ntpd(8)ntpd(8)NAMEntpd - Network Time Protocol (NTP) daemon
SYNOPSISntpd [ -46aAbdDgLmnNqx ] [ -c conffile ] [ -f driftfile ] [ -i jaildir
] [ -I iface ] [ -k keyfile ] [ -l logfile ] [ -p pidfile ] [ -P prior‐
ity ] [ -r broadcastdelay ] [ -s statsdir ] [ -t key ] [ -u
user[:group] ] [ -U interface_update_interval ] [ -v variable ] [ -V
The ntpd program is an operating system daemon that synchronises the
system clock with remote NTP time servers or local reference clocks. 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. The program can operate in any of several modes, as
described on the Association Management page, and with both symmetric
key and public key cryptography, as described on the Authentication
The ntpd program ordinarily requires a configuration file as desccribe
on the Configuration Commands and Options collection above. However a
client can discover remote servers and configure them automatically.
This makes it possible to deploy a fleet of workstations without speci‐
fying configuration details specific to the local environment. Further
details are on the Automatic Server Discovery page.
Once the NTP software distribution has been compiled and installed and
the configuration file constructed, the next step is to verify correct
operation and fix any bugs that may result. Usually, the command line
that starts the daemon is included in the system startup file, so it is
executed only at system boot time; however, the daemon can be stopped
and restarted from root at any time. Once started, the daemon will
begin sending and receiving messages, as specified in the configuration
SETTING THE TIME AND FREQUENCY
The ntpd program operates by exchanging messages with one or more
servers at designated intervals ranging from about one minute to about
17 minutes. When started, the program requires several exchanges while
the algorithms accumulate and groom the data before setting the clock.
The initial delay to set the clock can be reduced using options on the
Server Options page.
Most compters 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. In
case there is no TOY chip or the TOY time is more than 1000 s from the
server time, ntpd assumes something must be terribly wrong and exits
with a panic message to the system operator. With the -g option the
clock will be initially set to the server time regardless of the chip
time. However, once the clock has been set, an error greater than 1000
s will cause ntpd to exit anyway.
Under ordinary conditions, ntpd slews the clock so that the time is
effectively continuous and never runs backwards. If due to extreme net‐
work congestion an error spike exceeds the step threshold, by default
128 ms, the spike is discarded. However, if the error persists for more
than the stepout threshold, by default 900 s, the system clock is
stepped to the correct value. In practice the need for a step has is
extremely rare and almost always the result of a hardware failure. With
the -x option the step threshold is increased to 600 s. Other options
are available using the tinker command on the Miscellaneous Options
The issues should be carefully considered before using these options.
The maximum slew rate possible is limited to 500 parts-per-million
(PPM) by the Unix kernel. As a result, the clock can take 2000 s for
each second the clock is outside the acceptable range. During this
interval the 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.
The frequency file, usually called ntp.drift, contains the latest esti‐
mate of clock frequency. If this file does not exist when ntpd is
started, it enters a special mode designed to measure the particular
frequency directly. The measurement takes 15 minutes, after which the
frequency is set and ntpd resumes normal mode where the time and fre‐
quency are continuously adjusted. The frequency file is updated at
intervals of an hour or more depending on the measured clock stability.
The ntpd program normally operates continuously while adjusting the
time and frequency, but in some cases it may not be practical to run it
continuously. With the -q option ntpd operates as in continous mode,
but exits just after setting the clock for the first time with the con‐
figured servers. 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 10
s. If nothing is heard after a few minutes, the daemon times out and
POLL INTERVAL CONTROL
NTP uses an intricate heuristic algorithm to automatically control the
poll interval for maximum accuracy consistent with minimum network
overhead. The algorithm measures the incidental offset and jitter to
determine the best poll interval. When ntpd starts, the interval is the
default minimum 64 s. Under normal conditions when the clock discipline
has stabilized, the interval increases in steps to the default maximum
1024 s. In addition, should a server become unreachable after some
time, the interval increases in steps to the maximum in order to reduce
The default poll interval range is suitable for most conditions, but
can be changed using options on the Server Options and Miscellaneous
Options pages. However, when using maximum intervals much larger than
the default, the residual clock frequency error must be small enough
for the discipline loop to capture and correct. The capture range is
500 PPM with a 64-s interval decreasing by a factor of two for each
interval doubling. At a 36-hr interval, for example, the capture range
is only 0.24 PPM.
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.
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, such as during other than work hours. The
filter remembers the minimum delay over the most recent interval mea‐
sured usually in hours. Under conditions of severe delay, the filter
corrects the apparent offset using the sign of the offset and the dif‐
ference 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 huffpuff command, as described in the Miscellaneous Options
LEAP SECOND PROCESSING
As provided by international agreement, an extra second is sometimes
inserted in Coordinated Universal Time (UTC) at the end of a selected
month, usually June or December. The National Institutes of Standards
and Technology (NIST) provides an historic leapseconds file at
time.nist.gov for retrieval via FTP. When this file, usually called
ntp-leapseconds.list, is copied and installed in a directory. The leap‐
file configuration command specifies the path to this file. At startup,
ntpd reads it and initializes three leapsecond values: the NTP seconds
at the next leap event, the offset of UTC relative to International
Atomic Time (TAI) after the leap and the NTP seconds when the leapsec‐
onds file expires and should be retrieved again.
If a host does not have the leapsecond values, they can be obtained
over the net using the Autokey security protocol. Ordinarily, the
leapseconds file is installed on the primary servers and the values
flow from them via secondary servers to the clients. When multiple
servers are involved, the values with the latest expiration time are
If the latest leap is in the past, nothing further is done other than
to install the TAI offset. If the leap is in the future less than 28
days, the leap warning bits are set. If in the future less than 23
hours, the kernel is armed to insert one second at the end of the cur‐
rent day. If the kernel is enabled, the leap is done automatically at
that time; otherwise, the clock is effectively stopped for one second
at the leap. Additional details are in the The NTP Timescale and Leap
Seconds white paper
If none of the above provisions are available, dsependent servers and
clients tally the leap warning bits of surviving servers and reference
clocks. When a majority of the survivors show warning, a leap is pro‐
grammed at the end of the current month. During the month and day of
insertion, they operate as above. In this way the leap is is propagated
at all dependent servers and clients.
A new experimental feature called interleaved modes can be used in NTP
symmetric or broadcast modes. It is designed to improve accuracy by
avoiding kernel latency and queueing delay, as described on the NTP
Interleaved Modes page. It is activated by the xleave option with the
peer or broadcast configuration commands. The NTP protocol automati‐
cally reconfigures in normal or interleaved mode as required. Ordinary
broadcast clients can use the same servers as interleaved clients at
the same time. Further details are in the white paper NTP Interleaved
On-Wire Protocol and the briefing Interleaved Synchronization Protocols
for LANs and Space Data Links.
If ntpd, is configured with NetInfo support, it will attempt to read
its configuration from the NetInfo service 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, -d or -D option is used, ntpd changes the current work‐
ing directory to the root directory, so any options or commands speci‐
fying paths need to use an absolute path or a path relative to the
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
same operation as the enable auth command and is the default.
-A Do not require cryptographic authentication for broadcast
client, multicast client and symmetric passive associations.
This is the same operation as the disable auth command and
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 command.
-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.
-I [address | interface name]
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.
Specify the name and path of the symmetric key file. This is
the same operation as the keys keyfile 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‐
-L 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 Raise scheduler precision to its maximum (1 msec) using timeBe‐
ginPeriod. (Windows only)
-m Lock memory.
-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. This is the same operation as the trustedkey
Specify a user, and optionally a group, to switch to. This
option is only available if the OS supports running the server
without full root privileges. Currently, this option is sup‐
ported under NetBSD (configure with --enable-clockctl) and
Linux (configure with --enable-linuxcaps).
-U interface update interval
Number of seconds to wait between interface list scans to pick
up new and delete network interface. Set to 0 to disable
dynamic interface list updating. The default is to scan every 5
-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 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 client, with servers determined by listening to
broadcasts at run time.
Usually, the configuration file is installed as/etc/ntp.conf, 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 command keyword followed
by a list of option keywords separated by whitespace. Commands may not
be continued over multiple lines. Options may be host names, host
addresses written in numeric, dotted-quad form, integers, floating
point numbers (when specifying times in seconds) and text strings.
Optional arguments are delimited by [ ] in the options pages, while
alternatives are separated by |. The notation [ ... ] means an
optional, indefinite repetition of the last item before the [ ... ].
│File │ Default │ Option │ Command │
│configuration file │ /etc/ntp.conf │ -c │ none │
│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/ntpstats/ │ -s │ statsdir │
│keys path │ /etc/ntp/crypto │ none │ keysdir │
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.
SEE ALSOntp.conf(5), ntpq(8), ntpdc(8)
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