1ntp.conf(5) File Formats Manual ntp.conf(5)
2
6 ntp.conf - Server Options
7 Following is a description of the configuration commands in NTPv4.
10 There are two classes of commands, configuration commands that config‐
11 ure an association with a remote server, peer or reference clock, and
12 auxilliary commands that specify environmental variables that control
13 various related operations.
14 The various modes described on the Association Management page are
16 determined by the command keyword and the DNS name or IP address.
17 Addresses are classed by type as (s) a remote server or peer (IPv4
18 class A, B and C), (b) the IP broadcast address of a local interface,
19 (m) a multicast address (IPv4 class D), or (r) a reference clock
20 address (127.127.x.x). For type m addresses the IANA has assigned the
21 multicast group address IPv4 224.0.1.1 and IPv6 ff05::101 (site local)
22 exclusively to NTP, but other nonconflicting addresses can be used.
23 If the Basic Socket Interface Extensions for IPv6 (RFC-2553) is
25 detected, support for the IPv6 address family is generated in addition
26 to the default IPv4 address family. IPv6 addresses can be identified by
27 the presence of colons ":" in the address field. IPv6 addresses can be
28 used almost everywhere where IPv4 addresses can be used, with the
29 exception of reference clock addresses, which are always IPv4. Note
30 that in contexts where a host name is expected, a -4 qualifier preced‐
31 ing the host name forces DNS resolution to the IPv4 namespace, while a
32 -6 qualifier forces DNS resolution to the IPv6 namespace.
33
36 server address [options ...]
37 peer address [options ...]
39 broadcast address [options ...]
41 manycastclient address [options ...]
43 pool address [options ...]
45 unpeer [address | associd]
47 These commands specify the time server name or address to be
48 used and the mode in which to operate. The address can be
49 either a DNS name or a IPv4 or IPv6 address in standard nota‐
50 tion. In general, multiple commands of each type can be used
51 for different server and peer addresses or multicast groups.
52 server For type s and r addresses (only), this command mobi‐
54 lizes a persistent client mode association with the
55 specified remote server or local reference clock. If
56 the preempt flag is specified, a preemptable client
57 mode association is mobilized instead.
58 peer For type s addresses (only), this command mobilizes a
60 persistent symmetric-active mode association with the
61 specified remote peer.
62 broadcast
64 For type b and m addressees (only), this command mobi‐
65 lizes a persistent broadcast or multicast server mode
66 association. Note that type b messages go only to the
67 interface specified, but type m messages go to all
68 interfaces.
69 manycastclient
71 For type m addresses (only), this command mobilizes a
72 manycast client mode association for the multicast
73 group address specified. In this mode the address must
74 match the address specified on the manycastserver com‐
75 mand of one or more designated manycast servers.
76 pool For type s messages (only) this command mobilizes a
78 client mode association for servers implementing the
79 pool automatic server discovery scheme described on the
80 Association Management page. The address is a DNS name
81 in the form area.pool.ntp.org, where area is a quali‐
82 fier designating the server geographic area such as us
83 or europe.
84 unpeer This command removes a previously configured associa‐
86 tion. An address or association ID can be used to iden‐
87 tify the association. Either an IP address or DNS name
88 can be used. This command is most useful when supplied
89 via ntpq runtime configuration commands :config and
90 config-from-file.
91
95 autokey Send and receive packets authenticated by the Autokey scheme
96 described in the Authentication Options page. This option is
97 mutually exclusive with the key option.
98 burst When the server is reachable, send a burst of eight packets
100 instead of the usual one. The packet spacing is normally 2 s;
101 however, the spacing between the first and second packets can
102 be changed with the calldelay command to allow additional time
103 for a modem or ISDN call to complete. This option is valid only
104 with the server command and type s addressesa. It is a recom‐
105 mended option when the maxpoll option is greater than 10 (1024
106 s).
107 iburst When the server is unreachable, send a burst of eight packets
109 instead of the usual one. The packet spacing is normally 2 s;
110 however, the spacing between the first and second packets can
111 be changed with the calldelay command to allow additional time
112 for a modem or ISDN call to complete. This option is valid only
113 with the server command and type s addresses. It is a recom‐
114 mended option with this command.
115 key key Send and receive packets authenticated by the symmetric key
117 scheme described in the Authentication Options page. The key
118 specifies the key identifier with values from 1 to 65534,
119 inclusive. This option is mutually exclusive with the autokey
120 option.
121 minpoll minpoll
123 maxpoll maxpoll
125 These options specify the minimum and maximum poll intervals
126 for NTP messages, in seconds as a power of two. The maximum
127 poll interval defaults to 10 (1024 s), but can be increased by
128 the maxpoll option to an upper limit of 17 (36 h). The minimum
129 poll interval defaults to 6 (64 s), but can be decreased by the
130 minpoll option to a lower limit of 3 (8 s).
131 mode option
133 Pass the option to a reference clock driver, where option is an
134 integer in the range from 0 to 255, inclusive. This option is
135 valid only with type r addresses.
136 noselect
138 Marks the server or peer to be ignored by the selection algo‐
139 rithm but visible to the monitoring program. This option is
140 ignored with the broadcast command.
141 preempt Specifies the association as preemptable rather than the
143 default persistent. This option is ignored with the broadcast
144 command and is most useful with the manycastclient and pool
145 commands.
146 prefer Mark the server as preferred. All other things being equal,
148 this host will be chosen for synchronization among a set of
149 correctly operating hosts. See the Mitigation Rules and the
150 prefer Keyword page for further information. This option is
151 valid only with the server and peer commands.
152 true Mark the association to assume truechimer status; that is,
154 always survive the selection and clustering algorithms. This
155 option can be used with any association, but is most useful for
156 reference clocks with large jitter on the serial port and pre‐
157 cision pulse-per-second (PPS) signals. Caution: this option
158 defeats the algorithms designed to cast out falsetickers and
159 can allow these sources to set the system clock. This option is
160 valid only with the server and peer commands.
161 ttl ttl This option specifies the time-to-live ttl for the broadcast
163 command and the maximum ttl for the expanding ring search used
164 by the manycastclient command. Selection of the proper value,
165 which defaults to 127, is something of a black art and should
166 be coordinated with the network administrator. This option is
167 invalid with type r addresses.
168 version version
170 Specifies the version number to be used f or outgoing NTP pack‐
171 ets. Versions 1-4 are the choices, with version 4 the default.
172 xleave Operate in interleaved mode (symmetric and broadcast modes
174 only). (see NTP Interleaved Modes)
175
178 broadcastclient
179 Enable reception of broadcast server messages to any local
180 interface (type b address). Ordinarily, upon receiving a broad‐
181 cast message for the first time, the broadcast client measures
182 the nominal server propagation delay using a brief
183 client/server exchange, after which it continues in listen-only
184 mode. If a nonzero value is specified in the broadcastdelay
185 command, the value becomes the delay and the volley is not exe‐
186 cuted. Note: the novolley option has been deprecated for future
187 enhancements. Note that, in order to avoid accidental or mali‐
188 cious disruption in this mode, both the server and client
189 should operate using symmetric key or public key authentication
190 as described in the Authentication Options page. Note that the
191 novolley keyword is incompatible with public key authentica‐
192 tion.
193 manycastserver address [...]
195 Enable reception of manycast client messages (type m)to the
196 multicast group address(es) (type m) specified. At least one
197 address is required. Note that, in order to avoid accidental or
198 malicious disruption, both the server and client should operate
199 using symmetric key or public key authentication as described
200 in the Authentication Options page.
201 multicastclient address [...]
203 Enable reception of multicast server messages to the multicast
204 group address(es) (type m) specified. Upon receiving a message
205 for the first time, the multicast client measures the nominal
206 server propagation delay using a brief client/server exchange
207 with the server, then enters the broadcast client mode, in
208 which it synchronizes to succeeding multicast messages. Note
209 that, in order to avoid accidental or malicious disruption in
210 this mode, both the server and client should operate using sym‐
211 metric key or public key authentication as described in the
212 Authentication Options page.
213
216 The syntax checking is not picky; some combinations of ridiculous and
217 even hilarious options and modes may not be detected.
218
221 ntpd(8), ntp_auth(5), ntp_mon(5), ntp_acc(5), ntp_clock(5), ntp_misc(5)
222 The official HTML documentation.
224 This file was automatically generated from HTML source.
226 ntp.conf(5)