You can configure an NTP daemon to synchronize the time
of the machine with one or more NTP servers.
First, install the NTP package, which is not included with a minimal
installation. After you install the NTP package, select NTP servers,
create configuration files, and then enable and start the service.
Table 1. Configuring NTP
Configuration overview |
Configuration steps |
Install the NTP package |
If you have not done so already, install the
NTP package from your Linux distribution. |
Select NTP servers |
To synchronize your workstation's time, the
NTP daemon on your workstation contacts one or more NTP servers specified
in the configuration file /etc/ntp.conf .
If
an NTP server is not available on the local network, do one of the
following steps:
- Select a public NTP server (visit http://www.ntp.org/ and click Public Time Server Lists). If you select a
public NTP server, read the Rules of Engagement (click Rules of Engagement on
the main page of the NTP site).
- Use the NTP time server pool (visit http://www.pool.ntp.org/ and click How do I use pool.ntp.org).
As user root on the workstation, verify that
the machine can contact the selected NTP servers. Use the ntpdate command
with the query -q option. For example, to query an
NTP server whose IP address is 1.2.3.4 , use the following
command:
ntpdate -q 1.2.3.4
The output
must look like the following, which displays the contacted server
and the time difference between the local workstation and the server.
server 1.2.3.4, stratum 2, offset 150.695779, delay 0.03366
17 Nov 10:27:09 ntpdate[21597]: step time server 1.2.3.4 offset 150.695779 sec
If
the query fails, following output is likely to come:
server 1.2.3.4, stratum 0, offset 0.000000, delay 0.00000
17 Nov 10:29:04 ntpdate[21599]: no server suitable for synchronization found
|
Create configuration files |
Perform the following steps as user root
on the machine:
- Create file
/etc/ntp/step-tickers . You can back
up any existing version of the file. The file contains the host names
or IP addresses of the NTP servers to contact during startup to initially
set the time. If you use the NTP server pool, you must use host names,
which requires DNS.For example, if the two NTP servers to use are
1.2.3.4 and 5.6.7.8, then use the following commands to create the
file:
echo 1.2.3.4 > /etc/ntp/step-tickers
echo 5.6.7.8 >> /etc/ntp/step-tickers
- Create file
/etc/ntp.conf with the following
commands. You can back up any existing version of the file.echo restrict default ignore > /etc/ntp.conf
echo restrict 127.0.0.1 >> /etc/ntp.conf
echo driftfile /var/lib/ntp/drift >> /etc/ntp.conf
- Add entries by using the
restrict and server keywords
for each NTP server. The following example adds entries for the hypothetical
1.2.3.4 and 5.6.7.8 NTP servers. The mask, nomodify, notrap, and noquery
options prevent the server from modifying the NTP service on the Passive
Capture host machine.nullecho
restrict 1.2.3.4 mask 255.255.255.255 nomodify notrap noquery >> /etc/ntp.con
echo server 1.2.3.4 >> /etc/ntp.conf
echo restrict 5.6.7.8 mask 255.255.255.255 nomodify notrap noquery >> \
/etc/ntp.con
echo server 5.6.7.8 >> /etc/ntp.conf
|
Enable and start the NTP service |
Perform the following steps as user root
on the machine:
- Configure the service to start at boot time by using the following
command:
chkconfig ntpd on
- Start the service immediately by using the following command:
service ntpd start
- Verify that the service started and contacted a server by using
the following command:
ntpq -np
- View log messages for the NTP daemon in file /var/log/messages.
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