tinc/doc/PROTOCOL

This is the protocol documentation for tinc, a Virtual Private Network daemon.

   Copyright 2000,2001 Guus Sliepen <guus@sliepen.warande.net>,
             2000,2001 Ivo Timmmermans <itimmermans@bigfoot.com>

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

   Permission is granted to copy and distribute modified versions of
   this documentation 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.

   $Id: PROTOCOL,v 1.1.2.4 2001/01/07 17:08:02 guus Exp $


1.  Protocols used in tinc
-------------------------

tinc uses several protocols to function correctly.  To enter the
network of tinc daemons that make up the virtual private network, tinc
makes TCP connections to other tinc daemons.  It uses the "meta
protocol" for these connections.  To exchange packets on the virtual
network, UDP connections are made and the "packet protocol" is used.
Tinc also needs to exchange network packets with the kernel.  This is
done using the ethertap device or the universal TUN/TAP device that
can be found in various UNIX flavours.

2.  Packet protocol
------------------

Normal packets are sent without any state information, so the layout
is pretty basic.

A data packet can only be sent  if the encryption key is known to both
parties, and the connection is  activated. If the encryption key is not
known, a request is sent to the destination using the meta connection
to retreive it.

 0   1   2   3
| LEN  | DATA :   \
: DATA        .   } encrypted
. :               /
  .


3.  Meta protocol
----------------

The meta protocol is used to tie all tinc daemons together, and
exchange information about which tinc daemon serves which virtual
subnet.

The meta protocol consists of requests that can be sent to the other
side.  Each request has a unique number and several parameters.  All
requests are represented in the standard ASCII character set.  It is
possible to use tools such as telnet or netcat to connect to a tinc
daemon and to read and write requests by hand, provided that one
understands the numeric codes sent.

The authentication scheme is described in the SECURITY file. After a
succesful authentication, the server and the client will exchange all the
information about other tinc daemons and subnets they know of, so that both
sides (and all the other tinc daemons behind them) have their information
synchronised.

daemon	message
--------------------------------------------------------------------------
origin	ADD_HOST daemon a329e18c:655 0
                    |         |      +--> options
                    |         +---------> real address:port
                    +-------------------> name of new tinc daemon
origin	ADD_SUBNET daemon 1,0a010100/ffffff00
                      |   |     |       +--> netmask
                      |   |     +----------> vpn IPv4 network address
                      |   +----------------> subnet type (1=IPv4)
                      +--------------------> owner of this subnet
--------------------------------------------------------------------------

In case daemons leave the VPN, DEL_HOST and DEL_SUBNET messages with exactly
the same syntax are sent to inform the other daemons of the departure.

The keys used to encrypt VPN packets are not sent out directly. This is
because it would generate a lot of traffic on VPNs with many daemons, and
chances are that not every tinc daemon will ever send a packet to every
other daemon. Instead, if a daemon needs a key it sends a request for it
via the meta connection of the nearest hop in the direction of the
destination. If any hop on the way has already learned the key, it will
act as a proxy and forward it's copy back to the requestor.

daemon	message
--------------------------------------------------------------------------
daemon	REQ_KEY origin destination
                   |       +--> name of the tinc daemon it wants the key from
                   +----------> name of the daemon that wants the key      
daemon	ANS_KEY origin destination e4ae0b0a82d6e0078179b5290c62c7d0
                   |       |       \______________________________/
                   |       |                      +--> 128 bits key
                   |       +--> name of the daemon that wants the key
                   +----------> name of the daemon that uses this key
daemon	KEY_CHANGED origin
                      +--> daemon that has changed it's packet key
--------------------------------------------------------------------------

There is also a mechanism to check if hosts are still alive. Since network
failures or a crash can cause a daemon to be killed without properly
shutting down the TCP connection, this is necessary to keep an up to date
connection list. Pings are sent at regular intervals, except when there
is also some other traffic.

daemon	message
--------------------------------------------------------------------------
origin	PING
dest.	PONG
--------------------------------------------------------------------------

This basically covers everything that is sent over the meta connection by
tinc.