Configuring Linux Server for NTP Client to External NTP Server

NTP is used to synchronize the time on your Linux system with a centralized NTP server.

You can Install NTP Service using

$ sudo yum install ntp  
or
$ sudo apt-get install ntpd

To Configure NTP with External server

$ sudo ntpdate pool.ntp.org
31 Aug 19:07:11 ntpdate[10355]: adjust time server 46.29.176.115 offset -0.002893 sec

but sometimes you might receives errors if ntp daemon already up and running.

$ sudo ntpdate pool.ntp.org
31 Aug 19:05:55 ntpdate[8911]: the NTP socket is in use, exiting

to resolve this you need to restart NTP service & run the above commands.

$ sudo service ntpd stop
[ ok ] Stopping NTP server: ntpd.
$ sudo ntpdate pool.ntp.org
31 Aug 19:07:11 ntpdate[10355]: adjust time server 46.29.176.115 offset -0.002893 sec
$ sudo service ntpd start


or

$ sudo ntpdate -u pool.ntp.org

You can check the status of NTP using the ntpq command.

# sudo ntpq -p
   Remote           refid      st t when poll reach   delay   offset  jitter
=======================================================
*120.88.46.10    193.67.79.202    2 u  344  512  377    6.870   -7.724   2.512
If you get any connection refused errors then the time server is not responding or the NTP daemon/port is not started or listening.
You can also use the following command to get the current status of ntpd.

# sudo ntpdc -c sysinfo
system peer:                    120.88.46.10
system peer mode:              client
leap indicator:                 00
stratum:                           3
precision:                         -20
root distance:                  0.13884 s
root dispersion:               0.13063 s
reference ID:                   [120.88.46.10]
reference time:                d9c9d081.04ed3601  Thu, Oct 15 2015 12:43:05.019
system flags:                   auth monitor ntp kernel stats
jitter:                             0.013794 s
stability:                          0.000 ppm
broadcastdelay:               0.003998 s
authdelay:                       0.000000 s
Make sure the service starts at boot:
$ sudo chkconfig ntpd on

Note.
NTP servers communicate over port 123 UDP and unlike most UDP protocols the source port is NOT a high port, but uses 123 as well.  The firewall must be configured to allow UDP on both source and destination ports 123 for your servers.