[ntp:questions] Time reset

Richard B. Gilbert rgilbert88 at comcast.net
Fri Apr 4 18:29:27 UTC 2008


jkvbe wrote:
> On 3 apr, 23:10, "Richard B. Gilbert" <rgilber... at comcast.net> wrote:
> ...
> 
>>DOES the time step backward?
>>
>>If ntpd is working properly it should NOT need to step the time at all
>>with the possible exception of a single step when ntpd is first started.
>>
>>If ntpd is stepping time regularly, you have some other problem.  If you
>>find and fix that problem, ntpd should stop stepping the time.
>>
>>There are/were known issues with some Linux systems; during periods of
>>high disk usage, clock interrupts would be lost resulting in a FORWARD
>>step.  AFAIK these issues were related to EIDE disks used in PIO mode
>>rather than DMA mode.  ISTR reading that the problem has been fixed in
>>recent versions of Linux.  YMMV
> 
> 
> I agree that ntpd should not stepping time regularly and that it
> points to a problem if it happens regularly. But we develop an
> appliance and we don't control how customers deploy it. Given the
> adverse effects of stepping time (especially if it moves backwards),
> I'd would have liked to be protected against badly set-up NTP
> infrastructure or time servers that are compromised.
> 
> Jan

It seems to me that, in the circumstance you describe, supplying correct 
time is the customer's problem!

Having read this newsgroup for the last four or five years, I'm aware 
that people do some very strange things with computer clocks.  I'm 
thinking, in particlar, of at least one individual who deliberately set 
his clock to an incorrect time in order to see if Ntpd would correct it.
Ntpd did so, of course, but he was not happy with the way it was done or 
the amount of time it took!

If it's not under your control, it's not your responsibilty!  Your 
instructions for the appliance should point this out pretty explicitly;
e.g. "IF YOUR TIME SERVERS CAUSE TIME TO STEP, THE FOLLOWING ADVERSE 
CONSEQUENCES CAN BE EXPECTED TO OCCUR: <list of adverse consequences>
It is YOUR responsibility to ensure that this does not happen!"

The only halfway legitimate thing I can think of that would cause time 
to step would be a leap second.




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