Detailed specification of protocol in version 00000800
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Note: work in progress!!

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1. DNS protocol
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Quick alphabetical index / register:
	0-9	Data packet
	A-F	Data packet
	I	IP address
	L	Login
	N	Downstream fragsize	(NS.topdomain A-type reply)
	O	Options
	P	Ping
	R	Downstream fragsize probe
	S	Switch upstream codec
	V	Version
	W				(WWW.topdomain A-type reply)
	Y	Downstream codec check
	Z	Upstream codec check


CMC = 2 byte Cache Miss Counter, increased every time it is used

Version:
Client sends:
	First byte v or V
	Rest encoded with base32:
	4 bytes big endian protocol version
	CMC
Server replies:
	4 chars:
		VACK (version ok), followed by login challenge
		VNAK (version differs), followed by server protocol version
		VFUL (server has no free slots), followed by max users
	4 byte value: means login challenge/server protocol version/max users
	1 byte userid of the new user, or any byte if not VACK

Login:
Client sends:
	First byte l or L
	Rest encoded with base32:
	1 byte userid
	16 bytes MD5 hash of: (first 32 bytes of password) xor (8 repetitions of login challenge)
	CMC
Server replies:
	LNAK means not accepted
	x.x.x.x-y.y.y.y-mtu-netmask means accepted (server ip, client ip, mtu, netmask bits)

IP Request: (for where to try raw login or set data pipe mode)
Client sends:
	First byte i or I
	5 bits coded as Base32 char, meaning userid
	CMC as 3 Base32 chars
Server replies
	BADIP if bad userid
	First byte I
	Then comes external IP address of iodined server
	as 4 bytes (IPv4) or 16 bytes (IPv6)

Upstream codec check / bounce:
Client sends:
	First byte z or Z
	Lots of data that should not be decoded
Server replies:
	The requested domain copied raw, in the lowest-grade downstream codec
	available for the request type.

Downstream codec check:
Client sends:
	First byte y or Y
	1 char, meaning downstream codec to use
	5 bits coded as Base32 char, meaning check variant
	CMC as 3 Base32 chars
	Possibly extra data, depending on check variant
Server sends:
	Data encoded with requested downstream codec; data content depending
	on check variant number.
	BADCODEC if requested downstream codec not available.
	BADLEN if check variant is not available, or problem with extra data.

	Downstream codec chars are same as in 'O' Option request, below.

	Check variants:
	1: Send encoded DOWNCODECCHECK1 string as defined in encoding.h

	(Other variants reserved; possibly variant that sends a decoded-encoded
	copy of Base32-encoded extra data in the request)

Switch codec:
Client sends:
	First byte s or S
	5 bits coded as Base32 char, meaning userid
	5 bits coded as Base32 char, representing number of raw bits per
	encoded byte:
		5: Base32   (a-z0-5)
		6: Base64   (a-zA-Z0-9+-)
		26: Base64u (a-zA-Z0-9_-)
		7: Base128  (a-zA-Z0-9\274-\375)
	CMC as 3 Base32 chars
Server sends:
	Name of codec if accepted. After this all upstream data packets must
	be encoded with the new codec.
	BADCODEC if not accepted. Client must then revert to previous codec
	BADLEN if length of query is too short

Options:
Client sends:
	First byte o or O
	5 bits coded as Base32 char, meaning userid
	1 char, meaning option
	CMC as 3 Base32 chars
Server sends:
	Full name of option if accepted. After this, option immediately takes
	effect in server.
	BADCODEC if not accepted. Previous situation remains.
	All options affect only the requesting client.

	Option chars:
	t or T: Downstream encoding Base32, for TXT/CNAME/A/MX (default)
	s or S: Downstream encoding Base64, for TXT/CNAME/A/MX
	u or U: Downstream encoding Base64u, for TXT/CNAME/A/MX
	v or V: Downstream encoding Base128, for TXT/CNAME/A/MX
	r or R: Downstream encoding Raw, for PRIVATE/TXT/NULL (default for
		PRIVATE and NULL)
	If codec unsupported for request type, server will use Base32; note
	that server will answer any mix of request types that a client sends.
	Server may disregard this option; client must always use the downstream
	encoding type indicated in every downstream DNS packet.

	l or L: Lazy mode, server will keep one request unanswered until the
	next one comes in. Applies only to data transfer; handshake is always
	answered immediately.
	i or I: Immediate (non-lazy) mode, server will answer all requests
	(nearly) immediately.

Probe downstream fragment size:
Client sends:
	First byte r or R
	15 bits coded as 3 Base32 chars: UUUUF FFFFF FFFFF
		meaning 4 bits userid, 11 bits fragment size
	Then follows a long random query which contents does not matter
Server sends:
	Requested number of bytes as a response. The first two bytes contain
	the requested length. The third byte is 107 (0x6B). The fourth byte
	is a random value, and each following byte is incremented with 107.
	This is checked by the client to determine corruption.
	BADFRAG if requested length not accepted.

Set downstream fragment size:
Client sends:
	First byte n or N
	Rest encoded with base32:
	1 byte userid
	2 bytes new downstream fragment size
	CMC
Server sends:
	2 bytes new downstream fragment size. After this all downstream
	payloads will be max (fragsize + 2) bytes long.
	BADFRAG if not accepted.

Upstream data header:
	 76543 21076 54321076 54321076 5432
	+!----+!----+!----!--+--!----!+----+
	|0UUUU|UDCMC|SSSSSSSS|DDDDDDDD|ANFL|
	+-----+-----+--------+--------+----+

Downstream data header:        |=> only if P(ing) bit set
	 76543210 76543210 76543210 76543210 76543210
	+--------+--------+--------+--------+--------+
	|DDDDDDDD|SSSSSSSS|00PCANFL|ZZZZZZZZ|WWWWWWWW|
	+--------+--------+--------+--------+--------+

UUUU = Userid
L = Last fragment flag
A = ACK flag
N = NACK flag (unused)
F = First fragment flag
C = Compression enabled for downstream packet
P = is ping response
SSSSSSSS = Upstream packet sequence number/ACK
DDDDDDDD = Downstream packet sequence number/ACK
ZZZZZZZZ = Downstream window size
WWWWWWWW = Upstream window size
UDCMC = Upstream Data CMC char (actually base36 [a-z0-9]), case-insensitive

Upstream data packet starts with 1 byte ASCII hex coded user byte; then
1 char data-CMC; then 4 bytes Base32 encoded header; then comes the payload
data, encoded with the chosen upstream codec.

Downstream data starts with 3 byte header. Then payload data, which may be
compressed. If Ping flag is set, another 8 bits of downstream window size and
8 bits of upstream window size are appended to the header and the downstream or
upstream sequence IDs are the start of the remote windows. This occurs either
when the server wants to re-sync its window parameters or when the client has
requested a ping response.

In NULL and PRIVATE responses, downstream data is always raw. In all other
response types, downstream data is encoded (see Options above).
Encoding type is indicated by 1 prefix char:
TXT:
	End result is always DNS-chopped (series of len-prefixed strings
	<=255 bytes)
	t or T: Base32	 encoded before chop, decoded after un-chop
	s or S: Base64	 encoded before chop, decoded after un-chop
	u or U: Base64u	 encoded before chop, decoded after un-chop
	v or V: Base128	 encoded before chop, decoded after un-chop
	r or R: Raw	 no encoding, only DNS-chop
SRV/MX/CNAME/A:
	h or H: Hostname encoded with Base32
	i or I: Hostname encoded with Base64
	j or J: Hostname encoded with Base64u
	k or K: Hostname encoded with Base128
SRV and MX may reply with multiple hostnames, each encoded separately. Each
has a 10-multiple priority, and encoding/decoding is done in strictly
increasing priority sequence 10, 20, 30, etc. without gaps. Note that some DNS
relays will shuffle the answer records in the response.

Ping:
Client sends:
	First byte p or P
	Second byte CMC
	Rest encoded with Base32:
	 76543 21076543 21076543 21076543 21076543 210
	+!----+!----!--+--!----!+----!---+-!----!-+--------+
	|0UUUU|DDDDDDDD|SSSSSSSS|WWWWWWWW|ZZZZZZZZ|0000000R|
	+-----+--------+--------+--------+--------+--------+
	4 bits Userid
	1 byte Downstream seq ID of window start
	1 byte upStream seq ID of window start
	1 byte Window size (upstream)
	1 byte window siZe (downstream)
	1 byte "Respond with ping" flag
	2 bytes CMC

The server response to Ping and Data packets is a DNS NULL/TXT/.. type response,
always starting with the 3 bytes downstream data header as shown above. If R bit
set, server must respond with a ping downstream header.
If server has nothing to send, no data is added after the header. 
If server has something to send, it will add the downstream data packet
(or some fragment of it) after the header.


"Lazy-mode" operation
=====================

Client-server DNS traffic sequence has been reordered to provide increased
(interactive) performance and greatly reduced latency.

Idea taken from Lucas Nussbaum's slides (24th IFIP International Security
Conference, 2009) at http://www.loria.fr/~lnussbau/tuns.html. Current
implementation is original to iodine, no code or documentation from any other
project was consulted during development.

Server:
Upstream data is acked immediately*, to keep the slow upstream data flowing
as fast as possible (client waits for ack to send next frag).

Upstream pings are answered _only_ when 1) downstream data arrives from tun,
OR 2) new upstream ping/data arrives from client.
In most cases, this means we answer the previous DNS query instead of the
current one. The current query is kept in queue and used as soon as
downstream data has to be sent.

*: upstream data ack is usually done as reply on the previous ping packet,
and the upstream-data packet itself is kept in queue.

Client:
Downstream data is acked immediately, to keep it flowing fast (includes a
ping after last downstream frag).

Also, after all available upstream data is sent & acked by the server (which
in some cases uses up the last query), send an additional ping to prime the
server for the next downstream data.


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2. Raw UDP protocol
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All Raw UDP protcol messages start with a 3 byte header: 0x10d19e
This is not the start of a valid DNS message so it is easy to identify.
The fourth byte contains the command and the user id.

	 7654 3210
	+----+----+
	|CCCC|UUUU|
	+----+----+

Login message (command = 1):
The header is followed by a MD5 hash with the same password as in the DNS
login. The client starts the raw mode by sending this message, and uses
the login challenge +1, and the server responds using the login challenge -1.
After the login message has been exchanged, both the server and the client
switch to raw udp mode for the rest of the connection.

Data message (command = 2):
After the header comes the payload data, which may be compressed.

Ping message (command = 3):
Sent from client to server and back to keep session open. Has no payload.