ntpd manpage

Search topic Section
Get manual page for the search topic
List all commands matching the search topic
List all topics in the manpage index

ntpd(8)								       ntpd(8)

       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.

       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
       hourly intervals.

       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

       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,
       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.

       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
       utility programs.

       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.

       -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.

       -c conffile
	       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.

       -D level
	       Specify debugging level directly.

       -f driftfile
	       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.

       -i jaildir
	       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 iface
	       Listen on interface. This option may appear an unlimited number
	       of times.

       -k keyfile
	       Specify the name and path of the symmetric key  file.  This  is
	       the same operation as the keys keyfile configuration command.

       -l logfile
	       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.

       -p pidfile
	       Specify the name and path of the file used to record  the  ntpd
	       process	ID.  This is the same operation as the pidfile pidfile
	       configuration command.

       -P priority
	       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.

       -r broadcastdelay
	       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.

       -s statsdir
	       Specify	the directory path for files created by the statistics
	       facility. This is the same operation as the  statsdir  statsdir
	       configuration command.

       -t key  Add a key number to the trusted key list. This option can occur
	       more than once.

       -u user[:group]
	       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).

       -v variable

       -V variable
	       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

       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
       are ignored.

       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.

YoLinux.com Home Page
YoLinux Tutorial Index
Privacy Policy | Advertise with us | Feedback Form |
Unauthorized copying or redistribution prohibited.
    Bookmark and Share