[ntp:questions] ntpd on dial up machine: no DNS resolving => giving up on all servers

Danny Mayer mayer at ntp.org
Sun Jun 7 16:40:06 UTC 2009

David Woolley wrote:
> Richard B. Gilbert wrote:
>> Whether or not ntpd steps the clock depends on how far off it is.  If 
>> the clock was off by 15 minutes, most people would want it corrected 
>> immediately rather than having the offset amortized over three or four 
> More like three or four weeks, if the clock frequency error is << 500ppm.
>> days!  I believe that the default threshold is 128 seconds; less than 
>> that, the clock will slew.  If more than 128 seconds the clock will step.
> The threshold is 128ms.  Anything more than 0.5seconds disables the, 
> higher accuracy, kernel discipline.
> Note, if one doesn't mind backwards steps of less than many seconds, one 
> would need ntpd offline for many days to have problems.
>> If exact time and/or close agreement among a number of systems is 
>> wanted, it's best to try to keep the systems up 24x7; the reason being 
>> that NTPD may need as long as ten hours to synchronize the clock from a 
>> cold start.  Once synchronization has been achieved, NTPD should be able 
>> to maintain it with little effort.  Keeping the systems running also 
>> tends to stabilize the temperature which can be most helpful in keeping 
>> accurate time.
> I believe the issue is keeping the internet connection running, rather 
> than keeping the machine running.

Not really. NTP is designed to keep the clock disciplined even if it
cannot reach it's servers. That was part of the original design. The NTP
algorithm will keep the system clock steady at the same rate until a new
internet connection is established and it will then update it's information.


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