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22 <h1 class="settitle">NTP Configuration File User's Manual</h1>
25 <a name="Top"></a>Next: <a rel="next" accesskey="n" href="#ntp_002econf-Description">ntp.conf Description</a>,
26 Previous: <a rel="previous" accesskey="p" href="#dir">(dir)</a>,
27 Up: <a rel="up" accesskey="u" href="#dir">(dir)</a>
31 <h2 class="unnumbered">NTP's Configuration File User Manual</h2>
33 <p>This document describes the configuration file for the NTP Project's
34 <code>ntpd</code> program.
36 <p>This document applies to version 4.2.8p4 of <code>ntp.conf</code>.
38 <div class="shortcontents">
39 <h2>Short Contents</h2>
41 <a href="#Top">NTP's Configuration File User Manual</a>
46 <li><a accesskey="1" href="#ntp_002econf-Description">ntp.conf Description</a>
47 <li><a accesskey="2" href="#ntp_002econf-Notes">ntp.conf Notes</a>
52 <a name="ntp_002econf-Description"></a>Previous: <a rel="previous" accesskey="p" href="#Top">Top</a>,
53 Up: <a rel="up" accesskey="u" href="#Top">Top</a>
57 <!-- node-name, next, previous, up -->
58 <h3 class="section">Description</h3>
60 <p>The behavior of <code>ntpd</code> can be changed by a configuration file,
61 by default <code>ntp.conf</code>.
65 <a name="ntp_002econf-Notes"></a>
69 <h3 class="section">Notes about ntp.conf</h3>
71 <p><a name="index-ntp_002econf-1"></a><a name="index-Network-Time-Protocol-_0028NTP_0029-daemon-configuration-file-format-2"></a>
75 configuration file is read at initial startup by the
76 <code>ntpd(1ntpdmdoc)</code>
77 daemon in order to specify the synchronization sources,
78 modes and other related information.
79 Usually, it is installed in the
80 <span class="file">/etc</span>
82 but could be installed elsewhere
87 <p>The file format is similar to other
88 <span class="sc">unix</span>
92 character and extend to the end of the line;
93 blank lines are ignored.
94 Configuration commands consist of an initial keyword
95 followed by a list of arguments,
96 some of which may be optional, separated by whitespace.
97 Commands may not be continued over multiple lines.
98 Arguments may be host names,
99 host addresses written in numeric, dotted-quad form,
100 integers, floating point numbers (when specifying times in seconds)
103 <p>The rest of this page describes the configuration and control options.
105 "Notes on Configuring NTP and Setting up an NTP Subnet"
107 (available as part of the HTML documentation
109 <span class="file">/usr/share/doc/ntp</span>)
110 contains an extended discussion of these options.
111 In addition to the discussion of general
112 <a href="#Configuration-Options">Configuration Options</a>,
113 there are sections describing the following supported functionality
114 and the options used to control it:
116 <li><a href="#Authentication-Support">Authentication Support</a>
117 <li><a href="#Monitoring-Support">Monitoring Support</a>
118 <li><a href="#Access-Control-Support">Access Control Support</a>
119 <li><a href="#Automatic-NTP-Configuration-Options">Automatic NTP Configuration Options</a>
120 <li><a href="#Reference-Clock-Support">Reference Clock Support</a>
121 <li><a href="#Miscellaneous-Options">Miscellaneous Options</a>
124 <p>Following these is a section describing
125 <a href="#Miscellaneous-Options">Miscellaneous Options</a>.
126 While there is a rich set of options available,
127 the only required option is one or more
131 <code>broadcast</code>
133 <code>manycastclient</code>
137 <a name="Configuration-Support"></a>
141 <h4 class="subsection">Configuration Support</h4>
143 <p>Following is a description of the configuration commands in
145 These commands have the same basic functions as in NTPv3 and
146 in some cases new functions and new arguments.
148 classes of commands, configuration commands that configure a
149 persistent association with a remote server or peer or reference
150 clock, and auxiliary commands that specify environmental variables
151 that control various related operations.
153 <h5 class="subsubsection">Configuration Commands</h5>
155 <p>The various modes are determined by the command keyword and the
156 type of the required IP address.
157 Addresses are classed by type as
158 (s) a remote server or peer (IPv4 class A, B and C), (b) the
159 broadcast address of a local interface, (m) a multicast address (IPv4
160 class D), or (r) a reference clock address (127.127.x.x).
162 only those options applicable to each command are listed below.
164 of options not listed may not be caught as an error, but may result
165 in some weird and even destructive behavior.
167 <p>If the Basic Socket Interface Extensions for IPv6 (RFC-2553)
168 is detected, support for the IPv6 address family is generated
169 in addition to the default support of the IPv4 address family.
170 In a few cases, including the reslist billboard generated
171 by ntpdc, IPv6 addresses are automatically generated.
172 IPv6 addresses can be identified by the presence of colons
174 in the address field.
175 IPv6 addresses can be used almost everywhere where
176 IPv4 addresses can be used,
177 with the exception of reference clock addresses,
178 which are always IPv4.
180 <p>Note that in contexts where a host name is expected, a
183 the host name forces DNS resolution to the IPv4 namespace,
186 qualifier forces DNS resolution to the IPv6 namespace.
187 See IPv6 references for the
188 equivalent classes for that address family.
190 <dt><code>pool</code> <kbd>address</kbd> <code>[burst]</code> <code>[iburst]</code> <code>[version </code><kbd>version</kbd><code>]</code> <code>[prefer]</code> <code>[minpoll </code><kbd>minpoll</kbd><code>]</code> <code>[maxpoll </code><kbd>maxpoll</kbd><code>]</code><br><dt><code>server</code> <kbd>address</kbd> <code>[key </code><kbd>key</kbd> <kbd>|</kbd><code> autokey]</code> <code>[burst]</code> <code>[iburst]</code> <code>[version </code><kbd>version</kbd><code>]</code> <code>[prefer]</code> <code>[minpoll </code><kbd>minpoll</kbd><code>]</code> <code>[maxpoll </code><kbd>maxpoll</kbd><code>]</code><br><dt><code>peer</code> <kbd>address</kbd> <code>[key </code><kbd>key</kbd> <kbd>|</kbd><code> autokey]</code> <code>[version </code><kbd>version</kbd><code>]</code> <code>[prefer]</code> <code>[minpoll </code><kbd>minpoll</kbd><code>]</code> <code>[maxpoll </code><kbd>maxpoll</kbd><code>]</code><br><dt><code>broadcast</code> <kbd>address</kbd> <code>[key </code><kbd>key</kbd> <kbd>|</kbd><code> autokey]</code> <code>[version </code><kbd>version</kbd><code>]</code> <code>[prefer]</code> <code>[minpoll </code><kbd>minpoll</kbd><code>]</code> <code>[ttl </code><kbd>ttl</kbd><code>]</code><br><dt><code>manycastclient</code> <kbd>address</kbd> <code>[key </code><kbd>key</kbd> <kbd>|</kbd><code> autokey]</code> <code>[version </code><kbd>version</kbd><code>]</code> <code>[prefer]</code> <code>[minpoll </code><kbd>minpoll</kbd><code>]</code> <code>[maxpoll </code><kbd>maxpoll</kbd><code>]</code> <code>[ttl </code><kbd>ttl</kbd><code>]</code><dd></dl>
192 <p>These five commands specify the time server name or address to
193 be used and the mode in which to operate.
197 either a DNS name or an IP address in dotted-quad notation.
198 Additional information on association behavior can be found in the
199 "Association Management"
201 (available as part of the HTML documentation
203 <span class="file">/usr/share/doc/ntp</span>).
205 <dt><code>pool</code><dd>For type s addresses, this command mobilizes a persistent
206 client mode association with a number of remote servers.
207 In this mode the local clock can synchronized to the
208 remote server, but the remote server can never be synchronized to
210 <br><dt><code>server</code><dd>For type s and r addresses, this command mobilizes a persistent
211 client mode association with the specified remote server or local
213 In this mode the local clock can synchronized to the
214 remote server, but the remote server can never be synchronized to
220 <br><dt><code>peer</code><dd>For type s addresses (only), this command mobilizes a
221 persistent symmetric-active mode association with the specified
223 In this mode the local clock can be synchronized to
224 the remote peer or the remote peer can be synchronized to the local
226 This is useful in a network of servers where, depending on
227 various failure scenarios, either the local or remote peer may be
228 the better source of time.
229 This command should NOT be used for type
231 <br><dt><code>broadcast</code><dd>For type b and m addresses (only), this
232 command mobilizes a persistent broadcast mode association.
234 commands can be used to specify multiple local broadcast interfaces
235 (subnets) and/or multiple multicast groups.
237 broadcast messages go only to the interface associated with the
238 subnet specified, but multicast messages go to all interfaces.
239 In broadcast mode the local server sends periodic broadcast
240 messages to a client population at the
242 specified, which is usually the broadcast address on (one of) the
243 local network(s) or a multicast address assigned to NTP.
245 has assigned the multicast group address IPv4 224.0.1.1 and
246 IPv6 ff05::101 (site local) exclusively to
247 NTP, but other nonconflicting addresses can be used to contain the
248 messages within administrative boundaries.
250 specification applies only to the local server operating as a
251 sender; for operation as a broadcast client, see the
252 <code>broadcastclient</code>
254 <code>multicastclient</code>
257 <br><dt><code>manycastclient</code><dd>For type m addresses (only), this command mobilizes a
258 manycast client mode association for the multicast address
260 In this case a specific address must be supplied which
261 matches the address used on the
262 <code>manycastserver</code>
264 the designated manycast servers.
265 The NTP multicast address
266 224.0.1.1 assigned by the IANA should NOT be used, unless specific
267 means are taken to avoid spraying large areas of the Internet with
268 these messages and causing a possibly massive implosion of replies
271 <code>manycastserver</code>
272 command specifies that the local server
273 is to operate in client mode with the remote servers that are
274 discovered as the result of broadcast/multicast messages.
276 client broadcasts a request message to the group address associated
279 and specifically enabled
280 servers respond to these messages.
281 The client selects the servers
282 providing the best time and continues as with the
285 The remaining servers are discarded as if never
291 <dt><code>autokey</code><dd>All packets sent to and received from the server or peer are to
292 include authentication fields encrypted using the autokey scheme
294 <a href="#Authentication-Options">Authentication Options</a>.
295 <br><dt><code>burst</code><dd>when the server is reachable, send a burst of eight packets
296 instead of the usual one.
297 The packet spacing is normally 2 s;
298 however, the spacing between the first and second packets
299 can be changed with the calldelay command to allow
300 additional time for a modem or ISDN call to complete.
301 This is designed to improve timekeeping quality
304 command and s addresses.
305 <br><dt><code>iburst</code><dd>When the server is unreachable, send a burst of eight packets
306 instead of the usual one.
307 The packet spacing is normally 2 s;
308 however, the spacing between the first two packets can be
309 changed with the calldelay command to allow
310 additional time for a modem or ISDN call to complete.
311 This is designed to speed the initial synchronization
314 command and s addresses and when
315 <code>ntpd(1ntpdmdoc)</code>
319 <br><dt><code>key</code> <kbd>key</kbd><dd>All packets sent to and received from the server or peer are to
320 include authentication fields encrypted using the specified
322 identifier with values from 1 to 65534, inclusive.
324 default is to include no encryption field.
325 <br><dt><code>minpoll</code> <kbd>minpoll</kbd><br><dt><code>maxpoll</code> <kbd>maxpoll</kbd><dd>These options specify the minimum and maximum poll intervals
326 for NTP messages, as a power of 2 in seconds
328 interval defaults to 10 (1,024 s), but can be increased by the
330 option to an upper limit of 17 (36.4 h).
332 minimum poll interval defaults to 6 (64 s), but can be decreased by
335 option to a lower limit of 4 (16 s).
336 <br><dt><code>noselect</code><dd>Marks the server as unused, except for display purposes.
337 The server is discarded by the selection algroithm.
338 <br><dt><code>prefer</code><dd>Marks the server as preferred.
339 All other things being equal,
340 this host will be chosen for synchronization among a set of
341 correctly operating hosts.
343 "Mitigation Rules and the prefer Keyword"
345 (available as part of the HTML documentation
347 <span class="file">/usr/share/doc/ntp</span>)
348 for further information.
349 <br><dt><code>ttl</code> <kbd>ttl</kbd><dd>This option is used only with broadcast server and manycast
351 It specifies the time-to-live
354 use on broadcast server and multicast server and the maximum
356 for the expanding ring search with manycast
358 Selection of the proper value, which defaults to
359 127, is something of a black art and should be coordinated with the
360 network administrator.
361 <br><dt><code>version</code> <kbd>version</kbd><dd>Specifies the version number to be used for outgoing NTP
363 Versions 1-4 are the choices, with version 4 the
367 <h5 class="subsubsection">Auxiliary Commands</h5>
370 <dt><code>broadcastclient</code><dd>This command enables reception of broadcast server messages to
371 any local interface (type b) address.
372 Upon receiving a message for
373 the first time, the broadcast client measures the nominal server
374 propagation delay using a brief client/server exchange with the
375 server, then enters the broadcast client mode, in which it
376 synchronizes to succeeding broadcast messages.
378 to avoid accidental or malicious disruption in this mode, both the
379 server and client should operate using symmetric-key or public-key
380 authentication as described in
381 <a href="#Authentication-Options">Authentication Options</a>.
382 <br><dt><code>manycastserver</code> <kbd>address</kbd> <kbd>...</kbd><dd>This command enables reception of manycast client messages to
383 the multicast group address(es) (type m) specified.
385 address is required, but the NTP multicast address 224.0.1.1
386 assigned by the IANA should NOT be used, unless specific means are
387 taken to limit the span of the reply and avoid a possibly massive
388 implosion at the original sender.
389 Note that, in order to avoid
390 accidental or malicious disruption in this mode, both the server
391 and client should operate using symmetric-key or public-key
392 authentication as described in
393 <a href="#Authentication-Options">Authentication Options</a>.
394 <br><dt><code>multicastclient</code> <kbd>address</kbd> <kbd>...</kbd><dd>This command enables reception of multicast server messages to
395 the multicast group address(es) (type m) specified.
397 a message for the first time, the multicast client measures the
398 nominal server propagation delay using a brief client/server
399 exchange with the server, then enters the broadcast client mode, in
400 which it synchronizes to succeeding multicast messages.
402 in order to avoid accidental or malicious disruption in this mode,
403 both the server and client should operate using symmetric-key or
404 public-key authentication as described in
405 <a href="#Authentication-Options">Authentication Options</a>.
406 <br><dt><code>mdnstries</code> <kbd>number</kbd><dd>If we are participating in mDNS,
407 after we have synched for the first time
408 we attempt to register with the mDNS system.
409 If that registration attempt fails,
410 we try again at one minute intervals for up to
411 <code>mdnstries</code>
415 may be starting before mDNS.
416 The default value for
417 <code>mdnstries</code>
422 <a name="Authentication-Support"></a>
426 <h4 class="subsection">Authentication Support</h4>
428 <p>Authentication support allows the NTP client to verify that the
429 server is in fact known and trusted and not an intruder intending
430 accidentally or on purpose to masquerade as that server.
432 specification RFC-1305 defines a scheme which provides
433 cryptographic authentication of received NTP packets.
435 this was done using the Data Encryption Standard (DES) algorithm
436 operating in Cipher Block Chaining (CBC) mode, commonly called
438 Subsequently, this was replaced by the RSA Message Digest
439 5 (MD5) algorithm using a private key, commonly called keyed-MD5.
440 Either algorithm computes a message digest, or one-way hash, which
441 can be used to verify the server has the correct private key and
444 <p>NTPv4 retains the NTPv3 scheme, properly described as symmetric key
445 cryptography and, in addition, provides a new Autokey scheme
446 based on public key cryptography.
447 Public key cryptography is generally considered more secure
448 than symmetric key cryptography, since the security is based
449 on a private value which is generated by each server and
451 With Autokey all key distribution and
452 management functions involve only public values, which
453 considerably simplifies key distribution and storage.
454 Public key management is based on X.509 certificates,
455 which can be provided by commercial services or
456 produced by utility programs in the OpenSSL software library
457 or the NTPv4 distribution.
459 <p>While the algorithms for symmetric key cryptography are
460 included in the NTPv4 distribution, public key cryptography
461 requires the OpenSSL software library to be installed
462 before building the NTP distribution.
463 Directions for doing that
464 are on the Building and Installing the Distribution page.
466 <p>Authentication is configured separately for each association
474 <code>broadcast</code>
476 <code>manycastclient</code>
477 configuration commands as described in
478 <a href="#Configuration-Options">Configuration Options</a>
481 options described below specify the locations of the key files,
482 if other than default, which symmetric keys are trusted
483 and the interval between various operations, if other than default.
485 <p>Authentication is always enabled,
486 although ineffective if not configured as
488 If a NTP packet arrives
489 including a message authentication
490 code (MAC), it is accepted only if it
491 passes all cryptographic checks.
493 checks require correct key ID, key value
496 been modified in any way or replayed
497 by an intruder, it will fail one or more
498 of these checks and be discarded.
499 Furthermore, the Autokey scheme requires a
500 preliminary protocol exchange to obtain
501 the server certificate, verify its
502 credentials and initialize the protocol
506 flag controls whether new associations or
507 remote configuration commands require cryptographic authentication.
508 This flag can be set or reset by the
512 commands and also by remote
513 configuration commands sent by a
514 <code>ntpdc(1ntpdcmdoc)</code>
517 If this flag is enabled, which is the default
518 case, new broadcast client and symmetric passive associations and
519 remote configuration commands must be cryptographically
520 authenticated using either symmetric key or public key cryptography.
522 flag is disabled, these operations are effective
523 even if not cryptographic
525 It should be understood
526 that operating with the
528 flag disabled invites a significant vulnerability
529 where a rogue hacker can
530 masquerade as a falseticker and seriously
531 disrupt system timekeeping.
533 important to note that this flag has no purpose
534 other than to allow or disallow
535 a new association in response to new broadcast
536 and symmetric active messages
537 and remote configuration commands and, in particular,
538 the flag has no effect on
539 the authentication process itself.
541 <p>An attractive alternative where multicast support is available
542 is manycast mode, in which clients periodically troll
543 for servers as described in the
544 <a href="#Automatic-NTP-Configuration-Options">Automatic NTP Configuration Options</a>
546 Either symmetric key or public key
547 cryptographic authentication can be used in this mode.
548 The principle advantage
549 of manycast mode is that potential servers need not be
550 configured in advance,
551 since the client finds them during regular operation,
552 and the configuration
553 files for all clients can be identical.
555 <p>The security model and protocol schemes for
556 both symmetric key and public key
557 cryptography are summarized below;
558 further details are in the briefings, papers
559 and reports at the NTP project page linked from
560 <code>http://www.ntp.org/</code>.
562 <h5 class="subsubsection">Symmetric-Key Cryptography</h5>
564 <p>The original RFC-1305 specification allows any one of possibly
565 65,534 keys, each distinguished by a 32-bit key identifier, to
566 authenticate an association.
567 The servers and clients involved must
568 agree on the key and key identifier to
569 authenticate NTP packets.
571 related information are specified in a key
573 <span class="file">ntp.keys</span>,
574 which must be distributed and stored using
575 secure means beyond the scope of the NTP protocol itself.
576 Besides the keys used
577 for ordinary NTP associations,
578 additional keys can be used as passwords for the
579 <code>ntpq(1ntpqmdoc)</code>
581 <code>ntpdc(1ntpdcmdoc)</code>
585 <code>ntpd(1ntpdmdoc)</code>
586 is first started, it reads the key file specified in the
588 configuration command and installs the keys
591 individual keys must be activated with the
595 allows, for instance, the installation of possibly
596 several batches of keys and
597 then activating or deactivating each batch
599 <code>ntpdc(1ntpdcmdoc)</code>.
600 This also provides a revocation capability that can be used
601 if a key becomes compromised.
603 <code>requestkey</code>
604 command selects the key used as the password for the
605 <code>ntpdc(1ntpdcmdoc)</code>
607 <code>controlkey</code>
608 command selects the key used as the password for the
609 <code>ntpq(1ntpqmdoc)</code>
612 <h5 class="subsubsection">Public Key Cryptography</h5>
614 <p>NTPv4 supports the original NTPv3 symmetric key scheme
615 described in RFC-1305 and in addition the Autokey protocol,
616 which is based on public key cryptography.
617 The Autokey Version 2 protocol described on the Autokey Protocol
618 page verifies packet integrity using MD5 message digests
619 and verifies the source with digital signatures and any of several
620 digest/signature schemes.
621 Optional identity schemes described on the Identity Schemes
622 page and based on cryptographic challenge/response algorithms
624 Using all of these schemes provides strong security against
625 replay with or without modification, spoofing, masquerade
626 and most forms of clogging attacks.
628 <p>The Autokey protocol has several modes of operation
629 corresponding to the various NTP modes supported.
630 Most modes use a special cookie which can be
631 computed independently by the client and server,
632 but encrypted in transmission.
633 All modes use in addition a variant of the S-KEY scheme,
634 in which a pseudo-random key list is generated and used
636 These schemes are described along with an executive summary,
637 current status, briefing slides and reading list on the
638 <a href="#Autonomous-Authentication">Autonomous Authentication</a>
641 <p>The specific cryptographic environment used by Autokey servers
642 and clients is determined by a set of files
643 and soft links generated by the
644 <code>ntp-keygen(1ntpkeygenmdoc)</code>
646 This includes a required host key file,
647 required certificate file and optional sign key file,
648 leapsecond file and identity scheme files.
650 digest/signature scheme is specified in the X.509 certificate
651 along with the matching sign key.
652 There are several schemes
653 available in the OpenSSL software library, each identified
654 by a specific string such as
655 <code>md5WithRSAEncryption</code>,
656 which stands for the MD5 message digest with RSA
658 The current NTP distribution supports
659 all the schemes in the OpenSSL library, including
660 those based on RSA and DSA digital signatures.
662 <p>NTP secure groups can be used to define cryptographic compartments
663 and security hierarchies.
664 It is important that every host
665 in the group be able to construct a certificate trail to one
666 or more trusted hosts in the same group.
668 host runs the Autokey protocol to obtain the certificates
669 for all hosts along the trail to one or more trusted hosts.
670 This requires the configuration file in all hosts to be
671 engineered so that, even under anticipated failure conditions,
672 the NTP subnet will form such that every group host can find
673 a trail to at least one trusted host.
675 <h5 class="subsubsection">Naming and Addressing</h5>
677 <p>It is important to note that Autokey does not use DNS to
678 resolve addresses, since DNS can't be completely trusted
679 until the name servers have synchronized clocks.
680 The cryptographic name used by Autokey to bind the host identity
681 credentials and cryptographic values must be independent
682 of interface, network and any other naming convention.
683 The name appears in the host certificate in either or both
684 the subject and issuer fields, so protection against
685 DNS compromise is essential.
687 <p>By convention, the name of an Autokey host is the name returned
689 <code>gethostname(2)</code>
690 system call or equivalent in other systems.
692 model, there are no provisions to allow alternate names or aliases.
693 However, this is not to say that DNS aliases, different names
694 for each interface, etc., are constrained in any way.
696 <p>It is also important to note that Autokey verifies authenticity
697 using the host name, network address and public keys,
698 all of which are bound together by the protocol specifically
699 to deflect masquerade attacks.
700 For this reason Autokey
701 includes the source and destinatino IP addresses in message digest
702 computations and so the same addresses must be available
703 at both the server and client.
704 For this reason operation
705 with network address translation schemes is not possible.
706 This reflects the intended robust security model where government
707 and corporate NTP servers are operated outside firewall perimeters.
709 <h5 class="subsubsection">Operation</h5>
711 <p>A specific combination of authentication scheme (none,
712 symmetric key, public key) and identity scheme is called
713 a cryptotype, although not all combinations are compatible.
714 There may be management configurations where the clients,
715 servers and peers may not all support the same cryptotypes.
716 A secure NTPv4 subnet can be configured in many ways while
717 keeping in mind the principles explained above and
719 Note however that some cryptotype
720 combinations may successfully interoperate with each other,
721 but may not represent good security practice.
723 <p>The cryptotype of an association is determined at the time
724 of mobilization, either at configuration time or some time
725 later when a message of appropriate cryptotype arrives.
730 configuration command and no
734 subcommands are present, the association is not
735 authenticated; if the
737 subcommand is present, the association is authenticated
738 using the symmetric key ID specified; if the
740 subcommand is present, the association is authenticated
743 <p>When multiple identity schemes are supported in the Autokey
744 protocol, the first message exchange determines which one is used.
745 The client request message contains bits corresponding
746 to which schemes it has available.
747 The server response message
748 contains bits corresponding to which schemes it has available.
749 Both server and client match the received bits with their own
750 and select a common scheme.
752 <p>Following the principle that time is a public value,
753 a server responds to any client packet that matches
754 its cryptotype capabilities.
755 Thus, a server receiving
756 an unauthenticated packet will respond with an unauthenticated
757 packet, while the same server receiving a packet of a cryptotype
758 it supports will respond with packets of that cryptotype.
759 However, unconfigured broadcast or manycast client
760 associations or symmetric passive associations will not be
761 mobilized unless the server supports a cryptotype compatible
762 with the first packet received.
763 By default, unauthenticated associations will not be mobilized
764 unless overridden in a decidedly dangerous way.
766 <p>Some examples may help to reduce confusion.
767 Client Alice has no specific cryptotype selected.
768 Server Bob has both a symmetric key file and minimal Autokey files.
769 Alice's unauthenticated messages arrive at Bob, who replies with
770 unauthenticated messages.
771 Cathy has a copy of Bob's symmetric
772 key file and has selected key ID 4 in messages to Bob.
773 Bob verifies the message with his key ID 4.
775 same key and the message is verified, Bob sends Cathy a reply
776 authenticated with that key.
777 If verification fails,
778 Bob sends Cathy a thing called a crypto-NAK, which tells her
780 She can see the evidence using the
781 <code>ntpq(1ntpqmdoc)</code>
784 <p>Denise has rolled her own host key and certificate.
785 She also uses one of the identity schemes as Bob.
786 She sends the first Autokey message to Bob and they
787 both dance the protocol authentication and identity steps.
788 If all comes out okay, Denise and Bob continue as described above.
790 <p>It should be clear from the above that Bob can support
791 all the girls at the same time, as long as he has compatible
792 authentication and identity credentials.
793 Now, Bob can act just like the girls in his own choice of servers;
794 he can run multiple configured associations with multiple different
795 servers (or the same server, although that might not be useful).
796 But, wise security policy might preclude some cryptotype
797 combinations; for instance, running an identity scheme
798 with one server and no authentication with another might not be wise.
800 <h5 class="subsubsection">Key Management</h5>
802 <p>The cryptographic values used by the Autokey protocol are
803 incorporated as a set of files generated by the
804 <code>ntp-keygen(1ntpkeygenmdoc)</code>
805 utility program, including symmetric key, host key and
806 public certificate files, as well as sign key, identity parameters
807 and leapseconds files.
808 Alternatively, host and sign keys and
809 certificate files can be generated by the OpenSSL utilities
810 and certificates can be imported from public certificate
812 Note that symmetric keys are necessary for the
813 <code>ntpq(1ntpqmdoc)</code>
815 <code>ntpdc(1ntpdcmdoc)</code>
817 The remaining files are necessary only for the
820 <p>Certificates imported from OpenSSL or public certificate
821 authorities have certian limitations.
822 The certificate should be in ASN.1 syntax, X.509 Version 3
823 format and encoded in PEM, which is the same format
825 The overall length of the certificate encoded
826 in ASN.1 must not exceed 1024 bytes.
827 The subject distinguished
828 name field (CN) is the fully qualified name of the host
829 on which it is used; the remaining subject fields are ignored.
830 The certificate extension fields must not contain either
831 a subject key identifier or a issuer key identifier field;
832 however, an extended key usage field for a trusted host must
834 <code>trustRoot</code>;.
835 Other extension fields are ignored.
837 <h5 class="subsubsection">Authentication Commands</h5>
840 <dt><code>autokey</code> <code>[</code><kbd>logsec</kbd><code>]</code><dd>Specifies the interval between regenerations of the session key
841 list used with the Autokey protocol.
842 Note that the size of the key
843 list for each association depends on this interval and the current
845 The default value is 12 (4096 s or about 1.1 hours).
846 For poll intervals above the specified interval, a session key list
847 with a single entry will be regenerated for every message
849 <br><dt><code>controlkey</code> <kbd>key</kbd><dd>Specifies the key identifier to use with the
850 <code>ntpq(1ntpqmdoc)</code>
851 utility, which uses the standard
852 protocol defined in RFC-1305.
856 the key identifier for a trusted key, where the value can be in the
857 range 1 to 65,534, inclusive.
858 <br><dt><code>crypto</code> <code>[cert </code><kbd>file</kbd><code>]</code> <code>[leap </code><kbd>file</kbd><code>]</code> <code>[randfile </code><kbd>file</kbd><code>]</code> <code>[host </code><kbd>file</kbd><code>]</code> <code>[sign </code><kbd>file</kbd><code>]</code> <code>[gq </code><kbd>file</kbd><code>]</code> <code>[gqpar </code><kbd>file</kbd><code>]</code> <code>[iffpar </code><kbd>file</kbd><code>]</code> <code>[mvpar </code><kbd>file</kbd><code>]</code> <code>[pw </code><kbd>password</kbd><code>]</code><dd>This command requires the OpenSSL library.
859 It activates public key
860 cryptography, selects the message digest and signature
861 encryption scheme and loads the required private and public
862 values described above.
863 If one or more files are left unspecified,
864 the default names are used as described above.
865 Unless the complete path and name of the file are specified, the
866 location of a file is relative to the keys directory specified
870 <span class="file">/usr/local/etc</span>.
871 Following are the subcommands:
873 <dt><code>cert</code> <kbd>file</kbd><dd>Specifies the location of the required host public certificate file.
874 This overrides the link
875 <span class="file">ntpkey_cert_</span><kbd>hostname</kbd>
876 in the keys directory.
877 <br><dt><code>gqpar</code> <kbd>file</kbd><dd>Specifies the location of the optional GQ parameters file.
880 <span class="file">ntpkey_gq_</span><kbd>hostname</kbd>
881 in the keys directory.
882 <br><dt><code>host</code> <kbd>file</kbd><dd>Specifies the location of the required host key file.
885 <span class="file">ntpkey_key_</span><kbd>hostname</kbd>
886 in the keys directory.
887 <br><dt><code>iffpar</code> <kbd>file</kbd><dd>Specifies the location of the optional IFF parameters file.This
889 <span class="file">ntpkey_iff_</span><kbd>hostname</kbd>
890 in the keys directory.
891 <br><dt><code>leap</code> <kbd>file</kbd><dd>Specifies the location of the optional leapsecond file.
892 This overrides the link
893 <span class="file">ntpkey_leap</span>
894 in the keys directory.
895 <br><dt><code>mvpar</code> <kbd>file</kbd><dd>Specifies the location of the optional MV parameters file.
898 <span class="file">ntpkey_mv_</span><kbd>hostname</kbd>
899 in the keys directory.
900 <br><dt><code>pw</code> <kbd>password</kbd><dd>Specifies the password to decrypt files containing private keys and
902 This is required only if these files have been
904 <br><dt><code>randfile</code> <kbd>file</kbd><dd>Specifies the location of the random seed file used by the OpenSSL
906 The defaults are described in the main text above.
907 <br><dt><code>sign</code> <kbd>file</kbd><dd>Specifies the location of the optional sign key file.
910 <span class="file">ntpkey_sign_</span><kbd>hostname</kbd>
911 in the keys directory.
913 not found, the host key is also the sign key.
915 <br><dt><code>keys</code> <kbd>keyfile</kbd><dd>Specifies the complete path and location of the MD5 key file
916 containing the keys and key identifiers used by
917 <code>ntpd(1ntpdmdoc)</code>,
918 <code>ntpq(1ntpqmdoc)</code>
920 <code>ntpdc(1ntpdcmdoc)</code>
921 when operating with symmetric key cryptography.
922 This is the same operation as the
925 <br><dt><code>keysdir</code> <kbd>path</kbd><dd>This command specifies the default directory path for
926 cryptographic keys, parameters and certificates.
928 <span class="file">/usr/local/etc/</span>.
929 <br><dt><code>requestkey</code> <kbd>key</kbd><dd>Specifies the key identifier to use with the
930 <code>ntpdc(1ntpdcmdoc)</code>
931 utility program, which uses a
932 proprietary protocol specific to this implementation of
933 <code>ntpd(1ntpdmdoc)</code>.
936 argument is a key identifier
937 for the trusted key, where the value can be in the range 1 to
939 <br><dt><code>revoke</code> <kbd>logsec</kbd><dd>Specifies the interval between re-randomization of certain
940 cryptographic values used by the Autokey scheme, as a power of 2 in
942 These values need to be updated frequently in order to
943 deflect brute-force attacks on the algorithms of the scheme;
944 however, updating some values is a relatively expensive operation.
945 The default interval is 16 (65,536 s or about 18 hours).
947 intervals above the specified interval, the values will be updated
948 for every message sent.
949 <br><dt><code>trustedkey</code> <kbd>key</kbd> <kbd>...</kbd><dd>Specifies the key identifiers which are trusted for the
950 purposes of authenticating peers with symmetric key cryptography,
951 as well as keys used by the
952 <code>ntpq(1ntpqmdoc)</code>
954 <code>ntpdc(1ntpdcmdoc)</code>
956 The authentication procedures require that both the local
957 and remote servers share the same key and key identifier for this
958 purpose, although different keys can be used with different
962 arguments are 32-bit unsigned
963 integers with values from 1 to 65,534.
966 <h5 class="subsubsection">Error Codes</h5>
968 <p>The following error codes are reported via the NTP control
969 and monitoring protocol trap mechanism.
971 <dt>101<dd>(bad field format or length)
972 The packet has invalid version, length or format.
973 <br><dt>102<dd>(bad timestamp)
974 The packet timestamp is the same or older than the most recent received.
975 This could be due to a replay or a server clock time step.
976 <br><dt>103<dd>(bad filestamp)
977 The packet filestamp is the same or older than the most recent received.
978 This could be due to a replay or a key file generation error.
979 <br><dt>104<dd>(bad or missing public key)
980 The public key is missing, has incorrect format or is an unsupported type.
981 <br><dt>105<dd>(unsupported digest type)
982 The server requires an unsupported digest/signature scheme.
983 <br><dt>106<dd>(mismatched digest types)
985 <br><dt>107<dd>(bad signature length)
986 The signature length does not match the current public key.
987 <br><dt>108<dd>(signature not verified)
988 The message fails the signature check.
989 It could be bogus or signed by a
990 different private key.
991 <br><dt>109<dd>(certificate not verified)
992 The certificate is invalid or signed with the wrong key.
993 <br><dt>110<dd>(certificate not verified)
994 The certificate is not yet valid or has expired or the signature could not
996 <br><dt>111<dd>(bad or missing cookie)
997 The cookie is missing, corrupted or bogus.
998 <br><dt>112<dd>(bad or missing leapseconds table)
999 The leapseconds table is missing, corrupted or bogus.
1000 <br><dt>113<dd>(bad or missing certificate)
1001 The certificate is missing, corrupted or bogus.
1002 <br><dt>114<dd>(bad or missing identity)
1003 The identity key is missing, corrupt or bogus.
1007 <a name="Monitoring-Support"></a>
1011 <h4 class="subsection">Monitoring Support</h4>
1013 <p><code>ntpd(1ntpdmdoc)</code>
1014 includes a comprehensive monitoring facility suitable
1015 for continuous, long term recording of server and client
1016 timekeeping performance.
1018 <code>statistics</code>
1020 for a listing and example of each type of statistics currently
1022 Statistic files are managed using file generation sets
1024 <span class="file">./scripts</span>
1025 directory of this distribution.
1027 these facilities and
1028 <span class="sc">unix</span>
1029 <code>cron(8)</code>
1030 jobs, the data can be
1031 automatically summarized and archived for retrospective analysis.
1033 <h5 class="subsubsection">Monitoring Commands</h5>
1036 <dt><code>statistics</code> <kbd>name</kbd> <kbd>...</kbd><dd>Enables writing of statistics records.
1037 Currently, eight kinds of
1039 statistics are supported.
1041 <dt><code>clockstats</code><dd>Enables recording of clock driver statistics information.
1043 received from a clock driver appends a line of the following form to
1044 the file generation set named
1045 <code>clockstats</code>:
1046 <pre class="verbatim">
1047 49213 525.624 127.127.4.1 93 226 00:08:29.606 D
1050 <p>The first two fields show the date (Modified Julian Day) and time
1051 (seconds and fraction past UTC midnight).
1052 The next field shows the
1053 clock address in dotted-quad notation.
1054 The final field shows the last
1055 timecode received from the clock in decoded ASCII format, where
1057 In some clock drivers a good deal of additional information
1058 can be gathered and displayed as well.
1059 See information specific to each
1060 clock for further details.
1061 <br><dt><code>cryptostats</code><dd>This option requires the OpenSSL cryptographic software library.
1063 enables recording of cryptographic public key protocol information.
1064 Each message received by the protocol module appends a line of the
1065 following form to the file generation set named
1066 <code>cryptostats</code>:
1067 <pre class="verbatim">
1068 49213 525.624 127.127.4.1 message
1071 <p>The first two fields show the date (Modified Julian Day) and time
1072 (seconds and fraction past UTC midnight).
1073 The next field shows the peer
1074 address in dotted-quad notation, The final message field includes the
1075 message type and certain ancillary information.
1077 <a href="#Authentication-Options">Authentication Options</a>
1078 section for further information.
1079 <br><dt><code>loopstats</code><dd>Enables recording of loop filter statistics information.
1081 update of the local clock outputs a line of the following form to
1082 the file generation set named
1083 <code>loopstats</code>:
1084 <pre class="verbatim">
1085 50935 75440.031 0.000006019 13.778190 0.000351733 0.0133806
1088 <p>The first two fields show the date (Modified Julian Day) and
1089 time (seconds and fraction past UTC midnight).
1090 The next five fields
1091 show time offset (seconds), frequency offset (parts per million -
1092 PPM), RMS jitter (seconds), Allan deviation (PPM) and clock
1093 discipline time constant.
1094 <br><dt><code>peerstats</code><dd>Enables recording of peer statistics information.
1096 statistics records of all peers of a NTP server and of special
1097 signals, where present and configured.
1098 Each valid update appends a
1099 line of the following form to the current element of a file
1100 generation set named
1101 <code>peerstats</code>:
1102 <pre class="verbatim">
1103 48773 10847.650 127.127.4.1 9714 -0.001605376 0.000000000 0.001424877 0.000958674
1106 <p>The first two fields show the date (Modified Julian Day) and
1107 time (seconds and fraction past UTC midnight).
1109 show the peer address in dotted-quad notation and status,
1111 The status field is encoded in hex in the format
1112 described in Appendix A of the NTP specification RFC 1305.
1113 The final four fields show the offset,
1114 delay, dispersion and RMS jitter, all in seconds.
1115 <br><dt><code>rawstats</code><dd>Enables recording of raw-timestamp statistics information.
1117 includes statistics records of all peers of a NTP server and of
1118 special signals, where present and configured.
1120 received from a peer or clock driver appends a line of the
1121 following form to the file generation set named
1122 <code>rawstats</code>:
1123 <pre class="verbatim">
1124 50928 2132.543 128.4.1.1 128.4.1.20 3102453281.584327000 3102453281.58622800031 02453332.540806000 3102453332.541458000
1127 <p>The first two fields show the date (Modified Julian Day) and
1128 time (seconds and fraction past UTC midnight).
1130 show the remote peer or clock address followed by the local address
1131 in dotted-quad notation.
1132 The final four fields show the originate,
1133 receive, transmit and final NTP timestamps in order.
1135 values are as received and before processing by the various data
1136 smoothing and mitigation algorithms.
1137 <br><dt><code>sysstats</code><dd>Enables recording of ntpd statistics counters on a periodic basis.
1139 hour a line of the following form is appended to the file generation
1141 <code>sysstats</code>:
1142 <pre class="verbatim">
1143 50928 2132.543 36000 81965 0 9546 56 71793 512 540 10 147
1146 <p>The first two fields show the date (Modified Julian Day) and time
1147 (seconds and fraction past UTC midnight).
1148 The remaining ten fields show
1149 the statistics counter values accumulated since the last generated
1152 <dt>Time since restart <code>36000</code><dd>Time in hours since the system was last rebooted.
1153 <br><dt>Packets received <code>81965</code><dd>Total number of packets received.
1154 <br><dt>Packets processed <code>0</code><dd>Number of packets received in response to previous packets sent
1155 <br><dt>Current version <code>9546</code><dd>Number of packets matching the current NTP version.
1156 <br><dt>Previous version <code>56</code><dd>Number of packets matching the previous NTP version.
1157 <br><dt>Bad version <code>71793</code><dd>Number of packets matching neither NTP version.
1158 <br><dt>Access denied <code>512</code><dd>Number of packets denied access for any reason.
1159 <br><dt>Bad length or format <code>540</code><dd>Number of packets with invalid length, format or port number.
1160 <br><dt>Bad authentication <code>10</code><dd>Number of packets not verified as authentic.
1161 <br><dt>Rate exceeded <code>147</code><dd>Number of packets discarded due to rate limitation.
1163 <br><dt><code>statsdir</code> <kbd>directory_path</kbd><dd>Indicates the full path of a directory where statistics files
1164 should be created (see below).
1166 the (otherwise constant)
1167 <code>filegen</code>
1168 filename prefix to be modified for file generation sets, which
1169 is useful for handling statistics logs.
1170 <br><dt><code>filegen</code> <kbd>name</kbd> <code>[file </code><kbd>filename</kbd><code>]</code> <code>[type </code><kbd>typename</kbd><code>]</code> <code>[link | nolink]</code> <code>[enable | disable]</code><dd>Configures setting of generation file set name.
1172 file sets provide a means for handling files that are
1173 continuously growing during the lifetime of a server.
1174 Server statistics are a typical example for such files.
1175 Generation file sets provide access to a set of files used
1176 to store the actual data.
1177 At any time at most one element
1178 of the set is being written to.
1179 The type given specifies
1180 when and how data will be directed to a new element of the set.
1181 This way, information stored in elements of a file set
1182 that are currently unused are available for administrational
1183 operations without the risk of disturbing the operation of ntpd.
1184 (Most important: they can be removed to free space for new data
1187 <p>Note that this command can be sent from the
1188 <code>ntpdc(1ntpdcmdoc)</code>
1189 program running at a remote location.
1191 <dt><code>name</code><dd>This is the type of the statistics records, as shown in the
1192 <code>statistics</code>
1194 <br><dt><code>file</code> <kbd>filename</kbd><dd>This is the file name for the statistics records.
1196 members are built from three concatenated elements
1197 <code>prefix</code>,
1198 <code>filename</code>
1200 <code>suffix</code>:
1202 <dt><code>prefix</code><dd>This is a constant filename path.
1203 It is not subject to
1204 modifications via the
1207 It is defined by the
1208 server, usually specified as a compile-time constant.
1210 however, be configurable for individual file generation sets
1212 For example, the prefix used with
1213 <kbd>loopstats</kbd>
1215 <kbd>peerstats</kbd>
1216 generation can be configured using the
1218 option explained above.
1219 <br><dt><code>filename</code><dd>This string is directly concatenated to the prefix mentioned
1220 above (no intervening
1222 This can be modified using
1223 the file argument to the
1227 <span class="file">..</span>
1229 allowed in this component to prevent filenames referring to
1230 parts outside the filesystem hierarchy denoted by
1232 <br><dt><code>suffix</code><dd>This part is reflects individual elements of a file set.
1234 generated according to the type of a file set.
1236 <br><dt><code>type</code> <kbd>typename</kbd><dd>A file generation set is characterized by its type.
1238 types are supported:
1240 <dt><code>none</code><dd>The file set is actually a single plain file.
1241 <br><dt><code>pid</code><dd>One element of file set is used per incarnation of a ntpd
1243 This type does not perform any changes to file set
1244 members during runtime, however it provides an easy way of
1245 separating files belonging to different
1246 <code>ntpd(1ntpdmdoc)</code>
1247 server incarnations.
1248 The set member filename is built by appending a
1255 appending the decimal representation of the process ID of the
1256 <code>ntpd(1ntpdmdoc)</code>
1258 <br><dt><code>day</code><dd>One file generation set element is created per day.
1260 defined as the period between 00:00 and 24:00 UTC.
1262 member suffix consists of a
1264 and a day specification in
1266 <code>YYYYMMdd</code>.
1268 is a 4-digit year number (e.g., 1992).
1270 is a two digit month number.
1272 is a two digit day number.
1273 Thus, all information written at 10 December 1992 would end up
1276 <kbd>filename</kbd>.19921210.
1277 <br><dt><code>week</code><dd>Any file set member contains data related to a certain week of
1279 The term week is defined by computing day-of-year
1281 Elements of such a file generation set are
1282 distinguished by appending the following suffix to the file set
1283 filename base: A dot, a 4-digit year number, the letter
1285 and a 2-digit week number.
1286 For example, information from January,
1287 10th 1992 would end up in a file with suffix
1288 .No . Ns Ar 1992W1 .
1289 <br><dt><code>month</code><dd>One generation file set element is generated per month.
1291 file name suffix consists of a dot, a 4-digit year number, and
1293 <br><dt><code>year</code><dd>One generation file element is generated per year.
1295 suffix consists of a dot and a 4 digit year number.
1296 <br><dt><code>age</code><dd>This type of file generation sets changes to a new element of
1297 the file set every 24 hours of server operation.
1299 suffix consists of a dot, the letter
1301 and an 8-digit number.
1302 This number is taken to be the number of seconds the server is
1303 running at the start of the corresponding 24-hour period.
1304 Information is only written to a file generation by specifying
1305 <code>enable</code>;
1306 output is prevented by specifying
1307 <code>disable</code>.
1309 <br><dt><code>link</code> | <code>nolink</code><dd>It is convenient to be able to access the current element of a file
1310 generation set by a fixed name.
1311 This feature is enabled by
1315 <code>nolink</code>.
1316 If link is specified, a
1317 hard link from the current file set element to a file without
1319 When there is already a file with this name and
1320 the number of links of this file is one, it is renamed appending a
1323 and the pid of the ntpd server process.
1325 number of links is greater than one, the file is unlinked.
1327 allows the current file to be accessed by a constant name.
1328 <br><dt><code>enable</code> <code>|</code> <code>disable</code><dd>Enables or disables the recording function.
1334 <a name="Access-Control-Support"></a>
1338 <h4 class="subsection">Access Control Support</h4>
1341 <code>ntpd(1ntpdmdoc)</code>
1342 daemon implements a general purpose address/mask based restriction
1344 The list contains address/match entries sorted first
1345 by increasing address values and and then by increasing mask values.
1346 A match occurs when the bitwise AND of the mask and the packet
1347 source address is equal to the bitwise AND of the mask and
1348 address in the list.
1349 The list is searched in order with the
1350 last match found defining the restriction flags associated
1352 Additional information and examples can be found in the
1353 "Notes on Configuring NTP and Setting up a NTP Subnet"
1355 (available as part of the HTML documentation
1357 <span class="file">/usr/share/doc/ntp</span>).
1359 <p>The restriction facility was implemented in conformance
1360 with the access policies for the original NSFnet backbone
1362 Later the facility was expanded to deflect
1363 cryptographic and clogging attacks.
1364 While this facility may
1365 be useful for keeping unwanted or broken or malicious clients
1366 from congesting innocent servers, it should not be considered
1367 an alternative to the NTP authentication facilities.
1368 Source address based restrictions are easily circumvented
1369 by a determined cracker.
1371 <p>Clients can be denied service because they are explicitly
1372 included in the restrict list created by the restrict command
1373 or implicitly as the result of cryptographic or rate limit
1375 Cryptographic violations include certificate
1376 or identity verification failure; rate limit violations generally
1377 result from defective NTP implementations that send packets
1379 Some violations cause denied service
1380 only for the offending packet, others cause denied service
1381 for a timed period and others cause the denied service for
1382 an indefinate period.
1383 When a client or network is denied access
1384 for an indefinate period, the only way at present to remove
1385 the restrictions is by restarting the server.
1387 <h5 class="subsubsection">The Kiss-of-Death Packet</h5>
1389 <p>Ordinarily, packets denied service are simply dropped with no
1390 further action except incrementing statistics counters.
1392 more proactive response is needed, such as a server message that
1393 explicitly requests the client to stop sending and leave a message
1394 for the system operator.
1395 A special packet format has been created
1396 for this purpose called the "kiss-of-death" (KoD) packet.
1397 KoD packets have the leap bits set unsynchronized and stratum set
1398 to zero and the reference identifier field set to a four-byte
1401 <code>noserve</code>
1403 <code>notrust</code>
1404 flag of the matching restrict list entry is set,
1405 the code is "DENY"; if the
1406 <code>limited</code>
1407 flag is set and the rate limit
1408 is exceeded, the code is "RATE".
1409 Finally, if a cryptographic violation occurs, the code is "CRYP".
1411 <p>A client receiving a KoD performs a set of sanity checks to
1412 minimize security exposure, then updates the stratum and
1413 reference identifier peer variables, sets the access
1414 denied (TEST4) bit in the peer flash variable and sends
1415 a message to the log.
1416 As long as the TEST4 bit is set,
1417 the client will send no further packets to the server.
1418 The only way at present to recover from this condition is
1419 to restart the protocol at both the client and server.
1421 happens automatically at the client when the association times out.
1422 It will happen at the server only if the server operator cooperates.
1424 <h5 class="subsubsection">Access Control Commands</h5>
1427 <dt><code>discard</code> <code>[average </code><kbd>avg</kbd><code>]</code> <code>[minimum </code><kbd>min</kbd><code>]</code> <code>[monitor </code><kbd>prob</kbd><code>]</code><dd>Set the parameters of the
1428 <code>limited</code>
1429 facility which protects the server from
1432 <code>average</code>
1433 subcommand specifies the minimum average packet
1435 <code>minimum</code>
1436 subcommand specifies the minimum packet spacing.
1437 Packets that violate these minima are discarded
1438 and a kiss-o'-death packet returned if enabled.
1440 minimum average and minimum are 5 and 2, respectively.
1441 The monitor subcommand specifies the probability of discard
1442 for packets that overflow the rate-control window.
1443 <br><dt><code>restrict</code> <code>address</code> <code>[mask </code><kbd>mask</kbd><code>]</code> <code>[</code><kbd>flag</kbd> <kbd>...</kbd><code>]</code><dd>The
1445 argument expressed in
1446 dotted-quad form is the address of a host or network.
1449 argument can be a valid host DNS name.
1452 argument expressed in dotted-quad form defaults to
1453 <code>255.255.255.255</code>,
1456 is treated as the address of an individual host.
1457 A default entry (address
1458 <code>0.0.0.0</code>,
1460 <code>0.0.0.0</code>)
1461 is always included and is always the first entry in the list.
1462 Note that text string
1463 <code>default</code>,
1464 with no mask option, may
1465 be used to indicate the default entry.
1466 In the current implementation,
1469 restricts access, i.e., an entry with no flags indicates that free
1470 access to the server is to be given.
1471 The flags are not orthogonal,
1472 in that more restrictive flags will often make less restrictive
1474 The flags can generally be classed into two
1475 categories, those which restrict time service and those which
1476 restrict informational queries and attempts to do run-time
1477 reconfiguration of the server.
1478 One or more of the following flags
1481 <dt><code>ignore</code><dd>Deny packets of all kinds, including
1482 <code>ntpq(1ntpqmdoc)</code>
1484 <code>ntpdc(1ntpdcmdoc)</code>
1486 <br><dt><code>kod</code><dd>If this flag is set when an access violation occurs, a kiss-o'-death
1487 (KoD) packet is sent.
1488 KoD packets are rate limited to no more than one
1490 If another KoD packet occurs within one second after the
1491 last one, the packet is dropped.
1492 <br><dt><code>limited</code><dd>Deny service if the packet spacing violates the lower limits specified
1493 in the discard command.
1494 A history of clients is kept using the
1495 monitoring capability of
1496 <code>ntpd(1ntpdmdoc)</code>.
1497 Thus, monitoring is always active as
1498 long as there is a restriction entry with the
1499 <code>limited</code>
1501 <br><dt><code>lowpriotrap</code><dd>Declare traps set by matching hosts to be low priority.
1503 number of traps a server can maintain is limited (the current limit
1505 Traps are usually assigned on a first come, first served
1506 basis, with later trap requestors being denied service.
1508 modifies the assignment algorithm by allowing low priority traps to
1509 be overridden by later requests for normal priority traps.
1510 <br><dt><code>nomodify</code><dd>Deny
1511 <code>ntpq(1ntpqmdoc)</code>
1513 <code>ntpdc(1ntpdcmdoc)</code>
1514 queries which attempt to modify the state of the
1515 server (i.e., run time reconfiguration).
1516 Queries which return
1517 information are permitted.
1518 <br><dt><code>noquery</code><dd>Deny
1519 <code>ntpq(1ntpqmdoc)</code>
1521 <code>ntpdc(1ntpdcmdoc)</code>
1523 Time service is not affected.
1524 <br><dt><code>nopeer</code><dd>Deny packets which would result in mobilizing a new association.
1526 includes broadcast and symmetric active packets when a configured
1527 association does not exist.
1530 associations, so if you want to use servers from a
1532 directive and also want to use
1534 by default, you'll want a
1535 <code>restrict source ...</code> <code>line</code> <code>as</code> <code>well</code> <code>that</code> <code>does</code>
1536 <br><dt>not<dd>include the
1539 <br><dt><code>noserve</code><dd>Deny all packets except
1540 <code>ntpq(1ntpqmdoc)</code>
1542 <code>ntpdc(1ntpdcmdoc)</code>
1544 <br><dt><code>notrap</code><dd>Decline to provide mode 6 control message trap service to matching
1546 The trap service is a subsystem of the ntpdq control message
1547 protocol which is intended for use by remote event logging programs.
1548 <br><dt><code>notrust</code><dd>Deny service unless the packet is cryptographically authenticated.
1549 <br><dt><code>ntpport</code><dd>This is actually a match algorithm modifier, rather than a
1551 Its presence causes the restriction entry to be
1552 matched only if the source port in the packet is the standard NTP
1555 <code>ntpport</code>
1557 <code>non-ntpport</code>
1561 <code>ntpport</code>
1562 is considered more specific and
1563 is sorted later in the list.
1564 <br><dt><code>version</code><dd>Deny packets that do not match the current NTP version.
1567 <p>Default restriction list entries with the flags ignore, interface,
1568 ntpport, for each of the local host's interface addresses are
1569 inserted into the table at startup to prevent the server
1570 from attempting to synchronize to its own time.
1571 A default entry is also always present, though if it is
1572 otherwise unconfigured; no flags are associated
1573 with the default entry (i.e., everything besides your own
1574 NTP server is unrestricted).
1578 <a name="Automatic-NTP-Configuration-Options"></a>
1582 <h4 class="subsection">Automatic NTP Configuration Options</h4>
1584 <h5 class="subsubsection">Manycasting</h5>
1586 <p>Manycasting is a automatic discovery and configuration paradigm
1588 It is intended as a means for a multicast client
1589 to troll the nearby network neighborhood to find cooperating
1590 manycast servers, validate them using cryptographic means
1591 and evaluate their time values with respect to other servers
1592 that might be lurking in the vicinity.
1593 The intended result is that each manycast client mobilizes
1594 client associations with some number of the "best"
1595 of the nearby manycast servers, yet automatically reconfigures
1596 to sustain this number of servers should one or another fail.
1598 <p>Note that the manycasting paradigm does not coincide
1599 with the anycast paradigm described in RFC-1546,
1600 which is designed to find a single server from a clique
1601 of servers providing the same service.
1602 The manycast paradigm is designed to find a plurality
1603 of redundant servers satisfying defined optimality criteria.
1605 <p>Manycasting can be used with either symmetric key
1606 or public key cryptography.
1607 The public key infrastructure (PKI)
1608 offers the best protection against compromised keys
1609 and is generally considered stronger, at least with relatively
1611 It is implemented using the Autokey protocol and
1612 the OpenSSL cryptographic library available from
1613 <code>http://www.openssl.org/</code>.
1614 The library can also be used with other NTPv4 modes
1615 as well and is highly recommended, especially for broadcast modes.
1617 <p>A persistent manycast client association is configured
1618 using the manycastclient command, which is similar to the
1619 server command but with a multicast (IPv4 class
1624 The IANA has designated IPv4 address 224.1.1.1
1625 and IPv6 address FF05::101 (site local) for NTP.
1626 When more servers are needed, it broadcasts manycast
1627 client messages to this address at the minimum feasible rate
1628 and minimum feasible time-to-live (TTL) hops, depending
1629 on how many servers have already been found.
1630 There can be as many manycast client associations
1631 as different group address, each one serving as a template
1632 for a future ephemeral unicast client/server association.
1634 <p>Manycast servers configured with the
1635 <code>manycastserver</code>
1636 command listen on the specified group address for manycast
1638 Note the distinction between manycast client,
1639 which actively broadcasts messages, and manycast server,
1640 which passively responds to them.
1641 If a manycast server is
1642 in scope of the current TTL and is itself synchronized
1643 to a valid source and operating at a stratum level equal
1644 to or lower than the manycast client, it replies to the
1645 manycast client message with an ordinary unicast server message.
1647 <p>The manycast client receiving this message mobilizes
1648 an ephemeral client/server association according to the
1649 matching manycast client template, but only if cryptographically
1650 authenticated and the server stratum is less than or equal
1651 to the client stratum.
1652 Authentication is explicitly required
1653 and either symmetric key or public key (Autokey) can be used.
1654 Then, the client polls the server at its unicast address
1655 in burst mode in order to reliably set the host clock
1656 and validate the source.
1657 This normally results
1658 in a volley of eight client/server at 2-s intervals
1659 during which both the synchronization and cryptographic
1660 protocols run concurrently.
1661 Following the volley,
1662 the client runs the NTP intersection and clustering
1663 algorithms, which act to discard all but the "best"
1664 associations according to stratum and synchronization
1666 The surviving associations then continue
1667 in ordinary client/server mode.
1669 <p>The manycast client polling strategy is designed to reduce
1670 as much as possible the volume of manycast client messages
1671 and the effects of implosion due to near-simultaneous
1672 arrival of manycast server messages.
1673 The strategy is determined by the
1674 <code>manycastclient</code>,
1678 configuration commands.
1679 The manycast poll interval is
1680 normally eight times the system poll interval,
1681 which starts out at the
1682 <code>minpoll</code>
1683 value specified in the
1684 <code>manycastclient</code>,
1685 command and, under normal circumstances, increments to the
1686 <code>maxpolll</code>
1687 value specified in this command.
1688 Initially, the TTL is
1689 set at the minimum hops specified by the ttl command.
1690 At each retransmission the TTL is increased until reaching
1691 the maximum hops specified by this command or a sufficient
1692 number client associations have been found.
1693 Further retransmissions use the same TTL.
1695 <p>The quality and reliability of the suite of associations
1696 discovered by the manycast client is determined by the NTP
1697 mitigation algorithms and the
1698 <code>minclock</code>
1700 <code>minsane</code>
1701 values specified in the
1703 configuration command.
1705 <code>minsane</code>
1706 candidate servers must be available and the mitigation
1707 algorithms produce at least
1708 <code>minclock</code>
1709 survivors in order to synchronize the clock.
1710 Byzantine agreement principles require at least four
1711 candidates in order to correctly discard a single falseticker.
1712 For legacy purposes,
1713 <code>minsane</code>
1715 <code>minclock</code>
1717 For manycast service
1718 <code>minsane</code>
1719 should be explicitly set to 4, assuming at least that
1720 number of servers are available.
1723 <code>minclock</code>
1724 servers are found, the manycast poll interval is immediately
1726 <code>maxpoll</code>.
1728 <code>minclock</code>
1729 servers are found when the TTL has reached the maximum hops,
1730 the manycast poll interval is doubled.
1731 For each transmission
1732 after that, the poll interval is doubled again until
1733 reaching the maximum of eight times
1734 <code>maxpoll</code>.
1735 Further transmissions use the same poll interval and
1737 Note that while all this is going on,
1738 each client/server association found is operating normally
1739 it the system poll interval.
1741 <p>Administratively scoped multicast boundaries are normally
1742 specified by the network router configuration and,
1743 in the case of IPv6, the link/site scope prefix.
1744 By default, the increment for TTL hops is 32 starting
1745 from 31; however, the
1747 configuration command can be
1748 used to modify the values to match the scope rules.
1750 <p>It is often useful to narrow the range of acceptable
1751 servers which can be found by manycast client associations.
1752 Because manycast servers respond only when the client
1753 stratum is equal to or greater than the server stratum,
1754 primary (stratum 1) servers fill find only primary servers
1755 in TTL range, which is probably the most common objective.
1756 However, unless configured otherwise, all manycast clients
1757 in TTL range will eventually find all primary servers
1758 in TTL range, which is probably not the most common
1759 objective in large networks.
1762 command can be used to modify this behavior.
1763 Servers with stratum below
1766 <code>ceiling</code>
1769 command are strongly discouraged during the selection
1770 process; however, these servers may be temporally
1771 accepted if the number of servers within TTL range is
1773 <code>minclock</code>.
1775 <p>The above actions occur for each manycast client message,
1776 which repeats at the designated poll interval.
1777 However, once the ephemeral client association is mobilized,
1778 subsequent manycast server replies are discarded,
1779 since that would result in a duplicate association.
1780 If during a poll interval the number of client associations
1782 <code>minclock</code>,
1783 all manycast client prototype associations are reset
1784 to the initial poll interval and TTL hops and operation
1785 resumes from the beginning.
1786 It is important to avoid
1787 frequent manycast client messages, since each one requires
1788 all manycast servers in TTL range to respond.
1789 The result could well be an implosion, either minor or major,
1790 depending on the number of servers in range.
1791 The recommended value for
1792 <code>maxpoll</code>
1795 <p>It is possible and frequently useful to configure a host
1796 as both manycast client and manycast server.
1797 A number of hosts configured this way and sharing a common
1798 group address will automatically organize themselves
1799 in an optimum configuration based on stratum and
1800 synchronization distance.
1801 For example, consider an NTP
1802 subnet of two primary servers and a hundred or more
1804 With two exceptions, all servers
1805 and clients have identical configuration files including both
1806 <code>multicastclient</code>
1808 <code>multicastserver</code>
1809 commands using, for instance, multicast group address
1811 The only exception is that each primary server
1812 configuration file must include commands for the primary
1813 reference source such as a GPS receiver.
1815 <p>The remaining configuration files for all secondary
1816 servers and clients have the same contents, except for the
1818 command, which is specific for each stratum level.
1819 For stratum 1 and stratum 2 servers, that command is
1821 For stratum 3 and above servers the
1823 value is set to the intended stratum number.
1824 Thus, all stratum 3 configuration files are identical,
1825 all stratum 4 files are identical and so forth.
1827 <p>Once operations have stabilized in this scenario,
1828 the primary servers will find the primary reference source
1829 and each other, since they both operate at the same
1830 stratum (1), but not with any secondary server or client,
1831 since these operate at a higher stratum.
1833 servers will find the servers at the same stratum level.
1834 If one of the primary servers loses its GPS receiver,
1835 it will continue to operate as a client and other clients
1836 will time out the corresponding association and
1837 re-associate accordingly.
1839 <p>Some administrators prefer to avoid running
1840 <code>ntpd(1ntpdmdoc)</code>
1841 continuously and run either
1842 <code>sntp(1sntpmdoc)</code>
1844 <code>ntpd(1ntpdmdoc)</code>
1847 In either case the servers must be
1848 configured in advance and the program fails if none are
1849 available when the cron job runs.
1851 application of manycast is with
1852 <code>ntpd(1ntpdmdoc)</code>
1854 The program wakes up, scans the local landscape looking
1855 for the usual suspects, selects the best from among
1856 the rascals, sets the clock and then departs.
1857 Servers do not have to be configured in advance and
1858 all clients throughout the network can have the same
1861 <h5 class="subsubsection">Manycast Interactions with Autokey</h5>
1863 <p>Each time a manycast client sends a client mode packet
1864 to a multicast group address, all manycast servers
1865 in scope generate a reply including the host name
1867 The manycast clients then run
1868 the Autokey protocol, which collects and verifies
1869 all certificates involved.
1870 Following the burst interval
1871 all but three survivors are cast off,
1872 but the certificates remain in the local cache.
1873 It often happens that several complete signing trails
1874 from the client to the primary servers are collected in this way.
1876 <p>About once an hour or less often if the poll interval
1877 exceeds this, the client regenerates the Autokey key list.
1878 This is in general transparent in client/server mode.
1879 However, about once per day the server private value
1880 used to generate cookies is refreshed along with all
1881 manycast client associations.
1883 cryptographic values including certificates is refreshed.
1884 If a new certificate has been generated since
1885 the last refresh epoch, it will automatically revoke
1886 all prior certificates that happen to be in the
1888 At the same time, the manycast
1889 scheme starts all over from the beginning and
1890 the expanding ring shrinks to the minimum and increments
1891 from there while collecting all servers in scope.
1893 <h5 class="subsubsection">Manycast Options</h5>
1896 <dt><code>tos</code> <code>[ceiling </code><kbd>ceiling</kbd><code> | cohort { 0 | 1 } | floor </code><kbd>floor</kbd><code> | minclock </code><kbd>minclock</kbd><code> | minsane </code><kbd>minsane</kbd><code>]</code><dd>This command affects the clock selection and clustering
1898 It can be used to select the quality and
1899 quantity of peers used to synchronize the system clock
1900 and is most useful in manycast mode.
1901 The variables operate
1904 <dt><code>ceiling</code> <kbd>ceiling</kbd><dd>Peers with strata above
1905 <code>ceiling</code>
1906 will be discarded if there are at least
1907 <code>minclock</code>
1909 This value defaults to 15, but can be changed
1910 to any number from 1 to 15.
1911 <br><dt><code>cohort</code> <code>{0 | 1}</code><dd>This is a binary flag which enables (0) or disables (1)
1912 manycast server replies to manycast clients with the same
1914 This is useful to reduce implosions where
1915 large numbers of clients with the same stratum level
1917 The default is to enable these replies.
1918 <br><dt><code>floor</code> <kbd>floor</kbd><dd>Peers with strata below
1920 will be discarded if there are at least
1921 <code>minclock</code>
1923 This value defaults to 1, but can be changed
1924 to any number from 1 to 15.
1925 <br><dt><code>minclock</code> <kbd>minclock</kbd><dd>The clustering algorithm repeatedly casts out outlier
1926 associations until no more than
1927 <code>minclock</code>
1928 associations remain.
1929 This value defaults to 3,
1930 but can be changed to any number from 1 to the number of
1932 <br><dt><code>minsane</code> <kbd>minsane</kbd><dd>This is the minimum number of candidates available
1933 to the clock selection algorithm in order to produce
1934 one or more truechimers for the clustering algorithm.
1935 If fewer than this number are available, the clock is
1936 undisciplined and allowed to run free.
1938 for legacy purposes.
1939 However, according to principles of
1940 Byzantine agreement,
1941 <code>minsane</code>
1942 should be at least 4 in order to detect and discard
1943 a single falseticker.
1945 <br><dt><code>ttl</code> <kbd>hop</kbd> <kbd>...</kbd><dd>This command specifies a list of TTL values in increasing
1946 order, up to 8 values can be specified.
1947 In manycast mode these values are used in turn
1948 in an expanding-ring search.
1949 The default is eight
1950 multiples of 32 starting at 31.
1954 <a name="Reference-Clock-Support"></a>
1958 <h4 class="subsection">Reference Clock Support</h4>
1960 <p>The NTP Version 4 daemon supports some three dozen different radio,
1961 satellite and modem reference clocks plus a special pseudo-clock
1962 used for backup or when no other clock source is available.
1963 Detailed descriptions of individual device drivers and options can
1965 "Reference Clock Drivers"
1967 (available as part of the HTML documentation
1969 <span class="file">/usr/share/doc/ntp</span>).
1970 Additional information can be found in the pages linked
1971 there, including the
1972 "Debugging Hints for Reference Clock Drivers"
1974 "How To Write a Reference Clock Driver"
1976 (available as part of the HTML documentation
1978 <span class="file">/usr/share/doc/ntp</span>).
1979 In addition, support for a PPS
1980 signal is available as described in the
1981 "Pulse-per-second (PPS) Signal Interfacing"
1983 (available as part of the HTML documentation
1985 <span class="file">/usr/share/doc/ntp</span>).
1987 drivers support special line discipline/streams modules which can
1988 significantly improve the accuracy using the driver.
1991 "Line Disciplines and Streams Drivers"
1993 (available as part of the HTML documentation
1995 <span class="file">/usr/share/doc/ntp</span>).
1997 <p>A reference clock will generally (though not always) be a radio
1998 timecode receiver which is synchronized to a source of standard
1999 time such as the services offered by the NRC in Canada and NIST and
2001 The interface between the computer and the timecode
2002 receiver is device dependent, but is usually a serial port.
2004 device driver specific to each reference clock must be selected and
2005 compiled in the distribution; however, most common radio, satellite
2006 and modem clocks are included by default.
2007 Note that an attempt to
2008 configure a reference clock when the driver has not been compiled
2009 or the hardware port has not been appropriately configured results
2010 in a scalding remark to the system log file, but is otherwise non
2013 <p>For the purposes of configuration,
2014 <code>ntpd(1ntpdmdoc)</code>
2016 reference clocks in a manner analogous to normal NTP peers as much
2018 Reference clocks are identified by a syntactically
2019 correct but invalid IP address, in order to distinguish them from
2021 Reference clock addresses are of the form
2022 <code>127.127.</code><kbd>t</kbd>.<kbd>u</kbd>,
2026 denoting the clock type and
2029 number in the range 0-3.
2030 While it may seem overkill, it is in fact
2031 sometimes useful to configure multiple reference clocks of the same
2032 type, in which case the unit numbers must be unique.
2036 command is used to configure a reference
2039 argument in that command
2040 is the clock address.
2043 <code>version</code>
2046 options are not used for reference clock support.
2049 option is added for reference clock support, as
2053 option can be useful to
2054 persuade the server to cherish a reference clock with somewhat more
2055 enthusiasm than other reference clocks or peers.
2057 information on this option can be found in the
2058 "Mitigation Rules and the prefer Keyword"
2059 (available as part of the HTML documentation
2061 <span class="file">/usr/share/doc/ntp</span>)
2064 <code>minpoll</code>
2066 <code>maxpoll</code>
2068 meaning only for selected clock drivers.
2069 See the individual clock
2070 driver document pages for additional information.
2074 command is used to provide additional
2075 information for individual clock drivers and normally follows
2076 immediately after the
2081 argument specifies the clock address.
2085 <code>stratum</code>
2086 options can be used to
2087 override the defaults for the device.
2088 There are two optional
2089 device-dependent time offsets and four flags that can be included
2094 <p>The stratum number of a reference clock is by default zero.
2096 <code>ntpd(1ntpdmdoc)</code>
2097 daemon adds one to the stratum of each
2098 peer, a primary server ordinarily displays an external stratum of
2100 In order to provide engineered backups, it is often useful to
2101 specify the reference clock stratum as greater than zero.
2103 <code>stratum</code>
2104 option is used for this purpose.
2106 involving both a reference clock and a pulse-per-second (PPS)
2107 discipline signal, it is useful to specify the reference clock
2108 identifier as other than the default, depending on the driver.
2111 option is used for this purpose.
2113 these options apply to all clock drivers.
2115 <h5 class="subsubsection">Reference Clock Commands</h5>
2118 <dt><code>server</code> <code>127.127.</code><kbd>t</kbd>.<kbd>u</kbd> <code>[prefer]</code> <code>[mode </code><kbd>int</kbd><code>]</code> <code>[minpoll </code><kbd>int</kbd><code>]</code> <code>[maxpoll </code><kbd>int</kbd><code>]</code><dd>This command can be used to configure reference clocks in
2120 The options are interpreted as follows:
2122 <dt><code>prefer</code><dd>Marks the reference clock as preferred.
2123 All other things being
2124 equal, this host will be chosen for synchronization among a set of
2125 correctly operating hosts.
2127 "Mitigation Rules and the prefer Keyword"
2129 (available as part of the HTML documentation
2131 <span class="file">/usr/share/doc/ntp</span>)
2132 for further information.
2133 <br><dt><code>mode</code> <kbd>int</kbd><dd>Specifies a mode number which is interpreted in a
2134 device-specific fashion.
2135 For instance, it selects a dialing
2136 protocol in the ACTS driver and a device subtype in the
2139 <br><dt><code>minpoll</code> <kbd>int</kbd><br><dt><code>maxpoll</code> <kbd>int</kbd><dd>These options specify the minimum and maximum polling interval
2140 for reference clock messages, as a power of 2 in seconds
2142 most directly connected reference clocks, both
2143 <code>minpoll</code>
2145 <code>maxpoll</code>
2146 default to 6 (64 s).
2147 For modem reference clocks,
2148 <code>minpoll</code>
2149 defaults to 10 (17.1 m) and
2150 <code>maxpoll</code>
2151 defaults to 14 (4.5 h).
2152 The allowable range is 4 (16 s) to 17 (36.4 h) inclusive.
2154 <br><dt><code>fudge</code> <code>127.127.</code><kbd>t</kbd>.<kbd>u</kbd> <code>[time1 </code><kbd>sec</kbd><code>]</code> <code>[time2 </code><kbd>sec</kbd><code>]</code> <code>[stratum </code><kbd>int</kbd><code>]</code> <code>[refid </code><kbd>string</kbd><code>]</code> <code>[mode </code><kbd>int</kbd><code>]</code> <code>[flag1 0 | 1]</code> <code>[flag2 0 | 1]</code> <code>[flag3 0 | 1]</code> <code>[flag4 0 | 1]</code><dd>This command can be used to configure reference clocks in
2156 It must immediately follow the
2158 command which configures the driver.
2159 Note that the same capability
2160 is possible at run time using the
2161 <code>ntpdc(1ntpdcmdoc)</code>
2163 The options are interpreted as
2166 <dt><code>time1</code> <kbd>sec</kbd><dd>Specifies a constant to be added to the time offset produced by
2167 the driver, a fixed-point decimal number in seconds.
2169 as a calibration constant to adjust the nominal time offset of a
2170 particular clock to agree with an external standard, such as a
2171 precision PPS signal.
2172 It also provides a way to correct a
2173 systematic error or bias due to serial port or operating system
2174 latencies, different cable lengths or receiver internal delay.
2176 specified offset is in addition to the propagation delay provided
2177 by other means, such as internal DIPswitches.
2179 for an individual system and driver is available, an approximate
2180 correction is noted in the driver documentation pages.
2181 Note: in order to facilitate calibration when more than one
2182 radio clock or PPS signal is supported, a special calibration
2183 feature is available.
2184 It takes the form of an argument to the
2186 command described in
2187 <a href="#Miscellaneous-Options">Miscellaneous Options</a>
2188 page and operates as described in the
2189 "Reference Clock Drivers"
2191 (available as part of the HTML documentation
2193 <span class="file">/usr/share/doc/ntp</span>).
2194 <br><dt><code>time2</code> <kbd>secs</kbd><dd>Specifies a fixed-point decimal number in seconds, which is
2195 interpreted in a driver-dependent way.
2196 See the descriptions of
2197 specific drivers in the
2198 "Reference Clock Drivers"
2200 (available as part of the HTML documentation
2202 <span class="file">/usr/share/doc/ntp</span>).
2203 <br><dt><code>stratum</code> <kbd>int</kbd><dd>Specifies the stratum number assigned to the driver, an integer
2205 This number overrides the default stratum number
2206 ordinarily assigned by the driver itself, usually zero.
2207 <br><dt><code>refid</code> <kbd>string</kbd><dd>Specifies an ASCII string of from one to four characters which
2208 defines the reference identifier used by the driver.
2210 overrides the default identifier ordinarily assigned by the driver
2212 <br><dt><code>mode</code> <kbd>int</kbd><dd>Specifies a mode number which is interpreted in a
2213 device-specific fashion.
2214 For instance, it selects a dialing
2215 protocol in the ACTS driver and a device subtype in the
2218 <br><dt><code>flag1</code> <code>0</code> <code>|</code> <code>1</code><br><dt><code>flag2</code> <code>0</code> <code>|</code> <code>1</code><br><dt><code>flag3</code> <code>0</code> <code>|</code> <code>1</code><br><dt><code>flag4</code> <code>0</code> <code>|</code> <code>1</code><dd>These four flags are used for customizing the clock driver.
2220 interpretation of these values, and whether they are used at all,
2221 is a function of the particular clock driver.
2225 is used to enable recording monitoring
2227 <code>clockstats</code>
2228 file configured with the
2229 <code>filegen</code>
2231 Further information on the
2232 <code>filegen</code>
2233 command can be found in
2234 <a href="#Monitoring-Options">Monitoring Options</a>.
2239 <a name="Miscellaneous-Options"></a>
2243 <h4 class="subsection">Miscellaneous Options</h4>
2246 <dt><code>broadcastdelay</code> <kbd>seconds</kbd><dd>The broadcast and multicast modes require a special calibration
2247 to determine the network delay between the local and remote
2249 Ordinarily, this is done automatically by the initial
2250 protocol exchanges between the client and server.
2252 the calibration procedure may fail due to network or server access
2253 controls, for example.
2254 This command specifies the default delay to
2255 be used under these circumstances.
2256 Typically (for Ethernet), a
2257 number between 0.003 and 0.007 seconds is appropriate.
2259 when this command is not used is 0.004 seconds.
2260 <br><dt><code>calldelay</code> <kbd>delay</kbd><dd>This option controls the delay in seconds between the first and second
2261 packets sent in burst or iburst mode to allow additional time for a modem
2262 or ISDN call to complete.
2263 <br><dt><code>driftfile</code> <kbd>driftfile</kbd><dd>This command specifies the complete path and name of the file used to
2264 record the frequency of the local clock oscillator.
2268 command line option.
2269 If the file exists, it is read at
2270 startup in order to set the initial frequency and then updated once per
2271 hour with the current frequency computed by the daemon.
2273 specified, but the file itself does not exist, the starts with an initial
2274 frequency of zero and creates the file when writing it for the first time.
2275 If this command is not given, the daemon will always start with an initial
2278 <p>The file format consists of a single line containing a single
2279 floating point number, which records the frequency offset measured
2280 in parts-per-million (PPM).
2281 The file is updated by first writing
2282 the current drift value into a temporary file and then renaming
2283 this file to replace the old version.
2285 <code>ntpd(1ntpdmdoc)</code>
2286 must have write permission for the directory the
2287 drift file is located in, and that file system links, symbolic or
2288 otherwise, should be avoided.
2289 <br><dt><code>dscp</code> <kbd>value</kbd><dd>This option specifies the Differentiated Services Control Point (DSCP) value,
2290 a 6-bit code. The default value is 46, signifying Expedited Forwarding.
2291 <br><dt><code>enable</code> <code>[auth | bclient | calibrate | kernel | mode7 | monitor | ntp | stats]</code><br><dt><code>disable</code> <code>[auth | bclient | calibrate | kernel | mode7 | monitor | ntp | stats]</code><dd>Provides a way to enable or disable various server options.
2292 Flags not mentioned are unaffected.
2293 Note that all of these flags
2294 can be controlled remotely using the
2295 <code>ntpdc(1ntpdcmdoc)</code>
2298 <dt><code>auth</code><dd>Enables the server to synchronize with unconfigured peers only if the
2299 peer has been correctly authenticated using either public key or
2300 private key cryptography.
2301 The default for this flag is
2302 <code>enable</code>.
2303 <br><dt><code>bclient</code><dd>Enables the server to listen for a message from a broadcast or
2304 multicast server, as in the
2305 <code>multicastclient</code>
2306 command with default
2308 The default for this flag is
2309 <code>disable</code>.
2310 <br><dt><code>calibrate</code><dd>Enables the calibrate feature for reference clocks.
2313 <code>disable</code>.
2314 <br><dt><code>kernel</code><dd>Enables the kernel time discipline, if available.
2315 The default for this
2318 if support is available, otherwise
2319 <code>disable</code>.
2320 <br><dt><code>mode7</code><dd>Enables processing of NTP mode 7 implementation-specific requests
2321 which are used by the deprecated
2322 <code>ntpdc(1ntpdcmdoc)</code>
2324 The default for this flag is disable.
2325 This flag is excluded from runtime configuration using
2326 <code>ntpq(1ntpqmdoc)</code>.
2328 <code>ntpq(1ntpqmdoc)</code>
2329 program provides the same capabilities as
2330 <code>ntpdc(1ntpdcmdoc)</code>
2331 using standard mode 6 requests.
2332 <br><dt><code>monitor</code><dd>Enables the monitoring facility.
2334 <code>ntpdc(1ntpdcmdoc)</code>
2337 <code>monlist</code>
2338 command or further information.
2340 default for this flag is
2341 <code>enable</code>.
2342 <br><dt><code>ntp</code><dd>Enables time and frequency discipline.
2343 In effect, this switch opens and
2344 closes the feedback loop, which is useful for testing.
2347 <code>enable</code>.
2348 <br><dt><code>stats</code><dd>Enables the statistics facility.
2350 <a href="#Monitoring-Options">Monitoring Options</a>
2351 section for further information.
2352 The default for this flag is
2353 <code>disable</code>.
2355 <br><dt><code>includefile</code> <kbd>includefile</kbd><dd>This command allows additional configuration commands
2356 to be included from a separate file.
2358 be nested to a depth of five; upon reaching the end of any
2359 include file, command processing resumes in the previous
2361 This option is useful for sites that run
2362 <code>ntpd(1ntpdmdoc)</code>
2363 on multiple hosts, with (mostly) common options (e.g., a
2365 <br><dt><code>leapsmearinterval</code> <kbd>seconds</kbd><dd>This EXPERIMENTAL option is only available if
2366 <code>ntpd(1ntpdmdoc)</code>
2368 <code>--enable-leap-smear</code>
2370 <code>configure</code>
2372 It specifies the interval over which a leap second correction will be applied.
2373 Recommended values for this option are between
2374 7200 (2 hours) and 86400 (24 hours).
2375 .Sy DO NOT USE THIS OPTION ON PUBLIC-ACCESS SERVERS!
2376 See http://bugs.ntp.org/2855 for more information.
2377 <br><dt><code>logconfig</code> <kbd>configkeyword</kbd><dd>This command controls the amount and type of output written to
2379 <code>syslog(3)</code>
2380 facility or the alternate
2381 <code>logfile</code>
2383 By default, all output is turned on.
2385 <kbd>configkeyword</kbd>
2386 keywords can be prefixed with
2394 <code>syslog(3)</code>
2401 <code>syslog(3)</code>
2402 messages can be controlled in four
2404 (<code>clock</code>, <code>peer</code>, <code>sys</code> and <code>sync</code>).
2405 Within these classes four types of messages can be
2406 controlled: informational messages
2407 (<code>info</code>),
2409 (<code>events</code>),
2411 (<code>statistics</code>)
2414 (<code>status</code>).
2416 <p>Configuration keywords are formed by concatenating the message class with
2420 prefix can be used instead of a message class.
2422 message class may also be followed by the
2424 keyword to enable/disable all
2425 messages of the respective message class.Thus, a minimal log configuration
2426 could look like this:
2427 <pre class="verbatim">
2428 logconfig =syncstatus +sysevents
2431 <p>This would just list the synchronizations state of
2432 <code>ntpd(1ntpdmdoc)</code>
2433 and the major system events.
2434 For a simple reference server, the
2435 following minimum message configuration could be useful:
2436 <pre class="verbatim">
2437 logconfig =syncall +clockall
2440 <p>This configuration will list all clock information and
2441 synchronization information.
2442 All other events and messages about
2443 peers, system events and so on is suppressed.
2444 <br><dt><code>logfile</code> <kbd>logfile</kbd><dd>This command specifies the location of an alternate log file to
2445 be used instead of the default system
2446 <code>syslog(3)</code>
2448 This is the same operation as the -l command line option.
2449 <br><dt><code>setvar</code> <kbd>variable</kbd> <code>[default]</code><dd>This command adds an additional system variable.
2451 variables can be used to distribute additional information such as
2453 If the variable of the form
2454 <code>name</code><code>=</code><kbd>value</kbd>
2456 <code>default</code>
2458 variable will be listed as part of the default system variables
2459 (<code>rv</code> command)).
2460 These additional variables serve
2461 informational purposes only.
2462 They are not related to the protocol
2463 other that they can be listed.
2464 The known protocol variables will
2465 always override any variables defined via the
2468 There are three special variables that contain the names
2469 of all variable of the same group.
2471 <code>sys_var_list</code>
2473 the names of all system variables.
2475 <code>peer_var_list</code>
2477 the names of all peer variables and the
2478 <code>clock_var_list</code>
2479 holds the names of the reference clock variables.
2480 <br><dt><code>tinker</code> <code>[allan </code><kbd>allan</kbd><code> | dispersion </code><kbd>dispersion</kbd><code> | freq </code><kbd>freq</kbd><code> | huffpuff </code><kbd>huffpuff</kbd><code> | panic </code><kbd>panic</kbd><code> | step </code><kbd>step</kbd><code> | stepback </code><kbd>stepback</kbd><code> | stepfwd </code><kbd>stepfwd</kbd><code> | stepout </code><kbd>stepout</kbd><code>]</code><dd>This command can be used to alter several system variables in
2481 very exceptional circumstances.
2482 It should occur in the
2483 configuration file before any other configuration options.
2485 default values of these variables have been carefully optimized for
2486 a wide range of network speeds and reliability expectations.
2488 general, they interact in intricate ways that are hard to predict
2489 and some combinations can result in some very nasty behavior.
2491 rarely is it necessary to change the default values; but, some
2492 folks cannot resist twisting the knobs anyway and this command is
2494 Emphasis added: twisters are on their own and can expect
2495 no help from the support group.
2497 <p>The variables operate as follows:
2499 <dt><code>allan</code> <kbd>allan</kbd><dd>The argument becomes the new value for the minimum Allan
2500 intercept, which is a parameter of the PLL/FLL clock discipline
2502 The value in log2 seconds defaults to 7 (1024 s), which is also the lower
2504 <br><dt><code>dispersion</code> <kbd>dispersion</kbd><dd>The argument becomes the new value for the dispersion increase rate,
2505 normally .000015 s/s.
2506 <br><dt><code>freq</code> <kbd>freq</kbd><dd>The argument becomes the initial value of the frequency offset in
2508 This overrides the value in the frequency file, if
2509 present, and avoids the initial training state if it is not.
2510 <br><dt><code>huffpuff</code> <kbd>huffpuff</kbd><dd>The argument becomes the new value for the experimental
2511 huff-n'-puff filter span, which determines the most recent interval
2512 the algorithm will search for a minimum delay.
2514 900 s (15 m), but a more reasonable value is 7200 (2 hours).
2516 is no default, since the filter is not enabled unless this command
2518 <br><dt><code>panic</code> <kbd>panic</kbd><dd>The argument is the panic threshold, normally 1000 s.
2520 the panic sanity check is disabled and a clock offset of any value will
2522 <br><dt><code>step</code> <kbd>step</kbd><dd>The argument is the step threshold, which by default is 0.128 s.
2524 be set to any positive number in seconds.
2525 If set to zero, step
2526 adjustments will never occur.
2527 Note: The kernel time discipline is
2528 disabled if the step threshold is set to zero or greater than the
2530 <br><dt><code>stepback</code> <kbd>stepback</kbd><dd>The argument is the step threshold for the backward direction,
2531 which by default is 0.128 s.
2533 be set to any positive number in seconds.
2534 If both the forward and backward step thresholds are set to zero, step
2535 adjustments will never occur.
2536 Note: The kernel time discipline is
2538 each direction of step threshold are either
2539 set to zero or greater than .5 second.
2540 <br><dt><code>stepfwd</code> <kbd>stepfwd</kbd><dd>As for stepback, but for the forward direction.
2541 <br><dt><code>stepout</code> <kbd>stepout</kbd><dd>The argument is the stepout timeout, which by default is 900 s.
2543 be set to any positive number in seconds.
2544 If set to zero, the stepout
2545 pulses will not be suppressed.
2547 <br><dt><code>rlimit</code> <code>[memlock </code><kbd>Nmegabytes</kbd><code> | stacksize </code><kbd>N4kPages</kbd><code> filenum </code><kbd>Nfiledescriptors</kbd><code>]</code><dd>
2549 <dt><code>memlock</code> <kbd>Nmegabytes</kbd><dd>Specify the number of megabytes of memory that should be
2550 allocated and locked.
2551 Probably only available under Linux, this option may be useful
2552 when dropping root (the
2555 The default is 32 megabytes on non-Linux machines, and -1 under Linux.
2556 -1 means "do not lock the process into memory".
2557 0 means "lock whatever memory the process wants into memory".
2558 <br><dt><code>stacksize</code> <kbd>N4kPages</kbd><dd>Specifies the maximum size of the process stack on systems with the
2559 <code>mlockall()</code>
2561 Defaults to 50 4k pages (200 4k pages in OpenBSD).
2562 <br><dt><code>filenum</code> <kbd>Nfiledescriptors</kbd><dd>Specifies the maximum number of file descriptors ntpd may have open at once. Defaults to the system default.
2564 <br><dt><code>trap</code> <kbd>host_address</kbd> <code>[port </code><kbd>port_number</kbd><code>]</code> <code>[interface </code><kbd>interface_address</kbd><code>]</code><dd>This command configures a trap receiver at the given host
2565 address and port number for sending messages with the specified
2566 local interface address.
2567 If the port number is unspecified, a value
2569 If the interface address is not specified, the
2570 message is sent with a source address of the local interface the
2571 message is sent through.
2572 Note that on a multihomed host the
2573 interface used may vary from time to time with routing changes.
2575 <p>The trap receiver will generally log event messages and other
2576 information from the server in a log file.
2578 programs may also request their own trap dynamically, configuring a
2579 trap receiver will ensure that no messages are lost when the server
2581 <br><dt><code>hop</code> <kbd>...</kbd><dd>This command specifies a list of TTL values in increasing order, up to 8
2582 values can be specified.
2583 In manycast mode these values are used in turn in
2584 an expanding-ring search.
2585 The default is eight multiples of 32 starting at
2589 <p>This section was generated by <strong>AutoGen</strong>,
2590 using the <code>agtexi-cmd</code> template and the option descriptions for the <code>ntp.conf</code> program.
2591 This software is released under the NTP license, <http://ntp.org/license>.
2594 <li><a accesskey="1" href="#ntp_002econf-Files">ntp.conf Files</a>: Files
2595 <li><a accesskey="2" href="#ntp_002econf-See-Also">ntp.conf See Also</a>: See Also
2596 <li><a accesskey="3" href="#ntp_002econf-Bugs">ntp.conf Bugs</a>: Bugs
2597 <li><a accesskey="4" href="#ntp_002econf-Notes">ntp.conf Notes</a>: Notes
2602 <a name="ntp_002econf-Files"></a>
2606 <h4 class="subsection">ntp.conf Files</h4>
2609 <dt><span class="file">/etc/ntp.conf</span><dd>the default name of the configuration file
2610 <br><dt><span class="file">ntp.keys</span><dd>private MD5 keys
2611 <br><dt><span class="file">ntpkey</span><dd>RSA private key
2612 <br><dt><span class="file">ntpkey_</span><kbd>host</kbd><dd>RSA public key
2613 <br><dt><span class="file">ntp_dh</span><dd>Diffie-Hellman agreement parameters
2617 <a name="ntp_002econf-See-Also"></a>
2621 <h4 class="subsection">ntp.conf See Also</h4>
2623 <p><code>ntpd(1ntpdmdoc)</code>,
2624 <code>ntpdc(1ntpdcmdoc)</code>,
2625 <code>ntpq(1ntpqmdoc)</code>
2627 <p>In addition to the manual pages provided,
2628 comprehensive documentation is available on the world wide web
2630 <code>http://www.ntp.org/</code>.
2631 A snapshot of this documentation is available in HTML format in
2632 <span class="file">/usr/share/doc/ntp</span>.
2636 David L. Mills, <em>Network Time Protocol (Version 4)</em>, RFC5905
2639 <a name="ntp_002econf-Bugs"></a>
2643 <h4 class="subsection">ntp.conf Bugs</h4>
2645 <p>The syntax checking is not picky; some combinations of
2646 ridiculous and even hilarious options and modes may not be
2650 <span class="file">ntpkey_</span><kbd>host</kbd>
2651 files are really digital
2653 These should be obtained via secure directory
2654 services when they become universally available.
2657 <a name="ntp_002econf-Notes"></a>
2661 <h4 class="subsection">ntp.conf Notes</h4>
2663 <p>This document was derived from FreeBSD.