Provided by: ntpsec_1.2.2+dfsg1-4build2_amd64 
NAME
ntpd - Network Time Protocol service daemon
SYNOPSIS
ntpd
[-46agGhLmnNqx] [assert] [-c conffile] [-f driftfile]
[-i jaildir] [-k keyfile] [-l logfile] [-p pidfile]
[-P priority] [-s statsdir] [-t key]
[-u user[:'group']] [-U interface_update_interval]
[-v variable] [-V variable] [server...]
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 can synchronize time to a theoretical precision of about 232 picoseconds.
In practice, this limit is unattainable due to quantum limits on the clock speed of
ballistic-electron logic.
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.
The ntpd program normally operates continuously while adjusting the system time and
frequency, but in some cases this might not be practical. With the -q option ntpd operates
as in continuous mode, but exits just after setting the clock for the first time. Most
applications will probably want to specify the iburst option with the server command. With
this option, a volley of messages is exchanged to groom the data and set the clock in
about ten seconds. With -q, if nothing is heard after a few minutes, the daemon times out
and exits without setting the clock.
Various internal ntpd variables can be displayed and configuration options altered while
the ntpd is running using the ntpq(1) utility program. The state of ntpd can be
continuously monitored using ntpmon(1).
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, --assert
REQUIRE(false) to test assert handler.
-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, e.g. /var/lib/ntpsec/ntp.drift. This is the
same operation as the 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
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.
-h, --help
Print a usage message summarizing options and exit.
-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 Linux, NetBSD, and Solaris.
-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
the path to symmetric keys.
Specify the name and path of the symmetric key file. /etc/ntpsec/ntp.keys is the
default location. This is the same operation as the keys configuration file directive.
-l string, --logfile=string
the 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 configuration file directive. See ntp.conf(5) for
more info.
-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, --mdns
Register with mDNS as an NTP server.
Registers as an NTP server with the local mDNS server which allows the server to be
discovered via mDNS client lookup.
-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
the 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 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
pthread_setschedparam(SCHED_FIFO) priority.
-q, --quit
Set the time and quit. This option must not appear in combination with wait-sync.
ntpd will not daemonize and will exit after the clock is first synchronized. This
behavior mimics that of the old ntpdate program, which has been 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.
-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 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. The user and group may be
specified by name or numeric id. If no group is specified, then the default group for
userid is used. This option is only available if the OS supports adjusting the clock
without full root privileges. This option is supported under Linux, NetBSD, Solaris
and other OS.
-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.
-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. 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.
-z nvar, --var=nvar
make ARG an ntp variable (RW). This option may appear an unlimited number of times.
-Z nvar, --dvar=ndvar
make ARG an ntp variable (RW|DEF). This option may appear an unlimited number of
times.
-V, --version
Output version of program and exit.
Any arguments given after options are interpreted as server addresses or hostnames, with
the iburst option implied. Associations with these are formed before any associations
implied by the configuration file.
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 an 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 ms 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 at 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,
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 a separate utility 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 best current practice <#starting>
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 ntpdig(1).
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 and pool 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 ntpwait(8) as late as possible in the boot sequence
(perhaps with the -v flag) and after ntpwait(8) exits successfully it is as safe as it
will ever be to start any processes 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
ntpd 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.
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. The -q option is
provided to support running ntpd periodically from a cron(8) job. 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 sec. If nothing is heard after a couple of minutes, the daemon times out and
exits.
When kernel support is available to discipline the clock frequency, which is the case for
stock Solaris, 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 the 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).
FILES
┌───────────────────┬───────────────────┬────────┬─────────────┐
│ │ │ │ │
│File │ Default │ Option │ Option │
├───────────────────┼───────────────────┼────────┼─────────────┤
│ │ │ │ │
│configuration file │ /etc/ntp.conf │ -c │ conffile │
├───────────────────┼───────────────────┼────────┼─────────────┤
│ │ │ │ │
│configuration │ /etc/ntpsec/ntp.d │ -c │ conffile │
│directory │ │ │ │
├───────────────────┼───────────────────┼────────┼─────────────┤
│ │ │ │ │
│frequency file │ none │ -f │ driftfile │
├───────────────────┼───────────────────┼────────┼─────────────┤
│ │ │ │ │
│leapseconds file │ none │ │ leapfile │
├───────────────────┼───────────────────┼────────┼─────────────┤
│ │ │ │ │
│process ID file │ none │ -p │ pidfile │
├───────────────────┼───────────────────┼────────┼─────────────┤
│ │ │ │ │
│log file │ system log │ -l │ logfile │
├───────────────────┼───────────────────┼────────┼─────────────┤
│ │ │ │ │
│include file │ none │ none │ includefile │
├───────────────────┼───────────────────┼────────┼─────────────┤
│ │ │ │ │
│statistics path │ /var/log/ntpsec │ -s │ statsdir │
├───────────────────┼───────────────────┼────────┼─────────────┤
│ │ │ │ │
│keys file │ none │ -k │ keys │
└───────────────────┴───────────────────┴────────┴─────────────┘
Configuration files are parsed according to the following rules:
1. The plain config file (normally /etc/ntp.conf but the path can be overridden by the -c
option) is read first if it exists.
2. Then the configuration directory, if it exists, is scanned. Normally this directory is
/etc/ntpsec/ntp.d, but if the -c option is specified the /etc/ntpsec will be specified
by the directory name of the -c argument.
3. Each file beneath the configuration directory with the extension ".conf" is
interpreted. Files are interpreted in ASCII sort order of their pathnames. Files with
other extensions or no extensions are ignored.
SIGNALS
SIGQUIT, SIGINT, and SIGTERM will cause ntpd to clean up and exit.
SIGHUP checks various things that would otherwise require restarting ntpd.
It will reopen the log file if it has changed and check for a new leapseconds file if one
was specified.
If the NTS server is enabled, it will reload the certificate file if it has changed. (It
doesn’t check for a new key file, but reloads it when it reloads the certificate file.)
It will also retry any pending DNS or NTS lookups.
On most systems, you can send SIGHUP to ntpd with
# killall -HUP ntpd
If built with debugging enabled (waf configured with --enable-debug) SIGUSR1 will increase
the debug level by 1 and SIGUSR2 will decrease it by 1. This may be helpful if you are
running with -n, either just to see the logging on your screen or with gdb.
BUGS
The -V option is not backward-compatible with its use (as the equivalent of -Z) in older
versions.
STANDARDS
RFC 1059
David L. Mills, Network Time Protocol (Version 1), RFC 1059
RFC 1119
David L. Mills, Network Time Protocol (Version 2), RFC 1119
RFC 1305
David L. Mills, Network Time Protocol (Version 3), RFC 1305
RFC 5905
David L. Mills and J. Martin, Ed. and J. Burbank and W. Kasch, Network Time Protocol
Version 4: Protocol and Algorithms Specification, RFC 5905
RFC 5907
H. Gerstung and C. Elliott and B. Haberman, Ed., Definitions of Managed Objects
for Network Time Protocol Version 4: (NTPv4), RFC 5907
RFC 5908
R. Gayraud and B. Lourdelet, Network Time Protocol (NTP) Server Option for
DHCPv6, RFC 5908
EXIT STATUS
One of the following exit values will be returned:
0 (EXIT_SUCCESS)
Successful program execution.
1 (EXIT_FAILURE)
Execution failed - examine system logfiles.
SEE ALSO
ntp.conf(5), ntpq(1), ntpdig(1).