6. Configuring NTP

First, ntpd needs a majority of servers to agree on the time before it can sync. If you only have one server, that server WILL be believed. If you have two or three servers and they disagree, no majority will be found.

If you are going to have server or peer lines in ntp.conf I recommend having at least four or five of them, and use iburst for each. See SelectingOffsiteNTPServers for more.

Be very careful with burst as it is only rarely needed and will significantly increase the traffic. Do not use burst unless you have a very good reason for doing so. (I'd like to have a URL to the pages that describe the rare cases where burst is appropriate.)

6.9. Minimal configuration

A minimum comfiguration contains just a list of servers to get the time from but to avoid a lengthy startup you must define a path to a drift-file where the drift og the local clock can be stored. Example:

   server ntp.your.isp.com
   server 0.europe.pool.ntp.org
   server 1.europe.pool.ntp.org
   server 2.europe.pool.ntp.org
   driftfile /var/lib/ntp/ntp.drift

This asumes you are in Europe. With four servers ntpd will still work even if one server fails.

6.10. DHCP Auto-configuration

6.10.1. ISC dhcp 3

ISC's dhcp is able to automatically configure the servers used by ntpd. Here's how to get it working:

1. The dhcp server you are using must be configured to provide the ntp-servers option
2. Configure your dhclient to request ntp-servers (it doesn't by default). To do this add ntp-servers to the default request line in /etc/dhcp3/dhclient.conf
3. Create an /etc/ntp.conf with all of the other settings that you wish to use. This file will be used to create /etc/ntp.conf.dhcp, it won't be over written.
4. Your ntpd must be told to use /etc/ntp.conf.dhcp if it exists. This is usually accomplished in the ntp init script (e.g. /etc/init.d/ntp).

6.11. How dynamic IP addresses affect ntpd

Currently, ntpd does not rescan the network interfaces after it has started. If you are have a dynamic IP (use DHCP), this means ntpd will no longer be able to communicate with "outside" sources after your IP changes.

The script in Bug #51 will not help you unless it's the remote site's IP address that has changed.

6.12. Using the NIST Leap Second File

The U.S. National Institute of Standards and Technology (NIST) is publishing a file which contains a table of past and upcoming leap seconds. This file can be used by ntpd to become aware of leap second announcements and thus be able to handle leap seconds gracefully.

ntpd evaluates the leap second file only if the autokey feature has been enabled. The autokey feature also provides a mechanism to forward the information from the leap second file to NTP clients.

If the top level NTP server is unable to obtain the leap second file automatically then the file can be provided manually via FTP.

The file is available at ftp://time.nist.gov/pub/

The file name is: leap-seconds.nnnnnnnnnn where nnnnnnnnnn is a kind of "serial number" which is derived from a time stamp in order to simplify automatic updates.

As of this writing the name of the file is leap-seconds.3331497600.

This file must be copied to the crypto directory configured for ntpd, and inside that directory a link must be created which points to that file. The standard name for the link is ntpkey_leap. However, this can be overridden by a configuration parameter.

So assuming the name of the leap second file as mentioned above, the standard link to the leap second file inside the NTP crypto directory should look like this:

ntpkey_leap -> leap-seconds.3331497600