[ntp:questions] Isolated network

Steve Kostecke kostecke at ntp.org
Thu Nov 20 03:24:58 UTC 2008

On 2008-11-19, Mike Griffin <mcgriffin at gmail.com> wrote:

> I have an isolated network which has no connections to the outside
> world. This includes any type of modem access.
> The machines are running Red Hat Enterprise Linux 5.2 and I also have
> Cisco routers involved. What I would like to do is to assign one of my
> RHEL machines as my NTP server

This is trivial to set up.

# Server /etc/ntp.conf
driftfile /path/to/ntp.drift
server minpoll 4
fudge stratum 10

The ntpd using this configuration file will be able to answer client
queries approximately 55 seconds after is starts up.

If at all possible I would set up the time server in a location with
similar ambient conditions as your time-island and allow that ntpd to
poll some real time sources for 24 hours so that it has ample time to
calculate a resonable frequency correction to be stored in the drift

I would also give serious consideration to using a dedicated real
physical time server system (i.e. not in a VM) which does not run _any_
services other than ntpd (and the minimum number of administrative cron

> and have it synch off its local system clock.

ntpd can not "sync off its local system clock".

ntpd can use either the Undisciplined Local Clock or Orphan Mode to
_claim_ to be synced to a time source even though it is actually

Orphan Mode is the designated replacement for the Undisciplined Local
Clock. However, in certain configurations (e.g. a time-island) the
Undisciplined Local Clock aparently must be used.

> I'm not concerned with time accuracy to the real world, I just need
> time relevance within the network.

You _should_ be concerned about clock stability.

ntpd is not a magic box which can produce stable clocks out of thin air.
It needs a reference standard towards which it will steer the clocks.
Without a reference standard your client systems will be following a
moving target.

This reference standard could be, and usually is, UTC acquired over a
network or via a GPS/WWVB/etc receiver or via a POTS modem.

This reference standard could also be generated locally using a high
quality (e.g. rubidium) oscillator to provide a PPS or 10MHz reference

Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/

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