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Configuring NTP Server on Ubuntu

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As part of a few changes happening I’ve been asked to look into how difficult would it be to configure our own local NTP server. From looking at very useful Ubuntu Man Pages and finding great articles on The Geek Stuff (one of my favourites sites) and Blogging Dragon, it appears that it’s more straightforward than I expected! Soooo this post will note down how to install and configure a NTP Server.

Network Time Protocol (NTP) is a network protocol for clock synchronization between servers, network devices and desktops. NTP is defined in RFC5905 and is described as:

Network Time Protocol version 4 (NTPv4), which is widely used to synchronize system clocks among a set of distributed timeservers and clients…. The NTP subnet model includes a number of widely accessible primary time servers synchronized by wire or radio to national standards. The purpose of the NTP protocol is to convey timekeeping information from these primary servers to secondary time servers and clients via both private networks and the public Internet.

For this test, I’m using ESXi Ubuntu 14.04LTS host as the local NTP server and will be configuring both a Juniper SRX220 and another ESXi Ubuntu 14.04LTS host as a NTP clients. The local NTP server’s IP address is

You will need root and/or sudo privileges

With all the background done, let’s get cracking 😀

As always, when getting anything from Ubuntu’s apt-get repository we’ll need to make sure to run update; to get all the newest version of packages currently installed and run dist-upgrade; to ensure the most important packages are updated as it has a “smart” conflict resolution system.

sudo apt-get update
sudo apt-get dist-upgrade

Having ensured the server’s packages are up to date, we can install the ntp and ntpdate packages by using apt-get install ntp ntpdate

sudo apt-get install ntp ntpdate

Before making any changes we need to make sure that the default time zone to Universal Time Coordinated (UTC). This is because UTC is regarded as the primary time standard by which the world regulates clocks and time. You can check/change the default time zone by running the command dpkg-reconfigure tzdata. You will be prompted to these screens where you can select the time zone: Screen #1 and Screen #2.

Once the time zone has been set, you will get the output below confirming the Time Zone:

[email protected]:~$ sudo dpkg-reconfigure tzdata

Current default time zone: 'Etc/UTC'
Local time is now:      Mon Jan 11 14:56:57 UTC 2016.
Universal Time is now:  Mon Jan 11 14:56:57 UTC 2016.

Next create a backup of the ntp.conf file

sudo cp /etc/ntp.conf /etc/ntp.conf.old

Use a text editor (I prefer nano) to open up the ntp.conf file and find the following lines below:

# Use servers from the NTP Pool Project. Approved by Ubuntu Technical Board
# on 2011-02-08 (LP: #104525). See http://www.pool.ntp.org/join.html for
# more information.
server 0.ubuntu.pool.ntp.org
server 1.ubuntu.pool.ntp.org
server 2.ubuntu.pool.ntp.org
server 3.ubuntu.pool.ntp.org

You can comment them out, delete, keep or replace the lines. The lines are specifying multiple servers to act as a timeserver, which is helpful when one of the timeservers fail. You can use regional pool ntp severs from ntp.org’s Regional Pools. In my example, as I live in the UK, I’ve used the regional pool of servers closest to the UK

server 0.uk.pool.ntp.org iburst dynamic
server 1.uk.pool.ntp.org iburst dynamic
server 2.uk.pool.ntp.org iburst dynamic
server 3.uk.pool.ntp.org iburst dynamic

The sytax iburst and dynamic are optional commands but can be useful to have set, depending on your environment.

  • iburst: After every poll a burst of eight packets is sent instead of one. When the server is not responding, packets are sent at 16 seconds intervals. When the server responds, packets are sent every 2 seconds. This means that after reboot or restart ntp synchronizations are established quicker.
  • dynamic: This option tells NTP it can try a configured server again later if it’s unavailable at some point, which can be useful when the server doesn’t always have Internet connectivity.
You can see my full example ntp.conf file here

Having saved and close the updated ntp.conf file, we’ll need to restart the daemon running service ntp restart

[email protected]:~$ sudo service ntp restart
 * Stopping NTP server ntpd                 [ OK ] 
 * Starting NTP server ntpd                 [ OK ]

After restarting the daemon, the server will take around 10-15 minutes for NTP to synchronize with the timeservers and it will automatically set the system clock. By using the command ntpq -p we’ll be able to check the status of the NTP servers that we are synchronized with

[email protected]:~$  ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
 time.rdg.uk.as4    2 u   21   64    3    3.138    0.047   0.506
 resntp-a-vip.lo     3 u   22   64    3    1.723    2.038   0.526
 2a03:b0c0:1:d0:      3 u   19   64    3    2.870    1.632   0.337
 neon.trippett.o    2 u   26   64    3    2.928    1.863   0.473
 golem.canonical   2 u   24   64    3    9.211    3.418   0.387

The table below explains the different parameters from the ntpq -p output:

Parameters Function
Remote Specifics the hostname of the timeserver
Refid This is a 32-bit code identifying the particular reference clock.
St (Stratus) This indicates your physical GPS closeness to the timeserver. Anything under 3 is seen as good
When Number of Seconds passed since the last poll or time check
Poll This is the minimum interval between transmitted messages, in seconds as a power of two. For instance, a value of six indicates a minimum interval of 64 seconds.
Reach How well a clock can maintain a constant frequency.
Delay Provides the capability to launch a message to arrive at the reference clock at a specified time. Relative to a selected reference clock.
Offset The time difference between two clocks, relative to a selected reference clock. Represents the amount to adjust the local clock to bring it into correspondence with the reference clock.
Jitter Short-term variations in Frequency with components greater than 10 Hz. The estimated time error of the system clock measured as an exponential average of RMS time differences.

You will be able start, stop, restart and/or check ntp status by using these commands

service ntp status
service ntp start
service ntp stop
service ntp restart

And with that we have a NTP server configured!

Enabling NTP Client

For testing, I configured a Juniper SRX220 to be the NTP client. It’s quite straightforward to enable ntp on a SRX; you’ll need to set and commit the commands to below and with that you will have NTP enabled. Simple Right! 🙂

set system ntp server prefer
set system ntp server version 4

For verification of NTP on the SRX we can run show ntp associations, show ntp status and show system uptime

show ntp associationsshow system uptimeshow ntp status
[email protected]> show ntp associations no-resolve 
     remote           refid      st t when poll reach   delay   offset  marquk01
*     3 -    8   64    1    1.849    2.498   0.160
[email protected]> show system uptime 
Current time: 2016-01-11 10:15:27 UTC
System booted: 2016-01-08 10:02:36 UTC (3d 00:12 ago)
Protocols started: 2016-01-08 10:05:13 UTC (3d 00:10 ago)
Last configured: 2016-01-11 10:13:34 UTC (00:01:53 ago) by marquk01
10:15AM  up 3 days, 13 mins, 2 users, load averages: 0.05, 0.11, 0.04
[email protected]> show ntp status                     
status=0664 leap_none, sync_ntp, 6 events, event_peer/strat_chg,
version="ntpd 4.2.0-a Fri Nov 13 15:40:48 UTC 2015 (1)",
processor="octeon", system="JUNOS12.1X47-D30.4", leap=00, stratum=4,
precision=-17, rootdelay=13.781, rootdispersion=2.340, peer=31708,
reftime=da3dff68.c47f84ec  Mon, Jan 11 2016 10:16:08.767, poll=6,
clock=da3dff6b.8ee12a6f  Mon, Jan 11 2016 10:16:11.558, state=3,
offset=0.000, frequency=0.000, jitter=0.213, stability=0.000

And on the Ubuntu host, it’s exactly the same as I described above, but in the /etc/ntp.conf file you’ll need to set the server as your local NTP server

# Use servers from the NTP Pool Project. Approved by Ubuntu Technical Board
# on 2011-02-08 (LP: #104525). See http://www.pool.ntp.org/join.html for
# more information.
#server 0.ubuntu.pool.ntp.org
#server 1.ubuntu.pool.ntp.org
#server 2.ubuntu.pool.ntp.org
#server 3.ubuntu.pool.ntp.org

server prefer iburst dynamic

We can then run ntpq -p to check the ntp server is the local server!

[email protected]:~$ ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
*  3 u   28   64    1    0.454    0.380   0.315

For more in-depth detailed information on how to use NTP pools see ntp.org and the Ubuntu Man page for more detail on ntp.conf file.

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