14.6. Basic NTP configuration
The NTP program is configured using either the
/etc/ntp.conf or /etc/xntp.conf
file depending on what distribution of Linux you have. I won't go
into too much detail on how to configure NTP. Instead I'll just
cover the basics.
An example of a basic ntp.conf file would look like:
# --- GENERAL CONFIGURATION ---
fudge 127.127.1.0 stratum 10
# Drift file.
The most basic ntp.conf file will simply list 2 servers, one
that it wishes to synchronize with, and a pseudo IP address for
itself (in this case 127.127.1.0). The pseudo IP is used in case of
network problems or if the remote NTP server goes down. NTP will
synchronize against itself until the it can start synchronizing with
the remote server again. It is recommended that you list at
least 2 remote servers that you can synchronize against. One will
act as a primary server and the other as a backup.
You should also list a location for a drift file. Over time
NTP will "learn" the system clock's error rate and automatically
adjust for it.
The restrict option can be used to provide better control and
security over what NTP can do, and who can effect it. For example:
# Prohibit general access to this service.
restrict default ignore
# Permit systems on this network to synchronize with this
# time service. But not modify our time.
restrict aaa.bbb.ccc.ddd nomodify
# Allow the following unrestricted access to ntpd
It is advised that you wait until you have NTP working properly before
adding the restrict option. You can accidental restrict yourself from
synchronizing and waste time debugging why.
NTP slowly corrects your systems time. Be patient! A simple test
is to change your system clock by 10 minutes before you go to bed and then
check it when you get up. The time should be correct.
Many people get the idea that instead of running the NTP
daemon, they should just setup a cron job
job to periodically run the ntpdate command.
There are 2 main disadvantages of using using this method.
The first is that ntpdate does a "brute force"
method of changing the time. So if your computer's time is off my 5
minutes, it immediately corrects it. In some environments, this can
cause problems if time drastically changes. For example, if you are
using time sensitive security software, you can inadvertently kill
someones access. The NTP daemon slowly changes the time to avoid
causing this kind of disruption.
The other reason is that the NTP daemon can be configured to
try to learn your systems time drift and
then automatically adjust for it.