Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!ATHENA.MIT.EDU!mar
From: mar@ATHENA.MIT.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Need advice configuring ntpd on an isolated network
Message-ID: <8909101951.AA03218@TOTO.MIT.EDU>
Date: 10 Sep 89 19:51:10 GMT
References: <252@cs.nps.navy.mil>
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Organization: The Internet
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   From: cs!acrux!jxxl@ames.arc.nasa.gov  (John Locke)

   We had the same problem. We didn't use ntp, though. We used rdate. You define
   one machine as a server and run rdate on the rest. Sun advises running it
   from /etc/rc.local but there's no reason you couldn't run it from the
   crontab once a day.

There are good reasons for not running something like this out of
crontab if you have more than a handful of machines.  Since you're
running a clock synchronization protocol, your machines are probably
all within a couple of seconds of each other.  That means that each
machine's cron will attempt to check the time at the same time,
causing a large number of simultaneous requests first to page in the
binary of rdate from the file server, then all of the requests to the
master timeserver.  Things like this can cause massive collisions on
an ethernet, and tie up the network for a couple of minutes.

One solution is to make sure that each machine has a different crontab
to check the time at a different time, but that makes for a headache
to manage many machines and make sure that they all use different
times.  Another solution is to use a command like this in your
crontab:
    sleep `echo $ADDR | awk -F. '{ print $4 * 7 }' `; rdate
(anything started by our /etc/rc has $ADDR set to the machine's IP
address, but you could just as easily use "host `hostname`" to get the
IP address.

I don't want to go too much into Unix specifics on TCP-IP, but the
point is that you need to avoid certain kinds of synchronization in
networks, particularly when there are large numbers of similar
machines.
					-Mark Rosenstein