ntpdate

ntpdate(8)                  System Manager's Manual                 ntpdate(8)

NAME
       ntpdate - set the date and time via NTP

SYNOPSIS
       ntpdate  [-46bBdqsuv] [-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 re-
       liability 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 de-
       termines 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 adj-
       time()  routine. The latter technique is less disruptive and more accu-
       rate when the error is small, and works quite well when ntpdate is  run
       by cron every hour or two.

       ntpdate will, if the -u flag was not specified, 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 run-
       ning a daemon, doing so once every hour or two will result  in  precise
       enough timekeeping to avoid stepping the clock.

       Note  that  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 qualifier forces DNS resolution to the IPv6 namespace.

OPTIONS
       -4     Force DNS resolution of following host names on the command line
              to the IPv4 namespace.

       -6     Force DNS resolution of following host names on the command line
              to the IPv6 namespace.

       -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 +-500 ms.  The
              default  is  to step the time using settimeofday() if the offset
              is greater than +-500 ms. Note  that,  if  the  offset  is  much
              greater than +-500 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 and using an unprivi-
              leged port. 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, 2, 3 or 4. The default is 4.  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  de-
              fault 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 2 seconds, 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 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>

SEE ALSO
       ntpdate-debian(8)

                                                                    ntpdate(8)