1ntp_clock(5)                  File Formats Manual                 ntp_clock(5)
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NAME

6       ntp_clock - Reference Clock Options
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REFERENCE CLOCK SUPPORT

10       The  NTP  Version  4  daemon supports some three dozen different radio,
11       satellite and modem reference clocks plus a special  pseudo-clock  used
12       for  backup  or  when  no  other  clock  source  is available. Detailed
13       descriptions of individual device drivers and options can be  found  in
14       the  Reference  Clock Drivers page. Additional information can be found
15       in the pages linked there, including the Debugging Hints for  Reference
16       Clock Drivers and How To Write a Reference Clock Driver pages. In addi‐
17       tion, support for a PPS signal is available as described in  Pulse-per-
18       second (PPS) Signal Interfacing page. Many drivers support special line
19       discipline/streams modules which can significantly improve the accuracy
20       using  the  driver.  These  are  described  in the Line Disciplines and
21       Streams Drivers page.
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23       A reference clock will generally (though not always) be a  radio  time‐
24       code  receiver  which is synchronized to a source of standard time such
25       as the services offered by the NRC in Canada and NIST and USNO  in  the
26       US.  The  interface  between  the computer and the timecode receiver is
27       device dependent, but is usually a serial port. A  device  driver  spe‐
28       cific to each reference clock must be selected and compiled in the dis‐
29       tribution; however, most common radio, satellite and modem  clocks  are
30       included  by  default.  Note  that  an attempt to configure a reference
31       clock when the driver has not been compiled or the  hardware  port  has
32       not  been  appropriately configured results in a scalding remark to the
33       system log file, but is otherwise non hazardous.
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35       For the purposes of configuration, ntpd treats reference  clocks  in  a
36       manner  analogous  to  normal  NTP peers as much as possible. Reference
37       clocks are  identified  by  a  syntactically  correct  but  invalid  IP
38       address,  in order to distinguish them from normal NTP peers. Reference
39       clock addresses are of the form 127.127.t.u,  where  t  is  an  integer
40       denoting  the  clock  type and u indicates the unit number in the range
41       0-3. While it may seem overkill, it is in fact sometimes useful to con‐
42       figure  multiple  reference  clocks of the same type, in which case the
43       unit numbers must be unique.
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45       The server command is used to configure a reference  clock,  where  the
46       address argument in that command is the clock address. The key, version
47       and ttl options are not used for  reference  clock  support.  The  mode
48       option  is  added  for reference clock support, as described below. The
49       prefer option can be useful to persuade the server to cherish a  refer‐
50       ence clock with somewhat more enthusiasm than other reference clocks or
51       peers. Further information on this option can be found in  the  Mitiga‐
52       tion Rules and the prefer Keyword page. The minpoll and maxpoll options
53       have meaning only for selected clock drivers. See the individual  clock
54       driver document pages for additional information.
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56       The  fudge  command is used to provide additional information for indi‐
57       vidual clock drivers and normally follows immediately after the  server
58       command.  The  address  argument specifies the clock address. The refid
59       and stratum options control can be used to override  the  defaults  for
60       the  device.  There  are two optional device-dependent time offsets and
61       four flags that can be included in the fudge command as well.
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63       The stratum number of a reference clock is by default zero.  Since  the
64       ntpd  daemon  adds  one  to  the stratum of each peer, a primary server
65       ordinarily displays an external stratum of one.  In  order  to  provide
66       engineered  backups,  it is often useful to specify the reference clock
67       stratum as greater than zero. The stratum option is used for this  pur‐
68       pose.  Also, in cases involving both a reference clock and a pulse-per-
69       second (PPS) discipline signal, it is useful to specify  the  reference
70       clock  identifier  as  other than the default, depending on the driver.
71       The refid option is used for this purpose. Except  where  noted,  these
72       options apply to all clock drivers.
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REFERENCE CLOCK COMMANDS

76       server 127.127.t.u [prefer] [mode int] [minpoll int] [maxpoll int]
77               This  command can be used to configure reference clocks in spe‐
78               cial ways. The options are interpreted as follows:
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80               prefer  Marks the  reference  clock  as  preferred.  All  other
81                       things  being  equal, this host will be chosen for syn‐
82                       chronization among a set of correctly operating  hosts.
83                       See  the  Mitigation  Rules and the prefer Keyword page
84                       for further information.
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86               mode int
87                       Specifies a mode  number  which  is  interpreted  in  a
88                       device-specific  fashion.  For  instance,  it selects a
89                       dialing protocol in the ACTS driver and a  device  sub‐
90                       type in the parse drivers.
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92               minpoll int
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94               maxpoll int
95                       These  options  specify the minimum and maximum polling
96                       interval  for  reference  clock  messages  in  seconds,
97                       interpreted  as  dual  logarithms  (2  ^  x).  For most
98                       directly connected reference clocks, both  minpoll  and
99                       maxpoll default to 6 (2^16 = 64 s). For modem reference
100                       clocks, minpoll defaults to 10 (2^10 = 1024 s = 17.1 m)
101                       and  maxpoll  defaults  to 14 (2^14 = 16384 s = 4.5 h).
102                       The allowable range is 4 (16 s) to 17 (36.4  h)  inclu‐
103                       sive.
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106       fudge  127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string]
107       [mode int] [flag1 0|1] [flag2 0|1] [flag3 0|1] [flag4 0|1]
108               This command can be used to configure reference clocks in  spe‐
109               cial  ways. It must immediately follow the server command which
110               configures the driver. Note that the same capability is  possi‐
111               ble at run time using the ntpdc program. The options are inter‐
112               preted as follows:
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114               time1 sec
115                       Specifies a constant to be added  to  the  time  offset
116                       produced by the driver, a fixed-point decimal number in
117                       seconds. This is used  as  a  calibration  constant  to
118                       adjust the nominal time offset of a particular clock to
119                       agree with an external standard, such  as  a  precision
120                       PPS signal. It also provides a way to correct a system‐
121                       atic error or bias due to serial port or operating sys‐
122                       tem  latencies,  different  cable  lengths  or receiver
123                       internal delay. The specified offset is in addition  to
124                       the  propagation delay provided by other means, such as
125                       internal DIPswitches. Where a calibration for an  indi‐
126                       vidual  system  and driver is available, an approximate
127                       correction is noted in the driver documentation  pages.
128                       Note: in order to facilitate calibration when more than
129                       one radio clock or PPS signal is supported,  a  special
130                       calibration  feature is available. It takes the form of
131                       an argument to the enable command described in the Mis‐
132                       cellaneous  Options  page  and operates as described in
133                       the Reference Clock Drivers page.
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135               time2 secs
136                       Specifies a  fixed-point  decimal  number  in  seconds,
137                       which is interpreted in a driver-dependent way. See the
138                       descriptions of specific drivers in the reference clock
139                       drivers page.
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141               stratum int
142                       Specifies the stratum number assigned to the driver, an
143                       integer between 0 and 15.  This  number  overrides  the
144                       default  stratum  number  ordinarily  assigned  by  the
145                       driver itself, usually zero.
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147               refid string
148                       Specifies an ASCII string of from one to  four  charac‐
149                       ters which defines the reference identifier used by the
150                       driver. This string overrides  the  default  identifier
151                       ordinarily assigned by the driver itself.
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153               mode int
154                       Specifies  a  mode  number  which  is  interpreted in a
155                       device-specific fashion. For  instance,  it  selects  a
156                       dialing  protocol  in the ACTS driver and a device sub‐
157                       type in the parse drivers.
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159               flag1 flag2 flag3 flag4
160                       These four flags are used  for  customizing  the  clock
161                       driver. The interpretation of these values, and whether
162                       they are used at all, is a function of  the  particular
163                       clock  driver.  However, by convention flag4 is used to
164                       enable recording monitoring data to the clockstats file
165                       configured  with  the filegen command. Further informa‐
166                       tion on the filegen command can be found in  the  Moni‐
167                       toring Options page.
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SEE ALSO

172       ntp.conf(5)
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174       Primary source of documentation: /usr/share/doc/ntp-*
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176       This file was automatically generated from HTML source.
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