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. A number of sam? ples 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 outgoint 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 finds a server and updates 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>


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