ntpd - Network Time Protocol (NTP) daemon
ntpd [ -46aAbdDgLnNqx ] [ -c conffile ] [ -f driftfile ] [ -i jaildir ]
[ -I iface ] [ -k keyfile ] [ -l logfile ] [ -p pidfile ] [ -P priority
] [ -r broadcastdelay ] [ -s statsdir ] [ -t key ] [ -u user[:group] ]
[ -v variable ] [ -V variable ]
The ntpd program 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, but also retains compatibility with version 3, as
defined by RFC-1305, and version 1 and 2, as defined by RFC-1059 and
RFC-1119, respectively. ntpd does most computations in 64-bit floating
point arithmetic and does relatively clumsy 64-bit fixed point opera-
tions only when necessary to preserve the ultimate precision, about 232
picoseconds. While the ultimate precision is not achievable with ordi-
nary workstations and networks of today, it may be required with future
gigahertz CPU clocks and gigabit LANs.
HOW NTP OPERATES
The ntpd program operates by exchanging messages with one or more con-
figured servers at 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 mitiga-
tion 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. The initial delay to set the clock can be
reduced using the iburst keyword with the server configuration command,
as described on the Configuration Options page.
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 operat-
ing system time. After the machine has synchronized to a NTP server,
the operating system corrects the chip from time to time. In case there
is no TOY chip or for some reason its time 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. 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. 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 discontinu-
ities. 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, the error might exceed 128 ms. This may on occasion 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. If the -x option is included on the command line,
the clock will never be stepped and only slew corrections will be used.
The issues should be carefully explored before deciding to use the -x
option. The maximum slew rate possible is limited to 500 parts-per-mil-
lion (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. Dur-
ing this interval the local clock will not be consistent with any other
network clock and the system cannot be used for distributed applica-
tions 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 fol-
lowing 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 (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 con-
figuring the server.
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 fre-
quency 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 fre-
quency is initialized from the file and enters normal mode immediately.
After that the current frequency offset is written to the file at
ntpd can operate in any of several modes, including symmetric
active/passive, client/server broadcast/multicast and manycast, as
described in the Association Management page. It normally operates con-
tinuously 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 automati-
cally. 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
In some cases it may not be practical for ntpd to run continuously. A
common workaround has been to run the ntpdate program from a cron job
at designated times. However, this program does not have the crafted
signal processing, error checking and 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 with
the configured servers. 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 program may 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
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
advised to carefully consider the consequences of changing the poll
adjustment range from the default minimum of 64 s to the default maxi-
mum of 1,024 s. The default minimum can be changed with the tinker min-
poll 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,
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
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 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
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 usu-
ally 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 the Miscellaneous Options page.
If NetInfo support is built into ntpd, then ntpd will attempt to read
its configuration from the NetInfo if the default ntp.conf file cannot
be read and no file is specified by the -c option.
In contexts where a host name is expected, a -4 qualifier preceding the
host name forces DNS resolution to the IPv4 namespace, while a -6 qual-
ifier forces DNS resolution to the IPv6 namespace.
Various internal ntpd variables can be displayed and configuration
options altered while the ntpd is running using the ntpq and ntpdc
When ntpd starts it looks at the value of umask, and if zero ntpd will
set the umask to 022.
Unless the -n or -d option is used, ntpd changes the current working
directory to the root directory, so any options or commands specifying
paths need to use an absolute path or a path relative to the root.
COMMAND LINE OPTIONS
-4 Force DNS resolution of host names to the IPv4 namespace.
-6 Force DNS resolution of host names to the IPv6 namespace.
-a Require cryptographic authentication for broadcast client, mul-
ticast client and symmetric passive associations. This is the
-A Do not require cryptographic authentication for broadcast
client, multicast client and symmetric passive associations.
This is almost never a good idea.
-b Enable the client to synchronize to broadcast servers.
Specify the name and path of the configuration file, default
-d Specify debugging mode. This option may occur more than once,
with each occurrence indicating greater detail of display.
Specify debugging level directly.
Specify the name and path of the frequency file. This is the
same operation as the driftfile driftfile configuration com-
-g 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 thresh-
old 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 command for other options.
Chroot the server to the directory jaildir. This option also
implies that the server attempts to drop root privileges at
startup (otherwise, chroot gives very little additional secu-
rity), and it is only available if the OS supports to run the
server without full root privileges. You may need to also spec-
ify a -u option.
Listen on interface. This option may appear an unlimited number
Specify the name and path of the symmetric key file. This is
the same operation as the keys keyfile configuration command.
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 command.
-L Do not listen to virtual IPs. The default is to listen.
-n Don't fork.
-N To the extent permitted by the operating system, run the ntpd
at the highest priority.
Specify the name and path of the file used to record the ntpd
process ID. This is the same operation as the pidfile pidfile
To the extent permitted by the operating system, run the ntpd
at the specified priority.
-q Exit the ntpd just after the first time the clock is set. This
behavior mimics that of the ntpdate program, which is to be
retired. The -g and -x options can be used with this option.
Note: The kernel time discipline is disabled with this option.
Specify the default propagation delay from the broadcast/multi-
cast server to this client. This is necessary only if the delay
cannot be computed automatically by the protocol.
Specify the directory path for files created by the statistics
facility. This is the same operation as the statsdir statsdir
-t key Add a key number to the trusted key list. This option can occur
more than once.
Specify a user, and optionally a group, to switch to. This
option is only available if the OS supports to run the server
without full root privileges. Currently, this option is sup-
ported under NetBSD (configure with --enable-clockctl) and
Linux (configure with --enable-linuxcaps).
Add a system variable listed by default.
-x 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 manu-
ally. Note: Since the slew rate of typical Unix kernels is lim-
ited to 0.5 ms/s, each second of adjustment requires an amorti-
zation 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 command for other
options. Note: The kernel time discipline is disabled with this
THE CONFIGURATION FILE
Ordinarily, ntpd reads the ntp.conf 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, configura-
tion entirely on the command line, obviating the need for a configura-
tion file. This may be particularly useful when the local host is to be
configured as a broadcast/multicast client, with all peers being deter-
mined by listening to broadcasts at run time.
Usually, the configuration file is installed in the /etc directory, but
could be installed elsewhere (see the -c conffile command line option).
The file format is similar to other Unix configuration files - comments
begin with a # character and extend to the end of the line; blank lines
Configuration commands consist of an initial keyword followed by a list
of arguments, some of which may be optional, separated by whitespace.
Commands may not be continued over multiple lines. Arguments may be
host names, host addresses written in numeric, dotted-quad form, inte-
gers, floating point numbers (when specifying times in seconds) and
text strings. Optional arguments are delimited by [ ] in the following
descriptions, while alternatives are separated by |. The notation [ ...
] means an optional, indefinite repetition of the last item before the
[ ... ].
A non-zero exit code indicates an error. Any error messages are logged
to the system log by default.
The exit code is 0 only when ntpd is terminated by a signal, or when
the -q option is used and ntpd successfully sets the system clock.
ntp.conf(5), ntpq(8), ntpdc(8)
Primary source of documentation: /usr/share/doc/ntp-*
This file was automatically generated from HTML source.