tinc/doc/PROTOCOL

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

   Copyright 2000 Guus Sliepen <guus@sliepen.warande.net>,
             2000 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 1.1.2.3 2000/09/10 15:07:41 zarq 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 in Linux.  Also planned is a generic
PPP interface, because it is supported on virtually all UNIX flavours.
The protocols for those interfaces will not be described in this
document.

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

Normal packets are sent without any state information, so the layout
is pretty basic.  An exception to this are the connections which only
use TCP (configured with the directive `TCPonly=yes').  An explanation
of this type of packet is given in the next chapter, when we explain
the meta protocol.

A data packet can only be sent  if the encryption key is known to both
parties, and the connection is  activated.  Normally, tinc opens a UDP
connection when it  receives an acknowledgement that the  newly set up
connection is properly initiated, and has been verified.

 0   1   2   3
|  SOURCE IP  |
| SEQUENCE ID |
| 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.

When tinc daemons connect to each other, they will have to
authenticate each other first.  This is done by exchanging BASIC_INFO,
PASSPHRASE, PUBLIC_KEY and ACK requests.  BASIC_INFO requests contain
the virtual address and netmask of the tinc daemon, protocol version,
port number and flags.  This identifies that tinc daemon, though it
still has to be verified.  To that end, passphrases and public keys are
exchanged.  The passphrases are known at both ends, but they are
encrypted with the public key before transmission.  This way, nobody
that sniffs the network can see what the passphrase actually was, and
at the same time this ensures that the other host really knows the
secret key that belongs to the public key it sends.  If both hosts are
satisfied, the connection is activated, the contents of each other's
connection lists are exchanged and other requests may be sent.  The
following diagram shows how authentication is done:

Client				Server
----------------------------------------------------------------


----------------------------------------------------------------

The client must never make a connection to a server that is already in
it's connection list.  Not only would it corrupt the connection list,
but it would also violate the tree property.  The meta connections must
always be so that there are no loops.  This is very important, because
certain requests are broadcast over the entire network of tinc
daemons.  If there were loops in the network topology, some packets
would be forwarded in a ring until the end of times (or until the ring
breaks, which probably happens before time ends).