[ntp:questions] NTP-PPS standalone operation: How to ignore network interface changes?

Joachim Fabini Joachim.Fabini at tuwien.ac.at
Fri Jun 12 12:31:44 UTC 2015

Perhaps my statement was ambiguous, here's a next try. Although in the
meanwhile we found the culprit and it's NOT ntp, I still consider it
worth to post the solution as others might experience similar problems.
Sorry for the noise.

Our Setup: Stand-alone (Ubuntu) Linux desktop computer, GPS/PPS clock
connected to serial port, kernel and ntp configured appropriately. We
use this box to do accurate delay measurements. I.e., only the local
clock must be synchronized to UTC, there's no timeserver functionality
provided to others. The ntp.conf file has only one single server entry
for the NMEA/PPS clock (server and corresponding fudge line).

Half an hour or an hour after system start, ntpq -p reports the offset
to be in the order of 1-5 microseconds - perfect for our measurements.
But whenever we changed the measurement setup (i.e., unplugged the
Ethernet cable from the network interface and plugged it in again) the
ntpq -p-reported offset suddenly increased to several hundreds of
microseconds and took again at least half an hour to settle to the
required value (below 10 microseconds). This delayed all of our
measurements. We suspected ntp to listen to the ifups and ifdowns and
restart timekeeping on such events.

To make it short, the culprit was Ubuntu's default NetworkManager
configuration. The file /etc/network/if-up.d/ntpdate explicitly
(re)starts ntp on any network interface that comes up (either by
plugging in the connector or by issuing an ifup command).

As simplest solution you can remove the rights on the file (sudo chmod
000 ntpdate) such that your ntp will no longer lose the time on network
interface status changes.

hope it helps and sorry for the noise,
br Joachim

Am 12.06.2015 um 12:26 schrieb Marco Marongiu:
> On 12/06/15 11:54, Joachim Fabini wrote:
>> The straight-forward solution that you propose was the one that I tried
>> first. Unfortunately it does not work.
> Then I guess I don't understand what you're after exactly. I'll re-read
> your message entirely and check what I misunderstood. Sorry for the noise.
> -- M
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