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1 .Dd June 23 2020 2 .Dt NTPD 1ntpdmdoc User Commands 3 .Os 4 .\" EDIT THIS FILE WITH CAUTION (ntpd-opts.mdoc) 5 .\" 6 .\" It has been AutoGen-ed June 23, 2020 at 02:20:30 AM by AutoGen 5.18.5 7 .\" From the definitions ntpd-opts.def 8 .\" and the template file agmdoc-cmd.tpl 9 .Sh NAME 10 .Nm ntpd 11 .Nd NTP daemon program 12 .Sh SYNOPSIS 13 .Nm 14 .\" Mixture of short (flag) options and long options 15 .Op Fl flags 16 .Op Fl flag Op Ar value 17 .Op Fl \-option\-name Ns Oo Oo Ns "=| " Oc Ns Ar value Oc 18 [ <server1> ... <serverN> ] 19 .Pp 20 .Sh DESCRIPTION 21 The 22 .Nm 23 utility is an operating system daemon which sets 24 and maintains the system time of day in synchronism with Internet 25 standard time servers. 26 It is a complete implementation of the 27 Network Time Protocol (NTP) version 4, as defined by RFC\-5905, 28 but also retains compatibility with 29 version 3, as defined by RFC\-1305, and versions 1 30 and 2, as defined by RFC\-1059 and RFC\-1119, respectively. 31 .Pp 32 The 33 .Nm 34 utility does most computations in 64\-bit floating point 35 arithmetic and does relatively clumsy 64\-bit fixed point operations 36 only when necessary to preserve the ultimate precision, about 232 37 picoseconds. 38 While the ultimate precision is not achievable with 39 ordinary workstations and networks of today, it may be required 40 with future gigahertz CPU clocks and gigabit LANs. 41 .Pp 42 Ordinarily, 43 .Nm 44 reads the 45 .Xr ntp.conf 5 46 configuration file at startup time in order to determine the 47 synchronization sources and operating modes. 48 It is also possible to 49 specify a working, although limited, configuration entirely on the 50 command line, obviating the need for a configuration file. 51 This may 52 be particularly useful when the local host is to be configured as a 53 broadcast/multicast client, with all peers being determined by 54 listening to broadcasts at run time. 55 .Pp 56 If NetInfo support is built into 57 .Nm , 58 then 59 .Nm 60 will attempt to read its configuration from the 61 NetInfo if the default 62 .Xr ntp.conf 5 63 file cannot be read and no file is 64 specified by the 65 .Fl c 66 option. 67 .Pp 68 Various internal 69 .Nm 70 variables can be displayed and 71 configuration options altered while the 72 .Nm 73 is running 74 using the 75 .Xr ntpq 1ntpqmdoc 76 and 77 .Xr ntpdc 1ntpdcmdoc 78 utility programs. 79 .Pp 80 When 81 .Nm 82 starts it looks at the value of 83 .Xr umask 2 , 84 and if zero 85 .Nm 86 will set the 87 .Xr umask 2 88 to 022. 89 .Sh "OPTIONS" 90 .Bl -tag 91 .It Fl 4 , Fl \-ipv4 92 Force IPv4 DNS name resolution. 93 This option must not appear in combination with any of the following options: 94 ipv6. 95 .sp 96 Force DNS resolution of following host names on the command line 97 to the IPv4 namespace. 98 .It Fl 6 , Fl \-ipv6 99 Force IPv6 DNS name resolution. 100 This option must not appear in combination with any of the following options: 101 ipv4. 102 .sp 103 Force DNS resolution of following host names on the command line 104 to the IPv6 namespace. 105 .It Fl a , Fl \-authreq 106 Require crypto authentication. 107 This option must not appear in combination with any of the following options: 108 authnoreq. 109 .sp 110 Require cryptographic authentication for broadcast client, 111 multicast client and symmetric passive associations. 112 This is the default. 113 .It Fl A , Fl \-authnoreq 114 Do not require crypto authentication. 115 This option must not appear in combination with any of the following options: 116 authreq. 117 .sp 118 Do not require cryptographic authentication for broadcast client, 119 multicast client and symmetric passive associations. 120 This is almost never a good idea. 121 .It Fl b , Fl \-bcastsync 122 Allow us to sync to broadcast servers. 123 .sp 124 .It Fl c Ar string , Fl \-configfile Ns = Ns Ar string 125 configuration file name. 126 .sp 127 The name and path of the configuration file, 128 \fI/etc/ntp.conf\fP 129 by default. 130 .It Fl d , Fl \-debug\-level 131 Increase debug verbosity level. 132 This option may appear an unlimited number of times. 133 .sp 134 .It Fl D Ar number , Fl \-set\-debug\-level Ns = Ns Ar number 135 Set the debug verbosity level. 136 This option may appear an unlimited number of times. 137 This option takes an integer number as its argument. 138 .sp 139 .It Fl f Ar string , Fl \-driftfile Ns = Ns Ar string 140 frequency drift file name. 141 .sp 142 The name and path of the frequency file, 143 \fI/etc/ntp.drift\fP 144 by default. 145 This is the same operation as the 146 \fBdriftfile\fP \fIdriftfile\fP 147 configuration specification in the 148 \fI/etc/ntp.conf\fP 149 file. 150 .It Fl g , Fl \-panicgate 151 Allow the first adjustment to be Big. 152 This option may appear an unlimited number of times. 153 .sp 154 Normally, 155 \fBntpd\fP 156 exits with a message to the system log if the offset exceeds the panic threshold, which is 1000 s by default. This option allows the time to be set to any value without restriction; however, this can happen only once. If the threshold is exceeded after that, 157 \fBntpd\fP 158 will exit with a message to the system log. This option can be used with the 159 \fB\-q\fP 160 and 161 \fB\-x\fP 162 options. 163 See the 164 \fBtinker\fP 165 configuration file directive for other options. 166 .It Fl G , Fl \-force\-step\-once 167 Step any initial offset correction.. 168 .sp 169 Normally, 170 \fBntpd\fP 171 steps the time if the time offset exceeds the step threshold, 172 which is 128 ms by default, and otherwise slews the time. 173 This option forces the initial offset correction to be stepped, 174 so the highest time accuracy can be achieved quickly. 175 However, this may also cause the time to be stepped back 176 so this option must not be used if 177 applications requiring monotonic time are running. 178 See the \fBtinker\fP configuration file directive for other options. 179 .It Fl i Ar string , Fl \-jaildir Ns = Ns Ar string 180 Jail directory. 181 .sp 182 Chroot the server to the directory 183 \fIjaildir\fP 184 . 185 This option also implies that the server attempts to drop root privileges at startup. 186 You may need to also specify a 187 \fB\-u\fP 188 option. 189 This option is only available if the OS supports adjusting the clock 190 without full root privileges. 191 This option is supported under NetBSD (configure with 192 \fB\-\-enable\-clockctl\fP) or Linux (configure with 193 \fB\-\-enable\-linuxcaps\fP) or Solaris (configure with \fB\-\-enable\-solarisprivs\fP). 194 .It Fl I Ar iface , Fl \-interface Ns = Ns Ar iface 195 Listen on an interface name or address. 196 This option may appear an unlimited number of times. 197 .sp 198 Open the network address given, or all the addresses associated with the 199 given interface name. This option may appear multiple times. This option 200 also implies not opening other addresses, except wildcard and localhost. 201 This option is deprecated. Please consider using the configuration file 202 \fBinterface\fP command, which is more versatile. 203 .It Fl k Ar string , Fl \-keyfile Ns = Ns Ar string 204 path to symmetric keys. 205 .sp 206 Specify the name and path of the symmetric key file. 207 \fI/etc/ntp.keys\fP 208 is the default. 209 This is the same operation as the 210 \fBkeys\fP \fIkeyfile\fP 211 configuration file directive. 212 .It Fl l Ar string , Fl \-logfile Ns = Ns Ar string 213 path to the log file. 214 .sp 215 Specify the name and path of the log file. 216 The default is the system log file. 217 This is the same operation as the 218 \fBlogfile\fP \fIlogfile\fP 219 configuration file directive. 220 .It Fl L , Fl \-novirtualips 221 Do not listen to virtual interfaces. 222 .sp 223 Do not listen to virtual interfaces, defined as those with 224 names containing a colon. This option is deprecated. Please 225 consider using the configuration file \fBinterface\fP command, which 226 is more versatile. 227 .It Fl M , Fl \-modifymmtimer 228 Modify Multimedia Timer (Windows only). 229 .sp 230 Set the Windows Multimedia Timer to highest resolution. This 231 ensures the resolution does not change while ntpd is running, 232 avoiding timekeeping glitches associated with changes. 233 .It Fl n , Fl \-nofork 234 Do not fork. 235 This option must not appear in combination with any of the following options: 236 wait\-sync. 237 .sp 238 .It Fl N , Fl \-nice 239 Run at high priority. 240 .sp 241 To the extent permitted by the operating system, run 242 \fBntpd\fP 243 at the highest priority. 244 .It Fl p Ar string , Fl \-pidfile Ns = Ns Ar string 245 path to the PID file. 246 .sp 247 Specify the name and path of the file used to record 248 \fBntpd\fP's 249 process ID. 250 This is the same operation as the 251 \fBpidfile\fP \fIpidfile\fP 252 configuration file directive. 253 .It Fl P Ar number , Fl \-priority Ns = Ns Ar number 254 Process priority. 255 This option takes an integer number as its argument. 256 .sp 257 To the extent permitted by the operating system, run 258 \fBntpd\fP 259 at the specified 260 \fBsched_setscheduler(SCHED_FIFO)\fP 261 priority. 262 .It Fl q , Fl \-quit 263 Set the time and quit. 264 This option must not appear in combination with any of the following options: 265 saveconfigquit, wait\-sync. 266 .sp 267 \fBntpd\fP 268 will not daemonize and will exit after the clock is first 269 synchronized. This behavior mimics that of the 270 \fBntpdate\fP 271 program, which will soon be replaced with a shell script. 272 The 273 \fB\-g\fP 274 and 275 \fB\-x\fP 276 options can be used with this option. 277 Note: The kernel time discipline is disabled with this option. 278 .It Fl r Ar string , Fl \-propagationdelay Ns = Ns Ar string 279 Broadcast/propagation delay. 280 .sp 281 Specify the default propagation delay from the broadcast/multicast server to this client. This is necessary only if the delay cannot be computed automatically by the protocol. 282 .It Fl \-saveconfigquit Ns = Ns Ar string 283 Save parsed configuration and quit. 284 This option must not appear in combination with any of the following options: 285 quit, wait\-sync. 286 .sp 287 Cause \fBntpd\fP to parse its startup configuration file and save an 288 equivalent to the given filename and exit. This option was 289 designed for automated testing. 290 .It Fl s Ar string , Fl \-statsdir Ns = Ns Ar string 291 Statistics file location. 292 .sp 293 Specify the directory path for files created by the statistics facility. 294 This is the same operation as the 295 \fBstatsdir\fP \fIstatsdir\fP 296 configuration file directive. 297 .It Fl t Ar tkey , Fl \-trustedkey Ns = Ns Ar tkey 298 Trusted key number. 299 This option may appear an unlimited number of times. 300 .sp 301 Add the specified key number to the trusted key list. 302 .It Fl u Ar string , Fl \-user Ns = Ns Ar string 303 Run as userid (or userid:groupid). 304 .sp 305 Specify a user, and optionally a group, to switch to. 306 This option is only available if the OS supports adjusting the clock 307 without full root privileges. 308 This option is supported under NetBSD (configure with 309 \fB\-\-enable\-clockctl\fP) or Linux (configure with 310 \fB\-\-enable\-linuxcaps\fP) or Solaris (configure with \fB\-\-enable\-solarisprivs\fP). 311 .It Fl U Ar number , Fl \-updateinterval Ns = Ns Ar number 312 interval in seconds between scans for new or dropped interfaces. 313 This option takes an integer number as its argument. 314 .sp 315 Give the time in seconds between two scans for new or dropped interfaces. 316 For systems with routing socket support the scans will be performed shortly after the interface change 317 has been detected by the system. 318 Use 0 to disable scanning. 60 seconds is the minimum time between scans. 319 .It Fl \-var Ns = Ns Ar nvar 320 make ARG an ntp variable (RW). 321 This option may appear an unlimited number of times. 322 .sp 323 .It Fl \-dvar Ns = Ns Ar ndvar 324 make ARG an ntp variable (RW|DEF). 325 This option may appear an unlimited number of times. 326 .sp 327 .It Fl w Ar number , Fl \-wait\-sync Ns = Ns Ar number 328 Seconds to wait for first clock sync. 329 This option must not appear in combination with any of the following options: 330 nofork, quit, saveconfigquit. 331 This option takes an integer number as its argument. 332 .sp 333 If greater than zero, alters \fBntpd\fP's behavior when forking to 334 daemonize. Instead of exiting with status 0 immediately after 335 the fork, the parent waits up to the specified number of 336 seconds for the child to first synchronize the clock. The exit 337 status is zero (success) if the clock was synchronized, 338 otherwise it is \fBETIMEDOUT\fP. 339 This provides the option for a script starting \fBntpd\fP to easily 340 wait for the first set of the clock before proceeding. 341 .It Fl x , Fl \-slew 342 Slew up to 600 seconds. 343 .sp 344 Normally, the time is slewed if the offset is less than the step threshold, which is 128 ms by default, and stepped if above the threshold. 345 This option sets the threshold to 600 s, which is well within the accuracy window to set the clock manually. 346 Note: Since the slew rate of typical Unix kernels is limited to 0.5 ms/s, each second of adjustment requires an amortization interval of 2000 s. 347 Thus, an adjustment as much as 600 s will take almost 14 days to complete. 348 This option can be used with the 349 \fB\-g\fP 350 and 351 \fB\-q\fP 352 options. 353 See the 354 \fBtinker\fP 355 configuration file directive for other options. 356 Note: The kernel time discipline is disabled with this option. 357 .It Fl \-usepcc 358 Use CPU cycle counter (Windows only). 359 .sp 360 Attempt to substitute the CPU counter for \fBQueryPerformanceCounter\fP. 361 The CPU counter and \fBQueryPerformanceCounter\fP are compared, and if 362 they have the same frequency, the CPU counter (RDTSC on x86) is 363 used directly, saving the overhead of a system call. 364 .It Fl \-pccfreq Ns = Ns Ar string 365 Force CPU cycle counter use (Windows only). 366 .sp 367 Force substitution the CPU counter for \fBQueryPerformanceCounter\fP. 368 The CPU counter (RDTSC on x86) is used unconditionally with the 369 given frequency (in Hz). 370 .It Fl m , Fl \-mdns 371 Register with mDNS as a NTP server. 372 .sp 373 Registers as an NTP server with the local mDNS server which allows 374 the server to be discovered via mDNS client lookup. 375 .It Fl \&? , Fl \-help 376 Display usage information and exit. 377 .It Fl \&! , Fl \-more\-help 378 Pass the extended usage information through a pager. 379 .It Fl \-version Op Brq Ar v|c|n 380 Output version of program and exit. The default mode is `v', a simple 381 version. The `c' mode will print copyright information and `n' will 382 print the full copyright notice. 383 .El 384 .Sh "OPTION PRESETS" 385 Any option that is not marked as \fInot presettable\fP may be preset 386 by loading values from environment variables named: 387 .nf 388 \fBNTPD_<option\-name>\fP or \fBNTPD\fP 389 .fi 390 .ad 391 .Sh USAGE 392 .Ss "How NTP Operates" 393 The 394 .Nm 395 utility operates by exchanging messages with 396 one or more configured servers over a range of designated poll intervals. 397 When 398 started, whether for the first or subsequent times, the program 399 requires several exchanges from the majority of these servers so 400 the signal processing and mitigation algorithms can accumulate and 401 groom the data and set the clock. 402 In order to protect the network 403 from bursts, the initial poll interval for each server is delayed 404 an interval randomized over a few seconds. 405 At the default initial poll 406 interval of 64s, several minutes can elapse before the clock is 407 set. 408 This initial delay to set the clock 409 can be safely and dramatically reduced using the 410 .Cm iburst 411 keyword with the 412 .Ic server 413 configuration 414 command, as described in 415 .Xr ntp.conf 5 . 416 .Pp 417 Most operating systems and hardware of today incorporate a 418 time\-of\-year (TOY) chip to maintain the time during periods when 419 the power is off. 420 When the machine is booted, the chip is used to 421 initialize the operating system time. 422 After the machine has 423 synchronized to a NTP server, the operating system corrects the 424 chip from time to time. 425 In the default case, if 426 .Nm 427 detects that the time on the host 428 is more than 1000s from the server time, 429 .Nm 430 assumes something must be terribly wrong and the only 431 reliable action is for the operator to intervene and set the clock 432 by hand. 433 (Reasons for this include there is no TOY chip, 434 or its battery is dead, or that the TOY chip is just of poor quality.) 435 This causes 436 .Nm 437 to exit with a panic message to 438 the system log. 439 The 440 .Fl g 441 option overrides this check and the 442 clock will be set to the server time regardless of the chip time 443 (up to 68 years in the past or future \(em 444 this is a limitation of the NTPv4 protocol). 445 However, and to protect against broken hardware, such as when the 446 CMOS battery fails or the clock counter becomes defective, once the 447 clock has been set an error greater than 1000s will cause 448 .Nm 449 to exit anyway. 450 .Pp 451 Under ordinary conditions, 452 .Nm 453 adjusts the clock in 454 small steps so that the timescale is effectively continuous and 455 without discontinuities. 456 Under conditions of extreme network 457 congestion, the roundtrip delay jitter can exceed three seconds and 458 the synchronization distance, which is equal to one\-half the 459 roundtrip delay plus error budget terms, can become very large. 460 The 461 .Nm 462 algorithms discard sample offsets exceeding 128 ms, 463 unless the interval during which no sample offset is less than 128 464 ms exceeds 900s. 465 The first sample after that, no matter what the 466 offset, steps the clock to the indicated time. 467 In practice this 468 reduces the false alarm rate where the clock is stepped in error to 469 a vanishingly low incidence. 470 .Pp 471 As the result of this behavior, once the clock has been set it 472 very rarely strays more than 128 ms even under extreme cases of 473 network path congestion and jitter. 474 Sometimes, in particular when 475 .Nm 476 is first started without a valid drift file 477 on a system with a large intrinsic drift 478 the error might grow to exceed 128 ms, 479 which would cause the clock to be set backwards 480 if the local clock time is more than 128 s 481 in the future relative to the server. 482 In some applications, this behavior may be unacceptable. 483 There are several solutions, however. 484 If the 485 .Fl x 486 option is included on the command line, the clock will 487 never be stepped and only slew corrections will be used. 488 But this choice comes with a cost that 489 should be carefully explored before deciding to use 490 the 491 .Fl x 492 option. 493 The maximum slew rate possible is limited 494 to 500 parts\-per\-million (PPM) as a consequence of the correctness 495 principles on which the NTP protocol and algorithm design are 496 based. 497 As a result, the local clock can take a long time to 498 converge to an acceptable offset, about 2,000 s for each second the 499 clock is outside the acceptable range. 500 During this interval the 501 local clock will not be consistent with any other network clock and 502 the system cannot be used for distributed applications that require 503 correctly synchronized network time. 504 .Pp 505 In spite of the above precautions, sometimes when large 506 frequency errors are present the resulting time offsets stray 507 outside the 128\-ms range and an eventual step or slew time 508 correction is required. 509 If following such a correction the 510 frequency error is so large that the first sample is outside the 511 acceptable range, 512 .Nm 513 enters the same state as when the 514 .Pa ntp.drift 515 file is not present. 516 The intent of this behavior 517 is to quickly correct the frequency and restore operation to the 518 normal tracking mode. 519 In the most extreme cases 520 (the host 521 .Cm time.ien.it 522 comes to mind), there may be occasional 523 step/slew corrections and subsequent frequency corrections. 524 It 525 helps in these cases to use the 526 .Cm burst 527 keyword when 528 configuring the server, but 529 ONLY 530 when you have permission to do so from the owner of the target host. 531 .Pp 532 Finally, 533 in the past many startup scripts would run 534 .Xr ntpdate 1ntpdatemdoc 535 or 536 .Xr sntp 1sntpmdoc 537 to get the system clock close to correct before starting 538 .Xr ntpd 1ntpdmdoc , 539 but this was never more than a mediocre hack and is no longer needed. 540 If you are following the instructions in 541 .Sx "Starting NTP (Best Current Practice)" 542 and you still need to set the system time before starting 543 .Nm , 544 please open a bug report and document what is going on, 545 and then look at using 546 .Xr sntp 1sntpmdoc 547 if you really need to set the clock before starting 548 .Nm . 549 .Pp 550 There is a way to start 551 .Xr ntpd 1ntpdmdoc 552 that often addresses all of the problems mentioned above. 553 .Ss "Starting NTP (Best Current Practice)" 554 First, use the 555 .Cm iburst 556 option on your 557 .Cm server 558 entries. 559 .Pp 560 If you can also keep a good 561 .Pa ntp.drift 562 file then 563 .Xr ntpd 1ntpdmdoc 564 will effectively "warm\-start" and your system's clock will 565 be stable in under 11 seconds' time. 566 .Pp 567 As soon as possible in the startup sequence, start 568 .Xr ntpd 1ntpdmdoc 569 with at least the 570 .Fl g 571 and perhaps the 572 .Fl N 573 options. 574 Then, 575 start the rest of your "normal" processes. 576 This will give 577 .Xr ntpd 1ntpdmdoc 578 as much time as possible to get the system's clock synchronized and stable. 579 .Pp 580 Finally, 581 if you have processes like 582 .Cm dovecot 583 or database servers 584 that require 585 monotonically\-increasing time, 586 run 587 .Xr ntp\-wait 1ntp\-waitmdoc 588 as late as possible in the boot sequence 589 (perhaps with the 590 .Fl v 591 flag) 592 and after 593 .Xr ntp\-wait 1ntp\-waitmdoc 594 exits successfully 595 it is as safe as it will ever be to start any process that require 596 stable time. 597 .Ss "Frequency Discipline" 598 The 599 .Nm 600 behavior at startup depends on whether the 601 frequency file, usually 602 .Pa ntp.drift , 603 exists. 604 This file 605 contains the latest estimate of clock frequency error. 606 When the 607 .Nm 608 is started and the file does not exist, the 609 .Nm 610 enters a special mode designed to quickly adapt to 611 the particular system clock oscillator time and frequency error. 612 This takes approximately 15 minutes, after which the time and 613 frequency are set to nominal values and the 614 .Nm 615 enters 616 normal mode, where the time and frequency are continuously tracked 617 relative to the server. 618 After one hour the frequency file is 619 created and the current frequency offset written to it. 620 When the 621 .Nm 622 is started and the file does exist, the 623 .Nm 624 frequency is initialized from the file and enters normal mode 625 immediately. 626 After that the current frequency offset is written to 627 the file at hourly intervals. 628 .Ss "Operating Modes" 629 The 630 .Nm 631 utility can operate in any of several modes, including 632 symmetric active/passive, client/server broadcast/multicast and 633 manycast, as described in the 634 .Qq Association Management 635 page 636 (available as part of the HTML documentation 637 provided in 638 .Pa /usr/share/doc/ntp ) . 639 It normally operates continuously while 640 monitoring for small changes in frequency and trimming the clock 641 for the ultimate precision. 642 However, it can operate in a one\-time 643 mode where the time is set from an external server and frequency is 644 set from a previously recorded frequency file. 645 A 646 broadcast/multicast or manycast client can discover remote servers, 647 compute server\-client propagation delay correction factors and 648 configure itself automatically. 649 This makes it possible to deploy a 650 fleet of workstations without specifying configuration details 651 specific to the local environment. 652 .Pp 653 By default, 654 .Nm 655 runs in continuous mode where each of 656 possibly several external servers is polled at intervals determined 657 by an intricate state machine. 658 The state machine measures the 659 incidental roundtrip delay jitter and oscillator frequency wander 660 and determines the best poll interval using a heuristic algorithm. 661 Ordinarily, and in most operating environments, the state machine 662 will start with 64s intervals and eventually increase in steps to 663 1024s. 664 A small amount of random variation is introduced in order to 665 avoid bunching at the servers. 666 In addition, should a server become 667 unreachable for some time, the poll interval is increased in steps 668 to 1024s in order to reduce network overhead. 669 .Pp 670 In some cases it may not be practical for 671 .Nm 672 to run continuously. 673 A common workaround has been to run the 674 .Xr ntpdate 1ntpdatemdoc 675 or 676 .Xr sntp 1sntpmdoc 677 programs from a 678 .Xr cron 8 679 job at designated 680 times. 681 However, these programs do not have the crafted signal 682 processing, error checking or mitigation algorithms of 683 .Nm . 684 The 685 .Fl q 686 option is intended for this purpose. 687 Setting this option will cause 688 .Nm 689 to exit just after 690 setting the clock for the first time. 691 The procedure for initially 692 setting the clock is the same as in continuous mode; most 693 applications will probably want to specify the 694 .Cm iburst 695 keyword with the 696 .Ic server 697 configuration command. 698 With this 699 keyword a volley of messages are exchanged to groom the data and 700 the clock is set in about 10 s. 701 If nothing is heard after a 702 couple of minutes, the daemon times out and exits. 703 After a suitable 704 period of mourning, the 705 .Xr ntpdate 1ntpdatemdoc 706 program will be 707 retired. 708 .Pp 709 When kernel support is available to discipline the clock 710 frequency, which is the case for stock Solaris, Tru64, Linux and 711 .Fx , 712 a useful feature is available to discipline the clock 713 frequency. 714 First, 715 .Nm 716 is run in continuous mode with 717 selected servers in order to measure and record the intrinsic clock 718 frequency offset in the frequency file. 719 It may take some hours for 720 the frequency and offset to settle down. 721 Then the 722 .Nm 723 is 724 stopped and run in one\-time mode as required. 725 At each startup, the 726 frequency is read from the file and initializes the kernel 727 frequency. 728 .Ss "Poll Interval Control" 729 This version of NTP includes an intricate state machine to 730 reduce the network load while maintaining a quality of 731 synchronization consistent with the observed jitter and wander. 732 There are a number of ways to tailor the operation in order enhance 733 accuracy by reducing the interval or to reduce network overhead by 734 increasing it. 735 However, the user is advised to carefully consider 736 the consequences of changing the poll adjustment range from the 737 default minimum of 64 s to the default maximum of 1,024 s. 738 The 739 default minimum can be changed with the 740 .Ic tinker 741 .Cm minpoll 742 command to a value not less than 16 s. 743 This value is used for all 744 configured associations, unless overridden by the 745 .Cm minpoll 746 option on the configuration command. 747 Note that most device drivers 748 will not operate properly if the poll interval is less than 64 s 749 and that the broadcast server and manycast client associations will 750 also use the default, unless overridden. 751 .Pp 752 In some cases involving dial up or toll services, it may be 753 useful to increase the minimum interval to a few tens of minutes 754 and maximum interval to a day or so. 755 Under normal operation 756 conditions, once the clock discipline loop has stabilized the 757 interval will be increased in steps from the minimum to the 758 maximum. 759 However, this assumes the intrinsic clock frequency error 760 is small enough for the discipline loop correct it. 761 The capture 762 range of the loop is 500 PPM at an interval of 64s decreasing by a 763 factor of two for each doubling of interval. 764 At a minimum of 1,024 765 s, for example, the capture range is only 31 PPM. 766 If the intrinsic 767 error is greater than this, the drift file 768 .Pa ntp.drift 769 will 770 have to be specially tailored to reduce the residual error below 771 this limit. 772 Once this is done, the drift file is automatically 773 updated once per hour and is available to initialize the frequency 774 on subsequent daemon restarts. 775 .Ss "The huff\-n'\-puff Filter" 776 In scenarios where a considerable amount of data are to be 777 downloaded or uploaded over telephone modems, timekeeping quality 778 can be seriously degraded. 779 This occurs because the differential 780 delays on the two directions of transmission can be quite large. 781 In 782 many cases the apparent time errors are so large as to exceed the 783 step threshold and a step correction can occur during and after the 784 data transfer is in progress. 785 .Pp 786 The huff\-n'\-puff filter is designed to correct the apparent time 787 offset in these cases. 788 It depends on knowledge of the propagation 789 delay when no other traffic is present. 790 In common scenarios this 791 occurs during other than work hours. 792 The filter maintains a shift 793 register that remembers the minimum delay over the most recent 794 interval measured usually in hours. 795 Under conditions of severe 796 delay, the filter corrects the apparent offset using the sign of 797 the offset and the difference between the apparent delay and 798 minimum delay. 799 The name of the filter reflects the negative (huff) 800 and positive (puff) correction, which depends on the sign of the 801 offset. 802 .Pp 803 The filter is activated by the 804 .Ic tinker 805 command and 806 .Cm huffpuff 807 keyword, as described in 808 .Xr ntp.conf 5 . 809 .Sh "ENVIRONMENT" 810 See \fBOPTION PRESETS\fP for configuration environment variables. 811 .Sh FILES 812 .Bl -tag -width /etc/ntp.drift -compact 813 .It Pa /etc/ntp.conf 814 the default name of the configuration file 815 .It Pa /etc/ntp.drift 816 the default name of the drift file 817 .It Pa /etc/ntp.keys 818 the default name of the key file 819 .El 820 .Sh "EXIT STATUS" 821 One of the following exit values will be returned: 822 .Bl -tag 823 .It 0 " (EXIT_SUCCESS)" 824 Successful program execution. 825 .It 1 " (EXIT_FAILURE)" 826 The operation failed or the command syntax was not valid. 827 .It 70 " (EX_SOFTWARE)" 828 libopts had an internal operational error. Please report 829 it to autogen\-firstname.lastname@example.org. Thank you. 830 .El 831 .Sh "SEE ALSO" 832 .Xr ntp.conf 5 , 833 .Xr ntpdate 1ntpdatemdoc , 834 .Xr ntpdc 1ntpdcmdoc , 835 .Xr ntpq 1ntpqmdoc , 836 .Xr sntp 1sntpmdoc 837 .Pp 838 In addition to the manual pages provided, 839 comprehensive documentation is available on the world wide web 840 at 841 .Li http://www.ntp.org/ . 842 A snapshot of this documentation is available in HTML format in 843 .Pa /usr/share/doc/ntp . 844 .Rs 845 .%A David L. Mills 846 .%T Network Time Protocol (Version 1) 847 .%O RFC1059 848 .Re 849 .Rs 850 .%A David L. Mills 851 .%T Network Time Protocol (Version 2) 852 .%O RFC1119 853 .Re 854 .Rs 855 .%A David L. Mills 856 .%T Network Time Protocol (Version 3) 857 .%O RFC1305 858 .Re 859 .Rs 860 .%A David L. Mills 861 .%A J. Martin, Ed. 862 .%A J. Burbank 863 .%A W. Kasch 864 .%T Network Time Protocol Version 4: Protocol and Algorithms Specification 865 .%O RFC5905 866 .Re 867 .Rs 868 .%A David L. Mills 869 .%A B. Haberman, Ed. 870 .%T Network Time Protocol Version 4: Autokey Specification 871 .%O RFC5906 872 .Re 873 .Rs 874 .%A H. Gerstung 875 .%A C. Elliott 876 .%A B. Haberman, Ed. 877 .%T Definitions of Managed Objects for Network Time Protocol Version 4: (NTPv4) 878 .%O RFC5907 879 .Re 880 .Rs 881 .%A R. Gayraud 882 .%A B. Lourdelet 883 .%T Network Time Protocol (NTP) Server Option for DHCPv6 884 .%O RFC5908 885 .Re 886 .Sh "AUTHORS" 887 The University of Delaware and Network Time Foundation 888 .Sh "COPYRIGHT" 889 Copyright (C) 1992\-2020 The University of Delaware and Network Time Foundation all rights reserved. 890 This program is released under the terms of the NTP license, <http://ntp.org/license>. 891 .Sh BUGS 892 The 893 .Nm 894 utility has gotten rather fat. 895 While not huge, it has gotten 896 larger than might be desirable for an elevated\-priority 897 .Nm 898 running on a workstation, particularly since many of 899 the fancy features which consume the space were designed more with 900 a busy primary server, rather than a high stratum workstation in 901 mind. 902 .Pp 903 Please send bug reports to: http://bugs.ntp.org, email@example.com 904 .Sh NOTES 905 Portions of this document came from FreeBSD. 906 .Pp 907 This manual page was \fIAutoGen\fP\-erated from the \fBntpd\fP 908 option definitions.