View Source: MulticastNtp

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Once you have MulticastRoutes set up, you can configure network time synchronization (via the Network Time Protocol, [NTP|http://www.ntp.org]) to use multicast.  Theoretically this could significantly reduce the load on your time server.  Also, it generalizes the configuration of your clients, since they no longer have to be pointed at a specific time server.

I have only been able to make this work totally correctly with ntp v4.0.99k and later.

* Add a 'broadcast' entry to /etc/ntp.conf on the ntp server(s):%%%
<pre>
	broadcast 224.0.1.1	# broadcast to the multicast address.
</pre>
* Remove all 'server' entries from /etc/ntp.conf on client systems.
* Add a multicast client entry to /etc/ntp.conf on each client:
<pre>
	multicastclient		# listen on the reserved ntp multicast address of 224.0.1.1
</pre>

''Optional'': The unix ntp client will refuse to start up if the system clock is more than 1 hour off the server clock.  If you know the server is always right, you can add a call to 'ntpdate' before ntpd starts.  This happens automatically under !RedHat if the file /etc/ntp/step-tickers exists and contains a list of servers.  If you place only the address '224.0.0.1' in this file, ntpdate will set the system clock to the time that is being multicasted.  I don't know why, but this address has to be 224.0.0.1, not 224.0.1.1.

Under other Unix-ish OSes, you can accomplish the same with a call to ntpdate in an rc script before ntpd starts:

<pre>
	ntpdate 224.0.0.1
</pre>

Now that you have this configured and ntpd restarted on all the systems, test it out:

* ##ntpq -p## should print the name of every multicast server that the system is synchronized to.
* ##ntpdate -q 224.0.0.1## should report the time on every multicast server.

(Let the server run a few minutes before trying these out, to give everything time to start up and stabilize)

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