Ubuntu Manpage: ntpd — NTP daemon program

Provided by: ntp_4.2.8p10+dfsg-5ubuntu7.3_amd64

NAME

       ntpd — NTP daemon program

SYNOPSIS

       ntpd [-flags] [-flag [value]] [--option-name[[=| ]value]] [ <server1> ... <serverN> ]

DESCRIPTION

The ntpd utility is an operating system daemon which sets and maintains the system time of day in
synchronism with Internet standard time servers. It is a complete implementation of the Network Time
Protocol (NTP) version 4, as defined by RFC-5905, but also retains compatibility with version 3, as
defined by RFC-1305, and versions 1 and 2, as defined by RFC-1059 and RFC-1119, respectively.

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.

Various internal ntpd variables can be displayed and configuration options altered while the ntpd is
running using the ntpq(1) and ntpdc(1) utility programs.

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

OPTIONS

-4, --ipv4
Force IPv4 DNS name resolution. This option must not appear in combination with any of the
following options: ipv6.

Force DNS resolution of following host names on the command line to the IPv4 namespace.

-6, --ipv6
Force IPv6 DNS name resolution. This option must not appear in combination with any of the
following options: ipv4.

Force DNS resolution of following host names on the command line to the IPv6 namespace.

-a, --authreq
Require crypto authentication. This option must not appear in combination with any of the
following options: authnoreq.

Require cryptographic authentication for broadcast client, multicast client and symmetric passive
associations. This is the default.

-A, --authnoreq
Do not require crypto authentication. This option must not appear in combination with any of the
following options: authreq.

Do not require cryptographic authentication for broadcast client, multicast client and symmetric
passive associations. This is almost never a good idea.

-b, --bcastsync
Allow us to sync to broadcast servers.

-c string, --configfile=string
configuration file name.

The name and path of the configuration file, /etc/ntp.conf by default.

-d, --debug-level
Increase debug verbosity level. This option may appear an unlimited number of times.

-D number, --set-debug-level=number
Set the debug verbosity level. This option may appear an unlimited number of times. This option
takes an integer number as its argument.

-f string, --driftfile=string
frequency drift file name.

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.

-g, --panicgate
Allow the first adjustment to be Big. This option may appear an unlimited number of times.

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.

-G, --force-step-once
Step any initial offset correction..

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.

-i string, --jaildir=string
Jail directory.

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).

-I iface, --interface=iface
Listen on an interface name or address. This option may appear an unlimited number of times.

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.

-k string, --keyfile=string
path to symmetric keys.

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.

-l string, --logfile=string
path to the log file.

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.

-L, --novirtualips
Do not listen to virtual interfaces.

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.

-M, --modifymmtimer
Modify Multimedia Timer (Windows only).

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.

-n, --nofork
Do not fork. This option must not appear in combination with any of the following options:
wait-sync.

-N, --nice
Run at high priority.

To the extent permitted by the operating system, run ntpd at the highest priority.

-p string, --pidfile=string
path to the PID file.

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.

-P number, --priority=number
Process priority. This option takes an integer number as its argument.

To the extent permitted by the operating system, run ntpd at the specified
sched_setscheduler(SCHED_FIFO) priority.

-q, --quit
Set the time and quit. This option must not appear in combination with any of the following
options: saveconfigquit, wait-sync.

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.

-r string, --propagationdelay=string
Broadcast/propagation delay.

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.

--saveconfigquit=string
Save parsed configuration and quit. This option must not appear in combination with any of the
following options: quit, wait-sync.

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.

-s string, --statsdir=string
Statistics file location.

Specify the directory path for files created by the statistics facility. This is the same
operation as the statsdir statsdir configuration file directive.

-t tkey, --trustedkey=tkey
Trusted key number. This option may appear an unlimited number of times.

Add the specified key number to the trusted key list.

-u string, --user=string
Run as userid (or userid:groupid).

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).

-U number, --updateinterval=number
interval in seconds between scans for new or dropped interfaces. This option takes an integer
number as its argument.

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.

--var=nvar
make ARG an ntp variable (RW). This option may appear an unlimited number of times.

--dvar=ndvar
make ARG an ntp variable (RW|DEF). This option may appear an unlimited number of times.

-w number, --wait-sync=number
Seconds to wait for first clock sync. This option must not appear in combination with any of the
following options: nofork, quit, saveconfigquit. This option takes an integer number as its
argument.

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.

-x, --slew
Slew up to 600 seconds.

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.

--usepcc
Use CPU cycle counter (Windows only).

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.

--pccfreq=string
Force CPU cycle counter use (Windows only).

Force substitution the CPU counter for QueryPerformanceCounter. The CPU counter (RDTSC on x86)
is used unconditionally with the given frequency (in Hz).

-m, --mdns
Register with mDNS as a NTP server.

Registers as an NTP server with the local mDNS server which allows the server to be discovered
via mDNS client lookup.

-?, --help
Display usage information and exit.

-!, --more-help
Pass the extended usage information through a pager.

--version [{v|c|n}]
Output version of program and exit. The default mode is `v', a simple version. The `c' mode
will print copyright information and `n' will print the full copyright notice.

OPTION PRESETS

       Any  option  that  is  not  marked  as  not  presettable may be preset by loading values from environment
       variables named:
         NTPD_<option-name> or NTPD

USAGE

How NTP Operates
The ntpd utility operates by exchanging messages with one or more configured servers over a range of
designated poll intervals. When started, whether for the first or subsequent times, the program requires
several exchanges from the majority of these servers so the signal processing and mitigation algorithms
can accumulate and groom the data and set the clock. In order to protect the network from bursts, the
initial poll interval for each server is delayed an interval randomized over a few seconds. At the
default initial poll interval of 64s, several minutes can elapse before the clock is set. This initial
delay to set the clock can be safely and dramatically reduced using the iburst keyword with the server
configuration command, as described in ntp.conf(5).

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(1) 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(1) 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.

Starting NTP (Best Current Practice)
First, use the iburst option on your server entries.

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.

Frequency Discipline
The ntpd behavior at startup depends on whether the frequency file, usually ntp.drift, exists. This file
contains the latest estimate of clock frequency error. When the ntpd is started and the file does not
exist, the ntpd enters a special mode designed to quickly adapt to the particular system clock oscillator
time and frequency error. This takes approximately 15 minutes, after which the time and frequency are
set to nominal values and the ntpd enters normal mode, where the time and frequency are continuously
tracked relative to the server. After one hour the frequency file is created and the current frequency
offset written to it. When the ntpd is started and the file does exist, the ntpd frequency is
initialized from the file and enters normal mode immediately. After that the current frequency offset is
written to the file at hourly intervals.

Operating Modes
The ntpd utility can operate in any of several modes, including symmetric active/passive, client/server
broadcast/multicast and manycast, as described in the "Association Management" page (available as part of
the HTML documentation provided in /usr/share/doc/ntp). It normally operates continuously while
monitoring for small changes in frequency and trimming the clock for the ultimate precision. However, it
can operate in a one-time mode where the time is set from an external server and frequency is set from a
previously recorded frequency file. A broadcast/multicast or manycast client can discover remote
servers, compute server-client propagation delay correction factors and configure itself automatically.
This makes it possible to deploy a fleet of workstations without specifying configuration details
specific to the local environment.

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(1) 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.

Poll Interval Control
This version of NTP includes an intricate state machine to reduce the network load while maintaining a
quality of synchronization consistent with the observed jitter and wander. There are a number of ways to
tailor the operation in order enhance accuracy by reducing the interval or to reduce network overhead by
increasing it. However, the user is advised to carefully consider the consequences of changing the poll
adjustment range from the default minimum of 64 s to the default maximum of 1,024 s. The default minimum
can be changed with the tinker minpoll command to a value not less than 16 s. This value is used for all
configured associations, unless overridden by the minpoll option on the configuration command. Note that
most device drivers will not operate properly if the poll interval is less than 64 s and that the
broadcast server and manycast client associations will also use the default, unless overridden.

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
In scenarios where a considerable amount of data are to be downloaded or uploaded over telephone modems,
timekeeping quality can be seriously degraded. This occurs because the differential delays on the two
directions of transmission can be quite large. In many cases the apparent time errors are so large as to
exceed the step threshold and a step correction can occur during and after the data transfer is in
progress.

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).

ENVIRONMENT

       See OPTION PRESETS for configuration environment variables.

FILES

       /etc/ntp.conf   the default name of the configuration file
       /etc/ntp.drift  the default name of the drift file
       /etc/ntp.keys   the default name of the key file

EXIT STATUS

       One of the following exit values will be returned:

       0  (EXIT_SUCCESS)
               Successful program execution.

       1  (EXIT_FAILURE)
               The operation failed or the command syntax was not valid.

       70  (EX_SOFTWARE)
               libopts    had    an     internal     operational     error.      Please     report     it     to
               autogen-users@lists.sourceforge.net.  Thank you.

AUTHORS

       The University of Delaware and Network Time Foundation

COPYRIGHT

       Copyright (C) 1992-2017 The University of Delaware and Network Time Foundation all rights reserved.  This
       program is released under the terms of the NTP license, <http://ntp.org/license>.

BUGS

       The ntpd utility has gotten rather fat.  While not huge, it has gotten larger than might be desirable for
       an  elevated-priority  ntpd running on a workstation, particularly since many of the fancy features which
       consume the space were designed more with a busy primary server, rather than a high  stratum  workstation
       in mind.

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

NOTES

       Portions of this document came from FreeBSD.

       This manual page was AutoGen-erated from the ntpd option definitions.

Debian                                            March 21 2017                                          NTPD(8)