[ntp:questions] NTP no internet connection

Richard B. Gilbert rgilbert88 at comcast.net
Sat Feb 9 20:28:45 UTC 2008


flyersix wrote:
> I have a network that I would like to cordinate the time in without
> exposing it to the internet for NTP servers.
> 
> Could I just setup NTP on one of my internal servers and set the
> clients to go to it for time syncs?  I know the time won't match the
> internet time but my thought is if I only need to change the time on
> the one server and then let the clients all go to it to update their
> time.
> 
> Is this possible?

Possible?  Yes.  Useful?  Maybe!

The problem with this is that your clock is not being disciplined.  This 
means that, in addition to not necessarily having the correct time, it 
may not be ticking at exactly 1 second per second.  Even worse, if the 
temperature is not controlled, the clock frequency and, therefore, the 
tick rate, can change from hour to hour.

Now, imagine other machines trying to synchronize with this server.  I 
think of it as one drunk driver trying to follow another.  Nobody is 
going to crash into anything but the various machines trying to 
synchronize to the server will probably be all over the map.

If  you need or want tight synchronization, get an inexpensive GPS 
timing receiver.  A Garmin GPS18LVC can be had for less than $100 US.
You will need a soldering iron, a five volt power supply and a suitable 
connector (probably DB9 or DB25) to plug into a serial port.

If you can site the antenna where it will have a good view of the sky, 
you should be able to synchronize your server to it with an accuracy 
that might be as good as 50 to 100 nanoseconds.  Thus synchronized, the 
server should be stable as a table and should synnchronize your other 
machines quite closely, say within 50 or 100 microseconds.  A lot will 
depend on the latencies within your LAN.  Lightly loaded gigabit 
ethernet should do very well indeed if the network is small.  Network 
switches can introduce unwanted random latencies.  100MB Ethernet also 
works quite well.  I don't recall ever having used NTP over 10MB 
Ethernet but it's been years since I've seen one of those!




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