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

Nils Brubaker ncb at us.ibm.com
Wed Aug 14 15:03:24 UTC 2013


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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