-General references for this configuration file are:
-
-- "Raspberry Pi as a Stratum-1 NTP Server - Updating to the final NTP
- configuration file" by David Taylor. [[http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html#ntp-conf][Link]]. Date: 2020-03-24. Note:
- An example of kernel PPS transfer from ~gpsd~ to ~ntp~ via
- ~127.127.22.0~.
-
-- "GPSD Time Service HOWTO" by Garry E. Miller. [[https://gpsd.gitlab.io/gpsd/gpsd-time-service-howto.html][Link]]. Date:
- 2020-06-28. Note: Explains how shared memory segments available at
- ~127.127.28.0~, ~127.127.28.1~, ~127.127.28.2~, etc. work in context
- of an ~ntp~ configuration file.
-
-- "Building a Stratum 1 NTP Server with a Raspberry Pi 4 and Adafruit
- Ultimate GPS Hat". [[http://www.gregledet.net/computers/building-a-stratum-1-ntp-server-with-a-raspberry-pi-4-and-adafruit-ultimate-gps-hat/][Link]]. Date: 2020-02-23. Note: A concise example
- between ~gpsd~ and ~ntp~ setup for a Raspberry Pi 4 using
- ~127.127.28.0~ and ~127.127.28.1~. Recommends use of ~ntpsec~
- instead of ~ntp~.
-
-Expalantions of some ~ntpsec~ [[https://docs.ntpsec.org/latest/comdex.html][commands and options]] used in the
-configuration file include:
-
-- ~refclock~ : An ~ntpsec~ keyword not present in ~ntp-4.2.8~ that is
- meant to simplify configuration of reference clocks that use certain
- [[https://docs.ntpsec.org/latest/refclock.html][drivers]] including ~pps~ ([[https://docs.ntpsec.org/latest/driver_pps.html][PPS Clock Discipline]]), ~shm~ ([[https://docs.ntpsec.org/latest/driver_shm.html][Shared Memory
- Driver]]) . In ~ntp-4.2.8~ and earlier, in order to configure a clock,
- instead of ~ntpsec~'s one keyword (~refclock~), two keywords would
- be required (~server~ and ~fudge~).
-
-- ~server [address]~ : Tells ~ntpsec~ the address to query for time
- information. May be IP address or URL. Additional options such as
- ~minpoll~, ~maxpoll~, ~prefer~, and ~true~ may be added.
-
-- ~prefer~ : Indicates the source should be prioritized in some
- way. How exactly the ~ntpsec~ mitigation rules (see [[https://docs.ntpsec.org/latest/prefer.html][ref]]) use the
- ~prefer~ option is complicated but in the context of dealing with a
- GPS source and a PPS source, the GPS source should have a ~prefer~
- option. The PPS source may have a ~prefer~ option but it is not
- necessary; adding ~prefer~ to a PPS source is useful in the corner
- case that the GPS source is less reliable than another source (ex: a
- remote clock specified by ~server~).
-
-- ~... shm ... flag1 1~ : For an ~shm~ driver, tells ~ntpsec~ to
- ignore the default sanity check that discards any time that is more
- than 4-hours off from the system clock. Note that ~ntpsec~ treats
- ~shm flag1 1~ in the opposite manner as ~ntp~ (see [[https://docs.ntpsec.org/latest/driver_shm.html][ref]]).
-
-- ~... shm ... time1 [float]~ : Tells ~ntpsec~ that a latency
- time of ~[float]~ seconds is required for data to travel from the
- address to ~ntpsec~ (ex: a USB hub in the case of a GPS device that
- connects via USB). (see [[https://gpsd.gitlab.io/gpsd/gpsd-time-service-howto.html#_feeding_ntpd_from_gpsd][ref]]) This number of seconds should be
- adjusted to reduce the `offset` time of the address indicated in the
- output of the ~ntpq -pn~ command to near ~0.000~. Note that the
- units of ~offset~ displayed by ~ntpq -pn~ are in milliseconds.
-
-- ~... refid [string]~ : Tells ~ntpsec~ a string used to label the source.
-
-- ~fudge~ : An ~ntp~ keyword that ~ntpsec~ developers do not recommend
- using; ~ntpsec~ documentation for ~fudge~ is widthdrawn. For local
- hardware providing GPS and PPS time information, ~fudge~ can mostly
- be replaced with use of the ~refclock~ keyword instead.
-
-- ~fudge [address]~ : Tells ~ntpsec~ to consider additional options
- when evaluating the accuracy of time from a given address.
-
-- ~fudge [address] ... flag3 1~ : An option that tells ~ntpsec~ to
- enable "kernel PPS discipline" when interpreting the PPS signal at
- the address. (see [[http://doc.ntp.org/4.2.8p7/drivers/driver22.html][ref]]).
-
-- ~stratum 1~ : Tells ~ntpsec~ to label the source as a "stratum 1"
- time server. This means that it is a clock using NTP that is a
- canonical (?) time source. For example, a stratum 0 NTP time server
- would be an atomic clock. A stratum 1 NTP time server could be a
- clock that uses a GPS receiver to synchronize to said atomic clock.
-
-- ~iburst~ : Tells ~ntpsec~ how often to query a source.
-
-- ~fudge [address] ... flag1 [float]~ : See description of ~... shm
- ... time1 [float]~.
-
-- ~tos mindist [float]~ : Tells ~ntpsec~ to increase the tolerance for
- outliers in the clock selection algorithm. Units of ~[float]~ are
- seconds. The default value is ~0.001~ seconds. This [[http://doc.ntp.org/4.2.6/miscopt.html][reference]]
- indicates that it is acceptable to raise this value for "clocks with
- high jitter and a PPS signal". For the device being configured in
- this guide, this is the case. ~ntpsec~ receives a kernel PPS signal
- on ~127.127.22.0~ indicating when each second occurs but without
- information about which second of the day is being ticked. ~ntpsec~
- receives a coarse time from ~127.127.28.0~ supplied by ~gpsd~ that
- is derived from the NMEA sentences generated by the GPS receiver;
- this coarse time (high jitter) lacks the precision of the PPS signal
- since NMEA sentence transmission times are not well-controlled.
-
-****** Disable DHCP services affecting ~ntpsec~
-Some processes that use [[https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol][DHCP]] (Dynamic Host Configuration Protocol) may
-cause configuration changes to configuration files of ~ntp~. Since
-~ntpsec~ is similar to ~ntp~, it is reasonable to take action to
-prevent this.
-
-Remove some files:
-
-: $ sudo rm /etc/dhcp/dhclient-exit-hooks.d/ntp
-: $ sudo rm /lib/dhcpcd/dhcpcd-hooks/50-ntp.conf
-
-In the "request" block of ~dhclient.conf~, remove ~dhcp6.sntp-servers~
-and ~ntp-servers~.
-
-: $ sudo nano /etc/dhcp/dhclient.conf
-
-For example, the following section of ~dhclient.conf~:
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