Linux.Desktop.Hacks [Electronic resources] نسخه متنی

Hack 71. Update Your Clock via the Internet

Staying in time is easier when you let someone
else set your clocks.

There is little doubt that time
is
an important part of your daily life. Everything from watching the
latest episode of your favorite TV program to having a meeting with
your boss is based around time, and your computer is no different.
Inside your computer, hardware and software rely on time to help keep
you up-to-date. If our computers didn't know the
time, our calendar, email, and personal-information management would
be a mess. Each of these tools relies heavily on having an accurate
clock, and the challenge is keeping this clock accurate.

This hack explores a little tool called the Network Time Protocol
(NTP) that can solve your inaccurate-clock problems. The NTP software
synchronizes your computer's clock with a central
server on the Internet with impressive accuracy, and if you set this
synchronization to occur when your system boots (or at regular
intervals), your clock will never be inaccurate again.

9.3.1. Getting NTP

The NTP software is available
for download from http://www.ntp.org, but it is probably best
to use your distributions package-management program to obtain the
software.

With the software installed, you need to find a server with which to
synchronize your clock. Many ISPs provide NTP services to their
subscribers; contact your ISP to see if it has a server to
synchronize with. If your ISP does not provide a server, a number of
public NTP servers are available for different regions of the world.
The easiest way to find these servers is to search on Google for your
location/country and the words public NTP
server. For quick synchronizations which lead to a more
accurate clock, choose a server as physically close to your
geographic location as possible. Also, because you are an end user,
you should not synchronize to a stratum 1 server. These servers are
reserved for use only when you are setting up your own NTP server
that will service thousands of clients.

9.3.2. Synchronizing Your Clock

Within the NTP software is
a tool called
ntpdate that is used to synchronize your clock
with an NTP server. With the server address you just obtained and
ntpdate, you can easily synchronize your clock
with this command:

foo@bar:~$ ntpdate 
ntp.yourserver.com

When you run this command, ntpdate will connect
to the server and perform the synchronization. For an even easier
method of running NTP, create a configuration file,
/etc/ntp.conf, with your servers included. Use
this format to add your server:

server ntp.yourserver.com

With the file created, you need to use the ntpd
daemon to read the file and synchronize your clock. You can start it
manually with this command:

foo@bar:~$ ntpd

You should use your distribution's startup services
software manager to load ntpd when the system
boots.

9.3.3. Automate NTP Synchronization

Apart from setting your clock, one of the best uses of NTP is to
synchronize your clock regularly to compensate for the natural drift
that occurs with a hardware clock. This drift is caused by various
issues such as power fluctuations and hardware problems, but if you
synchronize with NTP as often as possible, you can reduce this drift.

To schedule synchronization at regular intervals, use a cronjob to
automate these NTP updates. [Hack #70] fully explains the uses of
cron. You can create your cronjob by opening the
cron file with:

foo@bar:~$ crontab -e

Then add a line to define the cronjob. As an example, to synchronize
your clock once a day at 3:00 a.m., use this line:

00 03 * * * ntpdate ntp.yourserver.com > /dev/null

When you save and quit the editor, your cronjob will be enabled and
your clock will be updated regularly.