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ntp.conf(4)							   ntp.conf(4)

       ntp.conf - Network Time Protocol (NTP) configuration file

       The  xntpd  configuration  file,	 /etc/ntp.conf,	 is  read  by xntpd at

       The xntpd configuration file is relatively free	of  formatting.	  Com‐
       ments,  which  may  be  freely  inserted,  begin with a # character and
       extend to the end of the line.  Blank lines are ignored.	 Configuration
       commands consist of an initial keyword followed by a list of arguments,
       separated by white space.  Configuration commands may not be  continued
       over multiple lines.  Arguments may be host names, host addresses writ‐
       ten in numeric, dotted-quad  form,  integers,  floating	point  numbers
       (when  specifying  times	 in seconds) and text strings.	Optional argu‐
       ments are delimited by “[]” in the following descriptions, while alter‐
       natives are separated by “|”.

       The  /etc/ntp.conf file is defined as a Context-Dependent Symbolic Link
       (CDSL), and must be maintained as such.	See the System	Administration
       manual  for  more information.  If you manually edit the ntp.conf file,
       do not use the SysMan Menu to modify the configuration in  the  future.
       The  SysMan Menu recognizes only a small subset of the options that you
       can use in the ntp.conf file, and might overwrite your configuration.

   Configuration Options
       peer host_address [key #] [version  #]  [burst]	[prefer]  [minpoll  #]
       [maxpoll	 #]  server  host_address [key #] [burst] [version #] [prefer]
       [mode #] [minpoll #] [maxpoll #] broadcast host_address [key #] [burst]
       [version #] [minpoll #] [maxpoll #] [ttl #] manycastclient host_address
       [key #] [burst] [version #] [minpoll #] [maxpoll #] [ttl #]

       The following commands specify various time servers to be used or  time
       services	 to  be provided: This command specifies that the local server
       is to operate  in  “symmetric  active”  mode  with  the	remote	server
       host_address.   In  this	 mode, the local server can be synchronized to
       the remote server and, in addition, the remote server can  be  synchro‐
       nized  by  the  local  server.	This is useful in a network of servers
       where, depending on various failure  scenarios,	either	the  local  or
       remote  server  host  may  be  the better source of time.  This command
       specifies that the local server is to operate in “client” mode with the
       remote server named in the command.  In this mode, the local server can
       be synchronized to the remote server, but the remote server  can	 never
       be  synchronized	 to the local server.  This command specifies that the
       local server is to operate in broadcast mode  where  the	 local	server
       sends  periodic broadcast messages to a client population at the broad‐
       cast or multicast address named in the command.	Ordinarily, this spec‐
       ification  applies only to the local server operating as a transmitter;
       for operation as a broadcast client, see the broadcastclient or	multi‐
       castclient  commands.   In  this	 mode, the host_address is usually the
       broadcast address on one of the local networks or a  multicast  address
       assigned	 to  NTP.   Address	is  assigned  to  NTP; this is
       presently the only number that should be used.  This command  specifies
       that  the  local	 server	 is  to operate in client mode with the remote
       servers that are discovered as a	 result	 of  broadcast/multicast  mes‐
       sages.	The client broadcasts a request message to the specified group
       address and specifically-enabled servers	 respond  to  these  messages.
       The client selects the servers providing the best time and continues as
       with the server command. The remaining  servers	are  discarded	as  if
       never  heard.   In  this mode, the supplied host_address must match the
       address used on the manycastserver command for the designated  manycast
       servers.	 The  NTP  multicast  address  ( should not be used,
       unless specific means are taken to avoid spraying large	areas  of  the
       Internet	 with  these messages and causing a possibly massive implosion
       of replies at the sender.

       The following options can be specified with these  commands:  Indicates
       that,  at  each	poll  interval, the server is to send a burst of eight
       packets instead of one.	This option is useful for maintaining accuracy
       over the intermittent connections that are typical of PPP and ISDN ser‐
       vices.  Indicates that all packets sent to the address are  to  include
       authentication  fields  encrypted  using	 the specified key number (the
       range of which is that of an unsigned 32 bit integer).  The default  is
       to  not include an encryption field.  Allows you to specify the version
       number to be used for outgoing NTP packets.  Versions 1, 2, and	3  are
       the  choices;  version  3  is  the default.  Version 1 must be used for
       hosts running the University of Maryland ntpd daemon.  Specify the min‐
       imum  and  maximum polling interval for NTP messages, in seconds to the
       power of two.  The default range is 6 (64 seconds)  to  10  (1024  sec‐
       onds).  The allowable range is 4 (16 seconds) to 17 (36.4 h) inclusive.
       Marks the host as a preferred host.  All other things being equal, this
       host  will be chosen for synchronization among a set of correctly oper‐
       ating hosts.  Specifies the time-to-live	 (TTL)	to  use	 on  multicast
       packets	(broadcast  mode  only).  Selection of the proper value, which
       defaults to 127, must be coordinated with the network administrator(s).

   Additional Configuration Options
       Directs the local server to listen for broadcast messages on the	 local
       network,	 in  order to discover other servers on the same subnet.  Upon
       hearing a broadcast message for the first time, the local  server  mea‐
       sures  the  nominal  network delay using a brief client/server exchange
       with the remote server, then enters the broadcastclient mode, in	 which
       it listens for and synchronizes to succeeding broadcast messages.  Note
       that, in order to avoid accidental  or  malicious  disruption  in  this
       mode,  both the local and remote servers must operate using authentica‐
       tion and the same trusted key and key identifier.  Provides  a  way  to
       disable	various	 server	 options.  Flags not mentioned are unaffected.
       The flags presently available are described under the  enable  command.
       Specifies  the  name of the file used to record the frequency offset of
       the local clock oscillator.  If the file exists on startup, it is  read
       and the value used to set the initial frequency offset and then updated
       once every hour with the current offset computed by xntpd.  If the file
       does not exist or this command is not given, the initial frequency off‐
       set is assumed zero.  In this case, it may take some hours for the fre‐
       quency  to  stabilize  and  the residual timing errors to subside.  The
       file contains a single floating point value  equal  to  the  offset  in
       parts-per-million  (ppm).   The	file is updated by writing the current
       drift value into a temporary file and then using rename to replace  the
       old  version.   Therefore,  xntpd  must	have  write permission for the
       directory the drift file is located in, and file system links, symbolic
       or  otherwise,  should  be  avoided.   Provides a way to enable various
       server options.	Flags not mentioned are unaffected.  Causes the server
       to  synchronize	with unconfigured peers only if the peer has been cor‐
       rectly authenticated using a  trusted  key  and	key  identifier.   The
       default	for  this  flag is disable (off).  Causes the server to listen
       for a message from a broadcast or multicast server, following which  an
       association is automatically instantiated for that server.  The default
       for this flag is disable (off).	Enables the server to adjust its local
       clock, with default enable (on).	 If not set, the local clock free-runs
       at its intrinsic time and frequency offset.  This  flag	is  useful  in
       case the local clock is controlled by some other device or protocol and
       NTP is used only to provide synchronization to other clients.   Enables
       the monitoring facility (see "Monitoring Options"), with default enable
       (on).  Enables statistics facility filegen, with default	 enable	 (on).
       This command controls the amount and type of output written to the sys‐
       tem syslog facility or the alternate logfile log file. By default, only
       minimal output is written.

	      All  configkeyword  keywords  can	 be  prefixed with =, + and -,
	      where = sets the syslogmask, + adds and - removes messages.  The
	      syslog  messages can be controlled in four classes: clock, peer,
	      sys and sync. Within these classes, four types of	 messages  can
	      be controlled: info, events, statistics, and status.

	      Informational messages (info) control configuration information.
	      Event messages (events) control logging of events (reachability,
	      synchronization,	alarm conditions).  Statistical output is con‐
	      trolled with the statistics keyword. The final message group  is
	      for  status  messages, which mainly describe the synchronization

	      Configuration keywords are formed by concatenating  the  message
	      class  with  the event class. The all prefix can be used instead
	      of a message class. A message class may also be followed by  the
	      all  keyword  to	enable/disable	all messages of the respective
	      message class.  Thus, a minimal  log  configuration  could  look
	      like this: logconfig = syncstatus +sysevents

	      This  configuration, the default for the operating system, lists
	      only the synchronization state of xntpd and major system events.

	      For a simple reference server,  the  following  minimum  message
	      configuration could be useful: logconfig = syncall +clockall

	      This  configuration lists all clock information and synchroniza‐
	      tion information. All other events  and  messages	 about	peers,
	      system  events and so on are suppressed.	This command specifies
	      the location of an alternate log file to be used instead of  the
	      default  system syslog facility.	This command directs the local
	      server to listen for and respond to broadcast messages  received
	      on  any  local  interface, and in addition enables the server to
	      respond to client mode messages  sent  to	 the  multicast	 group
	      address(es) specified. At least one address is required, but the
	      NTP multicast address ( should	NOT  be	 used,	unless
	      specific	means  are  taken  to  limit the span of the reply and
	      avoid a possibly massive implosion at the original sender.  This
	      command  is used in the same way as the broadcastclient command,
	      but operates using IP multicasting.  Support for	this  function
	      requires	a  multicast kernel and the use of authentication.  If
	      one or more IP addresses are given, the server joins the respec‐
	      tive  multicast  group(s).   If  none  are given, the IP address
	      assigned to NTP ( is assumed.

   Authentication Options
       Specifies the key identifier to use with the  ntpq  program,  which  is
       useful  to  diagnose  and  repair problems that affect xntpd operation.
       The operation of the ntpq program and xntpd conform to those  specified
       in RFC 1305.  Requests from a remote ntpq program that affect the state
       of the local server must be  authenticated,  which  requires  both  the
       remote  program and local server share a common key and key identifier.
       The argument to this command is	a  32-bit  unsigned  integer.	If  no
       requestkey  command  is	included  in the configuration file, or if the
       keys do not match, such requests are ignored.  Specifies the name of  a
       file  that  contains  the  encryption  keys and key identifiers used by
       xntpd when  operating  in  authenticated	 mode.	 See  ntp.keys(4)  for
       description  of	the  key file format.  Specifies the key identifier to
       use with the xntpdc program, which is useful  to	 diagnose  and	repair
       problems that affect xntpd operation.  The operation of the xntpdc pro‐
       gram are specific to this particular implementation of xntpd and can be
       expected	 to  work  only with this and previous versions of the daemon.
       Requests from a remote xntpdc program that  affect  the	state  of  the
       local server must be authenticated, which requires both the remote pro‐
       gram and local server share a common key and key identifier.  The argu‐
       ment  to	 this  command is a 32-bit unsigned integer.  If no requestkey
       command is included in the configuration file, or if the	 keys  do  not
       match, such requests are ignored.  Specifies the encryption key identi‐
       fiers that are trusted for the purposes of authenticating  peers	 suit‐
       able  for  synchronization.  The authentication procedures require that
       both the local and remote servers share the same key and key identifier
       for  this  purpose,  although different keys can be used with different
       servers.	 The arguments are 32-bit unsigned integers.   Note,  however,
       that  NTP key 0 is fixed and globally known.  If meaningful authentica‐
       tion is to be performed the 0 key should not be trusted.

   Access Control Options
       Defines the number of clients from the same network that are allowed to
       use  the	 server.   The	default is 3.  Specifies the number of seconds
       after which a client is considered  inactive  and  thus	no  longer  is
       counted	for  client  limit  restriction.  The default is 3600 seconds.
       The xntpd daemon implements a general  purpose  address-and-mask	 based
       restriction  list.   The list is sorted by address and by mask, and the
       list is searched in this order for matches, with the last  match	 found
       defining	 the  restriction  flags associated with the incoming packets.
       The source address of incoming packets is used for the match, with  the
       32  bit address being and'ed with the mask associated with the restric‐
       tion entry and then compared with the entry's address (which  has  also
       been  and'ed  with  the	mask)  to look for a match.  The mask argument
       defaults to, meaning that the address is treated as the
       address	of an individual host.	A default entry (address, mask is always included and, given the sort	algorithm,  is	always
       the  first  entry  in  the  list.  Note that, while address is normally
       given in dotted-quad format, the text  string  default,	with  no  mask
       option, may be used to indicate the default entry.

	      In  the current implementation, flags always restrict access: an
	      entry with no flags indicates that free access to the server  is
	      to  be  given.   The  flags  are	not  orthogonal,  in that more
	      restrictive flags will often make less restrictive  ones	redun‐
	      dant.   The  flags can generally be classed into two categories:
	      those that restrict time service and those that restrict	infor‐
	      mational	queries.   One	or  more of the following flags may be
	      specified: Ignores all packets from hosts that match this entry.
	      If this flag is specified, queries and time server polls receive
	      no response.  Ignores all NTP mode 6 and 7 packets  (information
	      queries  and configuration requests) from the source.  Time ser‐
	      vice is not affected.  Ignores all NTP mode 6 and 7 packets that
	      attempt to modify the state of the server (run time reconfigura‐
	      tion).  Queries that return information are permitted.  Declines
	      to  provide  mode	 6  control  message  trap service to matching
	      hosts.  The trap service is a subsystem of the  mode  6  control
	      message protocol, which is intended for use by remote event log‐
	      ging programs.  Declares traps set by matching hosts to  be  low
	      priority.	  The number of traps a server can maintain is limited
	      (the current limit is 3).	 Traps are usually assigned on a first
	      come,  first  served  basis,  with  later	 trap requestors being
	      denied service.  This flag modifies the assignment algorithm  by
	      allowing	low  priority traps to be overridden by later requests
	      for normal priority traps.  Ignores NTP packets  whose  mode  is
	      other  than  6  or 7.  In effect, time service is denied, though
	      queries may still be permitted.  Provides stateless time service
	      to polling hosts, but does not allocate peer memory resources to
	      these hosts even if they otherwise might be considered useful as
	      future synchronization partners.	Treats these hosts normally in
	      other respects, but never uses them as synchronization  sources.
	      Limits  the  number of clients that can use these hosts from the
	      same net.	 Net in this context refers to the IP  notion  of  net
	      (class  A,  class B, class C, etc.).  Only the first clientlimit
	      hosts that have accessed the server and that  have  been	active
	      during  the  last	 clientperiod  seconds are accepted.  Requests
	      from other clients from the same net are	rejected.   Only  time
	      request  packets	are taken into account.	 Private, control, and
	      broadcast packets are  not  subject  to  client  limitation  and
	      therefore do not contribute to client count.  History of clients
	      is kept using the monitoring capability of xntpd.	  Thus,	 moni‐
	      toring  is  active  as long as there is a restriction entry with
	      the limited flag.	 Specifies a match algorithm modifier,	rather
	      than  a  restriction  flag.  Its presence causes the restriction
	      entry to be matched only if the source port in the packet is the
	      standard	NTP  UDP port (123).  Both ntpport and non-ntpport may
	      be specified.  The ntpport is considered more  specific  and  is
	      sorted later in the list.

	      Default  restriction  list  entries, with the flags ignore, ntp‐
	      port, for each of	 the  local  host's  interface	addresses  are
	      inserted	into  the  table at startup to prevent the server from
	      attempting to synchronize to its own time.  A default  entry  is
	      also  always present, though if it is otherwise unconfigured, no
	      flags are associated with the default entry (everything  besides
	      your own NTP server is unrestricted).

	      The  restriction	facility allows the current access policies of
	      the time servers running on the NSFnet  backbone	to  be	imple‐
	      mented with xntpd as well.  While this facility may be otherwise
	      useful for keeping unwanted time	servers	 from  affecting  your
	      own,  it should not be considered an alternative to the standard
	      NTP authentication facility.  Source address based  restrictions
	      can be circumvented by a determined cracker.

   Monitoring Options
       filegen	name  [file  filename]	[type typename] [flag flagval] [link |
       nolink] [enable | disable] Configures setting of	 generation  file  set
       name.  Generation file sets provide a means for handling files that are
       continuously growing during the lifetime of a server.   Server  statis‐
       tics  are  a typical example for such files.  Generation file sets pro‐
       vide access to a set of files used to store the actual  data.   At  any
       time,  only one element of the set is being written to.	The type given
       specifies when and how data will be directed to a new  element  of  the
       set.   This  way, information stored in elements of a file set that are
       currently unused are available for administrational operations  without
       the  risk  of  disturbing the operation of xntpd.  (The elements can be
       removed to free space for new data produced.) File names of set members
       are built from three elements: This is a constant filename path.	 It is
       not subject to modifications by the filegen statement.  It  is  defined
       by  the	server, usually specified as a compile time constant.  It may,
       however, be configurable for individual file generation sets via	 other
       commands.   For	example,  the prefix used with loopstats and peerstats
       filegens can be configured using the statsdir statement explained  pre‐
       viously.	  This	string	is directly concatenated to the prefix with no
       intervening slash character (/).	 This can be modified using  the  file
       argument	 to the filegen statement.  No .. elements are allowed in this
       component to prevent filenames referring to parts outside the  filesys‐
       tem  hierarchy  denoted	by prefix.  This part reflects individual ele‐
       ments of a file set.  It is generated according to the type of  a  file

	      A file generation set is characterized by its type.  The follow‐
	      ing types are supported: The file set is a  single  plain	 file.
	      One  element  of	file  set  is  used per incarnation of a xntpd
	      server.  This type does not perform any changes to file set mem‐
	      bers during runtime, however it provides an easy way of separat‐
	      ing files belonging to different xntpd server incarnations.  The
	      set  member filename is built by appending a dot (.) to concate‐
	      nated prefix and filename strings,  and  appending  the  decimal
	      representation  of  the  process id of the xntpd server process.
	      One file generation set element is created per  day.   The  term
	      day  is  based  on  UTC.	A day is defined as the period between
	      00:00 and 24:00 UTC.  The file set member suffix consists	 of  a
	      dot  (.) and a day specification in the following form: YYYYMMDD
	      YYYY is a four-digit year number (for example, 1992);  MM	 is  a
	      two-digit month number; and DD is a two-digit day number.	 Thus,
	      information written at December 10th, 1992 would be written to a
	      file  named  prefixfilename.19921210.   Any file set member con‐
	      tains data related to a certain week of a year.  The  term  week
	      is  defined  by  computing  “day of year” modulo 7.  Elements of
	      such a file generation set are distinguished  by	appending  the
	      following	 suffix	 to the file set filename base: A dot, a four-
	      digit year number, the letter W, and a  two-digit	 week  number.
	      For  example, information from January, 10th 1992 would be writ‐
	      ten to a file with suffix One generation	file  set  element  is
	      generated	 per month.  The file name suffix consists of a dot, a
	      four-digit year number, and a two-digit month.   One  generation
	      file  element  is	 generated per year.  The filename suffix con‐
	      sists of a dot and a four-digit year number.  This type of  file
	      generation  sets	changes to a new element of the file set every
	      24 hours of server operation.  The filename suffix consists of a
	      dot,  the	 letter	 a, and an eight-digit number.	This number is
	      taken to be the number of seconds the server is running  at  the
	      start of the corresponding 24 hour period.

	      Information  is  only written to a file generation set when this
	      set is enabled.  Output is prevented by specifying disable.

	      It is convenient to be able to access the current element	 of  a
	      file generation set by a fixed name.  This feature is enabled by
	      specifying link and disabled using nolink.  If  link  is	speci‐
	      fied,  a	hard  link from the current file set element to a file
	      without suffix is created.  When there is already	 a  file  with
	      this  name  and  the  number of links of this file is one, it is
	      renamed appending a dot, the letter C, and the pid of the	 xntpd
	      server  process.	 When the number of links is greater than one,
	      the file is unlinked.   This  allows  the	 current  file	to  be
	      accessed	by  a  constant	 name.	 Enables writing of statistics
	      records.	The  following	types  of  statistics  are  supported:
	      Enables  recording  of loop filter statistics information.  Each
	      update of the local clock outputs a line of the  following  form
	      to  the  file  generation set named “loopstats”: 48773 10847.650
	      0.0001307 17.3478 2

	      The first two fields show the date  (Modified  Julian  Day)  and
	      time  (seconds  and fraction past UTC midnight).	The next three
	      fields show time offset in seconds, frequency offset  in	parts-
	      per-million  and time constant of the clock-discipline algorithm
	      at each update of the clock.  Enables recording of peer  statis‐
	      tics information.	 This includes statistics records of all peers
	      of a NTP server and of the 1-pps signal, where present and  con‐
	      figured.	Each valid update appends a line of the following form
	      to the current element of a file	generation  set	 named	“peer‐
	      stats”:  48773  10847.650  9714  -0.001605  0.00000

	      The first two fields show the date  (Modified  Julian  Day)  and
	      time  (seconds  and  fraction  past UTC midnight).  The next two
	      fields show the peer address in dotted-quad notation and status,
	      respectively.   The status field is encoded in hex in the format
	      described in Appendix A of the NTP specification RFC 1305.   The
	      final three fields show the offset, delay and dispersion, all in
	      seconds.	Enables recording of clock driver statistics  informa‐
	      tion.   Each  update received from a clock driver outputs a line
	      of the following form to the file generation set	named  “clock‐
	      stats”: 49213 525.624 93 226 00:08:29.606 D

	      The  first  two  fields  show the date (Modified Julian Day) and
	      time (seconds and fraction past UTC midnight).  The  next	 field
	      shows the clock address in dotted-quad notation, The final field
	      shows the last timecode received from the clock in decoded ASCII
	      format,  where meaningful.  In some clock drivers a good deal of
	      additional information can be gathered and  displayed  as	 well.
	      See information specific to each clock for further details.

	      Statistic	 files are managed using file generation sets (see the
	      filegen description).  The information obtained by enabling sta‐
	      tistics  recording  allows  analysis of temporal properties of a
	      xntpd server.  It is usually only useful to primary  servers  or
	      maybe  main  campus servers.  Enables recording of raw-timestamp
	      statistics information. This includes statistics records of  all
	      peers  of a NTP server and of special signals, where present and
	      configured. Each NTP message  received  from  a  peer  or	 clock
	      driver  appends a line of the following form to the file genera‐
	      tion set named rawstats:	50928  2132.543
	      3102453281.584327000  3102453281.58622800031  02453332.540806000

	      The first two fields show the date  (Modified  Julian  Day)  and
	      time  (seconds  and  fraction  past  UTC midnight). The next two
	      fields show the remote peer or clock  address  followed  by  the
	      local  address  in  dotted-quad  notation. The final four fields
	      show the originate, receive, transmit and final  NTP  timestamps
	      in  order.  The timestamp values are as received and before pro‐
	      cessing by the various data smoothing and mitigation algorithms.
	      Indicates	 the  full  path of a directory where statistics files
	      should be created.  This keyword allows the (otherwise constant)
	      filegen  filename prefix to be modified for file generation sets
	      used for handling statistics logs (see the  description  of  the
	      filegen statement).

   Miscellaneous Options
       Specifies  the  default delay to be used in cases where the delay cali‐
       bration procedure between local and remote servers might	 fail  due  to
       network	or server access controls.  Typically (for Ethernet), a number
       between 0.003 and 0.007 seconds is appropriate.	The default when  this
       command is not used is 0.004 seconds.

	      The  broadcast and multicast modes require a special calibration
	      to determine the network delay  between  the  local  and	remote
	      servers.	 Ordinarily, this is done automatically by the initial
	      protocol exchanges between the local and remote  servers.	  Adds
	      an  additional  system variable.	These variables can be used to
	      distribute additional information such as the access policy.  If
	      the  variable  of the form name=value is followed by the default
	      keyword, the variable is listed as part of  the  default	system
	      variables	 (ntpq	rv command).  These additional variables serve
	      informational purposes only.  They are not related to the proto‐
	      col other that they can be listed.  The known protocol variables
	      always override any variables defined by the the	setvar	mecha‐

	      There  are three special variables that contain the names of all
	      variables of the same group.  The sys_var_list holds  the	 names
	      of  all  system variables.  The peer_var_list holds the names of
	      all peer variables and the clock_var_list hold the names of  the
	      reference clock variables.  trap host_address [port port_number]
	      [interface interface_address] Configures a trap receiver at  the
	      given  host  address  and port number, sending messages with the
	      specified local  interface  address.   If	 the  port  number  is
	      unspecified, a value of 18447 is used.  If the interface address
	      is not specified, the message is sent with a source address (the
	      local  interface	the message is sent through).  On a multihomed
	      host, the interface used may vary from time to time with routing

	      The trap receiver generally logs event messages and other infor‐
	      mation from the server in a log file.  While such	 monitor  pro‐
	      grams may also request their own trap dynamically, configuring a
	      trap receiver ensures that no messages are lost when the	server
	      is started.

       Most variables used by the NTP protocol can be examined with the xntpdc
       (mode 7 messages) and the ntpq (mode 6 messages).  Currently  very  few
       variables  can  be  modified by using mode 6 messages.  These variables
       are either created with the setvar directive or the leap warning	 vari‐
       ables.	The leap warning bits that can be set in the leapwarning vari‐
       able (up to one	month  ahead).	 Both,	the  leapwarning  and  in  the
       leapindication  variable,  have	a slightly different encoding than the
       usual leap bits interpretation: The daemon passes the leap bits of  its
       synchronization	source	(usual	mode  of operation).  A leap second is
       added/deleted (operator forced leap second).  Leap information from the
       synchronization source is ignored (thus LEAP_NOWARNING is passed on).

   Reference Clock Support
       The  xntpd  daemon  includes support for a number of types of reference
       clocks.	A reference clock is generally (though	not  always)  a	 radio
       timecode	 receiver  that	 is synchronized to a source of standard time,
       such as the services offered by the NRC in Canada and NIST in the  U.S.
       The  interface between the computer and the timecode receiver is device
       dependent and will vary, but is often a serial port.

       For configuration purposes, xntpd treats reference clocks  like	normal
       NTP peers.  However, unlike normal peers, reference clocks are referred
       to by an invalid IP address.

       Reference clock addresses are of the form 127.127.t.u, where  t	is  an
       integer	denoting the clock type and u indicates the type-specific unit
       number, in the range 0-3, that is used to identify  multiple  instances
       of  clocks  of  the same type.  Most of these clocks require support in
       the form of a serial port or special  bus  peripheral.  The  particular
       device is normally specified by adding a soft link /dev/device%d to the
       particular hardware device involved.  The device is compiled  in	 xntpd
       according to the clock type.

       The  following  table lists the supported reference clock types, device
       names, clock names, and descriptions:

       Type   Device   Name	   Description
       1      (none)   LOCAL	   Undisciplined Local Clock
       3      pst      WWV_PST	   PSTI/Traconex WWV/WWVH Receiver
       4      wwvb     WWVB_SPEC   Spectracom WWVB Receiver
       5      true     TRUETIME	   TrueTime GPS/GOES/OMEGA Receivers
       15     true     TRUETIME	   TrueTime Generic Receivers (alias)
       18     acts     NIST_ACTS   NIST Automated Computer Time Service
       25     true     TRUETIME	   TrueTime Generic Receivers (alias)
       Although clock types 15 and 25 still  exist  as	aliases	 for  TrueTime
       clocks,	it  is best to use type 5, because the aliases might change in
       the future.

       Reference clocks are configured using a server statement in the config‐
       uration file.  Typically, this is the only command necessary to config‐
       ure a reference clock.  The following is the format for	this  command:
       server 127.127.t.u [prefer] [mode m] [minpoll #] [maxpoll #]

       In  the	preceding command: Specifies the clock type number.  Specifies
       the unit number.	 This is typically 1, but can range from  0-3.	 Modi‐
       fies  the  clock selection algorithm.  Specifies a clock mode for those
       clock drivers that support multiple modes of operation.	Specifies  the
       minimum	and  maximum polling interval for reference clock messages, in
       seconds to the power of two.  For  most	directly  connected  reference
       clocks,	both  minpoll and maxpoll default to 6 (64 seconds). For modem
       reference clocks, minpoll defaults to 10 (147.1	minutes)  and  maxpoll
       defaults to 14 (4.5 hours). The allowable range is 4 (16 seconds) to 17
       (36.4 hours) inclusive.

       Reference clock support provides the fudge command, which can  be  used
       to  configure reference clocks in special ways.	This command must fol‐
       low the corresponding server command in the  configuration  file.   The
       following is the format for this command:

       fudge  127.127.t.u  [time1  secs]  [time2  secs] [stratum #] [refid id]
       [mode #] [flag1 0|1] [flag2 0|1] [flag4 0|1]  Specifies	a  fixed-point
       decimal	number (in seconds) to be added to the time offset produced by
       xntpd. This provides a way to correct a systematic error or bias	 by  a
       particular  clock.   Specifies  a  fixed-point  decimal	number that is
       interpreted in a clock-dependent way.  Specifies a mode number which is
       interpreted  in	a device-specific fashion.  For instance, it selects a
       dialing protocol in the ACTS driver and a device subtype in  the	 parse
       drivers.	  Specifies  a number in the range 0 (zero) to 15, if you want
       to override the default stratum assigned by xntpd.  Specifies  a	 four-
       character,  null-terminated  ASCII  string, if you want to override the
       default reference identifier assigned by xntpd.	A flag whose interpre‐
       tation  depends on the clock receiving it.  A flag whose interpretation
       depends on the clock receiving it.  Enables detailed status  monitoring
       and  event recording.  The data collected are written to the clockstats
       file maintained by the filegen utility (See xntpd(8)).	This  file  is
       normally	 processed  by	a cron job run once per day to produce summary
       statistics and performance data.

       The clock drivers, and  the  addresses  used  to	 configure  them,  are
       described as follows:

       127.127.1.u - Undisciplined Local Clock

       This driver can have the following applications: Allow a machine to use
       its own system clock as a reference clock, using no outside clock  dis‐
       cipline	source.	  This is useful if you want to use NTP in an isolated
       environment with no radio clock or  NIST	 modem	available.   Choose  a
       machine	that  has  a  good clock oscillator and configure it with this
       support.	 Set the clock using the best means available. Then, point all
       the other machines at this one or use broadcast (not multicast) mode to
       distribute time.	 You want to use a particular server's	clock  as  the
       clock of last resort when all other normal synchronization sources have
       gone away. This is useful if that server has  an	 ovenized  oscillator.
       For this you would configure this clock at a higher stratum (3 or 4) to
       prevent the server's stratum from falling below that.  An external dis‐
       cipline	source	is  available,	such  as the NIST "lockclock" program,
       which synchronizes the local clock using a telephone modem and the NIST
       Automated  Computer  Time  Service (ACTS), or the Digital Time Synchro‐
       nization Service (DTSS), which runs on DCE machines.  In this case, set
       the stratum to zero, indicating a bona fide stratum-1 source.  Use this
       with caution since there is no easy way to telegraph through  NTP  that
       something  might be wrong in the discipline source itself.  In the case
       of DTSS, the local clock can have a rather large jitter,	 depending  on
       the  interval  between corrections and the intrinsic frequency error of
       the clock oscillator.  In extreme cases,	 this  can  cause  clients  to
       exceed the 128-ms slew window and drop off the NTP subnet.

       In  the	default mode, the behavior of the clock selection algorithm is
       modified when this support is in use.  The  algorithm  is  designed  so
       that the local clock support is not selected unless no other discipline
       source is available.  This can be overridden with the prefer keyword of
       the  server  configuration  command, in which case only this support is
       selected for synchronization  and  all  other  discipline  sources  are
       ignored.	 This behavior is intended for use when an external discipline
       source controls the system clock.

       Fudge Factors

       By default, the stratum for this driver LCLSTRATUM is set at 5 and  the
       reference  ID  is set to LCL.  Both can be changed by the fudge command
       or the xntpdc utility.  Never configure this driver  to	operate	 at  a
       stratum	that might disrupt a client with access to a bona fide primary
       server, unless the local clock oscillator is  reliably  disciplined  by
       another	source.	  Never	 configure  a  server that might devolve to an
       undisciplined local clock to use multicast mode.

       This driver provides a mechanism to trim the local clock in  both  time
       and frequency, as well as a way to manipulate the leap bits.  The fudge
       time1 parameter adjusts the time	 (in  seconds)	and  the  fudge	 time2
       parameter  adjusts  the	frequency (in ppm).  Both parameters are addi‐
       tive; that is, they add increments in time or frequency to the  present
       values. Note: The frequency cannot be changed when the kernel modifica‐
       tions are in use.  The fudge flag1 and fudge flag2 bits set the	corre‐
       sponding leap bits.  For example, setting flag1 causes a leap second to
       be added at the end of the UTC day.  These bits are not reset automati‐
       cally when the leap takes place; they must be turned off manually after
       the leap event.

       127.127.3.u - PSTI/Traconex WWV/WWVH Receiver

       This  driver  supports  the  PSTI  1010	and  Traconex  1020   WWV/WWVH
       Receivers.  No  specific claim of accuracy is made for these receivers,
       but actual experience suggests that  10	ms  would  be  a  conservative

       The  DIP	 switches  should  be  set  for	 9600  bps line speed, 24-hour
       day-of-year format and UTC time	zone.  Automatic  correction  for  DST
       should be disabled. It is very important that the year be set correctly
       in the DIP switches; otherwise, the day of year will be incorrect after
       28  April  of  a normal or leap year. The propagation delay DIPswitches
       should be set according to the distance from the transmitter  for  both
       WWV  and WWVH, as described in the instructions. While the delay can be
       set only to within 11 ms, the fudge time1 parameter  can	 be  used  for
       vernier corrections.

       Using  the poll sequence QTQDQM, the response timecode is in three sec‐
       tions totaling 50 ASCII printing characters,  as	 concatenated  by  the
       driver,	in  the	 following format: ahh:mm:ss.fffs<cr> yy/dd/mm/ddd<cr>
       frdzycchhSSFTttttuuxx<cr> In the preceding format: Is an AM/PM  indica‐
       tor.   If  this	is  a space (" "), it indicates 24-hour mode.  Denotes
       hours, minutes, seconds, and milliseconds.  LI "s" Is a daylight-saving
       indicator.   If	this  is  a  space  ("	"), it indicates 24-hour mode.
       Denotes on-time.	 Denotes year (from DIP	 switches).   Denotes  day  of
       month,  month, and day of year.	Denotes frequency enable (O = all fre‐
       quencies enabled).  Denotes baud rate (3 = 1200, 6 = 9600).  Is a  fea‐
       tures  indicator (@ = month/day display enabled).  Denotes time zone (0
       = UTC).	Denotes year (5 = 91).	Denotes WWV propagation delay (52 = 22
       ms).   Denotes WWVH propagation delay (81 = 33 ms).  Denotes status (80
       or 82 = operating correctly).  Denotes current receive frequency	 (4  =
       15  MHz).  Denotes transmitter (C = WWV, H = WWVH).  Denotes time since
       last update (0000 = minutes).   Denotes	flush  character  (03  =  ^c).
       Denotes 94 (unknown).

       The  alarm  condition is indicated by other than '8' at A, which occurs
       during initial synchronization and when received signal is lost for  an
       extended	 period; unlock condition is indicated by other than "0000" in
       the tttt subfield at Q.

       Fudge Factors

       Only generic fudge factors apply.

       127.127.4.u - Spectracom WWVB Receiver

       This driver supports the Spectracom Model 8170 and Netclock/2 WWVB Syn‐
       chronized  Clock.  This	clock  has  proven  a reliable source of time,
       except in some cases of high ambient conductive	RF  interference.  The
       claimed	accuracy  of  the  clock is 100 usec relative to the broadcast
       signal; however, in most cases the actual accuracy is  limited  by  the
       precision of the timecode and the latencies of the serial interface and
       operating system.

       The DIP switches on this clock should be set to 24-hour	display,  AUTO
       DST off, time zone 0 (UTC), data format 0 or 2, and baud rate 9600.

       There are two timecode formats used by these clocks: format 0, which is
       available with both the Netclock/2 and 8170; and	 format	 2,  which  is
       available only with the Netclock/2 and specially modified 8170.

       Format 0 (22 ASCII printing characters)

       <cr><lf>i  ddd hh:mm:ss	TZ=zz<cr><lf>

       In  the preceding format: Denotes on-time.  Denotes hours, minutes, and
       seconds.	 Is a synchronization flag.  A space (" ") indicates  in  syn‐
       chronization;  a	 question  mark	 (?) indicates out of synchronization.
       The alarm condition is indicated by other than " " at A,	 which	occurs
       during  initial	synchronization	 and  when received signal is lost for
       about ten hours.

       Format 2 (24 ASCII printing characters)

       <cr><lf>iqyy ddd hh:mm:ss.fff ld

       In the preceding format: Denotes on-time.  Is a	synchronization	 flag.
       A  space	 (" ") indicates in synchronization; a question mark (?) indi‐
       cates out of synchronization.  Is a quality indicator.  A space	("  ")
       indicates  locked and A,B,C, or D indicates unlocked.  Denotes year (as
       broadcast).  Denotes day of year.  Denotes hours, minutes, seconds, and

       The  alarm  condition is indicated by other than " " at A, which occurs
       during initial synchronization and when received	 signal	 is  lost  for
       about ten hours. The unlock condition is indicated by other than " " at

       The Q is normally " " when the time error is less than 1 ms, but either
       A,B,C,  or D when the time error is less than 10 ms, 100 ms, 500 ms, or
       greater than 500 ms, respectively.  The L is normally " ", but  is  set
       to "L" early in the month of an upcoming UTC leap second and reset to '
       ' on the first day of the following month.  The D is  set  to  'S'  for
       standard	 time  'I' on the day preceding a switch to daylight time, 'D'
       for daylight time and 'O' on the day preceding  a  switch  to  standard
       time.  The start bit of the first <cr> is synchronized to the indicated
       time as returned.

       This driver interpolates the format in use from the length of the  mes‐
       sage.  A three-stage median filter is used to reduce jitter and provide
       a dispersion measure.  The driver makes no attempt to correct  for  the
       intrinsic jitter of the radio itself, which is a known problem with the
       older radios.

       Fudge Factors

       This driver can retrieve a table of quality data maintained  internally
       by  the	Netclock/2 receiver.  If flag4 of the fudge configuration com‐
       mand is set to 1, the driver retrieves this table and writes it to  the
       clockstats  file	 when  the  first  timecode  message  of  a new day is

       127.127.5.u - TrueTime GPS/GOES/OMEGA Receivers

       This driver supports  several  models  of  Kinemetrics/TrueTime	Timing
       Receivers, including the 468-DC MK III GOES Synchronized Clock, GPS- DC
       MK III and GPS/TM-TMD GPS Synchronized  Clock,  XL-DC  (a  151-602-210,
       reported	 by  the driver as a GPS/TM-TMD), GPS-800 TCU (an 805-957 with
       the RS232 Talker/Listener module), OM-DC OMEGA Synchronized Clock,  and
       very  likely others in the same model family that use the same timecode

       These clocks are connected to a serial port.  Up to  four  units,  with
       unit  numbers  in the range 0 through 3, can be configured.  The driver
       assumes the serial port device name is /dev/truex (for example, unit  1
       at	opens  the  clock at /dev/true1) and that the clock is
       configured for 9600-baud operation.

       TrueTime clocks have the following timecode format: ADDD:HH:MM:SSQCL

       In the preceding format: Denotes	 control-A  (^A),  which  is  stripped
       before the user sees it.	 Denotes day of year.  Denotes hours, minutes,
       and seconds.  Is a quality indicator.  Denotes carriage return,	or  on
       time.   Start  bit  begins  on  0  seconds  and	extends to 1 bit time.
       Denotes line feed.

       The quality codes report the following offsets for all  receivers:  +/-
       500 milliseconds +/- 50 milliseconds +/- 5 milliseconds +/- 1 millisec‐
       ond Less than 1 millsecond

       The OM-DC OMEGA Receiver adds the following codes: +/-  1  second  Less
       than  1	millisecond. Indicates which station is being received as fol‐
       lows: Norway Liberia Hawaii North Dakota La Reunion Argentina Australia

       Notes on 468-DC and OMEGA receiver:

       Send  the clock an R or C, and once per second a timestamp will appear.
       Send an R to get the satellite position once (GOES only).

       Notes on the 468-DC receiver:

       Since the old east/west satellite locations are	only  historical,  you
       cannot  set  your  clock propagation delay settings correctly and still
       use automatic mode. The manual says to use a  compromise	 when  setting
       the  switches.  This  results in significant errors. The solution is to
       use fudge time1 and time2 to incorporate corrections. If your clock  is
       set  for 50 and it should be 58 for using the west and 46 for using the
       east, use the following fudge  line:  fudge	 time1	+0.008
       time2 -0.004

       This  corrects  the  4  milliseconds  advance and 8 milliseconds retard
       needed. The software will ask the clock which satellite it sees.

       Fudge Factors

       Only generic fudge factors apply.

       127.127.18.u - NIST Automated Computer Time Service

       This driver supports the NIST Automated Computer Time  Service  (ACTS).
       It  periodically	 dials	a  prespecified telephone number, receives the
       NIST timecode data and  calculates  the	local  clock  correction.   It
       designed	 for use when neither a radio clock nor connectivity to Inter‐
       net time servers is available.  For the best accuracy,  the  individual
       telephone  line/modem  delay  needs  to	be  calibrated	using  outside

       The ACTS is located at NIST Boulder,  CO,  telephone  303-494-4774.   A
       toll call from Newark, DE, costs between three and four cents, although
       it is not clear what carrier and time  of  day  discounts  apply.   The
       modem  dial  string will differ depending on local telephone configura‐
       tion, and is specified by the phone command in the configuration	 file.
       The  argument  to  this command is an AT command for a Hayes compatible

       The accuracy produced by this driver should be in the range of  a  mil‐
       lisecond	 or two, but may need correction due to the delay characteris‐
       tics of the individual modem involved.  For undetermined reasons,  some
       modems  work  with  the	ACTS echo-delay measurement scheme and some do
       not.  This driver tries to do the best it can with what it  gets.  Ini‐
       tial  experiments with a Practical Peripherals 9600SA modem in Delaware
       suggest an accuracy of a millisecond or two can be achieved without the
       scheme  by  using  a fudge time1 value of 65.0 ms.  In either case, the
       dispersion for a single call involving ten samples is about 1.3 ms.

       The driver can operate in either of three modes, as determined  by  the
       mode  parameter	in the server configuration command.  In mode 0 (auto‐
       matic), the driver operates continuously at intervals depending on  the
       prediction  error,  as  measured by the driver, usually in the order of
       several hours.  In mode 1, (backup) the driver is enabled in  automatic
       mode only when no other source of synchronization is available and when
       more than MAXOUTAGE (3600 s) have elapsed since	last  synchronized  by
       other  sources.	 In  mode  2,  (manual)	 the driver operates only when
       enabled using a fudge flags switch (see Fudge Factors).

       For reliable call management, this driver  requires  a  1200-bps	 modem
       with  a	Hayes-compatible  command  set and control over the modem data
       terminal ready (DTR) control line. Present restrictions require the use
       of  a POSIX-compatible programming interface, although other interfaces
       may work as well. The ACTS telephone number and modem setup string  are
       hard-coded in the driver and may require changes for nonstandard modems
       or special circumstances.

       Fudge Factors

       Ordinarily, the propagation time correction is  computed	 automatically
       by  ACTS	 and  the  driver. When this is not possible or erratic due to
       individual modem characteristics, the fudge flag2 switch should be  set
       to  disable  the	 ACTS  echo-delay scheme. In any case, the fudge time1
       parameter can be used to adjust the propagation delay as required.

       The ACTS call interval is determined in one the following ways: In man‐
       ual  mode,  a  call  is	initiated by setting fudge flag1 using xntpdc,
       either manually or by a cron job.  In automatic mode, this flag is  set
       by  the	peer  timer,  which  is controlled by the sys_poll variable in
       response to measured errors.  In backup mode, the driver is  ordinarily
       asleep,	but  awakes  (in  automatic mode) if all other synchronization
       sources are lost.

       In either automatic or backup modes, the	 call  interval	 increases  as
       long  as the measured errors do not exceed the value of the fudge time2

       When the fudge flag1 is set, the ACTS  calling  program	is  activated.
       This program dials each number listed in the phones command of the con‐
       figuration file in turn. If a call attempt fails, the  next  number  in
       the list is dialed. The fudge flag1 and counter are reset and the call‐
       ing program terminated if a valid clock update has been determined,  no
       more  numbers  remain in the list, a device fault or timeout occurs, or
       fudge flag1 is reset manually using xntpdc.

       The NIST timecode message is transmitted at 1200 bps in	the  following
       format: jjjjj yy-mm-dd hh:mm:ss tt l uuu mmmmm UTC(NIST) *

       In the previous messages: Denotes the modified Julian day.  Denotes the
       year, month, and day.  Denotes the hours, minutes, and seconds.	Is the
       DST  indicator  (see  driver listing).  Is the leap-second warning (see
       driver  listing).   Denotes  DUT1  correction  (see  driver   listing).
       Denotes	modem  calibration  (see  driver listing).  Denotes an on-time

       The timecode message is transmitted continuously after a signon banner,
       which  this driver ignores.  The driver also ignores all but the yy-mm-
       dd, hh:mm:ss and on-time character (*) fields, although it  checks  the
       format of all fields of the message.  A time stamp is captured at the *
       character, as required by the ACTS specification, and used as the  ref‐
       erence time of the timecode.  If a message with an on-time character of
       # is received, the driver updates the propagation  delay.   The	driver
       disconnects  when ten valid messages have been received, no message has
       been received for 15 seconds, or an # on-time  character	 is  received.
       These  messages are processed by a trimmed-mean filter to reduce timing
       noise and then by the usual NTP algorithms to develop the clock correc‐

       The  behavior  of  the  clock selection algorithm is modified when this
       driver is in use.  The algorithm is designed so that this  driver  will
       never be selected unless no other discipline source is available.  This
       can be overridden with the prefer keyword of the	 server	 configuration
       command,	 in  which case only this driver will be selected for synchro‐
       nization and all other discipline sources will be ignored.  Ordinarily,
       the  prefer keyword is used only in automatic mode when primary time is
       to be obtained through ACTS, and backup NTP peers used only  when  ACTS

       Call Management

       Since ACTS is a toll call in most areas of the country, it is necessary
       to carefully manage the calling interval.  The  ACTS  call  program  is
       initiated  by setting fudge flag1.  This flag can be set manually using
       xntpdc, by a cron job that calls xntpdc, or automatically by the driver
       itself.	 The  fudge flag1 is reset when the program terminates after a
       time determination is complete or when no more numbers  remain  in  the
       alternate  path	list;  a  device fault or timeout has occurred; or the
       fudge flag1 has been reset using xntpdc.

       In automatic and backup modes, the driver determines the call  interval
       using  a	 procedure  depending on the measured prediction error and the
       fudge time2 parameter.  If the error exceeds  time2  for	 a  number  of
       times depending on the current interval, the interval is decreased, but
       not less than about 1000 seconds.  If the error is less than time2  for
       some  number  of	 times,	 the  interval is increased, but not more than
       about 18 hours.	With the default value of zero for  fudge  time2,  the
       interval	 increases from 1000 seconds to the 4000-8000 second range, in
       which the expected accuracy should be in the  1-2  ms  range.   Setting
       fudge  time2 to a large value, like 0.1 second, may result in errors of
       that order, but increase the call interval to the maximum.   The	 exact
       value  for each configuration depends on the modem and operating system
       involved; you might have to experiment.

       Manual call attempts can be made at any time  by	 setting  fudge	 flag1
       using  xntpdc. For example, the following xntpdc command asks for a key
       identifier and password and,  if	 authenticated	by  the	 server,  sets
       flag1: fudge flags 1

       There  might  be a short delay until the expiration of the current poll

       The flag1 can be set from a cron job using the following steps:	Create
       a  file	with  the  following  contents:	 keyid	11 passwd dialup fudge flags 1 quit Run the following program at specified	 times
       as required: /usr/local/bin/xntpdc file

       Default	name  of the configuration file Conventional name of the drift
       file Conventional name of the key file

       Commands: ntp(1), ntpdate(8), ntpq(8), xntpd(8), xntpdc(8)

       Files: ntp.keys(4)

       Network Administration: Services delim off


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