[ntp:questions] Re: Linux NTP Polling

Richard B. Gilbert rgilbert88 at comcast.net
Sun May 22 12:27:26 UTC 2005


Geoff wrote:

>I am not trying to propogate large time changes really.  Although it appears
>this way.  I am trying to test if NTPD is working as it should.  I also have
>Cisco Devices and they accept the change of several years etc but not the
>Linux Switch.  I was just typing in years as I did not realise NTPD would
>drop out etc if the difference in time was more than 1000 Seconds.
>
>On that note, it appears that the NTPD Daemon syncronises on startup to the
>NTP server but does not maintain correct time.  It displays the difference
>in time, when the NTP server is omved forwards or backwards as in the Daemon
>displays 76s or -45s etc but it does not change the actuall time to reflect.
>
>I am still looking into this problem,
>
>Many thanks,
>
>Geoff
>
>
>"Darren Dunham" <ddunham at redwood.taos.com> wrote in message
>news:acpge.1401$Y81.215 at newssvr21.news.prodigy.com...
>  
>
>>Geoff <g_wynn at ozemail.com.au> wrote:
>>    
>>
>>>Darren, many thanks.
>>>      
>>>
>>>Actually the change in time is significant, by years, not seconds.  How
>>>      
>>>
>can
>  
>
>>>I test it is actually re-syncing if the max timeout is 1000 seconds (ok,
>>>1000 secs is ok but is it Secs or Msecs?)  I can change the time by a
>>>      
>>>
>few
>  
>
>>>mins to check if you think that is the better option, all I want to know
>>>      
>>>
>is
>  
>
>>>if it is re-syncing.  I waited 1/2 hour to an hour.
>>>      
>>>
>>Then run 'ntpq -p' and post the output.
>>
>>Can I back up real quick and ask why you're trying to propogate very
>>large time changes?  It's entirely possible that you should be using
>>something other than ntpd to process these changes.
>>
>>-- 
>>Darren Dunham                                           ddunham at taos.com
>>Senior Technical Consultant         TAOS            http://www.taos.com/
>>Got some Dr Pepper?                           San Francisco, CA bay area
>>         < This line left intentionally blank to confuse you. >
>>    
>>
>
>
>  
>
I think you may be working under a misapprehension!!  ntpd does not 
"set" the clock to whatever random time value it gets from a server.   
It assumes that time is monotonically increasing at a rate of one second 
per second and responds to large jumps with disbelief!

ntpd will not attempt to correct any error greater than 1024 seconds 
(about 17 minutes), the "panic threshold".  It assumes that someone has 
lost his mind and exits.

To start ntpd, you need to set your clock to approximately the correct 
time; the closer the better.  Use your wristwatch, your cellular phone, 
wall clock or whatever you have available.   If you are within seventeen 
minutes, ntpd will figure it out eventually.  The closer your clock is 
to the correct time, the faster ntpd will synchronize it.    The maximum 
rate at which ntpd will slew the clock is limited to 500 parts per 
million or 500 microseconds per second.   An error of 17 minutes will 
take days to correct!

Most people still use ntpdate to set the clock initially although 
ntpdate is "deprecated".   The supported way is to use nptd -g which 
allows a one time "step" though I believe the panic threshold still applies.

You should really try to read and understand the  Documentation 
<http://www.eecis.udel.edu/%7Emills/ntp/html/index.html>



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