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Zizzencs
Asked: 2011-05-06 22:32:44 +0800 CST

Synchronizing time with only one NTP server

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I have a linux machine on a network where the machine only has access to exactly one NTP server. After the time synchronization the status looks like this:

[root@test ~]# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 <ip address>    <ip address>     3 u    8   64   17    1.397    2.510   1.954
*LOCAL(0)        .LOCL.          10 l    6   64   17    0.000    0.000   0.001

My problem here is that even though the remote NTP server has way lower stratum and a decent jitter value, ntpd always picks the local clock as the time source, and in time its clock gets late. The situation remains the same, even after one day, so it's not about me not waiting enough for things to come together.

Questions:

  • is it possible to get reliable time synchronization with only one NTP server on the network? (my guess is no)
  • can I force ntpd somehow to prefer the NTP server over the local clock in this situation?
ntp ntpd

2 Answers

  1. Yes, reliable synchronisation to a single clock source is possible. It's not reliable, because you have no redundancy, but that's the only problem.

    In your place I would just remove the local time source. On my machines I usually just use external ntpd servers.

    You need the local clock only if you need to provide service to clients (think: ntpd server for your internal network) when you have no connection to internet and cannot sync to other servers.

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  2. Best Answer

    As Pawel has said, remove the local clock line in your ntp.conf. In fact, remove everything, pretty much. If you have a working, sync'ed NTP source on your local network that's willing to act as a server, then clients really only need one line in their ntp.conf, which should read

    server ntp.intranet.example.com

    or, for fastest syncing,

    server ntp.intranet.example.com burst

    (the latter puts more load on the server at service start time, but since it's your server, you can say "i permit that", if you want faster syncing at ntpd start time).

    Don't forget to put ntp.intranet.example.com in /etc/ntp/step-tickers, or wherever your distro keeps that file, so the clocks of clients can be hard-synced at startup time.

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