ntpd(newest.html) - phpMan

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

NAME
       ntpd - Network Time Protocol (NTP) daemon

SYNOPSIS
       ntpd  [ -46aAbdDgLmnNqx ] [ -c conffile ] [ -f driftfile ] [ -i jaildir
       ] [ -I iface ] [ -k keyfile ] [ -l logfile ] [ -p pidfile ] [ -P prior-
       ity  ]  [  -r  broadcastdelay  ]  [  -s  statsdir  ]  [  -t  key ] [ -u
       user[:group] ] [ -U interface_update_interval ] [ -v variable  ]  [  -V
       variable ]

DESCRIPTION
       The  ntpd  program  is an operating system daemon that synchronises the
       system clock with remote NTP time servers or local reference clocks. 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. The program can operate  in  any  of  several  modes,  as
       described  on  the Association Management page, and with both symmetric
       key and public key cryptography, as  described  on  the  Authentication
       Options page.
       The  ntpd program ordinarily requires a configuration file as described
       on the Configuration Commands and Options collection above.  However  a
       client  can  discover  remote servers and configure them automatically.
       This makes it possible to deploy a fleet of workstations without speci-
       fying  configuration details specific to the local environment. Further
       details are on the Automatic Server Discovery page.
       Once the NTP software distribution has been compiled and installed  and
       the  configuration file constructed, the next step is to verify correct
       operation and fix any bugs that may result. Usually, the  command  line
       that starts the daemon is included in the system startup file, so it is
       executed only at system boot time; however, the daemon can  be  stopped
       and  restarted  from  root  at  any time. Once started, the daemon will
       begin sending and receiving messages, as specified in the configuration
       file.

SETTING THE TIME AND FREQUENCY
       The  ntpd  program  operates  by  exchanging  messages with one or more
       servers at designated intervals ranging from about one minute to  about
       17  minutes. When started, the program requires several exchanges while
       the algorithms accumulate and groom the data before setting the  clock.
       The  initial delay to set the clock can be reduced using options on the
       Server Options page.
       Most computers 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 operating  system  time.  In
       case  there is no TOY chip or the TOY time is more than 1000 s from the
       server time, ntpd assumes something must be terribly  wrong  and  exits
       with  a  panic  message  to the system operator. With the -g option the
       clock will be initially set to the server time regardless of  the  chip
       time.  However, once the clock has been set, an error greater than 1000
       s will cause ntpd to exit anyway.
       Under ordinary conditions, ntpd slews the clock so  that  the  time  is
       effectively continuous and never runs backwards. If due to extreme net-
       work congestion an error spike exceeds the step threshold,  by  default
       128 ms, the spike is discarded. However, if the error persists for more
       than the stepout threshold, by default  900  s,  the  system  clock  is
       stepped  to  the  correct  value.  In  practice  the need for a step is
       extremely rare and is almost always the result of a  hardware  failure.
       With  the  -x  option  the  step threshold is increased to 600 s. Other
       options are available using the tinker  command  on  the  Miscellaneous
       Options page.
       The  issues  should be carefully considered before using these options.
       The maximum slew rate possible  is  limited  to  500  parts-per-million
       (PPM)  by  the  Unix kernel. As a result, the clock can take 2000 s for
       each second the clock is outside  the  acceptable  range.  During  this
       interval  the clock will not be consistent with any other network clock
       and the system cannot be used for distributed applications that require
       correctly synchronized network time.
       The frequency file, usually called ntp.drift, contains the latest esti-
       mate of clock frequency. If this file  does  not  exist  when  ntpd  is
       started,  it  enters  a special mode designed to measure the particular
       frequency directly. The measurement takes 15 minutes, after  which  the
       frequency  is  set and ntpd resumes normal mode where the time and fre-
       quency are continuously adjusted. The  frequency  file  is  updated  at
       intervals of an hour or more depending on the measured clock stability.

OPERATING MODES
       The  ntpd  program  normally  operates continuously while adjusting the
       time and frequency, but in some cases it may not be practical to run it
       continuously.  With  the  -q option ntpd operates as in continous mode,
       but exits just after setting the clock for the first time with the con-
       figured  servers.  Most  applications will probably want to specify the
       iburst option with the server command. With this  option  a  volley  of
       messages  is  exchanged to groom the data and set the clock in about 10
       s. If nothing is heard after a few minutes, the daemon  times  out  and
       exits.

POLL INTERVAL CONTROL
       NTP  uses an intricate heuristic algorithm to automatically control the
       poll interval for maximum  accuracy  consistent  with  minimum  network
       overhead.  The  algorithm  measures the incidental offset and jitter to
       determine the best poll interval. When ntpd starts, the interval is the
       default minimum 64 s. Under normal conditions when the clock discipline
       has stabilized, the interval increases in steps to the default  maximum
       1024  s.  In  addition,  should  a server become unreachable after some
       time, the interval increases in steps to the maximum in order to reduce
       network overhead.
       The  default  poll  interval range is suitable for most conditions, but
       can be changed using options on the Server  Options  and  Miscellaneous
       Options  pages.  However, when using maximum intervals much larger than
       the default, the residual clock frequency error must  be  small  enough
       for  the  discipline  loop to capture and correct. The capture range is
       500 PPM with a 64-s interval decreasing by a factor  of  two  for  each
       interval  doubling. At a 36-hr interval, for example, the capture range
       is only 0.24 PPM.

THE HUFF-N'-PUFF FILTER
       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.
       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, such as during other than work hours. The
       filter remembers the minimum delay over the most recent  interval  mea-
       sured  usually  in  hours. Under conditions of severe delay, the filter
       corrects the apparent offset using the sign of the offset and the  dif-
       ference  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 huffpuff command, as  described  in  the  Miscellaneous  Options
       page.

LEAP SECOND PROCESSING
       As  provided  by  international agreement, an extra second is sometimes
       inserted in Coordinated Universal Time (UTC) at the end of  a  selected
       month,  usually  June or December. The National Institutes of Standards
       and  Technology  (NIST)  provides  an  historic  leapseconds  file   at
       time.nist.gov  for  retrieval  via  FTP. When this file, usually called
       ntp-leapseconds.list, is copied and installed in a directory. The leap-
       file configuration command specifies the path to this file. At startup,
       ntpd reads it and initializes three leapsecond values: the NTP  seconds
       at  the  next  leap  event, the offset of UTC relative to International
       Atomic Time (TAI) after the leap and the NTP seconds when the  leapsec-
       onds file expires and should be retrieved again.
       If  a  host  does  not have the leapsecond values, they can be obtained
       over the net using  the  Autokey  security  protocol.  Ordinarily,  the
       leapseconds  file  is  installed  on the primary servers and the values
       flow from them via secondary servers  to  the  clients.  When  multiple
       servers  are  involved,  the values with the latest expiration time are
       used.
       If the latest leap is in the past, nothing further is done  other  than
       to  install  the  TAI offset. If the leap is in the future less than 28
       days, the leap warning bits are set. If in  the  future  less  than  23
       hours,  the kernel is armed to insert one second at the end of the cur-
       rent day. If the kernel is enabled, the leap is done  automatically  at
       that  time;  otherwise, the clock is effectively stopped for one second
       at the leap. Additional details are in the The NTP Timescale  and  Leap
       Seconds white paper
       If  none  of the above provisions are available, dsependent servers and
       clients tally the leap warning bits of surviving servers and  reference
       clocks.  When  a majority of the survivors show warning, a leap is pro-
       grammed at the end of the current month. During the month  and  day  of
       insertion, they operate as above. In this way the leap is propagated at
       all dependent servers and clients.

ADDITIONAL FEATURES
       A new experimental feature called interleaved modes can be used in  NTP
       symmetric  or  broadcast  modes.  It is designed to improve accuracy by
       avoiding kernel latency and queueing delay, as  described  on  the  NTP
       Interleaved  Modes  page. It is activated by the xleave option with the
       peer or broadcast configuration commands. The  NTP  protocol  automati-
       cally  reconfigures in normal or interleaved mode as required. Ordinary
       broadcast clients can use the same servers as  interleaved  clients  at
       the  same  time. Further details are in the white paper NTP Interleaved
       On-Wire Protocol and the briefing Interleaved Synchronization Protocols
       for LANs and Space Data Links.
       If  ntpd,  is  configured with NetInfo support, it will attempt to read
       its configuration from the NetInfo service 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, -d or -D option is used, ntpd changes the current  work-
       ing  directory to the root directory, so any options or commands speci-
       fying paths need to use an absolute path or  a  path  relative  to  the
       root.

COMMAND LINE OPTIONS
       -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
               same operation as the enable auth command and is the default.
       -A      Do  not  require  cryptographic  authentication  for  broadcast
               client, multicast client and  symmetric  passive  associations.
               This  is  the  same  operation  as the disable auth command and
               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
               /etc/ntp.conf.
       -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 command.
       -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 [address | interface name]
               Open the network address given, or all the addresses associated
               with  the given interface name. This option may appear multiple
               times. This option also implies not  opening  other  addresses,
               except  wildcard  and  localhost.  This  option  is deprecated.
               Please consider using the configuration file interface command,
               which is more versatile.
       -k keyfile
               Specify  the  name  and path of the symmetric key file. This is
               the same operation as the keys keyfile 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 command.
       -L      Do not listen to virtual  interfaces,  defined  as  those  with
               names  containing  a  colon.  This option is deprecated. Please
               consider using the configuration file interface command,  which
               is more versatile.
       -M      Raise scheduler precision to its maximum (1 msec) using timeBe-
               ginPeriod. (Windows only)
       -m      Lock memory.
       -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
               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
               command.
       -t key  Add a key number to the trusted key list. This option can occur
               more than once. This is the same operation  as  the  trustedkey
               key command.
       -u user[:group]
               Specify  a  user,  and  optionally  a group, to switch to. This
               option is only available if the OS supports running the  server
               without  full  root  privileges. Currently, this option is sup-
               ported under  NetBSD  (configure  with  --enable-clockctl)  and
               Linux (configure with --enable-linuxcaps).
       -U interface update interval
               Number  of seconds to wait between interface list scans to pick
               up new and delete  network  interface.  Set  to  0  to  disable
               dynamic interface list updating. The default is to scan every 5
               minutes.
       -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
               option and the step threshold is applied also  to  leap  second
               corrections.

THE CONFIGURATION FILE
       Ordinarily,  ntpd  reads  the ntp.conf configuration file at startup in
       order to determine the synchronization sources and operating modes.  It
       is  also possible to specify a working, although limited, configuration
       entirely on the command line, obviating the need  for  a  configuration
       file. This may be particularly useful when the local host is to be con-
       figured as a broadcast client, with servers determined by listening  to
       broadcasts at run time.
       Usually, the configuration file is installed as/etc/ntp.conf, 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 command keyword followed
       by a list of option keywords separated by whitespace. Commands may  not
       be  continued  over  multiple  lines.  Options  may be host names, host
       addresses written in  numeric,  dotted-quad  form,  integers,  floating
       point  numbers  (when  specifying  times  in seconds) and text strings.
       Optional arguments are delimited by [ ] in  the  options  pages,  while
       alternatives  are  separated  by  |.  The  notation  [  ...  ] means an
       optional, indefinite repetition of the last item before the [ ... ].

FILES
       +---------------------+----------------------+----------+--------------+
       |File                 | Default              |  Option  |  Command     |
       +---------------------+----------------------+----------+--------------+
       |configuration file   | /etc/ntp.conf        |  -c      |  none        |
       +---------------------+----------------------+----------+--------------+
       |frequency file       | none                 |  -f      |  driftfile   |
       +---------------------+----------------------+----------+--------------+
       |leapseconds file     | none                 |          |  leapfile    |
       +---------------------+----------------------+----------+--------------+
       |process ID file      | none                 |  -p      |  pidfile     |
       +---------------------+----------------------+----------+--------------+
       |log file             | system log           |  -l      |  logfile     |
       +---------------------+----------------------+----------+--------------+
       |include file         | none                 |  none    |  includefile |
       +---------------------+----------------------+----------+--------------+
       |statistics path      | /var/log/ntpstats/   |  -s      |  statsdir    |
       +---------------------+----------------------+----------+--------------+
       |keys path            | /etc/ntp/crypto      |  none    |  keysdir     |
       +---------------------+----------------------+----------+--------------+
EXIT CODES
       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.

SEE ALSO
       ntp.conf(5), ntpq(8), ntpdc(8)
       The official HTML documentation.
       This file was automatically generated from HTML source.


                                                                       ntpd(8)