how to install NTP on Centos 6.8

ntp-on-centos-6.8.md

Intall NTP Server and Client on CentOS 6.8

Sources

• http://www.thegeekstuff.com/2014/06/linux-ntp-server-client
• https://www.server-world.info/en/note?os=CentOS_6&p=ntp
• https://www.server-world.info/en/note?os=CentOS_6&p=ntp&f=2

On the server

Install NTP Server

[root@apaternina ~]# yum -y install ntp

Setup values in ntp.conf

[root@apaternina ~]# vi /etc/ntp.conf 

Add or modify the following lines:

# allow access by other clients
restrict default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery

# give access to specific hosts (in this network)
restrict 192.168.56.0 mask 255.255.255.0 nomodify notrap

# use the local clock as a backup
server 127.127.1.0
fudge 127.127.1.0 stratum 5

# specify the drift file and log file location.
driftfile /var/lib/ntp/drift
logfile /var/log/ntp.log

This is what those parameters mean:

• noquery: prevents dumping status data from ntpd.
• notrap: prevents control message trap service.
• nomodify: prevents all ntpq queries that attempts to modify the server.
• nopeer: prevents all packets that attempts to establish a peer association.
• Kod: – Kiss-o-death packet is to be sent to reduce unwanted queries

Then start the NTP daemon program:

service ntpd start

Setup autostart:

[root@apaternina ~]# chkconfig ntpd on

Allow 123 DUP port in iptables

[root@apaternina ~]# iptables -I INPUT 5 -p udp -m state --state NEW -m udp --dport 123 -j ACCEPT
[root@apaternina ~]# service iptables save

On the client

To use multiple NTP servers (in case one of them fails) do the following:

[root@apaternina ~]# vi /etc/ntp.conf 

Add/modify the following lines:

# 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

server 192.168.56.100 prefer

Then start the NTP daemon program:

service ntpd start

Setup autostart:

[root@apaternina ~]# chkconfig ntpd on

if you want to poll the NTP server directly (the NTP daemon must be stopped -> service ntpd stop or use ntpdate -u 192.168.56.100)

[root@apaternina ~]# ntpdate 192.168.56.100
 7 Apr 12:09:50 ntpdate[3716]: adjust time server 192.168.56.100 offset -0.007696 sec

On Mac OS

to sync time with a NTP server:

sudo ntpdate -u time.apple.com

or with a custom NTP server:

sudo ntpdate -u 192.168.56.100

Tools to Debug

to check the status of NTP do this:

[root@apaternina ~]# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 server1.laplata .INIT.          16 u    -   64    0    0.000    0.000   0.000
 200-89-75-197-L .INIT.          16 u    -   64    0    0.000    0.000   0.000
 ntp3.0x00.lv    .INIT.          16 u    -   64    0    0.000    0.000   0.000
 aquila.init7.ne .INIT.          16 u    -   64    0    0.000    0.000   0.000
 192.168.56.100  LOCAL(0)         6 u   14   64    1    1.370  1326.19   0.000

to get the current status of NTP

[root@apaternina ~]# ntpdc -c sysinfo
system peer:          LOCAL(0)
system peer mode:     client
leap indicator:       00
stratum:              6
precision:            -22
root distance:        0.00000 s
root dispersion:      0.07420 s
reference ID:         [127.127.1.0]
reference time:       dc92464f.3ee29679  Fri, Apr  7 2017 12:09:35.245
system flags:         auth monitor ntp kernel stats 
jitter:               0.000000 s
stability:            0.000 ppm
broadcastdelay:       0.000000 s
authdelay:            0.000000 s

Very good & useful info