[ntp:questions] Order of servers in ntp.conf

Brian Utterback brian.utterback at oracle.com
Wed Aug 14 16:06:48 UTC 2013


I don't understand how you get the idea that your system is 
synchronizing with only one server when the messages you posted show it 
synchronizing with 6 servers. Do you mean that it is synchronizing with 
one server at a time? That is what it is supposed to do. There is a 
combining step in the algorithms that can adjust the actual offset used 
by combining with offsets from others in a selected group, but there is 
always a single primary sync source.

On 8/14/2013 11:03 AM, Nils Brubaker wrote:
> Thank you, unruh at invalid.ca, for your response to my question.
>
> Couple follow-up questions.  My ntp.conf running on Linux has 4 servers
> defined:
>
> server 0.rhel.pool.ntp.org
> server 1.rhel.pool.ntp.org
> server 2.rhel.pool.ntp.org
>
> These are public servers from the NTP pool project.  In my
> /var/log/diag-log file, I see messages indicating that my ntpd is synching
> with individual servers, for example:
>
> Aug  7 09:53:58 yellowstone ntpd[3272]: synchronized to 69.50.219.51,
> stratum 2
> Aug  7 10:04:52 yellowstone ntpd[3272]: synchronized to 184.82.112.110,
> stratum 2
> Aug  7 10:39:05 yellowstone ntpd[3272]: synchronized to 69.50.219.51,
> stratum 2
> Aug  7 11:28:17 yellowstone ntpd[3272]: synchronized to 184.82.112.110,
> stratum 2
> Aug  7 12:47:06 yellowstone ntpd[3272]: synchronized to 128.10.19.24,
> stratum 1
> ...
> Aug  8 11:37:43 yellowstone ntpd[3254]: synchronized to 50.116.55.65,
> stratum 2
> Aug  8 12:18:46 yellowstone ntpd[3254]: synchronized to 50.116.55.161,
> stratum 2
> Aug  8 13:01:27 yellowstone ntpd[3254]: synchronized to 38.101.77.21,
> stratum 2
> Aug  8 15:01:00 yellowstone ntpd[3254]: synchronized to 50.116.55.161,
> stratum 2
> Aug  8 16:09:20 yellowstone ntpd[3254]: synchronized to 38.101.77.21,
> stratum 2
>
> These log messages suggest that ntpd is synchronizing with one and only
> one NTP server.  Is that the correct interpretation?  Is this single
> server selected for synchronization only after performing all the
> calculations described below?
>
> Also, I see long time periods in the diag-log where there is no
> synchronization message.   What does that signify?  No agreement between
> the servers on the correct time?  No need to adjust system clock because
> it is already in sync?
>
> Thanks,
>   Nils Brubaker
>
>
> On 2013-08-13, Nils Brubaker <ncb at us.ibm.com> wrote:
>> For ntpd 4.2.4p6 running on Linux, is there any significance to the
> order
>> of servers in the ntp.conf file?  Will ntpd synchronize with the first
>> available/good time server in the list?
> No, no.
> ntp gets the data from all the servers. It then looks at the times it
> gets from each server, and groups them into "classes" according to its
> estimate of the max time error from the server-- ie whether the error
> intervals overlap or not. It then looks for the largest group of servers
> all of whose error intervals overlap and uses the average of those times
> as the time to send on the the ntp engine. The others are "false
> tickers". It estimates the error by looking at the round trip time and
> the other machine's estimate of its own max error.
>
>
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