ntpd.mdoc.in revision 1.1.1.1
1.Dd December 24 2013 2.Dt NTPD @NTPD_MS@ User Commands 3.Os SunOS 5.10 4.\" EDIT THIS FILE WITH CAUTION (ntpd-opts.mdoc) 5.\" 6.\" It has been AutoGen-ed December 24, 2013 at 11:38:28 AM by AutoGen 5.18.3pre5 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 21The 22.Nm 23utility is an operating system daemon which sets 24and maintains the system time of day in synchronism with Internet 25standard time servers. 26It is a complete implementation of the 27Network Time Protocol (NTP) version 4, as defined by RFC\-5905, 28but also retains compatibility with 29version 3, as defined by RFC\-1305, and versions 1 30and 2, as defined by RFC\-1059 and RFC\-1119, respectively. 31.Pp 32The 33.Nm 34utility does most computations in 64\-bit floating point 35arithmetic and does relatively clumsy 64\-bit fixed point operations 36only when necessary to preserve the ultimate precision, about 232 37picoseconds. 38While the ultimate precision is not achievable with 39ordinary workstations and networks of today, it may be required 40with future gigahertz CPU clocks and gigabit LANs. 41.Pp 42Ordinarily, 43.Nm 44reads the 45.Xr ntp.conf 5 46configuration file at startup time in order to determine the 47synchronization sources and operating modes. 48It is also possible to 49specify a working, although limited, configuration entirely on the 50command line, obviating the need for a configuration file. 51This may 52be particularly useful when the local host is to be configured as a 53broadcast/multicast client, with all peers being determined by 54listening to broadcasts at run time. 55.Pp 56If NetInfo support is built into 57.Nm , 58then 59.Nm 60will attempt to read its configuration from the 61NetInfo if the default 62.Xr ntp.conf 5 63file cannot be read and no file is 64specified by the 65.Fl c 66option. 67.Pp 68Various internal 69.Nm 70variables can be displayed and 71configuration options altered while the 72.Nm 73is running 74using the 75.Xr ntpq @NTPQ_MS@ 76and 77.Xr ntpdc @NTPDC_MS@ 78utility programs. 79.Pp 80When 81.Nm 82starts it looks at the value of 83.Xr umask 2 , 84and if zero 85.Nm 86will set the 87.Xr umask 2 88to 022. 89.Sh "OPTIONS" 90.Bl -tag 91.It Fl 4 , Fl \-ipv4 92Force IPv4 DNS name resolution. 93This option must not appear in combination with any of the following options: 94ipv6. 95.sp 96Force DNS resolution of following host names on the command line 97to the IPv4 namespace. 98.It Fl 6 , Fl \-ipv6 99Force IPv6 DNS name resolution. 100This option must not appear in combination with any of the following options: 101ipv4. 102.sp 103Force DNS resolution of following host names on the command line 104to the IPv6 namespace. 105.It Fl a , Fl \-authreq 106Require crypto authentication. 107This option must not appear in combination with any of the following options: 108authnoreq. 109.sp 110Require cryptographic authentication for broadcast client, 111multicast client and symmetric passive associations. 112This is the default. 113.It Fl A , Fl \-authnoreq 114Do not require crypto authentication. 115This option must not appear in combination with any of the following options: 116authreq. 117.sp 118Do not require cryptographic authentication for broadcast client, 119multicast client and symmetric passive associations. 120This is almost never a good idea. 121.It Fl b , Fl \-bcastsync 122Allow us to sync to broadcast servers. 123.sp 124.It Fl c Ar string , Fl \-configfile Ns = Ns Ar string 125configuration file name. 126.sp 127The name and path of the configuration file, 128\fI/etc/ntp.conf\fP 129by default. 130.It Fl d , Fl \-debug\-level 131Increase debug verbosity level. 132This option may appear an unlimited number of times. 133.sp 134.It Fl D Ar number , Fl \-set\-debug\-level Ns = Ns Ar number 135Set the debug verbosity level. 136This option may appear an unlimited number of times. 137This option takes an integer number as its argument. 138.sp 139.It Fl f Ar string , Fl \-driftfile Ns = Ns Ar string 140frequency drift file name. 141.sp 142The name and path of the frequency file, 143\fI/etc/ntp.drift\fP 144by default. 145This is the same operation as the 146\fBdriftfile\fP \fIdriftfile\fP 147configuration specification in the 148\fI/etc/ntp.conf\fP 149file. 150.It Fl g , Fl \-panicgate 151Allow the first adjustment to be Big. 152This option may appear an unlimited number of times. 153.sp 154Normally, 155\fBntpd\fP 156exits 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 158will exit with a message to the system log. This option can be used with the 159\fB\-q\fP 160and 161\fB\-x\fP 162options. 163See the 164\fBtinker\fP 165configuration file directive for other options. 166.It Fl i Ar string , Fl \-jaildir Ns = Ns Ar string 167Jail directory. 168.sp 169Chroot the server to the directory 170\fIjaildir\fP 171. 172This option also implies that the server attempts to drop root privileges at startup. 173You may need to also specify a 174\fB\-u\fP 175option. 176This option is only available if the OS supports adjusting the clock 177without full root privileges. 178This option is supported under NetBSD (configure with 179\fB\-\-enable\-clockctl\fP) or Linux (configure with 180\fB\-\-enable\-linuxcaps\fP) or Solaris (configure with \fB\-\-enable\-solarisprivs\fP). 181.It Fl I Ar iface , Fl \-interface Ns = Ns Ar iface 182Listen on an interface name or address. 183This option may appear an unlimited number of times. 184.sp 185Open the network address given, or all the addresses associated with the 186given interface name. This option may appear multiple times. This option 187also implies not opening other addresses, except wildcard and localhost. 188This option is deprecated. Please consider using the configuration file 189\fBinterface\fP command, which is more versatile. 190.It Fl k Ar string , Fl \-keyfile Ns = Ns Ar string 191path to symmetric keys. 192.sp 193Specify the name and path of the symmetric key file. 194\fI/etc/ntp.keys\fP 195is the default. 196This is the same operation as the 197\fBkeys\fP \fIkeyfile\fP 198configuration file directive. 199.It Fl l Ar string , Fl \-logfile Ns = Ns Ar string 200path to the log file. 201.sp 202Specify the name and path of the log file. 203The default is the system log file. 204This is the same operation as the 205\fBlogfile\fP \fIlogfile\fP 206configuration file directive. 207.It Fl L , Fl \-novirtualips 208Do not listen to virtual interfaces. 209.sp 210Do not listen to virtual interfaces, defined as those with 211names containing a colon. This option is deprecated. Please 212consider using the configuration file \fBinterface\fP command, which 213is more versatile. 214.It Fl M , Fl \-modifymmtimer 215Modify Multimedia Timer (Windows only). 216.sp 217Set the Windows Multimedia Timer to highest resolution. This 218ensures the resolution does not change while ntpd is running, 219avoiding timekeeping glitches associated with changes. 220.It Fl n , Fl \-nofork 221Do not fork. 222This option must not appear in combination with any of the following options: 223wait\-sync. 224.sp 225.It Fl N , Fl \-nice 226Run at high priority. 227.sp 228To the extent permitted by the operating system, run 229\fBntpd\fP 230at the highest priority. 231.It Fl p Ar string , Fl \-pidfile Ns = Ns Ar string 232path to the PID file. 233.sp 234Specify the name and path of the file used to record 235\fBntpd\fP's 236process ID. 237This is the same operation as the 238\fBpidfile\fP \fIpidfile\fP 239configuration file directive. 240.It Fl P Ar number , Fl \-priority Ns = Ns Ar number 241Process priority. 242This option takes an integer number as its argument. 243.sp 244To the extent permitted by the operating system, run 245\fBntpd\fP 246at the specified 247\fBsched_setscheduler(SCHED_FIFO)\fP 248priority. 249.It Fl q , Fl \-quit 250Set the time and quit. 251This option must not appear in combination with any of the following options: 252saveconfigquit, wait\-sync. 253.sp 254\fBntpd\fP 255will not daemonize and will exit after the clock is first 256synchronized. This behavior mimics that of the 257\fBntpdate\fP 258program, which will soon be replaced with a shell script. 259The 260\fB\-g\fP 261and 262\fB\-x\fP 263options can be used with this option. 264Note: The kernel time discipline is disabled with this option. 265.It Fl r Ar string , Fl \-propagationdelay Ns = Ns Ar string 266Broadcast/propagation delay. 267.sp 268Specify 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. 269.It Fl \-saveconfigquit Ns = Ns Ar string 270Save parsed configuration and quit. 271This option must not appear in combination with any of the following options: 272quit, wait\-sync. 273.sp 274Cause \fBntpd\fP to parse its startup configuration file and save an 275equivalent to the given filename and exit. This option was 276designed for automated testing. 277.It Fl s Ar string , Fl \-statsdir Ns = Ns Ar string 278Statistics file location. 279.sp 280Specify the directory path for files created by the statistics facility. 281This is the same operation as the 282\fBstatsdir\fP \fIstatsdir\fP 283configuration file directive. 284.It Fl t Ar tkey , Fl \-trustedkey Ns = Ns Ar tkey 285Trusted key number. 286This option may appear an unlimited number of times. 287.sp 288Add the specified key number to the trusted key list. 289.It Fl u Ar string , Fl \-user Ns = Ns Ar string 290Run as userid (or userid:groupid). 291.sp 292Specify a user, and optionally a group, to switch to. 293This option is only available if the OS supports adjusting the clock 294without full root privileges. 295This option is supported under NetBSD (configure with 296\fB\-\-enable\-clockctl\fP) or Linux (configure with 297\fB\-\-enable\-linuxcaps\fP) or Solaris (configure with \fB\-\-enable\-solarisprivs\fP). 298.It Fl U Ar number , Fl \-updateinterval Ns = Ns Ar number 299interval in seconds between scans for new or dropped interfaces. 300This option takes an integer number as its argument. 301.sp 302Give the time in seconds between two scans for new or dropped interfaces. 303For systems with routing socket support the scans will be performed shortly after the interface change 304has been detected by the system. 305Use 0 to disable scanning. 60 seconds is the minimum time between scans. 306.It Fl \-var Ns = Ns Ar nvar 307make ARG an ntp variable (RW). 308This option may appear an unlimited number of times. 309.sp 310.It Fl \-dvar Ns = Ns Ar ndvar 311make ARG an ntp variable (RW|DEF). 312This option may appear an unlimited number of times. 313.sp 314.It Fl w Ar number , Fl \-wait\-sync Ns = Ns Ar number 315Seconds to wait for first clock sync. 316This option must not appear in combination with any of the following options: 317nofork, quit, saveconfigquit. 318This option takes an integer number as its argument. 319.sp 320If greater than zero, alters \fBntpd\fP's behavior when forking to 321daemonize. Instead of exiting with status 0 immediately after 322the fork, the parent waits up to the specified number of 323seconds for the child to first synchronize the clock. The exit 324status is zero (success) if the clock was synchronized, 325otherwise it is \fBETIMEDOUT\fP. 326This provides the option for a script starting \fBntpd\fP to easily 327wait for the first set of the clock before proceeding. 328.It Fl x , Fl \-slew 329Slew up to 600 seconds. 330.sp 331Normally, 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. 332This option sets the threshold to 600 s, which is well within the accuracy window to set the clock manually. 333Note: 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. 334Thus, an adjustment as much as 600 s will take almost 14 days to complete. 335This option can be used with the 336\fB\-g\fP 337and 338\fB\-q\fP 339options. 340See the 341\fBtinker\fP 342configuration file directive for other options. 343Note: The kernel time discipline is disabled with this option. 344.It Fl \-usepcc 345Use CPU cycle counter (Windows only). 346.sp 347Attempt to substitute the CPU counter for \fBQueryPerformanceCounter\fP. 348The CPU counter and \fBQueryPerformanceCounter\fP are compared, and if 349they have the same frequency, the CPU counter (RDTSC on x86) is 350used directly, saving the overhead of a system call. 351.It Fl \-pccfreq Ns = Ns Ar string 352Force CPU cycle counter use (Windows only). 353.sp 354Force substitution the CPU counter for \fBQueryPerformanceCounter\fP. 355The CPU counter (RDTSC on x86) is used unconditionally with the 356given frequency (in Hz). 357.It Fl m , Fl \-mdns 358Register with mDNS as a NTP server. 359.sp 360Registers as an NTP server with the local mDNS server which allows 361the server to be discovered via mDNS client lookup. 362.It Fl \&? , Fl \-help 363Display usage information and exit. 364.It Fl \&! , Fl \-more\-help 365Pass the extended usage information through a pager. 366.It Fl \-version Op Brq Ar v|c|n 367Output version of program and exit. The default mode is `v', a simple 368version. The `c' mode will print copyright information and `n' will 369print the full copyright notice. 370.El 371.Sh "OPTION PRESETS" 372Any option that is not marked as \fInot presettable\fP may be preset 373by loading values from environment variables named: 374.nf 375 \fBNTPD_<option\-name>\fP or \fBNTPD\fP 376.fi 377.ad 378cvt_prog='/usr/local/gnu/share/autogen/texi2mdoc' 379cvt_prog=`cd \`dirname "$cvt_prog"\` >/dev/null && pwd 380 `/`basename "$cvt_prog"` 381cd $tmp_dir 382test \-x "$cvt_prog" || die "'$cvt_prog' is not executable" 383{ 384 list='synopsis description options option\-presets' 385 for f in $list ; do cat $f ; echo ; done 386 rm \-f $list name 387 list='implementation\-notes environment files examples exit\-status errors 388 compatibility see\-also conforming\-to history authors copyright bugs 389 notes' 390 for f in $list ; do cat $f ; echo ; done > .end\-doc 391 rm \-f $list 392 list=`ls \-1 *`' .end\-doc' 393 for f in $list ; do cat $f ; echo ; done 394 rm \-f $list 395} 1>.doc 2>/dev/null 396sed \-f .cmds .doc | /usr/local/gnu/bin/grep \-E \-v '^[ ]*$' | $cvt_prog 397.Sh USAGE 398.Ss "How NTP Operates" 399The 400.Nm 401utility operates by exchanging messages with 402one or more configured servers over a range of designated poll intervals. 403When 404started, whether for the first or subsequent times, the program 405requires several exchanges from the majority of these servers so 406the signal processing and mitigation algorithms can accumulate and 407groom the data and set the clock. 408In order to protect the network 409from bursts, the initial poll interval for each server is delayed 410an interval randomized over a few seconds. 411At the default initial poll 412interval of 64s, several minutes can elapse before the clock is 413set. 414This initial delay to set the clock 415can be safely and dramatically reduced using the 416.Cm iburst 417keyword with the 418.Ic server 419configuration 420command, as described in 421.Xr ntp.conf 5 . 422.Pp 423Most operating systems and hardware of today incorporate a 424time\-of\-year (TOY) chip to maintain the time during periods when 425the power is off. 426When the machine is booted, the chip is used to 427initialize the operating system time. 428After the machine has 429synchronized to a NTP server, the operating system corrects the 430chip from time to time. 431In the default case, if 432.Nm 433detects that the time on the host 434is more than 1000s from the server time, 435.Nm 436assumes something must be terribly wrong and the only 437reliable action is for the operator to intervene and set the clock 438by hand. 439(Reasons for this include there is no TOY chip, 440or its battery is dead, or that the TOY chip is just of poor quality.) 441This causes 442.Nm 443to exit with a panic message to 444the system log. 445The 446.Fl g 447option overrides this check and the 448clock will be set to the server time regardless of the chip time 449(up to 68 years in the past or future \(em 450this is a limitation of the NTPv4 protocol). 451However, and to protect against broken hardware, such as when the 452CMOS battery fails or the clock counter becomes defective, once the 453clock has been set an error greater than 1000s will cause 454.Nm 455to exit anyway. 456.Pp 457Under ordinary conditions, 458.Nm 459adjusts the clock in 460small steps so that the timescale is effectively continuous and 461without discontinuities. 462Under conditions of extreme network 463congestion, the roundtrip delay jitter can exceed three seconds and 464the synchronization distance, which is equal to one\-half the 465roundtrip delay plus error budget terms, can become very large. 466The 467.Nm 468algorithms discard sample offsets exceeding 128 ms, 469unless the interval during which no sample offset is less than 128 470ms exceeds 900s. 471The first sample after that, no matter what the 472offset, steps the clock to the indicated time. 473In practice this 474reduces the false alarm rate where the clock is stepped in error to 475a vanishingly low incidence. 476.Pp 477As the result of this behavior, once the clock has been set it 478very rarely strays more than 128 ms even under extreme cases of 479network path congestion and jitter. 480Sometimes, in particular when 481.Nm 482is first started without a valid drift file 483on a system with a large intrinsic drift 484the error might grow to exceed 128 ms, 485which would cause the clock to be set backwards 486if the local clock time is more than 128 s 487in the future relative to the server. 488In some applications, this behavior may be unacceptable. 489There are several solutions, however. 490If the 491.Fl x 492option is included on the command line, the clock will 493never be stepped and only slew corrections will be used. 494But this choice comes with a cost that 495should be carefully explored before deciding to use 496the 497.Fl x 498option. 499The maximum slew rate possible is limited 500to 500 parts\-per\-million (PPM) as a consequence of the correctness 501principles on which the NTP protocol and algorithm design are 502based. 503As a result, the local clock can take a long time to 504converge to an acceptable offset, about 2,000 s for each second the 505clock is outside the acceptable range. 506During this interval the 507local clock will not be consistent with any other network clock and 508the system cannot be used for distributed applications that require 509correctly synchronized network time. 510.Pp 511In spite of the above precautions, sometimes when large 512frequency errors are present the resulting time offsets stray 513outside the 128\-ms range and an eventual step or slew time 514correction is required. 515If following such a correction the 516frequency error is so large that the first sample is outside the 517acceptable range, 518.Nm 519enters the same state as when the 520.Pa ntp.drift 521file is not present. 522The intent of this behavior 523is to quickly correct the frequency and restore operation to the 524normal tracking mode. 525In the most extreme cases 526(the host 527.Cm time.ien.it 528comes to mind), there may be occasional 529step/slew corrections and subsequent frequency corrections. 530It 531helps in these cases to use the 532.Cm burst 533keyword when 534configuring the server, but 535ONLY 536when you have permission to do so from the owner of the target host. 537.Pp 538Finally, 539in the past many startup scripts would run 540.Xr ntpdate @NTPDATE_MS@ 541to get the system clock close to correct before starting 542.Xr ntpd @NTPD_MS@ , 543but this was never more than a mediocre hack and is no longer needed. 544.Pp 545There is a way to start 546.Xr ntpd @NTPD_MS@ 547that often addresses all of the problems mentioned above. 548.Ss "Starting NTP (Best Current Practice)" 549First, use the 550.Cm iburst 551option on your 552.Cm server 553entries. 554.Pp 555If you can also keep a good 556.Pa ntp.drift 557file then 558.Xr ntpd @NTPD_MS@ 559will effectively "warm\-start" and your system's clock will 560be stable in under 11 seconds' time. 561.Pp 562As soon as possible in the startup sequence, start 563.Xr ntpd @NTPD_MS@ 564with at least the 565.Fl g 566and perhaps the 567.Fl N 568options. 569Then, 570start the rest of your "normal" processes. 571This will give 572.Xr ntpd @NTPD_MS@ 573as much time as possible to get the system's clock synchronized and stable. 574.Pp 575Finally, 576if you have processes like 577.Cm dovecot 578or database servers 579that require 580monotonically\-increasing time, 581run 582.Xr ntp\-wait 1ntp\-waitmdoc 583as late as possible in the boot sequence 584(perhaps with the 585.Fl v 586flag) 587and after 588.Xr ntp\-wait 1ntp\-waitmdoc 589exits successfully 590it is as safe as it will ever be to start any process that require 591stable time. 592.Ss "Frequency Discipline" 593The 594.Nm 595behavior at startup depends on whether the 596frequency file, usually 597.Pa ntp.drift , 598exists. 599This file 600contains the latest estimate of clock frequency error. 601When the 602.Nm 603is started and the file does not exist, the 604.Nm 605enters a special mode designed to quickly adapt to 606the particular system clock oscillator time and frequency error. 607This takes approximately 15 minutes, after which the time and 608frequency are set to nominal values and the 609.Nm 610enters 611normal mode, where the time and frequency are continuously tracked 612relative to the server. 613After one hour the frequency file is 614created and the current frequency offset written to it. 615When the 616.Nm 617is started and the file does exist, the 618.Nm 619frequency is initialized from the file and enters normal mode 620immediately. 621After that the current frequency offset is written to 622the file at hourly intervals. 623.Ss "Operating Modes" 624The 625.Nm 626utility can operate in any of several modes, including 627symmetric active/passive, client/server broadcast/multicast and 628manycast, as described in the 629.Qq Association Management 630page 631(available as part of the HTML documentation 632provided in 633.Pa /usr/share/doc/ntp ) . 634It normally operates continuously while 635monitoring for small changes in frequency and trimming the clock 636for the ultimate precision. 637However, it can operate in a one\-time 638mode where the time is set from an external server and frequency is 639set from a previously recorded frequency file. 640A 641broadcast/multicast or manycast client can discover remote servers, 642compute server\-client propagation delay correction factors and 643configure itself automatically. 644This makes it possible to deploy a 645fleet of workstations without specifying configuration details 646specific to the local environment. 647.Pp 648By default, 649.Nm 650runs in continuous mode where each of 651possibly several external servers is polled at intervals determined 652by an intricate state machine. 653The state machine measures the 654incidental roundtrip delay jitter and oscillator frequency wander 655and determines the best poll interval using a heuristic algorithm. 656Ordinarily, and in most operating environments, the state machine 657will start with 64s intervals and eventually increase in steps to 6581024s. 659A small amount of random variation is introduced in order to 660avoid bunching at the servers. 661In addition, should a server become 662unreachable for some time, the poll interval is increased in steps 663to 1024s in order to reduce network overhead. 664.Pp 665In some cases it may not be practical for 666.Nm 667to run 668continuously. 669A common workaround has been to run the 670.Xr ntpdate @NTPDATE_MS@ 671program from a 672.Xr cron 8 673job at designated 674times. 675However, this program does not have the crafted signal 676processing, error checking and mitigation algorithms of 677.Nm . 678The 679.Fl q 680option is intended for this purpose. 681Setting this option will cause 682.Nm 683to exit just after 684setting the clock for the first time. 685The procedure for initially 686setting the clock is the same as in continuous mode; most 687applications will probably want to specify the 688.Cm iburst 689keyword with the 690.Ic server 691configuration command. 692With this 693keyword a volley of messages are exchanged to groom the data and 694the clock is set in about 10 s. 695If nothing is heard after a 696couple of minutes, the daemon times out and exits. 697After a suitable 698period of mourning, the 699.Xr ntpdate @NTPDATE_MS@ 700program may be 701retired. 702.Pp 703When kernel support is available to discipline the clock 704frequency, which is the case for stock Solaris, Tru64, Linux and 705.Fx , 706a useful feature is available to discipline the clock 707frequency. 708First, 709.Nm 710is run in continuous mode with 711selected servers in order to measure and record the intrinsic clock 712frequency offset in the frequency file. 713It may take some hours for 714the frequency and offset to settle down. 715Then the 716.Nm 717is 718stopped and run in one\-time mode as required. 719At each startup, the 720frequency is read from the file and initializes the kernel 721frequency. 722.Ss "Poll Interval Control" 723This version of NTP includes an intricate state machine to 724reduce the network load while maintaining a quality of 725synchronization consistent with the observed jitter and wander. 726There are a number of ways to tailor the operation in order enhance 727accuracy by reducing the interval or to reduce network overhead by 728increasing it. 729However, the user is advised to carefully consider 730the consequences of changing the poll adjustment range from the 731default minimum of 64 s to the default maximum of 1,024 s. 732The 733default minimum can be changed with the 734.Ic tinker 735.Cm minpoll 736command to a value not less than 16 s. 737This value is used for all 738configured associations, unless overridden by the 739.Cm minpoll 740option on the configuration command. 741Note that most device drivers 742will not operate properly if the poll interval is less than 64 s 743and that the broadcast server and manycast client associations will 744also use the default, unless overridden. 745.Pp 746In some cases involving dial up or toll services, it may be 747useful to increase the minimum interval to a few tens of minutes 748and maximum interval to a day or so. 749Under normal operation 750conditions, once the clock discipline loop has stabilized the 751interval will be increased in steps from the minimum to the 752maximum. 753However, this assumes the intrinsic clock frequency error 754is small enough for the discipline loop correct it. 755The capture 756range of the loop is 500 PPM at an interval of 64s decreasing by a 757factor of two for each doubling of interval. 758At a minimum of 1,024 759s, for example, the capture range is only 31 PPM. 760If the intrinsic 761error is greater than this, the drift file 762.Pa ntp.drift 763will 764have to be specially tailored to reduce the residual error below 765this limit. 766Once this is done, the drift file is automatically 767updated once per hour and is available to initialize the frequency 768on subsequent daemon restarts. 769.Ss "The huff\-n'\-puff Filter" 770In scenarios where a considerable amount of data are to be 771downloaded or uploaded over telephone modems, timekeeping quality 772can be seriously degraded. 773This occurs because the differential 774delays on the two directions of transmission can be quite large. 775In 776many cases the apparent time errors are so large as to exceed the 777step threshold and a step correction can occur during and after the 778data transfer is in progress. 779.Pp 780The huff\-n'\-puff filter is designed to correct the apparent time 781offset in these cases. 782It depends on knowledge of the propagation 783delay when no other traffic is present. 784In common scenarios this 785occurs during other than work hours. 786The filter maintains a shift 787register that remembers the minimum delay over the most recent 788interval measured usually in hours. 789Under conditions of severe 790delay, the filter corrects the apparent offset using the sign of 791the offset and the difference between the apparent delay and 792minimum delay. 793The name of the filter reflects the negative (huff) 794and positive (puff) correction, which depends on the sign of the 795offset. 796.Pp 797The filter is activated by the 798.Ic tinker 799command and 800.Cm huffpuff 801keyword, as described in 802.Xr ntp.conf 5 . 803.Sh "ENVIRONMENT" 804See \fBOPTION PRESETS\fP for configuration environment variables. 805.Sh FILES 806.Bl -tag -width /etc/ntp.drift -compact 807.It Pa /etc/ntp.conf 808the default name of the configuration file 809.It Pa /etc/ntp.drift 810the default name of the drift file 811.It Pa /etc/ntp.keys 812the default name of the key file 813.El 814.Sh "EXIT STATUS" 815One of the following exit values will be returned: 816.Bl -tag 817.It 0 " (EXIT_SUCCESS)" 818Successful program execution. 819.It 1 " (EXIT_FAILURE)" 820The operation failed or the command syntax was not valid. 821.It 70 " (EX_SOFTWARE)" 822libopts had an internal operational error. Please report 823it to autogen\-users@lists.sourceforge.net. Thank you. 824.El 825.Sh "SEE ALSO" 826.Xr ntp.conf 5 , 827.Xr ntpdate @NTPDATE_MS@ , 828.Xr ntpdc @NTPDC_MS@ , 829.Xr ntpq @NTPQ_MS@ 830.Pp 831In addition to the manual pages provided, 832comprehensive documentation is available on the world wide web 833at 834.Li http://www.ntp.org/ . 835A snapshot of this documentation is available in HTML format in 836.Pa /usr/share/doc/ntp . 837.Rs 838.%A David L. Mills 839.%T Network Time Protocol (Version 1) 840.%O RFC1059 841.Re 842.Rs 843.%A David L. Mills 844.%T Network Time Protocol (Version 2) 845.%O RFC1119 846.Re 847.Rs 848.%A David L. Mills 849.%T Network Time Protocol (Version 3) 850.%O RFC1305 851.Re 852.Rs 853.%A David L. Mills 854.%A J. Martin, Ed. 855.%A J. Burbank 856.%A W. Kasch 857.%T Network Time Protocol Version 4: Protocol and Algorithms Specification 858.%O RFC5905 859.Re 860.Rs 861.%A David L. Mills 862.%A B. Haberman, Ed. 863.%T Network Time Protocol Version 4: Autokey Specification 864.%O RFC5906 865.Re 866.Rs 867.%A H. Gerstung 868.%A C. Elliott 869.%A B. Haberman, Ed. 870.%T Definitions of Managed Objects for Network Time Protocol Version 4: (NTPv4) 871.%O RFC5907 872.Re 873.Rs 874.%A R. Gayraud 875.%A B. Lourdelet 876.%T Network Time Protocol (NTP) Server Option for DHCPv6 877.%O RFC5908 878.Re 879.Sh "AUTHORS" 880The University of Delaware 881.Sh "COPYRIGHT" 882Copyright (C) 1970\-2013 The University of Delaware all rights reserved. 883This program is released under the terms of the NTP license, <http://ntp.org/license>. 884.Sh BUGS 885The 886.Nm 887utility has gotten rather fat. 888While not huge, it has gotten 889larger than might be desirable for an elevated\-priority 890.Nm 891running on a workstation, particularly since many of 892the fancy features which consume the space were designed more with 893a busy primary server, rather than a high stratum workstation in 894mind. 895.Pp 896Please send bug reports to: http://bugs.ntp.org, bugs@ntp.org 897.Sh NOTES 898This document corresponds to version 4.2.7p404 of NTP. 899Portions of this document came from FreeBSD. 900.Pp 901This manual page was \fIAutoGen\fP\-erated from the \fBntpd\fP 902option definitions. 903