Manual Pages  — NTPD

Ordinarily, ntpd reads the ntp.conf(5) configuration file at startup time in order to determine the synchronization sources and operating modes. It is also possible to specify a working, although limited, configuration entirely on the command line, obviating the need for a configuration file. This may be particularly useful when the local host is to be configured as a broadcast/multicast client, with all peers being determined by listening to broadcasts at run time.

If NetInfo support is built into ntpd, then ntpd will attempt to read its configuration from the NetInfo if the default ntp.conf(5) file cannot be read and no file is specified by the -c option.

Various internal ntpd variables can be displayed and configuration options altered while the ntpd is running using the ntpq(8) and ntpdc(8) utility programs.

When ntpd starts it looks at the value of umask(2), and if zero ntpd will set the umask(2) to 022.

Most operating systems and hardware of today incorporate a time-of-year (TOY) chip to maintain the time during periods when the power is off. When the machine is booted, the chip is used to initialize the operating system time. After the machine has synchronized to a NTP server, the operating system corrects the chip from time to time. In the default case, if ntpd detects that the time on the host is more than 1000s from the server time, ntpd assumes something must be terribly wrong and the only reliable action is for the operator to intervene and set the clock by hand. (Reasons for this include there is no TOY chip, or its battery is dead, or that the TOY chip is just of poor quality.) This causes ntpd to exit with a panic message to the system log. The -g option overrides this check and the clock will be set to the server time regardless of the chip time (up to 68 years in the past or future — this is a limitation of the NTPv4 protocol). However, and to protect against broken hardware, such as when the CMOS battery fails or the clock counter becomes defective, once the clock has been set an error greater than 1000s will cause ntpd to exit anyway.

Under ordinary conditions, ntpd adjusts the clock in small steps so that the timescale is effectively continuous and without discontinuities. Under conditions of extreme network congestion, the roundtrip delay jitter can exceed three seconds and the synchronization distance, which is equal to one-half the roundtrip delay plus error budget terms, can become very large. The ntpd algorithms discard sample offsets exceeding 128 ms, unless the interval during which no sample offset is less than 128 ms exceeds 900s. The first sample after that, no matter what the offset, steps the clock to the indicated time. In practice this reduces the false alarm rate where the clock is stepped in error to a vanishingly low incidence.

As the result of this behavior, once the clock has been set it very rarely strays more than 128 ms even under extreme cases of network path congestion and jitter. Sometimes, in particular when ntpd is first started without a valid drift file on a system with a large intrinsic drift the error might grow to exceed 128 ms, which would cause the clock to be set backwards if the local clock time is more than 128 s in the future relative to the server. In some applications, this behavior may be unacceptable. There are several solutions, however. If the -x option is included on the command line, the clock will never be stepped and only slew corrections will be used. But this choice comes with a cost that should be carefully explored before deciding to use the -x option. The maximum slew rate possible is limited to 500 parts-per-million (PPM) as a consequence of the correctness principles on which the NTP protocol and algorithm design are based. As a result, the local clock can take a long time to converge to an acceptable offset, about 2,000 s for each second the clock is outside the acceptable range. During this interval the local clock will not be consistent with any other network clock and the system cannot be used for distributed applications that require correctly synchronized network time.

In spite of the above precautions, sometimes when large frequency errors are present the resulting time offsets stray outside the 128-ms range and an eventual step or slew time correction is required. If following such a correction the frequency error is so large that the first sample is outside the acceptable range, ntpd enters the same state as when the ntp.drift file is not present. The intent of this behavior is to quickly correct the frequency and restore operation to the normal tracking mode. In the most extreme cases (the host time.ien.it comes to mind), there may be occasional step/slew corrections and subsequent frequency corrections. It helps in these cases to use the burst keyword when configuring the server, but ONLY when you have permission to do so from the owner of the target host.

Finally, in the past many startup scripts would run ntpdate(8) or sntp(8) to get the system clock close to correct before starting ntpd(8), but this was never more than a mediocre hack and is no longer needed. If you are following the instructions in Starting NTP (Best Current Practice) and you still need to set the system time before starting ntpd, please open a bug report and document what is going on, and then look at using sntp(8) if you really need to set the clock before starting ntpd.

There is a way to start ntpd(8) that often addresses all of the problems mentioned above.

If you can also keep a good ntp.drift file then ntpd(8) will effectively "warm-start" and your system's clock will be stable in under 11 seconds' time.

As soon as possible in the startup sequence, start ntpd(8) with at least the -g and perhaps the -N options. Then, start the rest of your "normal" processes. This will give ntpd(8) as much time as possible to get the system's clock synchronized and stable.

Finally, if you have processes like dovecot or database servers that require monotonically-increasing time, run ntp-wait(1ntp-waitmdoc) as late as possible in the boot sequence (perhaps with the -v flag) and after ntp-wait(1ntp-waitmdoc) exits successfully it is as safe as it will ever be to start any process that require stable time.

In some cases it may not be practical for ntpd to run continuously. A common workaround has been to run the ntpdate(8) or sntp(8) programs from a cron(8) job at designated times. However, these programs do not have the crafted signal processing, error checking or mitigation algorithms of ntpd. The -q option is intended for this purpose. Setting this option will cause ntpd to exit just after setting the clock for the first time. The procedure for initially setting the clock is the same as in continuous mode; most applications will probably want to specify the iburst keyword with the server configuration command. With this keyword a volley of messages are exchanged to groom the data and the clock is set in about 10 s. If nothing is heard after a couple of minutes, the daemon times out and exits. After a suitable period of mourning, the ntpdate(8) program will be retired.

When kernel support is available to discipline the clock frequency, which is the case for stock Solaris, Tru64, Linux and FreeBSD , a useful feature is available to discipline the clock frequency. First, ntpd is run in continuous mode with selected servers in order to measure and record the intrinsic clock frequency offset in the frequency file. It may take some hours for the frequency and offset to settle down. Then the ntpd is stopped and run in one-time mode as required. At each startup, the frequency is read from the file and initializes the kernel frequency.

The filter is activated by the tinker command and huffpuff keyword, as described in ntp.conf(5).

In addition to the manual pages provided, comprehensive documentation is available on the world wide web at http://www.ntp.org/. A snapshot of this documentation is available in HTML format in /usr/share/doc/ntp.

David L. Mills, RFC1059, Network Time Protocol (Version 1),

David L. Mills, RFC1119, Network Time Protocol (Version 2),

David L. Mills, RFC1305, Network Time Protocol (Version 3),

David L. Mills, J. Martin, Ed., J. Burbank, W. Kasch, RFC5905, Network Time Protocol Version 4: Protocol and Algorithms Specification,

David L. Mills, B. Haberman, Ed., RFC5906, Network Time Protocol Version 4: Autokey Specification,

H. Gerstung, C. Elliott, B. Haberman, Ed., RFC5907, Definitions of Managed Objects for Network Time Protocol Version 4: (NTPv4),

R. Gayraud, B. Lourdelet, RFC5908, Network Time Protocol (NTP) Server Option for DHCPv6,

Please send bug reports to: http://bugs.ntp.org, bugs@ntp.org