ntp

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NTP uses hierarchies of servers with top-level servers synchronizing time with atomic clocks. This hierarchy levels are known as stratum, and the level can range between 1 and 15, both inclusive. The highest stratum level is 1 and is determined by the accuracy of the clock the server synchronizes with. If a server synchronizes with other NTP server with stratum level 3, then the stratum level for this server is automatically set to 4.
Another time synchronization tool provided by Ubuntu is ntpdate, which comes preinstalled with Ubuntu. It executes once at boot time and synchronizes the local time with Ubuntu's NTP servers. The problem with ntpdate is that it matches server time with central time without considering the big drifts in local time, whereas the NTP daemon ntpd continuously adjusts the server time to match it with the reference clock. As mentioned in the ntpdate manual pages (man ntpdate), you can use ntpdatemultiple times throughout a day to keep time drifts low and get more accurate results, but it does not match the accuracy and reliability provided by ntpd.
In this recipe, we will set up a standalone time server for an internal network. Our time server will synchronize its time with public time servers and provide a time service to internal NTP clients.
Following are the steps to install and configure NTP daemon:
  1. First, synchronize the server's time with any Internet time server using the ntpdate command:
    $ ntpdate -s ntp.ubuntu.com
  2. To install ntpd, enter the following command in the terminal:
    $ sudo apt-get install ntp
  3. Edit the /etc/ntp.conf NTP configuration file to add/remove external NTP servers:
    $ sudo nano /etc/ntp.conf
  4. Set a fallback NTP server:
    server ntp.ubuntu.com
  5. Block any external access to the server, comment the first restrict line, and add the following command:
    restrict default noquery notrust nomodify
  6. Allow the clients on local network to use the NTP service:
    restrict 192.168.1.0 mask 255.255.255.0
  7. Save changes with Ctrl + O and exit nano with Ctrl + X.
  8. Reload the NTP daemon with the following command:
    $ sudo service ntp restart

Sometimes, the NTP daemon refuses to work if the time difference between local time and central time is too big. To avoid this problem, we have synchronized the local time and central time before installing ntpd. As ntpd and ntpdate both use the same UDP port, 123, the ntpdate command will not work when the ntpd service is in use.
A central DHCP server can be configured to provide NTP settings to all DHCP clients. For this to work, your clients should also be configured to query NTP details from DHCP. A DHCP client configuration on Ubuntu already contains the query for network time servers.
Add the following line to your DHCP configuration to provide NTP details to the clients:
subnet 192.168.1.0 netmask 255.255.255.0 {
    ...
    option ntp-servers  your_ntp_host;
}


On the clientside, make sure that your dhclient.conf contains ntp-servers in its default request:
request subnet-mask, broadcast-address, time-offset, routers,
        ...
        rfc3442-classless-static-routes, ntp-servers,

See also

  • Check the default /etc/ntp.conf configuration file. It contains a short explanation for each setting.


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