[LEAPSECS] stale leap second information

[Poul-Henning Kamp]

I played a bit with the idea this morning, and I think this is how I would
do it:

   +-------+-----------+-----+---------------------+---------------+
   |1 1 1 1|M M M M M M M M M|L L L L L L L L L|D D|C C C C C C C C|
   +-------+-----------+-----+---------------------+---------------+
    3 3 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 
    1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0

The first four bits are constant 0xf putting the resulting IPv4 in
the "Class-E" segment which is reserved.  This should give us 100%
certainty in detecting meddling DNS resolvers.

The "M" field is nine bit unsigned month number:

	(year - 1972) * 12 + month - 1

The "L" field the eight bit signed(!) TAI-UTC difference before the
start of the indicated month.

The "D" field is two bit signed TAI-UTC change before the start of
the indicated month.

The "C" field is a CRC8 with poly=0xcf (found by exhaustive search)

Here are how recent Bulletin-C's would look:

YYYY MM before  after  encoded crc IP               Decoded
-------------------------------------------------------------------------
2012  1     34     34 f7808800  65 247.128.136.101 -> OK  2012  1  +34 +0
2012  7     34     35 f7988900  66 247.152.137.102 -> OK  2012  7  +34 +1
2013  1     35     35 f7b08c00  0b  247.176.140.11 -> OK  2013  1  +35 +0
2013  7     35     35 f7c88c00  58  247.200.140.88 -> OK  2013  7  +35 +0
2014  1     35     35 f7e08c00  69 247.224.140.105 -> OK  2014  1  +35 +0
2014  7     35     35 f7f88c00  f3 247.248.140.243 -> OK  2014  7  +35 +0
2015  1     35     35 f8108c00  be  248.16.140.190 -> OK  2015  1  +35 +0
2015  7     35     36 f8288d00  fa  248.40.141.250 -> OK  2015  7  +35 +1
-------------------------------------------------------------------------

Attached is a proof of concept in python.

You can poke the result at my test-domain "leap.net-tid.dk"

Poul-Henning

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.
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