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The Network Time Protocol (NTP) is an Internet protocol designed and
developed to synchronise network time clients to an accurate reference
clock. NTP defines a number of algorithms and specifies a message
structure to pass accurate timing information from a server reference
to a client machine. This article describes how to install the freely
available public NTP server software distribution available from
'ntp.org'. It shows how to configure NTP to synchronise time with a
internet based public time reference.
Internet based NTP time server systems fall into two categories:
primary and secondary reference servers. Primary reference servers
utilise a highly accurate external timing reference, such as GPS or
radio clocks, to provide precise time. Secondart reference servers
synchronise their time with primary reference servers and therefore
offer slightly reduced accuracy. Primarty reference servers are
designated stratum 1, while other servers have a higher stratum, ie 2
or above.
The NTP source code is freely downloadable under the GNU public license
from the NTP web site at 'ntp.org'. NTP was originally developed for
the LINUX operating system, however, a port to Windows NT is also
available. Once the source code has been obtained, it should be
installed, compiled and configured on the host computer. This process
is automated with the installation and configuration scripts supplied
in the distribution. Many Linux based operating systems have the NTP
package pre-installed. However, it may be wise to download the most
recent version, which is currently 4.2.4.
The NTP daemon is configured via a configuration file 'ntp.conf'. This
configuration file may contain a list of public NTP server references
that can be used by the host to synchronise time. The 'server' command
can be used to specify public NTP time references, characters specified
after the # symbol are comments:
server time-a.nist.gov # Public NTP server: NIST, Gaithersburg,
Maryland server time-c.timefreq.bldrdoc.gov # Public NTP Server: NIST,
Boulder, Colorado
When the configuration file is complete the NTP daemon can be started
using the 'ntpd start' script. Other scripts are vailable that can be
used to control the daemon: 'ntpd stop', 'ntpd restart'. There is also
a query script available that shows the current synchronisation status
of the daemon: 'ntpq -p'.
The 'restrict' command can be used to control access premissions to the
NTP server. There are a number of permutations or the 'restrict'
command; a few examples are listed below.
restrict default ignore #Restrict all access to the time server
restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap # Only allow
local computers access
Multiple commands can be specified to restrict or disallow access to a range of computers.
NTP provides an additional level of security by utilising
authentication codes. This is a mechanism where matching key codes can
be specified on a client and server which are encrypted and passed
between the server and client. This allows a client to confirm the
source of a time message. Authetication keywords are specified in the
'ntp.keys' file. They are specified using a key reference, encryption
code and authentication key. Additionally, trusted authentication keys
can be specified using the 'trusted key' command is the 'ntpd.conf'
configuration file.
About the Author
Dave Evans is a technical author who specialises in Linux and GPS
NTP Server synchronisation systems to ensure accurate network time.
Please click here to find out more about NTP Servers.
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