doc/time/README.org

   1 #+TITLE: Ninfacyzga-1 Time Tracking
   2 #+AUTHOR: Steven Baltakatei Sandoval
   3 #+EMAIL: baltakatei@gmail.com
   4 * Time Tracking
   5 ** About
   6 This document was created by Steven Baltakatei Sandoval on
   7 ~2020-07-23T22:27Z~ under a [[https://creativecommons.org/licenses/by-sa/4.0/][Creative Commons BY-SA 4.0 license]]. It
   8 was updated by Steven Baltakatei Sandoval on ~2020-10-16T22:26Z~
   9
  10 ** Narrative
  11 The ~ninfacyzga-01~ device is equipped with an Ozzmaker BerryGPS-IMU
  12 module which provides time and location data to ~gpsd~ and ~ntpsec~. The
  13 time is provided by GPS satellites which themselves are
  14 equipped [fn:nasa_20020408_atomicclock] with atomic clocks. This
  15 extremely accurate set of clocks are needed since a GPS receiver
  16 calculates its position in space using a General Relativity
  17 calculation that uses the small variations in the time stamps received
  18 from each satellite. This means that ~gpsd~ may be used to set the
  19 system clock without a need for an internet connection to a default
  20 Debian time server; ~ninfacyzga-01~ can be its own time server.
  21
  22 [fn:nasa_20020408_atomicclock] Title:[[https://science.nasa.gov/science-news/science-at-nasa/2002/08apr_atomicclock/][Tick-Tock Atomic Clock]];
  23 Date:2002-04-08; Website:NASA.gov; [[https://web.archive.org/web/20100429141752/http://science.nasa.gov/science-news/science-at-nasa/2002/08apr_atomicclock/][Archive-link]]; Archive-date:
  24 2010-04-29
  25
  26 ** Description
  27 *** Hardware
  28 Ozzmaker BerryGPS-IMU, Version 3 (see [[https://ozzmaker.com/berrygps-berrygps-imu-quick-start-guide/][ref]]).
  29 *** Software
  30 - [[https://tracker.debian.org/pkg/gpsd][~gpsd~]] : A background daemon app capable of interfacing with the
  31   [[https://ozzmaker.com/berrygps-berrygps-imu-quick-start-guide/][Ozzmaker BerryGPS-IMU]]'s GPS submodule. Installed and initialized by
  32   ~apt~. Should be installed along with the ~gpsd-clients~
  33   package. This procedure was developed with ~gpsd~ version
  34
  35 - [[https://tracker.debian.org/pkg/ntpsec][~ntpsec~]] : A security-hardned version of [[https://tracker.debian.org/pkg/ntp][~ntp~]] which is a "[[https://en.wikipedia.org/wiki/Network_Time_Protocol][Network
  36   Time Protocol]]" app that synchronizes the system clock with local or
  37   remote time servers or devices. It is capable of extracting time
  38   data from a GPS signal handled by ~gpsd~. It is also capable of
  39   using the high precision PPS (pulse-per-second) signal handled by
  40   ~gpsd~. Installed via ~apt~. This procedure was developed with
  41   ~ntpsec~ version ~1.1.3~.
  42
  43 ** Operating Procedures
  44 *** Initial Startup
  45 **** Perform initial setup.
  46 See [[file:../setup/README.org][Main Setup]] procedure.
  47 **** Install Hardware for time tracking
  48 See [[https://ozzmaker.com/forums/topic/connecting-gps-pps-pin/][this]] Ozzmaker forum topic about connecting the BerryGPS-IMU
  49 ~T_PULSE~ pin to GPIO 18.
  50
  51 #+CAPTION: An image showing how to connect the PPS signal from an Ozzmaker BerryGPS-IMU board to a Raspberry Pi Zero W.
  52 #+NAME: fig:PPS_BERRYGPS_RASPIZW
  53 [[../../img/Compact_Stratum_1_NTP_time_server_hardware,_October_2020.jpg]]
  54
  55 Connect the ~T_PULSE~ connection on the BerryGPS-IMU-3 to GPIO pin 18
  56 (ex: with solder and wire) in order to provide the PPS data signal
  57 generated by the BerryGPS-IMU to the Raspberry Pi. Processing of this
  58 data signal is handled by adding a line to ~/boot/config.txt~ in the
  59 next section ("Install Software").
  60
  61 Note: If it is desired to specify a custom GPIO pin besides the one
  62 recommended, see this [[https://raspberryautomation.com/connect-multiple-ds18b20-temperature-sensors-to-a-raspberry-pi/][Raspberry Autom]] article.
  63
  64 **** Install Software for time tracking
  65 The time tracking function can be performed by two programs: ~gpsd~
  66 and ~ntpsec~.
  67
  68 Basically, two things need to happen:
  69
  70 1. ~gpsd~ needs to be pointed towards the correct device files for
  71    incoming GPS data (in NMEA format) and the PPS signal ("pulse per
  72    second"; a high precision time signal).
  73
  74 2. ~ntpsec~ needs to be pointed towards the correct local IP addresses
  75    where ~gpsd~ provides GPS data and the PPS signal.
  76
  77 ~gpsd~ then will provide GPS and PPS data to ~ntpsec~ via a "shared
  78 memory driver".
  79
  80 ***** Install packages via ~apt~
  81 Run the following command to install the required packages.
  82 : $ sudo apt install gpsd gpsd-clients python-gps pps-tools ntpsec
  83
  84 ***** Enable PPS device
  85 Modify the ~/boot/config.txt~ file in order to tell the Raspberry Pi
  86 to expect PPS data on ~BCM 18~ (pin number 12; see [[https://pinout.xyz/][link]]). This is done
  87 by adding the following line to ~/boot/config.txt~ as described on
  88 [[https://ozzmaker.com/forums/topic/problems-with-pps-on-a-pi0w-running-raspian-and-attached-to-a-berrygps-imuv3/][this Ozzmaker page]]:
  89
  90 : dtoverlay=pps-gpio,gpiopin=18
  91
  92 The ~/boot/config.txt~ file can be modified via:
  93
  94 : $ sudo nano /boot/config.txt
  95
  96 PPS data can be confirmed by running:
  97
  98 #+BEGIN_EXAMPLE
  99 $ sudo su -
 100 # ppstest /dev/pps0
 101 trying PPS source "/dev/pps0"
 102 found PPS source "/dev/pps0"
 103 ok, found 1 source(s), now start fetching data...
 104 source 0 - assert 1595708074.003644641, sequence: 219 - clear  0.000000000, sequence: 0
 105 source 0 - assert 1595708075.003709620, sequence: 220 - clear  0.000000000, sequence: 0
 106 source 0 - assert 1595708076.003779580, sequence: 221 - clear  0.000000000, sequence: 0
 107 source 0 - assert 1595708077.003850580, sequence: 222 - clear  0.000000000, sequence: 0
 108 #+END_EXAMPLE
 109
 110 Note: For older Raspberry Pi models, it may be necessary to enable
 111 ~pps-gpio~ via modifications to ~/etc/modules~ (see [[https://www.raspberrypi.org/forums/viewtopic.php?p=757747#p757747][link]]).
 112
 113 ***** Enable GPS device
 114 The Ozzmaker BerryGPS-IMU makes NMEA sentences available via the
 115 serial "UART" device ~/dev/ttyAMA0~. If bluetooth has not been
 116 disabled, the Raspberry Pi OS automatically creates a software "UART"
 117 device at ~/dev/serial0~. See the "[[file:~/git-OC/ninfacyzga-01/doc/setup/README.org::*Disable%20Bluetooth][Disable Bluetooth]]" section in the
 118 [[file:../setup/README.org][Main Setup]] Initial Startup procedure for instructions on how to
 119 disable bluetooth to free up ~/dev/ttyAMA0~ for use by ~gpsd~.
 120
 121 Note: Older ~ntp~ documentation indicates that ~ntp~ (and presumably
 122 ~ntpsec~ might be able to access GPS data (ex: NMEA sentences) without
 123 the aid of ~gpsd~ itself if the data is available at a device named
 124 ~/dev/ggp0~. Similarly, the same might apply to PPS data and the
 125 ~/dev/gpspps0~ device. However, since the Raspberry Pi OS overlay
 126 system automatically creates ~/dev/pps0~ using the modifications to
 127 ~/boot/config.txt~ (described [[*Enable PPS device][elsewhere]] in this procedure).
 128
 129 ***** Setup ~gpsd~
 130 See the "[[file:~/git-OC/ninfacyzga-01/doc/location/README.org::*Setup%20~gpsd~][Setup ~gpsd~]]" subsection within the "Initial Startup" section
 131 of the Location Logging [[file:~/git-OC/ninfacyzga-01/doc/location/README.org][~README.org~]] file. There is one additional
 132 change that must be made which is to add a ~/dev/pps0~ item to the
 133 ~DEVICES=~ line in ~/etc/default/gpsd~ like so:
 134
 135 : DEVICES="/dev/ttyAMA0 /dev/pps0"
 136
 137 ~/dev/ttyAMA0~ is where ~gpsd~ can get NMEA data from the GPS unit.
 138
 139 ~/dev/pps0~ is where ~gpsd~ can get a PPS signal.
 140
 141 As an example, the following lines will be present in
 142 ~/etc/default/gpsd~ if both location and time tracking are set up:
 143
 144 #+BEGIN_EXAMPLE
 145 START_DAEMON="true"
 146 USBAUTO="false"
 147 DEVICES="/dev/ttyAMA0 /dev/pps0"
 148 GPSD_OPTIONS="-n"
 149 #+END_EXAMPLE
 150
 151 ***** Setup ~ntpsec~
 152 The ~ntpsec~ configuration file at ~/etc/ntpsec/ntp.conf~ must be
 153 modified to tell ~ntpsec~ how to retrieve and interpret the GPS and
 154 PPS data provided by ~gpsd~.
 155
 156 In summary, lines need to be added telling ~ntpsec~ how often to poll
 157 certain sources (local or remote) for time information as well as how
 158 to prioritize and label information from each source.
 159
 160 ~ntpsec~ uses syntax simplified from that of ~ntp~. Specifically, it
 161 recommends use of the ~refclock~ keyword for configuring local
 162 hardware clocks such as GPS and PPS devices.
 163
 164 Below is an example of lines that may be added to
 165 ~/etc/ntpsec/ntp.conf~ in order to ensure that the system clock will always have
 166
 167 #+BEGIN_EXAMPLE
 168 # Local time sources
 169
 170 # Kernel PPS (precision seconds; driver 22)
 171 refclock pps unit 0 refid kPPS flag3 1 minpoll 4 maxpoll 4 stratum 1
 172
 173 # GPS PPS reference (precision seconds; driver 28; root) (NTP1)
 174 refclock shm unit 1 refid PPS minpoll 4 maxpoll 4 iburst stratum 1
 175
 176 # GPS Serial data reference (coarse time; driver28; root) (NTP0)
 177 refclock shm unit 0 refid GPS prefer time1 0.109 minpoll 4 maxpoll 4 iburst stratum 1
 178
 179 # GPS PPS reference (precision seconds; driver 28; user) (NTP2)
 180 refclock shm unit 2 refid PPSuser minpoll 4 maxpoll 4 iburst stratum 1
 181
 182 # Tolerate outliers (useful for coarse clocks)
 183 tos mindist 0.500
 184 #+END_EXAMPLE
 185
 186 A complete working example of an ~ntp.conf~ file is found in Appendix
 187 A of this document.
 188
 189 Commands useful for examining the shared memory driver used by ~gpsd~
 190 and ~ntpsec~ are:
 191
 192 - ~sudo ntpshmmon~ : Shows live output of data using the shared memory
 193   driver. ([[https://gpsd.gitlab.io/gpsd/gpsd-time-service-howto.html][ref]])
 194
 195 - ~sudo ipcs -m~ : Show live segments of the shared memory. ([[https://gpsd.gitlab.io/gpsd/gpsd-time-service-howto.html][ref]])
 196
 197 General references for this configuration file are:
 198
 199 - "Raspberry Pi as a Stratum-1 NTP Server - Updating to the final NTP
 200   configuration file" by David Taylor. [[http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html#ntp-conf][Link]]. Date: 2020-03-24. Note:
 201   An example of kernel PPS transfer from ~gpsd~ to ~ntp~ via
 202   ~127.127.22.0~.
 203
 204 - "GPSD Time Service HOWTO" by Garry E. Miller. [[https://gpsd.gitlab.io/gpsd/gpsd-time-service-howto.html][Link]]. Date:
 205   2020-06-28. Note: Explains how shared memory segments available at
 206   ~127.127.28.0~, ~127.127.28.1~, ~127.127.28.2~, etc. work in context
 207   of an ~ntp~ configuration file.
 208
 209 - "Building a Stratum 1 NTP Server with a Raspberry Pi 4 and Adafruit
 210   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
 211   between ~gpsd~ and ~ntp~ setup for a Raspberry Pi 4 using
 212   ~127.127.28.0~ and ~127.127.28.1~. Recommends use of ~ntpsec~
 213   instead of ~ntp~.
 214
 215 Expalantions of some ~ntpsec~ [[https://docs.ntpsec.org/latest/comdex.html][commands and options]] used in the
 216 configuration file include:
 217
 218 - ~refclock~ : An ~ntpsec~ keyword not present in ~ntp-4.2.8~ that is
 219   meant to simplify configuration of reference clocks that use certain
 220   [[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
 221   Driver]]) . In ~ntp-4.2.8~ and earlier, in order to configure a clock,
 222   instead of ~ntpsec~'s one keyword (~refclock~), two keywords would
 223   be required (~server~ and ~fudge~).
 224
 225 - ~server [address]~ : Tells ~ntpsec~ the address to query for time
 226   information. May be IP address or URL. Additional options such as
 227   ~minpoll~, ~maxpoll~, ~prefer~, and ~true~ may be added.
 228
 229 - ~prefer~ : Indicates the source should be prioritized in some
 230   way. How exactly the ~ntpsec~ mitigation rules (see [[https://docs.ntpsec.org/latest/prefer.html][ref]]) use the
 231   ~prefer~ option is complicated but in the context of dealing with a
 232   GPS source and a PPS source, the GPS source should have a ~prefer~
 233   option. The PPS source may have a ~prefer~ option but it is not
 234   necessary; adding ~prefer~ to a PPS source is useful in the corner
 235   case that the GPS source is less reliable than another source (ex: a
 236   remote clock specified by ~server~).
 237
 238 - ~... shm ... flag1 1~ : For an ~shm~ driver, tells ~ntpsec~ to
 239   ignore the default sanity check that discards any time that is more
 240   than 4-hours off from the system clock. Note that ~ntpsec~ treats
 241   ~shm flag1 1~ in the opposite manner as ~ntp~ (see [[https://docs.ntpsec.org/latest/driver_shm.html][ref]]).
 242
 243 - ~... shm ... time1 [float]~ : Tells ~ntpsec~ that a latency
 244   time of ~[float]~ seconds is required for data to travel from the
 245   address to ~ntpsec~ (ex: a USB hub in the case of a GPS device that
 246   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
 247   adjusted to reduce the `offset` time of the address indicated in the
 248   output of the ~ntpq -pn~ command to near ~0.000~. Note that the
 249   units of ~offset~ displayed by ~ntpq -pn~ are in milliseconds.
 250
 251 - ~... refid [string]~ : Tells ~ntpsec~ a string used to label the source.
 252
 253 - ~fudge~ : An ~ntp~ keyword that ~ntpsec~ developers do not recommend
 254   using; ~ntpsec~ documentation for ~fudge~ is widthdrawn. For local
 255   hardware providing GPS and PPS time information, ~fudge~ can mostly
 256   be replaced with use of the ~refclock~ keyword instead.
 257
 258 - ~fudge [address]~ : Tells ~ntpsec~ to consider additional options
 259   when evaluating the accuracy of time from a given address.
 260
 261 - ~fudge [address] ... flag3 1~ : An option that tells ~ntpsec~ to
 262   enable "kernel PPS discipline" when interpreting the PPS signal at
 263   the address. (see [[http://doc.ntp.org/4.2.8p7/drivers/driver22.html][ref]]).
 264
 265 - ~stratum 1~ : Tells ~ntpsec~ to label the source as a "stratum 1"
 266   time server. This means that it is a clock using NTP that is a
 267   canonical (?) time source. For example, a stratum 0 NTP time server
 268   would be an atomic clock. A stratum 1 NTP time server could be a
 269   clock that uses a GPS receiver to synchronize to said atomic clock.
 270
 271 - ~iburst~ : Tells ~ntpsec~ how often to query a source.
 272
 273 - ~fudge [address] ... flag1 [float]~ : See description of ~... shm
 274   ... time1 [float]~.
 275
 276 - ~tos mindist [float]~ : Tells ~ntpsec~ to increase the tolerance for
 277   outliers in the clock selection algorithm. Units of ~[float]~ are
 278   seconds. The default value is ~0.001~ seconds. This [[http://doc.ntp.org/4.2.6/miscopt.html][reference]]
 279   indicates that it is acceptable to raise this value for "clocks with
 280   high jitter and a PPS signal". For the device being configured in
 281   this guide, this is the case. ~ntpsec~ receives a kernel PPS signal
 282   on ~127.127.22.0~ indicating when each second occurs but without
 283   information about which second of the day is being ticked. ~ntpsec~
 284   receives a coarse time from ~127.127.28.0~ supplied by ~gpsd~ that
 285   is derived from the NMEA sentences generated by the GPS receiver;
 286   this coarse time (high jitter) lacks the precision of the PPS signal
 287   since NMEA sentence transmission times are not well-controlled.
 288
 289 ****** Disable DHCP services affecting ~ntpsec~
 290 Some processes that use [[https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol][DHCP]] (Dynamic Host Configuration Protocol) may
 291 cause configuration changes to configuration files of ~ntp~. Since
 292 ~ntpsec~ is similar to ~ntp~, it is reasonable to take action to
 293 prevent this.
 294
 295 Remove some files:
 296
 297 : $ sudo rm /etc/dhcp/dhclient-exit-hooks.d/ntp
 298 : $ sudo rm /lib/dhcpcd/dhcpcd-hooks/50-ntp.conf
 299
 300 In the "request" block of ~dhclient.conf~, remove ~dhcp6.sntp-servers~
 301 and ~ntp-servers~.
 302
 303 : $ sudo nano /etc/dhcp/dhclient.conf
 304
 305 For example, the following section of ~dhclient.conf~:
 306
 307 #+BEGIN_EXAMPLE
 308 send host-name = gethostname();
 309 request subnet-mask, broadcast-address, time-offset, routers,
 310         domain-name, domain-name-servers, domain-search, host-name,
 311         dhcp6.name-servers, dhcp6.domain-search, dhcp6.fqdn, dhcp6.sntp-servers,
 312         netbios-name-servers, netbios-scope, interface-mtu,
 313         rfc3442-classless-static-routes, ntp-servers;
 314 #+END_EXAMPLE
 315
 316 should be changed to:
 317
 318 #+BEGIN_EXAMPLE
 319 send host-name = gethostname();
 320 request subnet-mask, broadcast-address, time-offset, routers,
 321         domain-name, domain-name-servers, domain-search, host-name,
 322         dhcp6.name-servers, dhcp6.domain-search, dhcp6.fqdn,
 323         netbios-name-servers, netbios-scope, interface-mtu,
 324         rfc3442-classless-static-routes;
 325 #+END_EXAMPLE
 326
 327 ***** Disable CPU power saving
 328 Power saving featurs of the Raspberry Pi Zero W may also be disabled
 329 in order to improve accuracy.
 330
 331 ****** Configure CPU ~scaling_governor~
 332 If additional precision is required, the PPS signal may be made more
 333 reliable at the cost of increasing CPU power by configuring the CPU to
 334 always run at maximum frequency.[fn:se_20180320_raspicpugov] This
 335 change can be performed by modifying the following file as root:
 336
 337 : /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
 338
 339 The file should consist of one line. Change
 340
 341 : ondemand
 342
 343 to
 344
 345 : performance
 346
 347 .
 348
 349 This change can be performed via the ~nano~ text editor by running the
 350 following commands:
 351
 352 : $ sudo su -
 353 : # nano /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
 354
 355 Additionally, in order to prevent the ~raspi-config~ init script from
 356 reverting this text file back to ~ondemand~ after a reboot, this
 357 script must be disabled via:
 358
 359 : $ sudo systemctl disable raspi-config
 360
 361 ****** Configure ~/boot/config.txt~
 362 Modify ~/boot/config.txt~ so that it contains these lines in order to
 363 disable power saving functions:
 364
 365 #+BEGIN_EXAMPLE
 366 # Disable power saving
 367 nohz=off
 368 #+END_EXAMPLE
 369
 370 [fn:se_20180320_raspicpugov] Title:[[https://raspberrypi.stackexchange.com/questions/9034/how-to-change-the-default-governor]["How to change the default governor?"]]; Author:[[https://raspberrypi.stackexchange.com/users/5538/goldilocks][goldilocks]]; Date: 2018-03-20; Website:stackexchange.com;
 371
 372 *** Normal Startup
 373 *** Normal Operation
 374 *** Normal Shutdown
 375 *** Unscheduled Shutdown
 376 ** Appendix A
 377 *** Example ~ntp.conf~ for ~ntpsec~
 378 Below is an example ~ntp.conf~ file for use with ~ntpsec~.
 379
 380 #+BEGIN_EXAMPLE
 381 # /etc/ntpsec/ntp.conf, configuration for ntpd; see ntp.conf(5) for help
 382
 383 # Drift file
 384 driftfile /var/lib/ntpsec/ntp.drift
 385
 386 # Leap seconds definition provided by tzdata
 387 leapfile /usr/share/zoneinfo/leap-seconds.list
 388
 389 # Log file
 390 #logfile /var/log/ntp.log
 391
 392
 393 # You must create /var/log/ntpsec (owned by ntpsec:ntpsec) to enable logging.
 394 #statsdir /var/log/ntpsec/
 395 #statistics loopstats peerstats clockstats
 396 #filegen loopstats file loopstats type day enable
 397 #filegen peerstats file peerstats type day enable
 398 #filegen clockstats file clockstats type day enable
 399
 400
 401 # Comment this out if you have a refclock and want it to be able to discipline
 402 # the clock by itself (e.g. if the system is not connected to the network).
 403 #tos minclock 4 minsane 3
 404
 405
 406 # Local time sources
 407
 408 # Kernel PPS (precision seconds; driver 22)
 409 refclock pps unit 0 refid kPPS flag3 1 minpoll 4 maxpoll 4 stratum 1
 410
 411 # GPS PPS reference (precision seconds; driver 28; root) (NTP1)
 412 refclock shm unit 1 refid PPS minpoll 4 maxpoll 4 iburst stratum 1
 413
 414 # GPS Serial data reference (coarse time; driver28; root) (NTP0)
 415 refclock shm unit 0 refid GPS prefer time1 0.109 minpoll 4 maxpoll 4 iburst stratum 1
 416
 417 # GPS PPS reference (precision seconds; driver 28; user) (NTP2)
 418 refclock shm unit 2 refid PPSuser minpoll 4 maxpoll 4 iburst stratum 1
 419
 420 # Tolerate outliers (useful for coarse clocks)
 421 tos mindist 0.500
 422
 423
 424 # Specify one or more NTP servers.
 425
 426 # pool.ntp.org maps to about 1000 low-stratum NTP servers.  Your server will
 427 # pick a different set every time it starts up.  Please consider joining the
 428 # pool: <https://www.pool.ntp.org/join.html>
 429 server time.nist.gov iburst
 430 server tick.usno.navy.mil iburst
 431 server 0.us.pool.ntp.org iburst
 432 #server 0.us.pool.ntp.org iburst
 433 #server 0.us.pool.ntp.org iburst
 434 #server 0.us.pool.ntp.org iburst
 435 pool 0.debian.pool.ntp.org iburst
 436 #pool 1.debian.pool.ntp.org iburst
 437 #pool 2.debian.pool.ntp.org iburst
 438 #pool 3.debian.pool.ntp.org iburst
 439
 440
 441 # Access control configuration; see /usr/share/doc/ntpsec-doc/html/accopt.html
 442 # for details.
 443 #
 444 # Note that "restrict" applies to both servers and clients, so a configuration
 445 # that might be intended to block requests from certain clients could also end
 446 # up blocking replies from your own upstream servers.
 447
 448 # By default, exchange time with everybody, but don't allow configuration.
 449 restrict default kod nomodify nopeer noquery limited
 450
 451 # Local users may interrogate the ntp server more closely.
 452 restrict 127.0.0.1
 453 restrict ::1
 454 #+END_EXAMPLE
 455
 456 * Discard
 457
 458 ** Transitional ntp to ntpsec config file snippet
 459 # Kernel PPS (precision seconds; driver 22)
 460 #server 127.127.22.0 minpoll 4 maxpoll 4 true
 461 #fudge 127.127.22.0 flag3 1 refid kPPS stratum 1
 462 refclock pps unit 0 refid kPPS flag3 1 minpoll 4 maxpoll 4 stratum 1
 463
 464 # GPS PPS reference (precision seconds; driver 28; root) (NTP1)
 465 #server 127.127.28.1 minpoll 4 maxpoll 4 iburst
 466 #fudge 127.127.28.1 flag1 1 refid PPS
 467 refclock shm unit 1 refid PPS minpoll 4 maxpoll 4 iburst stratum 1
 468
 469 # GPS Serial data reference (coarse time; driver28; root) (NTP0)
 470 #server 127.127.28.0 minpoll 4 maxpoll 4 iburst prefer
 471 #fudge 127.127.28.0 flag1 1 time1 0.109 refid GPS stratum 1
 472 refclock shm unit 0 refid GPS prefer time1 0.109 minpoll 4 maxpoll 4 iburst stratum 1
 473
 474 # GPS PPS reference (precision seconds; driver 28; user) (NTP2)
 475 #server 127.127.28.2 minpoll 4 maxpoll 4 iburst
 476 #fudge 127.127.28.2 flag1 1 refid PPSuser stratum 1
 477 refclock shm unit 2 refid PPSuser minpoll 4 maxpoll 4 iburst stratum 1
 478
 479 # Tolerate outliers (useful for coarse clocks)
 480 tos mindist 0.500