NTP is
protocol among the oldest Internet protocols (1985), used for synchronizing clocks of computer systems across networks, using packet switching (units of information transported between nodes over data links shared) and variable latency (time delay between the moment something starts and when its effect begins). NTP was originally designed and is being maintained, by Dave Mills of the University of Delaware.
The service works through port 123, only UDP.
strata.
NTP uses a hierarchical system of clock strata.
Stratum 0: are devices such as GPS clocks or radio clocks, which are not connected to networks, but computers.
Stratum 1: systems are synchronized to stratum 0 devices. Systems of this layer are referred to as time servers.
Stratum 2: systems to send their requests NTP stratum 1 server, using the Marzullo algorithm to obtain the best data sample, ruling that appear to provide incorrect information and data sharing systems in the same stratum 2.
systems this layer act as servers for Stratum 3. Stratum 3
: systems using functions similar to the stratum 2, serving as servers for stratum 4.
Stratum 4: systems using functions similar to those in stratum 3.
About UTC.
UTC (Coordinated Universal Time or Universal Time Coordinated) is a standard high-precision atomic time. Has uniform seconds defined by TAI (International Atomic Time, or International Atomic Time), with leap seconds announced additional or at irregular intervals to compensate for the slowdown in the Earth's rotation and other discrepancies. These extra seconds allow UTC to be almost on par with the Universal Time (UT, or Universal Time), which is another standard but
Procedures. Ntpdate tool
A simple way to synchronize your system clock with any time server is via ntpdate. It is a tool similar to rdate, and is used to set the system date and time using NTP. The following example performs a query NTP directly, using an unprivileged port (option-u, very useful if there is a firewall that prevents the output) to the server
2.pool.ntp.org.
ntpdate-u 2.pool.ntp.org
configuration file / etc / ntp.conf.
operating systems like Red Hat ™ Enterprise Linux 4 and CentOS 4, includes a configuration file / Etc / ntp.conf, demonstrative purposes. The recommendation is to support it for future reference, and start a file with a new configuration, the same as described below .
# It sets the default policy for any
# server time used: it allows synchronization
# of time to the sources, but without allowing the source
# check (noquery) or modify the Service in
# system (nomodify) and declining to provide
# log messages (notrap). Restrict default
nomodify notrap noquery
# Allow all access to the loopback interface of
# system.
restrict 127.0.0.1
# It allows local network to synchronize with the server without allowing
# modify
# system settings, and without using them as peers to synchronize.
restrict 192.168.1.0 mask 255.255.255.0 # nomodify notrap
undisciplined local clock.
# This is an emulated driver that is used only as
# backup when none of the actual sources are
# available.
fudge 127.127.1.0 stratum 10 server 127.127.1.0
# File of variations.
driftfile / var / lib / ntp / drift
broadcastdelay 0,008
# key file in case they were needed for consultations
# keys / etc / ntp / keys
# List of time servers stratum 1 or 2.
# You should have at least 3 servers listed.
# More servers:
# http://kopernix.com/?q=ntp
# http://www.eecis.udel.edu/ ~ mills / ntp / server servers.html
0.pool.ntp. org
1.pool.ntp.org
server server 2.pool.ntp.org
# Permits to be allocated for each time server.
# In the examples, not allowed to view source or
# modify the service on the system or send e-
# registration. 255,255,255,255 mask
0.pool.ntp.org restrict nomodify notrap noquery
1.pool.ntp.org restrict mask 255,255,255,255 nomodify notrap noqueryrestrict 2.pool.ntp.org 255,255,255,255 mask nomodify notrap noquery
Add service boot.
To make the ntp service is active with the next system boot in all run levels (2, 3, 4 and 5) is used following
chkconfig ntpd on
Start, stop and restart service.
first to run the service, use: service ntpd start
To make your changes after you change the settings to take effect, use:
service ntpd restart To stop the service, use:
service ntpd stop
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