Install NTP on CentOS

Written on May 28, 2016 | DevOps

Install NTP and its documentation

yum install ntp ntp-doc

Configure NTP servers to query from

Head to http://www.pool.ntp.org/en/ and select the country based on the geographical location of the server. For example, the following servers would be used for a server located in the U.S.:

server 0.us.pool.ntp.org
server 1.us.pool.ntp.org
server 2.us.pool.ntp.org
server 3.us.pool.ntp.org

Edit /etc/ntp.conf, replacing the default servers with the ones identified above.

Review security configuration

The default settings are most likely sufficient, but review them against the NTP access restrictions documentation and modify as needed.

Configure leap seconds

Download the leap seconds file from http://www.ietf.org/timezones/data/leap-seconds.list and save it to the server in a location like /var/lib/ntp/leapfile. Ensure that this file is readable by ntpd.

Modify /etc/ntp.conf and add the line:

leapfile /var/lib/ntp/leapfile

See: http://support.ntp.org/bin/view/Support/ConfiguringNTP#Section_6.14.

Configure logging

Log output is placed in /var/log/messages by default but can be overridden if desired. My preference is to leave it there as it doesn’t really make sense to have a separate log file for a specific service, but the following line in /etc/ntp.conf will log elsewhere.

logfile /var/log/ntp.log

See: https://www.redhat.com/archives/rhl-list/2007-December/msg03702.html

Start the ntpd service

service ntpd start

Check synchronization status with peers

# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
+host2.kingrst.c 198.60.73.8      2 u   38   64  377   31.327   -3.206  12.424
*lithium.constan 18.26.4.105      2 u   66   64  373   10.723  -11.870   3.080
-propjet.latt.ne 199.233.236.226  3 u    4   64  377  112.837    3.736  30.346
+F-Current.sjela 204.123.2.72     2 u    5   64  377   98.932    1.000  14.950

Sniff UDP port 123

Watch for NTP packets on UDP 123. It should take a minute or so to see some traffic to the NTP servers.

# tcpdump dst port 123
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
39:33.371065 IP localhost.ntp > F-Current.sjelab.net.ntp: NTPv4, Client, length 48
39:33.441244 IP F-Current.sjelab.net.ntp > localhost.ntp: NTPv4, Server, length 48
39:35.369876 IP localhost.ntp > propjet.latt.net.ntp: NTPv4, Client, length 48
39:35.516572 IP propjet.latt.net.ntp > localhost.ntp: NTPv4, Server, length 48
39:37.369878 IP localhost.ntp > lithium.constant.com.ntp: NTPv4, Client, length 48
39:37.389644 IP lithium.constant.com.ntp > localhost.ntp: NTPv4, Server, length 48
39:47.377980 IP localhost.ntp > host2.kingrst.com.ntp: NTPv4, Client, length 48
39:47.475338 IP host2.kingrst.com.ntp > localhost.ntp: NTPv4, Server, length 48

Query current status of ntpd

# ntpdc -c sysinfo
system peer:          lithium.constant.com
system peer mode:     client
leap indicator:       11
stratum:              3
precision:            -20
root distance:        0.05229 s
root dispersion:      0.09605 s
reference ID:         [108.61.56.35]
reference time:       daf4104b.6a0f384e  Sat, May 28 2016 12:40:43.414
system flags:         auth monitor ntp kernel stats
jitter:               0.010727 s
stability:            0.000 ppm
broadcastdelay:       0.000000 s
authdelay:            0.000000 s