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公司先后采购过多种GPS授时服务器。之前一直不是太明白ntpd是如何使用外接的这些硬件。
这两年树莓派比较流行，也有很大人利用外接的扩展件让树莓派成为一级时钟源。其中关键的ntp.conf配置如下

driftfile /var/lib/ntp/ntp.drift



# coarse time ref-clock, not really needed here as we have LAN & WAN servers

server 127.127.28.0  minpoll 4 maxpoll 4

fudge 127.127.28.0 time1 +0.350 refid GPS  stratum 15



# Kernel-mode PPS ref-clock for the precise seconds

server 127.127.22.0 minpoll 4 maxpoll 4

fudge 127.127.22.0  refid PPS



# LAN servers

server 192.168.0.3  minpoll 5 maxpoll 5 iburst prefer

server 192.168.0.2  minpoll 5 maxpoll 5 iburst

server 192.168.0.7  minpoll 5 maxpoll 5 iburst

对比了公司采购的GPS授时服务器的配置

NTP-SERVER:/#ntpq -pn

     remote           refid      st t when poll reach   delay   offset  jitter

==============================================================================

*127.127.20.1    .GPS.            0 l   16   16  377    0.000    0.002   0.001

 127.127.1.0     .LCK.            0 l    -   16  377    0.000    0.000   0.001

NTP-SERVER:/#cat /etc/ntp.conf|grep -v "^#"|grep '[a-z]'

server  127.127.20.1 prefer mode 17 minpoll 4

fudge   127.127.20.1 stratum 0 refid GPS flag2 1 flag3 1

server  127.127.1.0 minpoll 4   maxpoll 4

fudge   127.127.1.0 stratum 0 refid LCK

driftfile /var/drift

broadcastdelay  0.008

authenticate yes

keys        /etc/ntp/keys

logconfig =clockevents +syncevents +sysevents +clockall +syncall +sysall

server和fudge的相关说明如下

       The  fudge  command  is  used  to provide additional information for individual clock drivers and normally follows immediately after the server command. The address

       argument specifies the clock address. The refid and stratum options control can be used to override the defaults for the device.  There  are  two  optional  device-

       dependent time offsets and four flags that can be included in the fudge command as well.

       The  stratum  number  of  a  reference clock is by default zero. Since the ntpd daemon adds one to the stratum of each peer, a primary server ordinarily displays an

       external stratum of one. In order to provide engineered backups, it is often useful to specify the reference clock stratum as greater than zero. The stratum  option

       is  used  for  this  purpose.  Also, in cases involving both a reference clock and a pulse-per-second (PPS) discipline signal, it is useful to specify the reference

       clock identifier as other than the default, depending on the driver. The refid option is used for this purpose. Except where noted, these options apply to all clock

       drivers.

REFERENCE CLOCK COMMANDS

       server 127.127.t.u [prefer] [mode int] [minpoll int] [maxpoll int]

               This command can be used to configure reference clocks in special ways. The options are interpreted as follows:

               prefer  Marks  the reference clock as preferred. All other things being equal, this host will be chosen for synchronization among a set of correctly operat‐

                       ing hosts. See the Mitigation Rules and the prefer Keyword page for further information.

               mode int

                       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.

               minpoll int

               maxpoll int

                       These  options specify the minimum and maximum polling interval for reference clock messages in seconds, interpreted as dual logarithms (2 ^ x). For

                       most directly connected reference clocks, both minpoll and maxpoll default to 6 (2^16 = 64 s). For modem reference clocks, minpoll  defaults  to  10

                       (2^10 = 1024 s = 17.1 m) and maxpoll defaults to 14 (2^14 = 16384 s = 4.5 h). The allowable range is 4 (16 s) to 17 (36.4 h) inclusive.

       fudge 127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string] [flag1 0|1] [flag2 0|1] [flag3 0|1] [flag4 0|1]

               This command can be used to configure reference clocks in special ways. It must immediately follow the server command which configures the driver. Note that

               the same capability is possible at run time using the ntpdc program. The options are interpreted as follows:

               time1 sec

                       Specifies a constant to be added to the time offset produced by the driver, a fixed-point decimal number in seconds. This is used as  a  calibration

                       constant  to  adjust  the nominal time offset of a particular clock to agree with an external standard, such as a precision PPS signal. It also pro‐

                       vides a way to correct a systematic error or bias due to serial port or operating system latencies, different cable  lengths  or  receiver  internal

                       delay.  The  specified offset is in addition to the propagation delay provided by other means, such as internal DIPswitches. Where a calibration for

                       an individual system and driver is available, an approximate correction is noted in the driver documentation pages.  Note: in  order  to  facilitate

                       calibration  when more than one radio clock or PPS signal is supported, a special calibration feature is available. It takes the form of an argument

                       to the enable command described in the Miscellaneous Options page and operates as described in the Reference Clock Drivers page.

               time2 secs

                       Specifies a fixed-point decimal number in seconds, which is interpreted in a driver-dependent way. See the descriptions of specific drivers  in  the

                       reference clock drivers page.

               stratum int

                       Specifies  the  stratum  number  assigned  to  the  driver, an integer between 0 and 15. This number overrides the default stratum number ordinarily

                       assigned by the driver itself, usually zero.

               refid string

                       Specifies an ASCII string of from one to four characters which defines the reference identifier used  by  the  driver.  This  string  overrides  the

                       default identifier ordinarily assigned by the driver itself.

               flag1 flag2 flag3 flag4

                       These  four  flags are used for customizing the clock driver. The interpretation of these values, and whether they are used at all, is a function of

                       the particular clock driver. However, by convention flag4 is used to enable recording monitoring data to the clockstats  file  configured  with  the

                       filegen command. Further information on the filegen command can be found in the Monitoring Options page.

因此127.127.20.1实际是代表了type 20的时钟类型，1是代表单元号。当前ntpd支持的驱动如下