[ntp:questions] Re: Freeware for NTP time synchronization underWindows?

Martin Dorey mdorey at bluearc.com
Wed Oct 20 22:41:05 UTC 2004

Tapio Sokura wrote:
> Resolving the server name before sending 
> each individual query could result in needless clockhopping. Perhaps a 
> good compromise would be to do a DNS lookup at every client software 
> startup and after that periodically, like after running continuously for > a week, as well after a server is unreachable for a certain period of 
> time.

Our lowest stratum ntp servers do several lookups for pool.ntp.org at startup.  I sometimes find that they can only use one of the resulting IP addresses because the other is permanently stratum 16.

On another occasion, we had a problem once when we changed the IP address of one of our internal ntp servers.  Although we changed the DNS CNAME to point to the new server, our clients were left drifting.  We didn't realise that they wouldn't resolve the name again.  A number of the replies to this question have focused on the problems caused by routinely changing IP addresses.  If your IP address is always changing, it's clear that some script is necessary.  The problem I'm talking about here is of a different sort - where the IP address changes say once a year and it's not obvious that there's a need for a script until the problem happens.

The clock-hopping point is a compelling reason not to re-lookup on every poll interval.  Indeed, I don't see any need to re-lookup the server while it's reachable, even if the client has been running for a week.  However, when the server hasn't been reached in several poll intervals, surely repeating the lookup is both cheap and sensible?
Martin, BlueArc Engineering

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