NTP

To check if ntpd is configured:

$chkconfig –list ntpd

ntpd            0:off   1:off   2:on    3:on    4:on    5:on    6:off

Install and configure ntpd if not configured:

#yum install ntp ntpdate ntp-doc

#chkconfig ntpd on

Initial time synchronization:

#ntpdate <ntp-srv-IP-address>

Configure ntpd:

Into /etc/ntpd.conf add NTP server and comment out default servers if necessary:
server <…>

Start ntpd:
#/etc/init.d/ntpd start

or

#service ntpd start

Check if ntpd is working:

$ntpq -p

 remote refid st t when poll reach delay offset jitter
==============================================================================
+clock.util.phx2 .CDMA. 1 u 111 128 377 175.495 3.076 2.250
*clock02.util.ph .CDMA. 1 u 69 128 377 175.357 7.641 3.671
 ms21.snowflakeh .STEP. 16 u - 1024 0 0.000 0.000 0.000

The command lists connected time servers and displays information indicating when they were last polled and the stability of the replies. The column headings are as follows:

  • remote and refid: remote NTP server, and its NTP server
  • st: stratum of server
  • t: type of server (local, unicast, multicast, or broadcast)
  • poll: how frequently to query server (in seconds)
  • when: how long since last poll (in seconds)
  • reach: octal bitmask of success or failure of last 8 queries (left-shifted); 377 = 11111111 = all recent queries were successful; 257 = 10101111 = 4 most recent were successful, 5 and 7 failed
  • delay: network round trip time (in milliseconds)
  • offset: difference between local clock and remote clock (in milliseconds)
  • jitter: difference of successive time values from server (high jitter could be due to an unstable clock or, more likely, poor network performance)

To obtain summary:

$ntpstat
synchronised to NTP server (10.5.26.10) at stratum 2
 time correct to within 52 ms
 polling server every 1024 s

!! If there is a big gap between local and NTP time we need to stop ntpd daemon, perform synchronization with ntpdate and start ntpd again.