[Pool] Leap second is coming

Martin Burnicki martin.burnicki at meinberg.de
Wed Apr 15 17:09:02 UTC 2015

lst_hoe02 at 79365-rhs.de wrote:
> Sorry for maybe asking the obvious but
> - How to check if ntp is configured with a leap second file? Must it be
> listed in ntp.conf or is there some default set?

If you run "ntpq -c rv" and the output shows a "tai" value then ntpd has
read a leap second file. For example:

pc-martin:/etc # ntpq -c rv
associd=0 status=0414 leap_none, sync_uhf_radio, 1 event, freq_mode,
version="ntpd 4.2.8p2 at 1.3316-o Wed Apr  8 08:20:36 UTC 2015 (3)",
processor="x86_64", system="Linux/3.16.7-7-desktop", leap=00, stratum=1,
precision=-23, rootdelay=0.000, rootdisp=7937.673, refid=shm0,
reftime=d8d9160e.2c2467ac  Wed, Apr 15 2015 18:53:34.172,
clock=d8d91616.da49a196  Wed, Apr 15 2015 18:53:42.852, peer=55606, tc=4,
mintc=3, offset=-0.053, frequency=0.000, sys_jitter=0.000000,
clk_jitter=0.019, clk_wander=0.000, tai=35, leapsec=201507010000,

As you can see in the "version", the output above is from ntpd 4.2.8*,
which also prints the "leapsec" date, and the expiration date of the
installed leap second file.

> - Without a leap second file ntp will simply decide based on the
> majority of upstream ntp servers it it will pass the information or will
> it pass the information from the "*" upstream?

ntpd 4.2.6 and newer requires a majority of configured upstream servers
to accept and send a leap second warning, *or* a refclock which can
provide it, *or* a current leap second file which supersedes both other
types of source.

> - What is the expected timeframe of leap second file changes
> (Month/Years/Decade)??

The IERS bulletin C is published twice a year, usually a few days after
a potential leap second date, i.e. in the first days of July or January.
Leap seconds files are updated afterwards, hopefully soon after the

The file should even be updated if *no* leap second has been scheduled,
with a new expiration date, so ntpd can possibly distinguish whether no
leap second has been scheduled, or the file has just not been updated.

> What happens if the upstream get it wrong and omit the leap second? Are
> we wrong by one second or do we simply switch later and slower?

I'm not sure what you mean here, and I think you need to distinguish
between several cases.

If your server has a leap second file and inserts the leap second, but
the upstream servers you have configured don't insert the leap second
then your server will observe a 1 s offset after the leap second and,
and will re-synchronize to the upstream server(s) a few minutes later,
i.e. step the time to be wrong like the servers.

> Thanks for explainig it to mortals ;-)

You're welcome. ;-)

Martin Burnicki

MEINBERG Funkuhren GmbH & Co. KG
Email: martin.burnicki at meinberg.de
Phone: +49 (0)5281 9309-14
Fax: +49 (0)5281 9309-30

Lange Wand 9, 31812 Bad Pyrmont, Germany
Amtsgericht Hannover 17HRA 100322
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