1ntpdate(8) System Manager's Manual ntpdate(8)
2
6 ntpdate - set the date and time via NTP
7 Disclaimer: This program has known bugs and deficiencies and nobody has
10 volunteered to fix them in a long time. The good news is the function‐
11 ality originally intended for this program is available in the ntpd and
12 sntp programs. See the Deprecating ntpdate topic in the NTP Support
13 wiki for a thorough discussion and analysis of the issues. See the -q
14 command line option in the ntpd - Network Time Protocol (NTP) daemon
15 page and/or the sntp - Simple Network Time Protocol (SNTP) Client page.
16 After a suitable period of mourning, the ntpdate program will be
17 retired from this distribution.
18
21 ntpdate [ -46bBdqsuv ] [ -a key ] [ -e authdelay ] [ -k keyfile ] [ -o
22 version ] [ -p samples ] [ -t timeout ] server [ ... ]
23
26 ntpdate sets the local date and time by polling the Network Time Proto‐
27 col (NTP) server(s) given as the server arguments to determine the cor‐
28 rect time. It must be run as root on the local host. A number of sam‐
29 ples are obtained from each of the servers specified and a subset of
30 the NTP clock filter and selection algorithms are applied to select the
31 best of these. Note that the accuracy and reliability of ntpdate
32 depends on the number of servers, the number of polls each time it is
33 run and the interval between runs.
34 ntpdate can be run manually as necessary to set the host clock, or it
36 can be run from the host startup script to set the clock at boot time.
37 This is useful in some cases to set the clock initially before starting
38 the NTP daemon ntpd. It is also possible to run ntpdate from a cron
39 script. However, it is important to note that ntpdate with contrived
40 cron scripts is no substitute for the NTP daemon, which uses sophisti‐
41 cated algorithms to maximize accuracy and reliability while minimizing
42 resource use. Finally, since ntpdate does not discipline the host clock
43 frequency as does ntpd, the accuracy using ntpdate is limited.
44 Time adjustments are made by ntpdate in one of two ways. If ntpdate
46 determines the clock is in error more than 0.5 second it will simply
47 step the time by calling the system settimeofday() routine. If the
48 error is less than 0.5 seconds, it will slew the time by calling the
49 system adjtime() routine. The latter technique is less disruptive and
50 more accurate when the error is small, and works quite well when ntp‐
51 date is run by cron every hour or two.
52 ntpdate will, if the -u flag was not specified, decline to set the date
54 if an NTP server daemon (e.g., ntpd) is running on the same host. When
55 running ntpdate on a regular basis from cron as an alternative to run‐
56 ning a daemon, doing so once every hour or two will result in precise
57 enough timekeeping to avoid stepping the clock.
58 Note that in contexts where a host name is expected, a -4 qualifier
60 preceding the host name forces DNS resolution to the IPv4 namespace,
61 while a -6 qualifier forces DNS resolution to the IPv6 namespace.
62 If NetInfo support is compiled into ntpdate, then the server argument
64 is optional if ntpdate can find a time server in the NetInfo configura‐
65 tion for ntpd.
66
69 -4 Force DNS resolution of following host names on the command
70 line to the IPv4 namespace.
71 -6 Force DNS resolution of following host names on the command
73 line to the IPv6 namespace.
74 -a key Enable the authentication function and specify the key identi‐
76 fier to be used for authentication as the argument key. The
77 keys and key identifiers must match in both the client and
78 server key files. The default is to disable the authentication
79 function.
80 -B Force the time to always be slewed using the adjtime() system
82 call, even if the measured offset is greater than +-500 ms. The
83 default is to step the time using settimeofday() if the offset
84 is greater than +-500 ms. Note that, if the offset is much
85 greater than +-500 ms in this case, that it can take a long
86 time (hours) to slew the clock to the correct value. During
87 this time. the host should not be used to synchronize clients.
88 -b Force the time to be stepped using the settimeofday() system
90 call, rather than slewed (default) using the adjtime() system
91 call. This option should be used when called from a startup
92 file at boot time.
93 -d Enable the debugging mode, in which ntpdate will go through all
95 the steps, but not adjust the local clock and using an unprivi‐
96 leged port. Information useful for general debugging will also
97 be printed.
98 -e authdelay
100 Specify the processing delay to perform an authentication func‐
101 tion as the value authdelay, in seconds and fraction (see ntpd
102 for details). This number is usually small enough to be negli‐
103 gible for most purposes, though specifying a value may improve
104 timekeeping on very slow CPU's.
105 -k keyfile
107 Specify the path for the authentication key file as the string
108 keyfile. The default is /etc/ntp.keys. This file should be in
109 the format described in ntpd.
110 -o version
112 Specify the NTP version for outgoing packets as the integer
113 version, which can be 1, 2, 3 or 4. The default is 4. This
114 allows ntpdate to be used with older NTP versions.
115 -p samples
117 Specify the number of samples to be acquired from each server
118 as the integer samples, with values from 1 to 8 inclusive. The
119 default is 4.
120 -q Query only - don't set the clock.
122 -s Divert logging output from the standard output (default) to the
124 system syslog facility. This is designed primarily for conve‐
125 nience of cron scripts.
126 -t timeout
128 Specify the maximum time waiting for a server response as the
129 value timeout, in seconds and fraction. The value is is rounded
130 to a multiple of 0.2 seconds. The default is 2 seconds, a value
131 suitable for polling across a LAN.
132 -u Direct ntpdate to use an unprivileged port for outgoing pack‐
134 ets. This is most useful when behind a firewall that blocks
135 incoming traffic to privileged ports, and you want to synchro‐
136 nize with hosts beyond the firewall. Note that the -d option
137 always uses unprivileged ports.
138 -v Be verbose. This option will cause ntpdate's version identifi‐
140 cation string to be logged.
141
144 ntpdate's exit status is zero if it finds a server and updates the
145 clock, and nonzero otherwise.
146
149 /etc/ntp.keys - encryption keys used by ntpdate.
150
153 The slew adjustment is actually 50% larger than the measured offset,
154 since this (it is argued) will tend to keep a badly drifting clock more
155 accurate. This is probably not a good idea and may cause a troubling
156 hunt for some values of the kernel variables tick and tickadj.
157
159 ntpd(8)
160 The official HTML documentation.
162 This file was automatically generated from HTML source.
164 ntpdate(8)