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ntpdate(8)							    ntpdate(8)

NAME
       ntpdate - set the date and time via NTP

SYNOPSIS
       ntpdate	[-bBdoqsuv]  [-a key] [-e authdelay] [-k keyfile] [-o version]
       [-p samples] [-t timeout] server [...]

DESCRIPTION
       ntpdate sets the local date and time by polling the Network Time Proto‐
       col (NTP) server(s) given as the server arguments to determine the cor‐
       rect time. It must be run as root on the local host (unless the	option
       -q  is used). A number of samples are obtained from each of the servers
       specified and a subset of the NTP clock filter and selection algorithms
       are  applied  to	 select	 the best of these. Note that the accuracy and
       reliability of ntpdate depends on the number of servers, the number  of
       polls each time it is run and the interval between runs.

       ntpdate	can  be run manually as necessary to set the host clock, or it
       can be run from the host startup script to set the clock at boot	 time.
       This is useful in some cases to set the clock initially before starting
       the NTP daemon ntpd. It is also possible to run	ntpdate	 from  a  cron
       script.	However,  it  is important to note that ntpdate with contrived
       cron scripts is no substitute for the NTP daemon, which uses  sophisti‐
       cated  algorithms to maximize accuracy and reliability while minimizing
       resource use. Finally, since ntpdate does not discipline the host clock
       frequency as does ntpd, the accuracy using ntpdate is limited.

       Time  adjustments  are  made  by ntpdate in one of two ways. If ntpdate
       determines the clock is in error more than 0.5 second  it  will	simply
       step  the  time	by  calling  the system settimeofday() routine. If the
       error is less than 0.5 seconds, it will slew the time  by  calling  the
       system  adjtime()  routine. The latter technique is less disruptive and
       more accurate when the error is small, and works quite well  when  ntp‐
       date is run by cron every hour or two.

       ntpdate	will  decline  to  set the date if an NTP server daemon (e.g.,
       ntpd) is running on the same host. When running ntpdate	on  a  regular
       basis  from  cron  as an alternative to running a daemon, doing so once
       every hour or two will result in precise enough	timekeeping  to	 avoid
       stepping the clock.

OPTIONS
       -a key Enable  the  authentication function and specify the key identi‐
	      fier to be used for authentication as the	 argument  keyntpdate.
	      The  keys	 and key identifiers must match in both the client and
	      server key files. The default is to disable  the	authentication
	      function.

       -B     Force  the  time	to always be slewed using the adjtime() system
	      call, even if the measured offset is greater than +-128 ms.  The
	      default  is  to step the time using settimeofday() if the offset
	      is greater than +-128 ms. Note  that,  if	 the  offset  is  much
	      greater than +-128 ms in this case, that it can take a long time
	      (hours) to slew the clock to  the	 correct  value.  During  this
	      time, the host should not be used to synchronize clients.

       -b     Force  the  time	to  be stepped using the settimeofday() system
	      call, rather than slewed (default) using	the  adjtime()	system
	      call. This option should be used when called from a startup file
	      at boot time.

       -d     Enable the debugging mode, in which ntpdate will go through  all
	      the  steps,  but	not adjust the local clock. Information useful
	      for general debugging will also be printed.

       -e authdelay
	      Specify the processing delay to perform an authentication	 func‐
	      tion  as	the value authdelay, in seconds and fraction (see ntpd
	      for details). This number is usually small enough to be negligi‐
	      ble  for	most  purposes,	 though specifying a value may improve
	      timekeeping on very slow CPU's.

       -k keyfile
	      Specify the path for the authentication key file as  the	string
	      keyfile.	The  default  is /etc/ntp.keys. This file should be in
	      the format described in ntpd.

       -o version
	      Specify the NTP version for outgoing packets as the integer ver‐
	      sion, which can be 1 or 2. The default is 3. This allows ntpdate
	      to be used with older NTP versions.

       -p samples
	      Specify the number of samples to be acquired from each server as
	      the  integer  samples,  with  values  from 1 to 8 inclusive. The
	      default is 4.

       -q     Query only - don't set the clock.

       -s     Divert logging output from the standard output (default) to  the
	      system  syslog  facility.	 This is designed primarily for conve‐
	      nience of cron scripts.

       -t timeout
	      Specify the maximum time waiting for a server  response  as  the
	      value  timeout, in seconds and fraction. The value is is rounded
	      to a multiple of 0.2 seconds. The default is 1 second,  a	 value
	      suitable for polling across a LAN.

       -u     Direct ntpdate to use an unprivileged port for outgoing packets.
	      This is most useful when behind a firewall that blocks  incoming
	      traffic  to  privileged  ports, and you want to synchronise with
	      hosts beyond the firewall. Note that the -d option  always  uses
	      unprivileged ports.

       -v     Be verbose. This option will cause ntpdate's version identifica‐
	      tion string to be logged.

DIAGNOSTICS
       ntpdate's exit status is zero if it found a server and could update the
       clock, and nonzero otherwise.

FILES
       /etc/ntp.keys
	      - encryption keys used by ntpdate.

BUGS
       The  slew  adjustment  is actually 50% larger than the measured offset,
       since this (it is argued) will tend to keep a badly drifting clock more
       accurate.  This	is  probably not a good idea and may cause a troubling
       hunt for some values of the kernel variables tick and tickadj.

AUTHOR
       David L. Mills (mills@udel.edu)
       This manpage converted from html to  roff  by  Fabrizio	Polacco	 <fpo‐
       lacco@debian.org>

SEE ALSO
       ntpdate-debian(8)

								    ntpdate(8)
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