by Karthikeyan Sadhasivam
on June 25, 2014
NTP stands for Network Time Protocol.
It is used to synchronize the time on your Linux system with a centralized NTP server.
A local NTP server on the network can be synchronized with an
external timing source to keep all the servers in your organization
in-sync with an accurate time.
I. Configure NTP server
1. Install NTP Server
First, install NTP package on your server using the appropriate package management tool that is available on your Linux distro.
For example, on RedHat or CentOS, use yum to install ntp as shown below:
2. Setup Restrict values in ntp.conf
Modify the /etc/ntp.conf file to make sure it has the following two restrict lines.
# Permit time synchronization with our time source, but do not
# permit the source to query or modify the service on this system.
restrict default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery
The first restrict line allows other clients to query your time server. This restrict line has the following parameters
- 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
The value -6 in the second line allows forces the DNS resolution to
the IPV6 address resolution. For more information on the access
parameters list, Please refer to documentation on “man ntp_acc”
3. Allow Only Specific Clients
To only allow machines on your own network to synchronize with your
NTP server, add the following restrict line to your /etc/ntp.conf file:
restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
If the localhost needs to have the full access to query or modify, add the following line to /etc/ntp.conf
4. Add Local Clock as Backup
Add the local clock to the ntp.conf file so that if the NTP server is
disconnected from the internet, NTP server provides time from its local
system clock.
server 127.127.1.0 # local clock
fudge 127.127.1.0 stratum 10
In the above line, Stratum is used to synchronize the time with the
server based on distance. A stratum-1 time server acts as a primary
network time standard. A stratum-2 server is connected to the stratum-1
server over the network. Thus, a stratum-2 server gets its time via NTP
packet requests from a stratum-1 server. A stratum-3 server gets its
time via NTP packet requests from a stratum-2 server, and so on.
Also stratum 0 devices are always used as reference clock.
5. Setup NTP Log Parameters
Specify the drift file and the log file location in your ntp.conf file
driftfile /var/lib/ntp/ntp.drift
logfile /var/log/ntp.log
driftfile is used to log how far your clock is from what it should be, and slowly ntp should lower this value as time progress.
6. Start the NTP Serrver
After setting up appropriate values in the ntp.conf file, start the ntp service:
II. Configure NTP Client to Synchronize with NTP Server
7. Modify ntp.conf on NTP Client
This setup should be done on your NTP Client (Not on NTP-server)
To synchronize the time of your local Linux client machine with NTP
server, edit the /etc/ntp.conf file on the client side. Here is an
example of how the sample entries looks like. In the following example,
you are specifying multiple servers to act as time server, which is
helpful when one of the timeservers fails.
server 0.rhel.pool.ntp.org iburst
server 1.rhel.pool.ntp.org iburst
server 2.rhel.pool.ntp.org iburst
server 3.rhel.pool.ntp.org iburst
iburst: After every poll, a burst of eight packets is sent instead of
one. When the server is not responding, packets are sent 16s interval.
When the server responds, packets are sent every 2s.
Edit your NTP.conf to reflect appropriate entries for your own NTP server.
prefer: If this option is specified that server is preferred over
other servers. A response from the preferred server will be discarded if
it differs significantly different from other server’s responses.
8. Start the NTP Daemon
Once the ntp.conf is configured with correct settings, start the ntp daemon.
You will see the NTP will slowly start to synchronize the time of your linux machine with the NTP Server.
9. Check the NTP Status
Check the status of NTP using the ntpq command. If you get any
connection refused errors then the time server is not responding or the
NTP daemon/port is not started or listening.
# ntpq -p
remote refid st t when poll reach delay offset jitter
==============================================================================
*elserver1 19.168.1.1 3 u 300 1024 377 1.225 -0.071 4.606
10. Set Local Date and Time
The ntpdate command can be used to set the local date and time by
polling the NTP server. Typically, you’ll have to do this only one time.
Your jitter value should be low, else check the drift from the clock
in the driftfile. You may also need to change to some other NTP server
based on the difference. This command synchronizes the time with your
NTP server manually.
After this initial sync, NTP client will talk to the NTP server on an
on-going basis to make sure the local time reflects the accurate time.
You can also use the following command to get the current status of ntpd.
# ntpdc -c sysinfo
system peer: thegeekstuff.com
system peer mode: client
leap indicator: 00
stratum: 4
precision: -23
root distance: 0.00279 s
root dispersion: 0.06271 s
reference ID: [19.168.1.1]
reference time: d70bd07b.f4b5cf2b Wed, Apr 30 2014 15:41:47.955
system flags: auth monitor ntp kernel stats
jitter: 0.000000 s
stability: 0.000 ppm
broadcastdelay: 0.000000 s
authdelay: 0.000000 s
