NTP question

Wed May 1 23:03:54 UTC 2019

On Wed, May 1, 2019, at 18:46, Brandon Martin wrote:
> Think about what might happen if you lost time sync as a result of the 
> incident causing said connectivity outage.  Depending on your time 
> sources available, you might see rapid drift or, worst case, lose your 
> time reference entirely as a result of equipment restarts, etc.  GPS, as 
> long as you have a good view of the sky, provides extremely accurate 
> "lights out" time info, both absolute and relative, from a single source 
> with no (mostly) strings attached for that purpose.

Properly deployed NTP should calibrate the local hardware clocks to prevent drift even during connectivity outages. (I'm talking both the low resolution hardware clocks used for timing across power cycles and reboots, and the oscillators used while the OS is running). While most computer hardware is temperature sensitive, if your datacenter is suddenly changing temperature enough to cause clock drift, well, you have bigger problems. :)

I admit that this is an anecdote, but in our environment, I find that our GPSDO loses its GPS signal due to weather more often than we lose our connections to internet NTP servers.

On the other hand, we once had a site-wide Kerberos authentication outage because all of our Windows clients were using some windows NTP client that by default used two NTP sources owned by the software developer; when they both suddenly stepped by 20 minutes, Kerberos locked everyone out.

Time is hard :)

-- 
Harald Koch
chk at pobox.com