NTP configuration in RHEL 7

The Network Time Protocol (NTP) enables the accurate dissemination of time and date information in order to keep the time clocks on networked computer systems synchronized to a common reference over the network or the Internet. Many standards bodies around the world have atomic clocks which may be made available as a reference. The satellites that make up the Global Position System contain more than one atomic clock, making their time signals potentially very accurate. Their signals can be deliberately degraded for military reasons. An ideal situation would be where each site has a server, with its own reference clock attached, to act as a site-wide time server. Many devices which obtain the time and date via low frequency radio transmissions or the Global Position System (GPS) exist. However for most situations, a range of publicly accessible time servers connected to the Internet at geographically dispersed locations can be used. These NTP servers provide “Coordinated Universal Time” (UTC). Information about these time servers can found at www.pool.ntp.org.

Accurate time keeping is important for a number of reasons in IT. In networking for example, accurate time stamps in packets and logs are required. Logs are used to investigate service and security issues and so time stamps made on different systems must be made by synchronized clocks to be of real value. As systems and networks become increasingly faster, there is a corresponding need for clocks with greater accuracy and resolution. In some countries there are legal obligations to keep accurately synchronized clocks. Please see www.ntp.org for more information. In Linux systems, NTP is implemented by a daemon running in user space. The default NTP user space daemon in Red Hat Enterprise Linux 7 is chronyd. It must be disabled if you want to use the ntpd daemon.

[root@RHEL7 ~]# systemctl stop chronyd

[root@RHEL7 ~]# systemctl disable chronyd

[root@RHEL7 ~]# yum install ntp

Configure ntp.conf file like shown below

[root@RHEL7 ~]# cat /etc/ntp.conf

tinker panic 0

driftfile /var/lib/ntp/drift

restrict default nomodify notrap nopeer noquery

restrict 127.0.0.1

restrict ::1

server 10.18.17.32 iburst

server 10.18.17.33 iburst

server 10.18.11.30 iburst

server 10.18.11.31 iburst

includefile /etc/ntp/crypto/pw

keys /etc/ntp/keys

disable monitor

[root@RHEL7 ~]# systemctl restart ntpd

[root@RHEL7 ~]# systemctl status ntpd

[root@RHEL7 ~]# systemctl enable ntpd

[root@RHEL7 ~]# ntpq -np

     remote           refid      st t when poll reach   delay   offset  jitter

==============================================================================

 10.18.17.32    .INIT.          16 u    - 1024    0    0.000    0.000   0.000

 10.18.17.33    .INIT.          16 u    - 1024    0    0.000    0.000   0.000

*10.18.11.30    10.18.11.30     5 u  261  512  377    0.827   -8.280  10.270

+10.18.11.31    10.18.11.30     5 u  497  512  377    0.720  -23.474  11.101

[root@RHEL7 ~]# ntpstat

synchronised to NTP server (10.18.11.30) at stratum 6

   time correct to within 388 ms

   polling server every 512 s

[root@RHEL7 ~]#

timedatectl command gives a detailed info about system time and date

[root@RHEL7 ~]# timedatectl

      Local time: Thu 2015-12-31 00:27:00 CST

  Universal time: Thu 2015-12-31 06:27:00 UTC

        RTC time: Thu 2015-12-31 06:27:00

       Time zone: America/Chicago (CST, -0600)

     NTP enabled: yes

NTP synchronized: yes

 RTC in local TZ: no

      DST active: no

 Last DST change: DST ended at

                  Sun 2015-11-01 01:59:59 CDT

                  Sun 2015-11-01 01:00:00 CST

 Next DST change: DST begins (the clock jumps one hour forward) at

                  Sun 2016-03-13 01:59:59 CST

                  Sun 2016-03-13 03:00:00 CDT