tinc/doc/tinc.texi

\input texinfo   @c -*-texinfo-*-
@c %**start of header
@setfilename tinc.info
@settitle tinc Manual
@setchapternewpage odd
@c %**end of header

@ifinfo
@direntry
* tinc: (tinc).              The tinc Manual.
@end direntry

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

Copyright 1998,199,2000 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.

@end ifinfo

@titlepage
@title tinc Manual
@subtitle Setting up a Virtual Private Network with tinc
@author Ivo Timmermans <itimmermans@@bigfoot.com> and Guus Sliepen <guus@sliepen.warande.net>

@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1998,1999,2000 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.

@end titlepage

@c ==================================================================
@node Top, Introduction, (dir), (dir)

@menu
* Introduction::                Introduction
* Configuring a Linux system::  Before compiling tinc
* Installing tinc::             
* Configuring tinc::            
* Running tinc::                
* Technical information::       
* About us::                    
* Concept Index::               All used terms explained
@end menu

@c ==================================================================
@node    Introduction, Configuring a Linux system, Top, Top
@chapter Introduction

@c straight from the www page

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.

@menu
* VPNs::                        Virtual Private Networks in general
* tinc::                        about tinc
@end menu

@c ==================================================================
@node    VPNs, tinc, Introduction, Introduction
@section 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.

@cindex private
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 @emph{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.

@cindex virtual
This problem can be solved by using @emph{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.

@cindex virtual
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
@emph{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.


@c ==================================================================
@node    tinc,  , VPNs, Introduction
@section 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 @emph{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 @samp{vpnd}.

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

@cindex tincd
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.


@c ==================================================================
@node    Configuring a Linux system, Installing tinc, Introduction, Top
@chapter Configuring a Linux system

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

@menu
* Configuring the kernel::      
* Files Needed::                
* Setting up the devices::      
@end menu


@c ==================================================================
@node    Configuring the kernel, Files Needed, Configuring a Linux system, Configuring a Linux system
@section 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
@uref{http://howto.linuxberg.com/LDP/HOWTO/Kernel-HOWTO.html, Kernel
HOWTO} first. Do that now!

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

@example
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
@end example

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 @file{/etc/modules.conf}.

@example
alias tap0 ethertap
alias char-major-36 netlink_dev
@end example

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.


@c ==================================================================
@node    Files Needed, Setting up the devices, Configuring the kernel, Configuring a Linux system
@section Files Needed

@subsubheading Device files

First, you'll need the special device file(s) that form the interface
between the kernel and the daemon. If you are running the new 2.4 kernel and
you are using the devfs filesystem, then the tap device will be automatically
generated as @file{/dev/netlink/tap0}. Otherwise, you have to make it yourself:

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

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. Under devfs, these files will
be automatically generated.


@subsubheading @file{/etc/networks}

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

@example
myvpn 10.0.0.0
@end example

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.

@subsubheading @file{/etc/services}

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

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


@c ==================================================================
@node    Setting up the devices,  , Files Needed, Configuring a Linux system
@section 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. @xref{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:

@example
ifconfig tap@emph{n} hw ether fe:fd:@emph{xx}:@emph{xx}:@emph{xx}:@emph{xx}
@end example

The @emph{n} here is the number of the ethertap device you want to
use. It should be the same @emph{n} as the one you use for
@file{/dev/tap@emph{n}}. The @emph{xx}s 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 @emph{xx}s
match the numbers of the IP address you will give to the tap device
and to the MyOwnVPNIP configuration (which will be discussed later):

@example
ifconfig tap@emph{n} @emph{xx}.@emph{xx}.@emph{xx}.@emph{xx} netmask @emph{mask}
@end example

This will activate the device with an IP address @emph{IP} with network
mask @emph{mask}. The netmask is the mask of the @emph{entire} VPN network,
not just your own subnet. It is the same netmask you will have to specify
with the VpnMask configuration variable.


@c ==================================================================
@node    Installing tinc, Configuring tinc, Configuring a Linux system, Top
@chapter Installing tinc

First download it. This is the
@uref{http://tinc.nl.linux.org/download.html, download
page}, 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 @file{INSTALL}, which is
included in the source distribution.


@c ==================================================================
@node    Configuring tinc, Running tinc, Installing tinc, Top
@chapter Configuring tinc

@menu
* Multiple networks::           
* How connections work::        
* Configuration file::          
* Example::                     
@end menu


@c ==================================================================
@node    Multiple networks, How connections work, Configuring tinc, Configuring tinc
@section Multiple networks

@c from the manpage

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.


@c ==================================================================
@node    How connections work, Configuration file, Multiple networks, Configuring tinc
@section 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.


@c ==================================================================
@node    Configuration file, Example, How connections work, Configuring tinc
@section Configuration file

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

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

@example
Variable = Value.
@end example

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.

@menu
* Variables::                   
@end menu

@c ==================================================================
@node    Variables,  , Configuration file, Configuration file
@subsection Variables

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

@c straight from the manpage
@table @asis
@item 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.

@item 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.

@item 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.

@item 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.

@item 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.

@item 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.

@item 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.

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

@item 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.

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

@item Passphrases = <directory> (/etc/tinc/NETNAME/passphrases)
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.

@item 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.

@item 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.

@item 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.

@item 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.
@end table



@c ==================================================================
@node    Example,  , Configuration file, Configuring tinc
@section 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.

@example
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
@end example

``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.

@subsubheading For A

@emph{A} would be configured like this:

@example
#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
@end example

and in /etc/tinc/tinc.conf:

@example
TapDevice = /dev/tap0
MyVirtualIP = 10.1.54.1/16
VpnMask = 255.0.0.0
@end example

@subsubheading For B

@example
#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
@end example

and in /etc/tinc/tinc.conf:

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

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.

@subsubheading For C

@example
#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
@end example

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

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

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'.

@subsubheading For D

@example
#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
@end example

and in /etc/tinc/tinc.conf:

@example
MyVirtualIP = 10.4.3.32/16
ConnectTo = 3.4.5.6
ConnectPort = 2000
VpnMask=255.0.0.0
@end example

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.

@subsubheading 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

@subsubheading 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.



@c ==================================================================
@node    Running tinc, Technical information, Configuring tinc, Top
@chapter 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.

@menu
* Managing keys::               
* Runtime options::             
@end menu


@c ==================================================================
@node    Managing keys, Runtime options, Running tinc, Running tinc
@section 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 @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.


@c ==================================================================
@node    Runtime options,  , Managing keys, Running tinc
@section 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 from the manpage
@table @asis
@item -c, --config=FILE
Read configuration options from FILE. The default is
@file{/etc/tinc/nn/tinc.conf}.

@item -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.

@item -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.

@item -n, --net=NETNAME
Connect to net NETNAME. @xref{Multiple networks}.

@item -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.

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

@item --version
Output version information and exit.

@end table


@c ==================================================================
@node    Technical information, About us, Running tinc, Top
@chapter Technical information


@c ==================================================================
@menu
* The Connection::              
* Security::                    
@end menu

@node    The Connection, Security, Technical information, Technical information
@section The basic philosophy of the way tinc works
@cindex Connection

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

@menu
* Protocol Preview::            
* The Meta-connection::         
@end menu


@c ==================================================================
@node    Protocol Preview, The Meta-connection, The Connection, The Connection
@subsection A preview of the way the tinc works

@cindex ethertap
@cindex frame type
The data itself is read from a character device file, the so-called
@emph{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 @emph{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.

@cindex encapsulating
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
@emph{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.


@c ==================================================================
@node    The Meta-connection,  , Protocol Preview, The Connection
@subsection 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.

@cindex data-protocol
@cindex meta-protocol
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.

@c ==================================================================
@node    Security,  , The Connection, Technical information
@section About tinc's encryption and other security-related issues.

@cindex tinc
@cindex Cabal
tinc got its name from ``TINC,'' short for @emph{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 @emph{don't} want, we named
the tinc project after TINC.

@cindex SVPN
But in order to be ``immune'' to eavesdropping, you'll have to encrypt
your data. Because tinc is a @emph{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.

@menu
* Key Types::
* Key Management::              
* Authentication::              
* Protection::                  
@end menu

@c ==================================================================
@node    Key Types, Key Management, Security, Security
@subsection Key Types
@c FIXME: check if I'm not talking nonsense

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
@emph{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).

@c ==================================================================
@node    Key Management, Authentication, Key Types, Security
@subsection Key Management
@c FIXME: recheck
@c I did, it sounds sane :) [guus]

@cindex Diffie-Hellman
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
(@uref{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.

@cindex secret key
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.


@c ==================================================================
@node    Authentication, Protection, Key Management, Security
@subsection Authentication
@c FIXME: recheck

@cindex man-in-the-middle attack
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.

@cindex passphrase
@c ehrmz... but we only use 1024 bits passphrases ourselves? [guus]
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. @file{/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!


@c ==================================================================
@node    Protection,  , Authentication, Security
@subsection 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.


@c ==================================================================
@node    About us, Concept Index, Technical information, Top
@chapter About us


@menu
* Contact Information::         
* Authors::                     
@end menu


@c ==================================================================
@node    Contact Information, Authors, About us, About us
@section Contact information

tinc's main page is at @url{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
@uref{http://openprojects.nu/services/irc.html, irc.openprojects.net},
and join channel #tinc.


@c ==================================================================
@node    Authors,  , Contact Information, About us
@section Authors

@table @asis
@item Ivo Timmermans (zarq) (@email{itimmermans@@bigfoot.com})
Main coder/hacker and maintainer of the package.

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

@item Wessel Dankers (Ubiq)
General obfuscater of the code.

@end table

Thank you's to: Dekan, Emphyrio, vDong

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


@c ==================================================================
@node    Concept Index,  , About us, Top
@c        node-name,    next, previous,        up
@unnumbered Concept Index

@c ==================================================================
@printindex cp


@c ==================================================================
@contents
@bye