in a running kernel.

Configuring the FreeBSD kernel
------------------------------

This section will contain information on how to configure your FreeBSD kernel to support the universal TUN/TAP device.  For 5.0 and 4.1 systems, this is included in the kernel configuration, for earlier systems (4.0 and 3.x), you need to install the universal TUN/TAP driver yourself.

Unfortunately somebody still has to write the text.

Configuring the Solaris kernel
------------------------------

This section will contain information on how to configure your Solaris kernel to support the universal TUN/TAP device.  You need to install this driver yourself.

Unfortunately somebody still has to write the text.

Libraries
=========

Before you can configure or build tinc, you need to have the OpenSSL library installed on your system.  If you try to configure tinc without having installed it, configure will give you an error message, and stop.

OpenSSL
-------

For all cryptography-related functions, tinc uses the functions provided by the OpenSSL library.  We recommend using version 0.9.5 or 0.9.6 of this library.  Other versions may also work, but we can guarantee nothing.

If this library is not installed, you wil get an error when configuring tinc for build.  Support for running tinc without having OpenSSL installed may be added in the future.

You can use your operating system's package manager to install this if available.  Make sure you install the development AND runtime versions of this package.

If you have to install OpenSSL manually, you can get the source code from http://www.openssl.org/.  Instructions on how to configure, build and install this package are included within the package.  Please make sure you build development and runtime libraries (which is the default).

If you installed the OpenSSL libraries from source, it may be necessary to let configure know where they are, by passing configure one of the --with-openssl-* parameters.

     --with-openssl=DIR      OpenSSL library and headers prefix
     --with-openssl-include=DIR OpenSSL headers directory
                             (Default is OPENSSL_DIR/include)
     --with-openssl-lib=DIR  OpenSSL library directory
                             (Default is OPENSSL_DIR/lib)

Installing tinc: installation
*****************************

If you use Redhat or Debian, you may want to install one of the precompiled packages for your system.  These packages are equipped with system startup scripts and sample configurations.

If you don't run either of these systems, or you want to compile tinc for yourself, you can use the source.  The source is distributed under the GNU General Public License (GPL).  Download the source from the download page (http://tinc.nl.linux.org/download.html), which has the checksums of these files listed; you may wish to check these with md5sum before continuing.

tinc comes in a handy autoconf/automake package, which you can just treat the same as any other package.  Which is just untar it, type `configure' and then `make'.

More detailed instructions are in the file INSTALL, which is included in the source distribution.

Building tinc
=============

Detailed instructions on configuring the source and building tinc can be found in the file called INSTALL.

System files
============

Before you can run tinc, you

Device files
------------

First, you'll need the special device file(s) that form the interface between the kernel and the daemon.

The permissions for these files have to be such that only the super user may read/write to this file.  You'd want this, because otherwise eavesdropping would become a bit too easy.  This does, however, imply that you'd have to run tincd as root.

If you use the universal TUN/TAP driver, you have to create the following device files (unless they already exist):

     mknod -m 600 /dev/... c .. ..
     chown 0.0 /dev/...

If you want to have more devices, the device numbers will be .. .. ...

If you use Linux, and you run the new 2.4 kernel using the devfs filesystem, then the tap device will be automatically generated as /dev/netlink/tap0.

If you use Linux and have kernel 2.2.x, you have to make the ethertap devices:

     mknod -m 600 /dev/tap0 c 36 16
     chown 0.0 /dev/tap0

Any further ethertap devices have minor device number 16 through 31.

Other files
-----------

/etc/networks
.............

You may add a line to /etc/networks so that your VPN will get a symbolic name.  For example:

     myvpn 10.0.0.0

This has nothing to do with the MyVPNIP configuration variable that will be discussed later, it is only to make the output of the route command more legible.

/etc/services
.............

You may add this line to /etc/services.  The effect is that you may supply a tinc as a valid port number to some programs.  The number 655 is registered with the IANA.

     tinc            655/tcp    TINC
     tinc            655/udp    TINC
     #                          Ivo Timmermans <itimmermans@bigfoot.com>

Interfaces
==========

Before you can start transmitting data over the tinc tunnel, you must set up the ethertap network devices.

First, decide which IP addresses you want to have associated with these devices, and what network mask they must have.  You also need these numbers when you are going to configure tinc itself.  See Configuring tinc.

It doesn't matter much which part you do first, setting up the network devices or configure tinc.  But they both have to be done before you try to start a tincd.

The actual setup of the ethertap device is quite simple, just repeat after me:

     ifconfig tapn hw ether fe:fd:00:00:00:00

Note: Since version 1.0pre3, all interface addresses are set to this address, whereas previous versions required the MAC to match the actual IP address.

To activate the device, you have to assign an IP address to it.  To set an IP address IP with network mask mask, do the following:

     ifconfig tapn xx.xx.xx.xx netmask mask

The netmask is the mask of the entire VPN network, not just your own subnet.  It is the same netmask you will have to specify with the VpnMask configuration variable.

Configuring tinc
****************

Multiple networks
=================

It is perfectly OK for you to run more than one tinc daemon. However, in its default form, you will soon notice that you can't use two different configuration files without the -c option.

We have thought of another way of dealing with this: network names.  This means that you call tincd with the -n argument, which will assign a name to this daemon.

The effect of this is that the daemon will set its configuration ``root'' to /etc/tinc/nn/, where nn is your argument to the -n option.  You'll notice that it appears in syslog as ``tinc.nn''.

However, it is not strictly necessary that you call tinc with the -n option.  In this case, the network name would just be empty, and it will be used as such.  tinc now looks for files in /etc/tinc/, instead of /etc/tinc/nn/; the configuration file should be /etc/tinc/tinc.conf, and the passphrases are now expected to be in /etc/tinc/passphrases/.

But it is highly recommended that you use this feature of tinc, because it will be so much clearer whom your daemon talks to.  Hence, we will assume that you use it.

How connections work
====================

Before going on, first a bit on how tinc sees connections.

When tinc starts up, it reads in the configuration file and parses the command-line options.  If it sees a `ConnectTo' value in the file, it will try to connect to it, on the given port.  If this fails, tinc exits.

Configuration file
==================

The actual configuration of the daemon is done in the file /etc/tinc/nn/tinc.conf.

This file consists of comments (lines started with a #) or assignments in the form of

     Variable = Value.

The variable names are case insensitive, and any spaces, tabs, newlines and carriage returns are ignored.  Note: it is not required that you put in the `=' sign, but doing so improves readability.  If you leave it out, remember to replace it with at least one space character.

Variables
---------

Here are all valid variables, listed in alphabetical order.  The default value, required or optional is given between parentheses.

ConnectPort = <port> (655)
     Connect to the upstream host (given with the ConnectTo directive) on port port.  port may be given in decimal (default), octal (when preceded by a single zero) or hexadecimal (prefixed with 0x).  port is the port number for both the UDP and the TCP (meta) connections.

ConnectTo = <IP address|hostname> (optional)
     Specifies which host to connect to on startup.  Multiple ConnectTo variables may be specified, if connecting to the first one fails then tinc will try the next one, and so on.  It is possible to specify hostnames for dynamic IP addresses (like those given on dyndns.org), tinc will not cache the resolved IP address.

     If you don't specify a host with ConnectTo, regardless of whether a value for ConnectPort is given, tinc won't connect at all, and will instead just listen for incoming connections.

Hostnames = <yes|no> (no)
     This option selects whether IP addresses (both real and on the VPN) should be resolved.  Since DNS lookups are blocking, it might affect tinc's efficiency, even stopping the daemon for a few seconds everytime it does a lookup if your DNS server is not responding.

     This does not affect resolving hostnames to IP addresses from the configuration file.

IndirectData = <yes|no> (no)
     This option specifies whether other tinc daemons besides the one you specified with ConnectTo can make a direct connection to you.  This is especially useful if you are behind a firewall and it is impossible to make a connection from the outside to your tinc daemon.  Otherwise, it is best to leave this option out or set it to no.

Interface = <device> (optional)
     If you have more than one network interface in your computer, tinc will by default listen on all of them for incoming connections.  It is possible to bind tinc to a single interface like eth0 or ppp0 with this variable.

InterfaceIP = <local address> (optional)
     If your computer has more than one IP address on a single interface (for example if you are running virtual hosts), tinc will by default listen on all of them for incoming connections.  It is possible to bind tinc to a single IP address with this variable.  It is still possible to listen on several interfaces at the same time though, if they share the same IP address.

KeyExpire = <seconds> (3600)
     This option controls the time the encryption keys used to encrypt the data are valid.  It is common practice to change keys at regular intervals to make it even harder for crackers, even though it is thought to be nearly impossible to crack a single key.

ListenPort = <port> (655)
     Listen on local port port.  The computer connecting to this daemon should use this number as the argument for his ConnectPort.

MyOwnVPNIP = <local address[/maskbits]> (required)
     The local address is the number that the daemon will propagate to other daemons on the network when it is identifying itself.  Hence this will be the file name of the passphrase file that the other end expects to find the passphrase in.

     The local address is the IP address of the tap device, not the real IP address of the host running tincd.  Due to changes in recent kernels, it is also necessary that you make the ethernet (also known as MAC) address equal to the IP address (see the example).

     maskbits is the number of bits set to 1 in the netmask part.

MyVirtualIP = <local address[/maskbits]>
     This is an alias for MyOwnVPNIP.

PingTimeout = <seconds> (5)
     The number of seconds of inactivity that tinc will wait before sending a probe to the other end.  If that other end doesn't answer within that same amount of seconds, the connection is terminated, and the others will be notified of this.

PrivateKey = <key>
     This is a sequence of hexadecimal numbers, as generated by ``tincd --generate-keys''.  Please be careful with line breaking, the entire key should be on one line.

PublicKey = <key>
     This is a sequence of hexadecimal numbers, as generated by ``tincd --generate-keys''.  Please be careful with line breaking, the entire key should be on one line.

TapDevice = <device> (/dev/tap0)
     The ethertap device to use.  Note that you can only use one device per daemon.  The info pages of the tinc package contain more information about configuring an ethertap device for Linux.

TCPonly = <yes|no> (no, experimental)
     If this variable is set to yes, then the packets are tunnelled over a TCP connection instead of a UDP connection.  This is especially useful for those who want to run a tinc daemon from behind a masquerading firewall, or if UDP packet routing is disabled somehow.  This is experimental code, try this at your own risk.

VpnMask = <mask> (optional)
     The mask that defines the scope of the entire VPN.  This option is not used by the tinc daemon itself, but can be used by startup scripts to configure the ethertap devices correctly.

Example
=======

Imagine the following situation.  An A-based company wants to connect three branch offices in B, C and D using the internet.  All four offices have a 24/7 connection to the internet.

A is going to serve as the center of the network.  B and C will connect to A, and D will connect to C.  Each office will be assigned their own IP network, 10.x.0.0.

     A: net 10.1.0.0 mask 255.255.0.0 gateway 10.1.54.1 internet IP 1.2.3.4
     B: net 10.2.0.0 mask 255.255.0.0 gateway 10.2.1.12 internet IP 2.3.4.5
     C: net 10.3.0.0 mask 255.255.0.0 gateway 10.3.69.254 internet IP 3.4.5.6
     D: net 10.4.0.0 mask 255.255.0.0 gateway 10.4.3.32 internet IP 4.5.6.7

``gateway'' is the VPN IP address of the machine that is running the tincd.  ``internet IP'' is the IP address of the firewall, which does not need to run tincd, but it must do a port forwarding of TCP&UDP on port 655 (unless otherwise configured).

In this example, it is assumed that eth0 is the interface that points to the inner LAN of the office, although this could also be the same as the interface that leads to the internet.  The configuration of the real interface is also shown as a comment, to give you an idea of how these example host is set up.

For A
.....

A would be configured like this:

     #ifconfig eth0 10.1.54.1 netmask 255.255.0.0 broadcast 10.1.255.255
     ifconfig tap0 hw ether fe:fd:0a:01:36:01
     ifconfig tap0 10.1.54.1 netmask 255.0.0.0

and in /etc/tinc/tinc.conf:

     TapDevice = /dev/tap0
     MyVirtualIP = 10.1.54.1/16
     VpnMask = 255.0.0.0

For B
.....

     #ifconfig eth0 10.2.43.8 netmask 255.255.0.0 broadcast 10.2.255.255
     ifconfig tap0 hw ether fe:fd:0a:02:01:0c
     ifconfig tap0 10.2.1.12 netmask 255.0.0.0

and in /etc/tinc/tinc.conf:

     TapDevice = /dev/tap0
     MyVirtualIP = 10.2.1.12/16
     ConnectTo = 1.2.3.4
     VpnMask = 255.0.0.0

Note here that the internal address (on eth0) doesn't have to be the same as on the tap0 device.  Also, ConnectTo is given so that no-one can connect to this node.

For C
.....

     #ifconfig eth0 10.3.69.254 netmask 255.255.0.0 broadcast 10.3.255.255
     ifconfig tap0 hw ether fe:fd:0a:03:45:fe
     ifconfig tap0 10.3.69.254 netmask 255.0.0.0

and in /etc/tinc/A/tinc.conf:

     MyVirtualIP = 10.3.69.254/16
     TapDevice = /dev/tap1
     ConnectTo = 1.2.3.4
     ListenPort = 2000
     VpnMask = 255.0.0.0

C already has another daemon that runs on port 655, so they have to reserve another port for tinc.  It can connect to other tinc daemons on the regular port though, so no ConnectPort variable is needed. They also use the netname to distinguish between the two.  tinc is started with `tincd -n A'.

For D
.....

     #ifconfig tap0 10.4.3.32 netmask 255.255.0.0 broadcast 10.4.255.255
     ifconfig tap0 hw ether fe:fd:0a:04:03:20
     ifconfig tap0 10.4.3.32 netmask 255.0.0.0

and in /etc/tinc/tinc.conf:

     MyVirtualIP = 10.4.3.32/16
     ConnectTo = 3.4.5.6
     ConnectPort = 2000
     VpnMask=255.0.0.0

D will be connecting to C, which has a tincd running for this network on port 2000.  Hence they need to put in a ConnectPort, but it doesn't need to have a different ListenPort.

Authentication
..............

A, B, C and D all generate a passphrase with genauth 2048, the output is stored in /etc/tinc/passphrases/local, except for C, where it should be /etc/tinc/A/passphrases/local.

A stores a copy of B's passphrase in /etc/tinc/passphrases/10.2.1.12

A stores a copy of C's passphrase in /etc/tinc/passphrases/10.3.69.254

B stores a copy of A's passphrase in /etc/tinc/passphrases/10.1.54.1

C stores a copy of A's passphrase in /etc/tinc/A/passphrases/10.1.54.1

C stores a copy of D's passphrase in /etc/tinc/A/passphrases/10.4.3.32

D stores a copy of C's passphrase in /etc/tinc/passphrases/10.3.69.254

Starting
........

A has to start their tincd first.  Then come B and C, where C has to provide the option `-n A', because they have more than one tinc network.  Finally, D's tincd is started.

Running tinc
************

Running tinc isn't just as easy as typing `tincd' and hoping everything will just work out the way you wanted.  Instead, the use of tinc is a project that involves trust relations and more than one computer.

Managing keys
=============

Before attempting to start tinc, you have to create passphrases.  When tinc tries to make a connection, it exchanges some sensitive data.  Before doing so, it likes to know if the other end is trustworthy.

To do this, both ends must have some knowledge about the other.  In the case of tinc this is the authentication passphrase.

This passphrase is a number, which is chosen at random.  This number is then sent to the other computers which want to talk to us directly.  To avoid breaking security, this should be done over a known secure channel (such as ssh or similar).

All passphrases are stored in the passphrases directory, which is normally /etc/tinc/nn/passphrases/, but it may be changed using the `Passphrases' option in the config file.

To generate a passphrase, run `genauth'.  genauth takes one argument, which is the length of the passphrase in bits.  The length of the passphrase should be in the range 1024--2048 for a key length of 128 bits.  genauth creates a random number of the specified length, and puts it to stdout.

Every computer that wants to participate in the VPN should do this, and store the output in the passphrases directory, in the file local.

When every computer has his own local key, it should copy it to the computer that it wants to talk to directly.  (i.e.  the one it connects to during startup.) This should be done via a secure channel, because it is sensitive information.  If this is not done securely, someone might break in on you later on.

Those non-local passphrase files must have the name of the VPN IP address that they will advertise to you.  For instance, if a computer tells us it likes to be 10.1.1.3 with netmask 255.255.0.0, the file should still be called 10.1.1.3, and not 10.1.0.0.

Runtime options
===============

Besides the settings in the configuration file, tinc also accepts some command line options.

This list is a longer version of that in the manpage.  The latter is generated automatically, so may be more up-to-date.

-c, --config=FILE
     Read configuration options from FILE.  The default is /etc/tinc/nn/tinc.conf.

-d
     Increase debug level.  The higher it gets, the more gets logged.  Everything goes via syslog.

     0 is the default, only some basic information connection attempts get logged.  Setting it to 1 will log a bit more, still not very disturbing.  With two -d's tincd will log protocol information, which can get pretty noisy.  Three or more -d's will output every single packet that goes out or comes in, which probably generates more data than the packets themselves.

-k, --kill
     Attempt to kill a running tincd and exit.  A TERM signal (15) gets sent to the daemon that his its PID in /var/run/tinc.nn.pid.

     Because it kills only one tincd, you should use -n here if you use it normally.

-n, --net=NETNAME
     Connect to net NETNAME.  See Multiple networks.

-t, --timeout=TIMEOUT
     Seconds to wait before giving a timeout.  Should not be set too low, because every time tincd senses a timeout, it disconnects and reconnects again, which will cause unnecessary network traffic and log messages.

--help
     Display a short reminder of these runtime options and terminate.

--version
     Output version information and exit.

Technical information
*********************

The basic philosophy of the way tinc works
==========================================

tinc is a daemon that takes VPN data and transmit that to another host computer over the existing Internet infrastructure.

A preview of the way the tinc works
-----------------------------------

The data itself is read from a character device file, the so-called ethertap device.  This device is associated with a network interface.  Any data sent to this interface can be read from the device, and any data written to the device gets sent from the interface.  Data to and from the device is formatted as if it were a normal ethernet card, so a frame is preceded by two MAC addresses and a frame type field.

So when tinc reads an ethernet frame from the device, it determines its type.  Right now, tinc can only handle Internet Protocol version 4 (IPv4) frames.  Plans to support other protocols are being made.  When tinc knows which type of frame it has read, it can also read the source and destination address from it.

Now it is time that the frame gets encrypted.  Currently the only encryption algorithm available is blowfish.

When the encryption is ready, time has come to actually transport the packet to the destination computer.  We do this by sending the packet over an UDP connection to the destination host.  This is called encapsulating, the VPN packet (though now encrypted) is encapsulated in another IP datagram.

When the destination receives this packet, the same thing happens, only in reverse.  So it does a decrypt on the contents of the UDP datagram, and it writes the decrypted information to its own ethertap device.

The meta-connection
-------------------

Having only an UDP connection available is not enough.  Though suitable for transmitting data, we want to be able to reliably send other information, such as routing and encryption information to somebody.

TCP is a better alternative, because it already contains protection against information being lost, unlike UDP.

So we establish two connections.  One for the encrypted VPN data, and one for other information, the meta-data.  Hence, we call the second connection the meta-connection.  We can now be sure that the meta-information doesn't get lost on the way to another computer.

Like with any communication, we must have a protocol, so that everybody knows what everything stands for, an how he should react.  Because we have two connections, we also have two protocols.  The protocol used for the UDP data is the ``data-protocol,'' the other one is the ``meta-protocol.''

The reason we don't use TCP for both protocols is that UDP is much better for encapsulation, even while it is less reliable.  The real problem is that when TCP would be used to encapsulate a TCP stream that's on the private network, for every packet sent there would be three ACK's sent instead of just one.  Furthermore, if there would be a timeout, both TCP streams would sense the timeout, and both would start resending packets.

About tinc's encryption and other security-related issues.
==========================================================

tinc got its name from ``TINC,'' short for There Is No Cabal; the alleged Cabal was/is an organization that was said to keep an eye on the entire Internet.  As this is exactly what you don't want, we named the tinc project after TINC.

But in order to be ``immune'' to eavesdropping, you'll have to encrypt your data.  Because tinc is a Secure VPN (SVPN) daemon, it does exactly that: encrypt.

This chapter is a mixture of ideas, reasoning and explanation, please don't take it too serious.

Key Types
---------

There are several types of encryption keys.  Tinc uses two of them, symmetric private keypairs and public/private keypairs.

Public/private keypairs are used in public key cryptography.  It enables someone to send out a public key with which other people can encrypt their data.  The encrypted data now can only be decrypted by the person who has the private key that matches the public key.  So, a public key only allows other people to send encrypted messages to you.  This is very useful in setting up private communications channels.  Just send out your public key and other people can talk to you in a secure way.  But how can you know the other person is who he says he is?

For authentication itself tinc uses symmetric private keypairs, referred to as a passphrase.  The identity of each tinc daemon is defined by it's passphrase (like you can be identified by your social security number). Every tinc daemon that is allowed to connect to you has a copy of your passphrase (hence symmetrical).

It would also be possible to use public/private keypairs for authentication, so that you could shout out your public key and don't need to keep it secret (like the passphrase you would have to send to someone else).  Also, no one else has to know a private key from you. Both forms have their pros and cons, and at the moment tinc just uses passphrases (which are computationaly more efficient and perhaps in some way more secure).

Key Management
--------------

You can't just send a private encryption key to your peer, because somebody else might already be listening to you.  So you'll have to negotiate over a shared but secret key.  One way to do this is by using the ``Diffie-Hellman key exchange'' protocol (http://www.rsa.com/rsalabs/faq/html/3-6-1.html).  The idea is as follows.

You have two participants A and B that want to agree over a shared secret encryption key.  Both parties have some large prime number p and a generator g.  These numbers may be known to the outside world, and hence may be included in the source distribution.

Both parties then generate a secret key.  A generates a, and computes g^a mod p.  This is then sent to B; while B computes g^b mod p, and transmits this to A, b being generated by B.  Both a and b must be smaller than p-1.

Both parties then calculate g^ab mod p = k.  k is the new, shared, but still secret key.

To obtain a key k of a sufficient length (128 bits in our vpnd), p should be 2^129-1 or more.

Authentication
--------------

Because the Diffie-Hellman protocol is in itself vulnerable to the ``man-in-the-middle attack,'' we should introduce an authentication system.

We will let A transmit a passphrase that is also known to B encrypted with g^a, before A sends this to B.  This way, B can check whether A is really A or just someone else. B will never receive the real passphrase though, because it was encrypted using public/private keypairs.  This way there is no way an imposter could steal A's passphrase.

This passphrase should be 2304 bits for a symmetric encryption system.  But since an asymmetric system is more secure, we could do with 2048 bits.  This only holds if the passphrase is very random.

These passphrases could be stored in a file that is non-readable by anyone else but root; e.g.  /etc/tinc/passphrases with UID 0 and permissions mode 700.

The only thing that needs to be taken care of is how A can securely send a copy of it's passphrase to B if B doesn't have it yet.  This could be done via mail with PGP, but you should be really convinced of the identity of the person who owns the email address you are sending this to. Swapping floppy disks in real life might be the best way to do this!

Protecting your data
--------------------

Now we have securely hidden our data.  But a malicious cracker may still bother you by randomly altering the encrypted data he intercepts.

About us
********

Contact information
===================

tinc's main page is at http://tinc.nl.linux.org/, this server is located in the Netherlands.

We have an IRC channel on the Open Projects IRC network.  Connect to irc.openprojects.net (http://openprojects.nu/services/irc.html), and join channel #tinc.

Authors
=======

Ivo Timmermans (zarq) (itimmermans@bigfoot.com)
     Main coder/hacker and maintainer of the package.

Guus Sliepen (guus)
     Originator of it all, co-author.

Wessel Dankers (Ubiq)
     General obfuscater of the code.

Thank you's to: Dekan, Emphyrio, vDong

Greetings to: braque, Fluor, giggles, macro, smoke, tribbel