View source for Use OpenNTPD for time synchronisation

<i>by Vincent Danen, TechRepublic</i>

Traditionally, the time synchronisation application used on Linux systems
has been ntpd (network time protocol daemon). However, for quite some time
now, the developers behind OpenSSH have created a secure, lean, and easy to
configure NTP daemon called OpenNTPD (http://www.openntpd.org/). Many Linux
distributions provide OpenNTPD, but few use it as the default ntpd service.

The traditional NTP service provided with Linux distributions comes in a
client/server flavor. The client, ntpdate, connects to an NTP service and
sets the system clock accordingly, to keep the system clock as accurate as
possible. Ntpd does the same thing, but keeps checking the time servers to
keep the system clock accurate, as well as acts as an ntpd server to other
clients.

However, if you look at the NTP package with most distributions, you will
see a handful of applications: ntpdate, ntpd, ntpq, ntp-keygen, ntpstat,
and more. Contrast that to what comes with OpenNTPD: ntpd -- one single
application that acts as both client and server. With the use of its very
straightforward configuration file, you can determine whether or not ntpd
acts as just a client, or as both server and client. By default, ntpd acts
as a client only.

Starting ntpd is extremely simple and has a small number of options. Most
distributions that provide OpenNTPD will also provide an initscript to
start it. If such an initscript is not available, simply add the following
to /etc/rc.d/rc.local or a similar one-time startup script:

/usr/sbin/ntpd

This will start the ntpd service, and it will immediately begin checking
the clock time against the configured remote NTP servers and adjust the
clock as necessary.

To have ntpd act as a server, edit /etc/ntpd.conf  and uncomment one of
the "listen" lines, such as:

listen 192.168.0.10

This will make ntpd listen for incoming requests on the IP address
192.168.0.10, port 123.

If you have multiple machines and wish to have one act as the primary NTP
server for your local network, use a configuration similar to the
following for the NTP server:

listen 192.168.0.10
servers pool.ntp.org

On the client systems, simply use:

server 192.168.0.10

This will cause all the client systems to contact the defined NTP server
on the local network, which will reduce extraneous traffic to the
world-wide NTP servers. The primary NTP server still sets the clock
according to the world-wide NTP servers, and that time information is then
easily propagated to the clients.

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