[ntp:questions] NTP makes a time jump

unruh unruh at invalid.ca
Thu Jul 4 17:19:40 UTC 2013

On 2013-07-04, karpekin at gmail.com <karpekin at gmail.com> wrote:
> Hello,
> I have a problem with NTP configuration when the client loses uplink connection for some time and then re-establishes it. 
> ntp.conf:
> tos maxdist 4
> server iburst
> server iburst
> server iburst
> server
> fudge stratum 12 

You have these last two lines in there why? They should NOT be there.
Get rid of them. 

> I do the following:
> 1. kill the NTPD process, 
> 2. disconnect the uplink to real servers, 
> 3. change the local clock time to +3 sec 

AGain, why in the world should ntpd behave rationally when you present
it with an irrational situation?

> 4. reboot the system.
> System comes up and after a while syncs to itself. After it starts to provide my "fake" time reference (+3sec) to clients downstream, I plug in the uplink, so my client is able to see the 3 servers defined in the config above. And as soon as it established the connection and peered to one of them, I see the -3 sec time jump. I was expecting that NTPD will slowly start to correct the time, but it jumps. 

ntpd jumps if the time is out by more than 128 ms ( that is thousandths
of a second). 

With ntpd's limit of 500PPMM, it would take over  2 hours to slew away a 3
second time difference. 

If you want different behaviour, use chrony instead (on Linux or BSD).
You can tell it to slew instead, and it does not have a 500PPM limit on
the slew rate.

> Can you please explain if that behavior is by NTP design? Maybe I have something misconfigured, and if so, how to force NTPD not to make jumps? 

Don't present it with irrational situations?

> Thank you,
> Igor.

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