1ntp_clock(5) File Formats Manual ntp_clock(5)
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6 ntp_clock - Reference Clock Options
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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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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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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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181 ntp_clock(5)