The ntpd utility does most computations in 64-bit floating point arithmetic and does relatively clumsy 64-bit fixed point operations only when necessary to preserve the ultimate precision, about 232 picoseconds. While the ultimate precision is not achievable with ordinary workstations and networks of today, it may be required with future gigahertz CPU clocks and gigabit LANs.
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.
Force DNS resolution of following host names on the command line to the IPv4 namespace.
Force DNS resolution of following host names on the command line to the IPv6 namespace.
Require cryptographic authentication for broadcast client, multicast client and symmetric passive associations. This is the default.
Do not require cryptographic authentication for broadcast client, multicast client and symmetric passive associations. This is almost never a good idea.
The name and path of the configuration file, /etc/ntp.conf by default.
The name and path of the frequency file, /etc/ntp.drift by default. This is the same operation as the driftfile driftfile configuration specification in the /etc/ntp.conf file.
Normally, ntpd exits with a message to the system log if the offset exceeds the panic threshold, which is 1000 s by default. This option allows the time to be set to any value without restriction; however, this can happen only once. If the threshold is exceeded after that, ntpd will exit with a message to the system log. This option can be used with the -q and -x options. See the tinker configuration file directive for other options.
Normally, ntpd steps the time if the time offset exceeds the step threshold, which is 128 ms by default, and otherwise slews the time. This option forces the initial offset correction to be stepped, so the highest time accuracy can be achieved quickly. However, this may also cause the time to be stepped back so this option must not be used if applications requiring monotonic time are running. See the tinker configuration file directive for other options.
Chroot the server to the directory jaildir This option also implies that the server attempts to drop root privileges at startup. You may need to also specify a -u option. This option is only available if the OS supports adjusting the clock without full root privileges. This option is supported under NetBSD (configure with --enable-clockctl) or Linux (configure with --enable-linuxcaps) or Solaris (configure with --enable-solarisprivs).
Open the network address given, or all the addresses associated with the given interface name. This option may appear multiple times. This option also implies not opening other addresses, except wildcard and localhost. This option is deprecated. Please consider using the configuration file interface command, which is more versatile.
Specify the name and path of the symmetric key file. /etc/ntp.keys is the default. This is the same operation as the keys keyfile configuration file directive.
Specify the name and path of the log file. The default is the system log file. This is the same operation as the logfile logfile configuration file directive.
Do not listen to virtual interfaces, defined as those with names containing a colon. This option is deprecated. Please consider using the configuration file interface command, which is more versatile.
Set the Windows Multimedia Timer to highest resolution. This ensures the resolution does not change while ntpd is running, avoiding timekeeping glitches associated with changes.
To the extent permitted by the operating system, run ntpd at the highest priority.
Specify the name and path of the file used to record ntpd's process ID. This is the same operation as the pidfile pidfile configuration file directive.
To the extent permitted by the operating system, run ntpd at the specified sched_setscheduler(SCHED_FIFO) priority.
ntpd will not daemonize and will exit after the clock is first synchronized. This behavior mimics that of the ntpdate program, which will soon be replaced with a shell script. The -g and -x options can be used with this option. Note: The kernel time discipline is disabled with this option.
Specify the default propagation delay from the broadcast/multicast server to this client. This is necessary only if the delay cannot be computed automatically by the protocol.
Cause ntpd to parse its startup configuration file and save an equivalent to the given filename and exit. This option was designed for automated testing.
Specify the directory path for files created by the statistics facility. This is the same operation as the statsdir statsdir configuration file directive.
Add the specified key number to the trusted key list.
Specify a user, and optionally a group, to switch to. This option is only available if the OS supports adjusting the clock without full root privileges. This option is supported under NetBSD (configure with --enable-clockctl) or Linux (configure with --enable-linuxcaps) or Solaris (configure with --enable-solarisprivs).
Give the time in seconds between two scans for new or dropped interfaces. For systems with routing socket support the scans will be performed shortly after the interface change has been detected by the system. Use 0 to disable scanning. 60 seconds is the minimum time between scans.
If greater than zero, alters ntpd's behavior when forking to daemonize. Instead of exiting with status 0 immediately after the fork, the parent waits up to the specified number of seconds for the child to first synchronize the clock. The exit status is zero (success) if the clock was synchronized, otherwise it is ETIMEDOUT. This provides the option for a script starting ntpd to easily wait for the first set of the clock before proceeding.
Normally, the time is slewed if the offset is less than the step threshold, which is 128 ms by default, and stepped if above the threshold. This option sets the threshold to 600 s, which is well within the accuracy window to set the clock manually. Note: Since the slew rate of typical Unix kernels is limited to 0.5 ms/s, each second of adjustment requires an amortization interval of 2000 s. Thus, an adjustment as much as 600 s will take almost 14 days to complete. This option can be used with the -g and -q options. See the tinker configuration file directive for other options. Note: The kernel time discipline is disabled with this option.
Attempt to substitute the CPU counter for QueryPerformanceCounter. The CPU counter and QueryPerformanceCounter are compared, and if they have the same frequency, the CPU counter (RDTSC on x86) is used directly, saving the overhead of a system call.
Force substitution the CPU counter for QueryPerformanceCounter. The CPU counter (RDTSC on x86) is used unconditionally with the given frequency (in Hz).
Registers as an NTP server with the local mDNS server which allows the server to be discovered via mDNS client lookup.
NTPD_<option-name> or NTPD
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(8) as late as possible in the boot sequence (perhaps with the -v flag) and after ntp-wait(8) exits successfully it is as safe as it will ever be to start any process that require stable time.
By default, ntpd runs in continuous mode where each of possibly several external servers is polled at intervals determined by an intricate state machine. The state machine measures the incidental roundtrip delay jitter and oscillator frequency wander and determines the best poll interval using a heuristic algorithm. Ordinarily, and in most operating environments, the state machine will start with 64s intervals and eventually increase in steps to 1024s. A small amount of random variation is introduced in order to avoid bunching at the servers. In addition, should a server become unreachable for some time, the poll interval is increased in steps to 1024s in order to reduce network overhead.
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.
In some cases involving dial up or toll services, it may be useful to increase the minimum interval to a few tens of minutes and maximum interval to a day or so. Under normal operation conditions, once the clock discipline loop has stabilized the interval will be increased in steps from the minimum to the maximum. However, this assumes the intrinsic clock frequency error is small enough for the discipline loop correct it. The capture range of the loop is 500 PPM at an interval of 64s decreasing by a factor of two for each doubling of interval. At a minimum of 1,024 s, for example, the capture range is only 31 PPM. If the intrinsic error is greater than this, the drift file ntp.drift will have to be specially tailored to reduce the residual error below this limit. Once this is done, the drift file is automatically updated once per hour and is available to initialize the frequency on subsequent daemon restarts.
The huff-n'-puff filter is designed to correct the apparent time offset in these cases. It depends on knowledge of the propagation delay when no other traffic is present. In common scenarios this occurs during other than work hours. The filter maintains a shift register that remembers the minimum delay over the most recent interval measured usually in hours. Under conditions of severe delay, the filter corrects the apparent offset using the sign of the offset and the difference between the apparent delay and minimum delay. The name of the filter reflects the negative (huff) and positive (puff) correction, which depends on the sign of the offset.
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, Network Time Protocol (Version 1), RFC1059
David L. Mills, Network Time Protocol (Version 2), RFC1119
David L. Mills, Network Time Protocol (Version 3), RFC1305
David L. Mills and J. Martin, Ed. and J. Burbank and W. Kasch, Network Time Protocol Version 4: Protocol and Algorithms Specification, RFC5905
David L. Mills and B. Haberman, Ed., Network Time Protocol Version 4: Autokey Specification, RFC5906
H. Gerstung and C. Elliott and B. Haberman, Ed., Definitions of Managed Objects for Network Time Protocol Version 4: (NTPv4), RFC5907
R. Gayraud and B. Lourdelet, Network Time Protocol (NTP) Server Option for DHCPv6, RFC5908
This manual page was AutoGen-erated from the ntpd option definitions.