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

Copyright 1998 Ivo Timmermans <itimmermans@bigfoot.com>

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.

For instance, a VPN can consist of a single stand-alone ethernet LAN. Or even two computers hooked up using a null-modem cable (Though discuss-able, I think it qualifies as a VPN.). In these cases, it is obvious that the network is private. But there is another type of VPN, the type 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.

Configuring a Linux system
**************************

This chapter contains information on how a Linux system is configured for the use of tinc.

Configuring the 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/off when configuring a new kernel.

     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

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

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.

Files Needed
============

Device files
............

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

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

The permissions now will 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 want to, you may also create more device files, which would be numbered 0...15, with minor device numbers 16...31. They all should be owned by root and have permission 600.

/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

/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>

Setting up the devices
======================

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:xx:xx:xx:xx

The n here is the number of the ethertap device you want to use. It should be the same n as the one you use for /dev/tapn. The xxs are four hexadecimal numbers (0--ff). With previous versions of tincd, it didn't matter what they were. But newer kernels require properly set up ethernet addresses. In fact, the old behavior was wrong. It is required that the xxs match MyOwnVPNIP.

     ifconfig tapn IP netmask mask

This will activate the device with an IP address IP with network mask mask.

Installing tinc
***************

First download it. This is 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.

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:

AllowConnect = (yes|no)
     If set to yes, anyone may try to connect to you. If you set this to no, no incoming connections will be accepted. This does not affect the outgoing connections.

ConnectPort = port
     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)
     Specifies which host to connect to on startup. If the ConnectPort variable is omitted, then tinc will try to connect to port 655.

     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. Only the initiator of a tinc VPN should need this.

ListenPort = port
     Listen on local port port. The computer connecting to this daemon should use this number as the argument for his ConnectPort. Again, the default is 655.

MyOwnVPNIP = local address[/maskbits]
     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.

Passphrases = directory
     The directory where tinc will look for passphrases when someone tries to connect. Please see the manpage for genauth(8) for more information about passphrases as used by tinc.

PingTimeout = number
     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.

TapDevice = device
     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.

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. This could be the same as the interface that leads to the internet.

For A
.....

A would be configured like this:

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

and in /etc/tinc/tinc.conf:

     TapDevice = /dev/tap0
     MyVirtualIP = 10.1.54.1/16

For B
.....

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

and in /etc/tinc/tinc.conf:

     TapDevice = /dev/tap0
     MyVirtualIP = 10.2.1.12/16
     ConnectTo = 1.2.3.4
     AllowConnect = no

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

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

     MyVirtualIP = 10.3.69.254/16
     ConnectTo = 1.2.3.4
     ListenPort = 2000

C already has another daemon that runs on port 655, so they have to reserve another port for tinc. They also use the netname to distinguish between the two. tinc is started with `tincd -n A'.

For D
.....

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

and in /etc/tinc/tinc.conf:

     MyVirtualIP = 10.4.3.32/16
     ConnectTo = 3.4.5.6
     ConnectPort = 2000
     AllowConnect = no

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.

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/10.2.0.0

A stores a copy of C's passphrase in /etc/tinc/passphrases/10.3.0.0

B stores a copy of A's passphrase in /etc/tinc/passphrases/10.1.0.0

C stores a copy of A's passphrase in /etc/tinc/A/passphrases/10.1.0.0

C stores a copy of D's passphrase in /etc/tinc/A/passphrases/10.4.0.0

D stores a copy of C's passphrase in /etc/tinc/passphrases/10.3.0.0

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 a 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.''

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 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.

These private keys are generated upon startup, and they are not changed while the connection exists. A possible feature in the future is to dynamically change the keys, every hour for example.

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.

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/vpn/passphrases.

The only thing that needs to be taken care of is how A announces its passphrase to B.

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