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 Protocol (NTP) server(s)  given  as  the
       server  arguments  to  determine  the  correct 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  sophisticated  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 ntpdate
       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 identifier 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 function  as  the  value  authdelay,  in
              seconds  and fraction (see ntpd for details). This number is usually small enough to be negligible
              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 version, 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 convenience 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 identification 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 <fpolacco@debian.org>

SEE ALSO

       ntpdate-debian(8)

                                                                                                      ntpdate(8)