Note: This is an RHCSA 7 exam objective and an RHCE 7 exam objective.

Presentation

NTP (Network Time Protocol) is a protocol to keep servers time synchronized: one or several master servers provide time to client servers that can themselves provide time to other client servers (notion of stratus).

This tutorial deals with client side configuration, even though server configuration is not entirely different.

Two main packages are used in RHEL 7 to set up the client side:

  • ntp: this is the classic package, already existing in RHEL 6, RHEL 5, etc. It can be used both as a NTP client or server.
  • chrony: this is a new solution better suited for portable PC or machines with network connection problems (time synchronization is quicker). It can only be used as a NTP client. chrony is the default package in RHEL 7.

Caution: ntpd and chronyd shouldn’t run at the same time. Choose one and only one of them! There are reports from RHCE candidates noting that one of them is purposely already running at the beginning of the exam.

Prerequisites

Before anything else, you need to assign the correct time zone. To get the current configuration, type:

# timedatectl
Local time: Sat 2015-11-07 08:17:33 EST
Universal time: Sat 2015-11-07 13:17:33 UTC
RTC time: Sat 2015-11-07 13:17:33
Timezone: America/New_York (EST, -0500)
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 EDT
Sun 2015-11-01 01:00:00 EST
Next DST change: DST begins (the clock jumps one hour forward) at
Sun 2016-03-13 01:59:59 EST
Sun 2016-03-13 03:00:00 EDT

To get the list of all the available time zones, type:

# timedatectl list-timezones
Africa/Abidjan
Africa/Accra
Africa/Addis_Ababa
...
America/La_Paz
America/Lima
America/Los_Angeles
...
Asia/Seoul
Asia/Shanghai
Asia/Singapore
...
Pacific/Tongatapu
Pacific/Wake
Pacific/Wallis

Finally, to set a specific time zone (here America/Los_Angeles), type:

# timedatectl set-timezone America/Los_Angeles

Then, to check your new configuration, type:

# timedatectl
      Local time: Sat 2015-11-07 05:32:43 PST
  Universal time: Sat 2015-11-07 13:32:43 UTC
        RTC time: Sat 2015-11-07 13:32:43
        Timezone: America/Los_Angeles (PST, -0800)
     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 PDT
                  Sun 2015-11-01 01:00:00 PST
 Next DST change: DST begins (the clock jumps one hour forward) at
                  Sun 2016-03-13 01:59:59 PST
                  Sun 2016-03-13 03:00:00 PDT

The NTP Package

Install the NTP package:

# yum install -y ntp

Activate the NTP service at boot:

# systemctl enable ntpd

Start the NTP service:

# systemctl start ntpd

The NTP configuration is in the /etc/ntp.conf file:

# For more information about this file, see the man pages
# ntp.conf(5), ntp_acc(5), ntp_auth(5), ntp_clock(5), ntp_misc(5), ntp_mon(5).

driftfile /var/lib/ntp/drift

# Permit time synchronization with our time source, but do not
# permit the source to query or modify the service on this system.
restrict default nomodify notrap nopeer noquery

# Permit all access over the loopback interface.  This could
# be tightened as well, but to do so would effect some of
# the administrative functions.
restrict 127.0.0.1 
restrict ::1

# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
server 0.centos.pool.ntp.org iburst
server 1.centos.pool.ntp.org iburst
server 2.centos.pool.ntp.org iburst
server 3.centos.pool.ntp.org iburst

includefile /etc/ntp/crypto/pw

# Key file containing the keys and key identifiers used when operating
# with symmetric key cryptography. 
keys /etc/ntp/keys

Note: For basic configuration purpose, only the server directives could need a change to point at a different set of master time servers than the defaults specified.

To get some information about the time synchronization process, type:

# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*y.ns.gin.ntt.ne 192.93.2.20      2 u   47   64  377   27.136    6.958  11.322
+ns1.univ-montp3 192.93.2.20      2 u   45   64  377   34.836   -0.009  11.463
+merlin.ensma.ne 193.204.114.232  2 u   48   64  377   34.586    4.443  11.370
+obsidian.ad-not 131.188.3.220    2 u   50   64  377   22.548    4.256  12.077

Alternatively, to get a basic report, type:

# ntpstat
synchronised to NTP server (129.250.35.251) at stratum 3
time correct to within 60 ms
polling server every 64 s

To quickly synchronize a server, type:

# systemctl stop ntpd
# ntpdate pool.ntp.org
 5 Jul 10:36:58 ntpdate[2190]: adjust time server 95.81.173.74 offset -0.005354 sec
# systemctl start ntpd

The Chrony Package

Alternatively, you can install the newChrony service that is quicker to synchronize clocks in mobile and virtual systems.

Install the Chrony service:

# yum install -y chrony

Activate the Chrony service at boot:

# systemctl enable chronyd

Start the Chrony service:

# systemctl start chronyd

The Chrony configuration is in the /etc/chrony.conf file:

# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
server 0.centos.pool.ntp.org iburst
server 1.centos.pool.ntp.org iburst
server 2.centos.pool.ntp.org iburst
server 3.centos.pool.ntp.org iburst

# Ignore stratum in source selection.
stratumweight 0

# Record the rate at which the system clock gains/losses time.
driftfile /var/lib/chrony/drift

# Enable kernel RTC synchronization.
rtcsync

# In first three updates step the system clock instead of slew
# if the adjustment is larger than 10 seconds.
makestep 10 3

# Listen for commands only on localhost.
bindcmdaddress 127.0.0.1
bindcmdaddress ::1

keyfile /etc/chrony.keys

# Specify the key used as password for chronyc.
commandkey 1

# Generate command key if missing.
generatecommandkey

# Disable logging of client accesses.
noclientlog

# Send a message to syslog if a clock adjustment is larger than 0.5 seconds.
logchange 0.5

logdir /var/log/chrony

Note: For basic configuration purpose, only the server directives could need a change to point at a different set of master time servers than the defaults specified.

To get information about the main time reference, type:

# chronyc tracking
Reference ID    : 94.23.44.157 (merzhin.deuza.net)
Stratum         : 3
Ref time (UTC)  : Thu Jul  3 22:26:27 2014
System time     : 0.000265665 seconds fast of NTP time
Last offset     : 0.000599796 seconds
RMS offset      : 3619.895751953 seconds
Frequency       : 0.070 ppm slow
Residual freq   : 0.012 ppm
Skew            : 0.164 ppm
Root delay      : 0.030609 seconds
Root dispersion : 0.005556 seconds
Update interval : 1026.9 seconds
Leap status     : Normal

To get equivalent information to the ntpq command, type:

# chronyc sources -v
210 Number of sources = 4

  .-- Source mode  '^' = server, '=' = peer, '#' = local clock.
 / .- Source state '*' = current synced, '+' = combined , '-' = not combined,
| /   '?' = unreachable, 'x' = time may be in error, '~' = time too variable.
||                                                 .- xxxx [ yyyy ] +/- zzzz
||                                                /   xxxx = adjusted offset,
||         Log2(Polling interval) -.             |    yyyy = measured offset,
||                                  \            |    zzzz = estimated error.
||                                   |           |
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
^+ merlin.ensma.fr               2   6    77    61   +295us[+1028us] +/-   69ms
^* lafkor.de                     2   6    77    61  -1371us[ -638us] +/-   65ms
^+ kimsuflol.iroqwa.org          3   6    77    61   -240us[ -240us] +/-   92ms
^+ merzhin.deuza.net             2   6    77    61    +52us[  +52us] +/-   48ms

# chronyc sourcestats -v
210 Number of sources = 4
                             .- Number of sample points in measurement set.
                            /    .- Number of residual runs with same sign.
                           |    /    .- Length of measurement set (time).
                           |   |    /      .- Est. clock freq error (ppm).
                           |   |   |      /           .- Est. error in freq.
                           |   |   |     |           /         .- Est. offset.
                           |   |   |     |          |          |   On the -.
                           |   |   |     |          |          |   samples. \
                           |   |   |     |          |          |             |
Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
==============================================================================
merlin.ensma.fr             7   5   200      0.106      6.541   +381us   176us
lafkor.de                   7   4   199      0.143     10.145   -916us   290us
kimsuflol.iroqwa.org        7   7   200     -0.298      6.717    +69us   184us
merzhin.deuza.net           7   5   200      0.585     11.293   +675us   314us

To quickly synchronize a server, type:

# ntpdate pool.ntp.org
 5 Jul 10:31:06 ntpdate[2135]: step time server 193.55.167.1 offset 121873.493146 sec

Note: You don’t need to stop the Chrony service to synchronize the server.

Additional Resources

You can read these Red Hat articles about leap seconds management, how to resolve leap second issues or the differences between NTP and PTP. The xmodulo website provides a tutorial on How to set up NTP server in CentOS. Documentation about NTP is available at the NTP Documentation Archive website and at the Tuxfamily website for Chrony.

Beyond the exam objectives, virtualization can trigger problems (see this thread) and it is useful to know How to avoid VM clock drift.

You can also be interested in converting a Raspberry PI into a stratum 1 NTP server.

Some specific services can need to wait until the clock is synchronized: read this thread to know how to configure them.