tinc/doc/PROTOCOL

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

   Copyright 2000 Guus Sliepen <guus@sliepen.warande.net>

   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 1.1.2.2 2000/07/01 14:32:24 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 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
------------------

See net.h for now.

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
----------------------------------------------------------------
Connects to server:
				Accepts connection.
                                Sends BASIC_INFO:
Verifies BASIC_INFO.
If server is already in
connection list, abort.
Else sends his own BASIC_INFO:
				Verifies BASIC_INFO.
                                If client is alread in
                                connection list, remove
                                old entry.
                                Sends PASSPHRASE:
Receives and stores PASSPHRASE.
Sends his own PASSPHRASE:
				Receives and stores PASSPHRASE.
                                Sends PUBLIC_KEY:
Verifies PUBLIC key and stored
PASSPHRASE. If wrong, abort.
Else sends his own PUBLIC_KEY:
				Verifies PUBLIC key and stored
                                PASSPHRASE. If wrong, abort.
                                Else activates connection and
                                sends ACK and ADD_HOSTs for all
                                known hosts.
Receives ACK and activates
connection.
Sends ADD_HOSTs for all known
hosts.
----------------------------------------------------------------

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, packets
would be sent infinitely.