NTPD(8) BSD System Manager's Manual NTPD(8)
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 arith-
metic 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 con-
figured 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 op-
tions 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, multi-
cast client and symmetric passive associations. This is the de-
fault.
-A, --authnoreq
Do not require crypto authentication. This option must not ap-
pear 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 al-
most 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 de-
fault.
-d, --debug-level
Increase debug verbosity level. This option may appear an unlim-
ited number of times.
-D number, --set-debug-level=number
Set the debug verbosity level. This option may appear an unlim-
ited 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 de-
fault. 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 off-
set exceeds the panic threshold, which is 1000 s by default. This
option allows the time to be set to any value without restric-
tion; however, this can happen only once. If the threshold is ex-
ceeded 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 im-
plies 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 --en-
able-linuxcaps) or Solaris (configure with --enable-solar-
isprivs).
-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, ex-
cept 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 log-
file 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 en-
sures 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 ar-
gument.
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 combina-
tion 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 dis-
cipline is disabled with this option.
-r string, --propagationdelay=string
Broadcast/propagation delay.
Specify the default propagation delay from the broadcast/multi-
cast 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 de-
signed 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 op-
tion 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-so-
larisprivs).
-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 in-
terfaces. 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 min-
imum time between scans.
--var=nvar
make ARG an ntp variable (RW). This option may appear an unlim-
ited number of times.
--dvar=ndvar
make ARG an ntp variable (RW|DEF). This option may appear an un-
limited number of times.
-w number, --wait-sync=number
Seconds to wait for first clock sync. This option must not ap-
pear 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 dae-
monize. 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 QueryPerformance-
Counter. The CPU counter and QueryPerformanceCounter are com-
pared, 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 al-
lows 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 config-
ured 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 in-
terval for each server is delayed an interval randomized over a few sec-
onds. 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 op-
erating 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 defec-
tive, 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. Un-
der 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, un-
less the interval during which no sample offset is less than 128 ms ex-
ceeds 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 fu-
ture 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 lo-
cal 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 re-
quire 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, usu-
ally 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 particu-
lar system clock oscillator time and frequency error. This takes approx-
imately 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 fre-
quency 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 de-
scribed 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 specify-
ing 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 jit-
ter and oscillator frequency wander and determines the best poll interval
using a heuristic algorithm. Ordinarily, and in most operating environ-
ments, the state machine will start with 64s intervals and eventually in-
crease in steps to 1024s. A small amount of random variation is intro-
duced 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 continu-
ous mode; most applications will probably want to specify the iburst key-
word 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) pro-
gram 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 fre-
quency 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 tai-
lor the operation in order enhance accuracy by reducing the interval or
to reduce network overhead by increasing it. However, the user is ad-
vised to carefully consider the consequences of changing the poll adjust-
ment 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 com-
mand to a value not less than 16 s. This value is used for all config-
ured associations, unless overridden by the minpoll option on the config-
uration 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 overrid-
den.
In some cases involving dial up or toll services, it may be useful to in-
crease the minimum interval to a few tens of minutes and maximum interval
to a day or so. Under normal operation conditions, once the clock disci-
pline 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 avail-
able 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 de-
graded. This occurs because the differential delays on the two direc-
tions 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 cor-
rection 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 appar-
ent 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.
SEE ALSO
ntp.conf(5), ntpdate(8), ntpdc(1), ntpq(1), sntp(1)
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.
AUTHORS
The University of Delaware and Network Time Foundation
COPYRIGHT
Copyright (C) 1992-2020 The University of Delaware and Network Time Foun-
dation 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.
BSD March 3 2020 BSD