NTPD(8) | System Manager's Manual (user) | NTPD(8) |
ntpd
—
ntpd |
[-flags ] [-flag
[value]] [--option-name [[=|
]value]] [ <server1> ... <serverN>
] |
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(1ntpqmdoc) and
ntpdc(1ntpdcmdoc)
utility programs.
When ntpd
starts it looks at the value of
umask(2), and if zero
ntpd
will set the
umask(2) to 022.
-4
,
--ipv4
Force DNS resolution of following host names on the command line to the IPv4 namespace.
-6
,
--ipv6
Force DNS resolution of following host names on the command line to the IPv6 namespace.
-a
,
--authreq
Require cryptographic authentication for broadcast client, multicast client and symmetric passive associations. This is the default.
-A
,
--authnoreq
Do not require cryptographic authentication for broadcast client, multicast client and symmetric passive associations. This is almost never a good idea.
-b
,
--bcastsync
-c
string,
--configfile
=stringThe name and path of the configuration file, /etc/ntp.conf by default.
-d
,
--debug-level
-D
number,
--set-debug-level
=number-f
string,
--driftfile
=stringThe 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
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
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
=stringChroot 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
=ifaceOpen 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
=stringSpecify 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
=stringSpecify 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, 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
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
-N
,
--nice
To the extent permitted by the operating system, run ntpd at the highest priority.
-p
string,
--pidfile
=stringSpecify 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
=numberTo the extent permitted by the operating system, run ntpd at the specified sched_setscheduler(SCHED_FIFO) priority.
-q
,
--quit
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
=stringSpecify 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
=stringCause 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
=stringSpecify 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
=tkeyAdd the specified key number to the trusted key list.
-u
string,
--user
=stringSpecify 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
=numberGive 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--dvar
=ndvar-w
number,
--wait-sync
=numberIf 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
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
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
=stringForce substitution the CPU counter for QueryPerformanceCounter. The CPU counter (RDTSC on x86) is used unconditionally with the given frequency (in Hz).
-m
,
--mdns
Registers as an NTP server with the local mDNS server which allows the server to be discovered via mDNS client lookup.
-?
,
--help
-!
,
--more-help
--version
[{v|c|n}]NTPD_<option-name> or NTPD
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(1ntpdatemdoc)
or sntp(1sntpmdoc) 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(1sntpmdoc) 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.
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.
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.
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(1ntpdatemdoc)
or sntp(1sntpmdoc)
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(1ntpdatemdoc)
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.
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 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, J. Martin, Ed., 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, 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.
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
This manual page was AutoGen-erated from the ntpd option definitions.
August 14 2018 | NetBSD 9.4 |