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How to configure NTP Server in RedHat / CentOS / Fedora

Introduction

Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks. NTP is a protocol designed to synchronize the clocks of computers over a network.

By Default Port # 123

Step 1: Install ntp Package

To configure NTP Server, firstly we need to install the ntp package:

# yum -y install ntp
Loaded plugins: amazon-id, rhui-lb
Resolving Dependencies
--> Running transaction check
---> Package ntp.x86_64 0:4.2.6p5-19.el7_0 will be installed
--> Processing Dependency: ntpdate = 4.2.6p5-19.el7_0 for package: ntp-4.2.6p5-19.el7_0.x86_64
--> Processing Dependency: libopts.so.25()(64bit) for package: ntp-4.2.6p5-19.el7_0.x86_64
--> Running transaction check
---> Package autogen-libopts.x86_64 0:5.18-5.el7 will be installed
---> Package ntpdate.x86_64 0:4.2.6p5-19.el7_0 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

================================================================================================================
 Package                 Arch           Version                  Repository                                Size
================================================================================================================
Installing:
 ntp                     x86_64         4.2.6p5-19.el7_0         rhui-REGION-rhel-server-releases         540 k
Installing for dependencies:
 autogen-libopts         x86_64         5.18-5.el7               rhui-REGION-rhel-server-releases          66 k
 ntpdate                 x86_64         4.2.6p5-19.el7_0         rhui-REGION-rhel-server-releases          82 k

Transaction Summary
================================================================================================================
Install  1 Package (+2 Dependent packages)

Total download size: 689 k
Installed size: 1.6 M
Downloading packages:
(1/3): autogen-libopts-5.18-5.el7.x86_64.rpm                                             |  66 kB  00:00:00
(2/3): ntp-4.2.6p5-19.el7_0.x86_64.rpm                                                   | 540 kB  00:00:00
(3/3): ntpdate-4.2.6p5-19.el7_0.x86_64.rpm                                               |  82 kB  00:00:00
----------------------------------------------------------------------------------------------------------------
Total                                                                           1.3 MB/s | 689 kB  00:00:00
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Installing : ntpdate-4.2.6p5-19.el7_0.x86_64                                                              1/3
  Installing : autogen-libopts-5.18-5.el7.x86_64                                                            2/3
  Installing : ntp-4.2.6p5-19.el7_0.x86_64                                                                  3/3
  Verifying  : autogen-libopts-5.18-5.el7.x86_64                                                            1/3
  Verifying  : ntp-4.2.6p5-19.el7_0.x86_64                                                                  2/3
  Verifying  : ntpdate-4.2.6p5-19.el7_0.x86_64                                                              3/3

Installed:
  ntp.x86_64 0:4.2.6p5-19.el7_0

Dependency Installed:
  autogen-libopts.x86_64 0:5.18-5.el7                     ntpdate.x86_64 0:4.2.6p5-19.el7_0

Step 2: Configuration

Make changes in ntp.conf file

# vi /etc/ntp.conf

restrict 10.0.0.0 mask 255.255.255.0 nomodify notrap
server 0.rhel.pool.ntp.org iburst
server 1.rhel.pool.ntp.org iburst
server 2.rhel.pool.ntp.org iburst
server 3.rhel.pool.ntp.org iburst

Now start the ntpd service

# /etc/rc.d/init.d/ntpd start
Starting ntpd:            [ OK ]

Enable ntpd in multi-user levels.

# chkconfig ntpd on

To verify NTP peers synchronization status, delay, offset, jitter use the following command:

# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*natasha.netwurx 209.242.224.11   2 u    -   64    1   67.464    6.336   0.038
 static-72-87-88 192.5.41.209     2 u    1   64    1   89.650   -3.463   0.000
 96.44.142.5     204.123.2.5      2 u    -   64    1   56.794    0.794   0.000
 ns.tx.primate.n .INIT.          16 u    -   64    0    0.000    0.000   0.000

How to Change Time in Linux

Check Date and Time

To check the date and current time of server,use date command:

# date

Wed Jun 17 14:39:35 CEST 2015

Step 1: Modify /etc/localtime File

Edit /etc/localtime to change the timezone data file.

# file /etc/localtime
/etc/localtime: timezone data# file /usr/share/zoneinfo/Asia/Kuala_Lumpur
/usr/share/zoneinfo/Asia/Kuala_Lumpur: timezone data
# date
Wed Jun 17 14:39:35 CEST 2015
# cp /usr/share/zoneinfo/Asia/Kuala_Lumpur /etc/localtime
cp: overwrite `/etc/localtime'? y
# date
Wed Jun 17 14:39:35 CEST 2015

There is also a nice way in order to change the server time using tzselect

# tzselect
Please identify a location so that time zone rules can be set correctly.
Please select a continent or ocean.
1) Africa
2) Americas
3) Antarctica
4) Arctic Ocean
5) Asia
6) Atlantic Ocean
7) Australia
8) Europe
9) Indian Ocean
10) Pacific Ocean
11) none - I want to specify the time zone using the Posix TZ format.
#? 5
Please select a country.
1) Afghanistan 18) Israel 35) Palestine
2) Armenia 19) Japan 36) Philippines
3) Azerbaijan 20) Jordan 37) Qatar
4) Bahrain 21) Kazakhstan 38) Russia
5) Bangladesh 22) Korea (North) 39) Saudi Arabia
6) Bhutan 23) Korea (South) 40) Singapore
7) Brunei 24) Kuwait 41) Sri Lanka
8) Cambodia 25) Kyrgyzstan 42) Syria
9) China 26) Laos 43) Taiwan
10) Cyprus 27) Lebanon 44) Tajikistan
11) East Timor 28) Macau 45) Thailand
12) Georgia 29) Malaysia 46) Turkmenistan
13) Hong Kong 30) Mongolia 47) United Arab Emirates
14) India 31) Myanmar (Burma) 48) Uzbekistan
15) Indonesia 32) Nepal 49) Vietnam
16) Iran 33) Oman 50) Yemen
17) Iraq 34) Pakistan
#? 29
Please select one of the following time zone regions.
1) peninsular Malaysia
2) Sabah & Sarawak
#? 1

Is the above information OK?
1) Yes
2) No
#? 1

You can make this change permanent for yourself by appending the line
TZ='Asia/Kuala_Lumpur'; export TZ
to the file '.profile' in your home directory; then log out and log in again.

Here is that TZ value again, this time on standard output so that you
can use the /usr/bin/tzselect command in shell scripts:
Asia/Kuala_Lumpur

TO set the Hardware Clock of System Time use the command:

# hwclock --systohc

OR

# hwclock -w