[packages/ntp.git] / ntpdate.8

.TH ntpdate 8
.SH NAME
ntpdate \- set the date and time via NTP
.SH SYNOPSIS
.B ntpdate 
.RB [\| \-bBdoqsuv \|] 
.RB [\| \-a 
.IR key \|] 
.RB [\| \-e 
.IR authdelay \|] 
.RB [\| \-k 
.IR keyfile \|]
.RB [\| \-o
.IR version \|]
.RB [\| \-p
.IR samples \|]
.RB [\| \-t
.IR timeout \|]
.IR server
.RB [\| ... \|]
.SH DESCRIPTION
.B ntpdate 
sets the local date and time by polling the Network Time
Protocol (NTP) server(s) given as the
.I server 
arguments to determine
the correct time. It must be run as root on the local host. A number
of samples are obtained from each of the servers specified and a
subset of the NTP clock filter and selection algorithms are applied to
select the best of these. Note that the accuracy and reliability of
ntpdate depends on the number of servers, the number of polls each
time it is run and the interval between runs.

ntpdate can be run manually as necessary to set the host clock, or it
can be run from the host startup script to set the clock at boot time.
This is useful in some cases to set the clock initially before
starting the NTP daemon ntpd. It is also possible to run ntpdate from
a cron script. However, it is important to note that ntpdate with
contrived cron scripts is no substitute for the NTP daemon, which uses
sophisticated algorithms to maximize accuracy and reliability while
minimizing resource use. Finally, since ntpdate does not discipline
the host clock frequency as does ntpd, the accuracy using ntpdate is
limited.

Time adjustments are made by ntpdate in one of two ways. If ntpdate
determines the clock is in error more than 0.5 second it will simply
step the time by calling the system settimeofday() routine. If the
error is less than 0.5 seconds, it will slew the time by calling the
system adjtime() routine. The latter technique is less disruptive and
more accurate when the error is small, and works quite well when
ntpdate is run by cron every hour or two.

ntpdate will decline to set the date if an NTP server daemon (e.g.,
ntpd) is running on the same host. When running ntpdate on a regular
basis from cron as an alternative to running a daemon, doing so once
every hour or two will result in precise enough timekeeping to avoid
stepping the clock.
.SH OPTIONS
.TP
.BI \-a \ key
Enable the authentication function and specify the key
identifier to be used for authentication as the argument
keyntpdate. The keys and key identifiers must match in both the
client and server key files. The default is to disable the
authentication function.
.TP
.B \-B
Force the time to always be slewed using the adjtime() system
call, even if the measured offset is greater than +-128 ms. The
default is to step the time using settimeofday() if the offset
is greater than +-128 ms. Note that, if the offset is much
greater than +-128 ms in this case, that it can take a long
time (hours) to slew the clock to the correct value. During
this time. the host should not be used to synchronize clients.
.TP
.B \-b
Force the time to be stepped using the settimeofday() system
call, rather than slewed (default) using the adjtime() system
call. This option should be used when called from a startup
file at boot time.
.TP
.B \-d
Enable the debugging mode, in which ntpdate will go through all
the steps, but not adjust the local clock. Information useful
for general debugging will also be printed.
.TP
.BI \-e \ authdelay
Specify the processing delay to perform an authentication
function as the value authdelay, in seconds and fraction (see
ntpd for details). This number is usually small enough to be
negligible for most purposes, though specifying a value may
improve timekeeping on very slow CPU's.
.TP
.BI \-k \ keyfile
Specify the path for the authentication key file as the string
keyfile. The default is /etc/ntp.keys. This file should be in
the format described in ntpd.
.TP
.BI \-o \ version
Specify the NTP version for outgoint packets as the integer
version, which can be 1 or 2. The default is 3. This allows
ntpdate to be used with older NTP versions.
.TP
.BI \-p \ samples
Specify the number of samples to be acquired from each server
as the integer samples, with values from 1 to 8 inclusive. The
default is 4.
.TP
.B \-q
Query only - don't set the clock.
.TP
.B \-s
Divert logging output from the standard output (default) to the
system syslog facility. This is designed primarily for
convenience of cron scripts.
.TP
.BI \-t \ timeout
Specify the maximum time waiting for a server response as the
value timeout, in seconds and fraction. The value is is rounded
to a multiple of 0.2 seconds. The default is 1 second, a value
suitable for polling across a LAN.
.TP
.B \-u
Direct ntpdate to use an unprivileged port or outgoing packets.
This is most useful when behind a firewall that blocks incoming
traffic to privileged ports, and you want to synchronise with
hosts beyond the firewall. Note that the -d option always uses
unprivileged ports.
.TP
.B \-v
Be verbose. This option will cause ntpdate's version
identification string to be logged.
.SH FILES
.TP
.I /etc/ntp.keys 
\- encryption keys used by ntpdate.
.SH BUGS
The slew adjustment is actually 50% larger than the measured offset,
since this (it is argued) will tend to keep a badly drifting clock
more accurate. This is probably not a good idea and may cause a
troubling hunt for some values of the kernel variables tick and
tickadj.
.SH AUTHOR
David L. Mills (mills@udel.edu)
.br
This manpage converted from html to roff by 
Fabrizio Polacco <fpolacco@debian.org>
Commit	Line	Data
c53ea2f8	1	.TH ntpdate 8
	2	.SH NAME
	3	ntpdate \- set the date and time via NTP
	4	.SH SYNOPSIS
	5	.B ntpdate
	6	.RB [\\| \-bBdoqsuv \\|]
	7	.RB [\\| \-a
	8	.IR key \\|]
	9	.RB [\\| \-e
	10	.IR authdelay \\|]
	11	.RB [\\| \-k
	12	.IR keyfile \\|]
	13	.RB [\\| \-o
	14	.IR version \\|]
	15	.RB [\\| \-p
	16	.IR samples \\|]
	17	.RB [\\| \-t
	18	.IR timeout \\|]
	19	.IR server
	20	.RB [\\| ... \\|]
	21	.SH DESCRIPTION
	22	.B ntpdate
	23	sets the local date and time by polling the Network Time
	24	Protocol (NTP) server(s) given as the
	25	.I server
	26	arguments to determine
	27	the correct time. It must be run as root on the local host. A number
	28	of samples are obtained from each of the servers specified and a
	29	subset of the NTP clock filter and selection algorithms are applied to
	30	select the best of these. Note that the accuracy and reliability of
	31	ntpdate depends on the number of servers, the number of polls each
	32	time it is run and the interval between runs.
	33
	34	ntpdate can be run manually as necessary to set the host clock, or it
	35	can be run from the host startup script to set the clock at boot time.
	36	This is useful in some cases to set the clock initially before
	37	starting the NTP daemon ntpd. It is also possible to run ntpdate from
	38	a cron script. However, it is important to note that ntpdate with
	39	contrived cron scripts is no substitute for the NTP daemon, which uses
	40	sophisticated algorithms to maximize accuracy and reliability while
	41	minimizing resource use. Finally, since ntpdate does not discipline
	42	the host clock frequency as does ntpd, the accuracy using ntpdate is
	43	limited.
	44
	45	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
	51	ntpdate is run by cron every hour or two.
	52
	53	ntpdate will decline to set the date if an NTP server daemon (e.g.,
	54	ntpd) is running on the same host. When running ntpdate on a regular
	55	basis from cron as an alternative to running a daemon, doing so once
	56	every hour or two will result in precise enough timekeeping to avoid
	57	stepping the clock.
	58	.SH OPTIONS
	59	.TP
	60	.BI \-a \ key
	61	Enable the authentication function and specify the key
	62	identifier to be used for authentication as the argument
	63	keyntpdate. The keys and key identifiers must match in both the
	64	client and server key files. The default is to disable the
65	authentication function.
66	.TP
67	.B \-B
68	Force the time to always be slewed using the adjtime() system
69	call, even if the measured offset is greater than +-128 ms. The
70	default is to step the time using settimeofday() if the offset
71	is greater than +-128 ms. Note that, if the offset is much
72	greater than +-128 ms in this case, that it can take a long
73	time (hours) to slew the clock to the correct value. During
74	this time. the host should not be used to synchronize clients.
75	.TP
76	.B \-b
77	Force the time to be stepped using the settimeofday() system
78	call, rather than slewed (default) using the adjtime() system
79	call. This option should be used when called from a startup
80	file at boot time.
81	.TP
82	.B \-d
83	Enable the debugging mode, in which ntpdate will go through all
84	the steps, but not adjust the local clock. Information useful
85	for general debugging will also be printed.
86	.TP
87	.BI \-e \ authdelay
88	Specify the processing delay to perform an authentication
89	function as the value authdelay, in seconds and fraction (see
90	ntpd for details). This number is usually small enough to be
91	negligible for most purposes, though specifying a value may
92	improve timekeeping on very slow CPU's.
93	.TP
94	.BI \-k \ keyfile
95	Specify the path for the authentication key file as the string
96	keyfile. The default is /etc/ntp.keys. This file should be in
97	the format described in ntpd.
98	.TP
99	.BI \-o \ version
100	Specify the NTP version for outgoint packets as the integer
101	version, which can be 1 or 2. The default is 3. This allows
102	ntpdate to be used with older NTP versions.
103	.TP
104	.BI \-p \ samples
105	Specify the number of samples to be acquired from each server
106	as the integer samples, with values from 1 to 8 inclusive. The
107	default is 4.
108	.TP
109	.B \-q
110	Query only - don't set the clock.
111	.TP
112	.B \-s
113	Divert logging output from the standard output (default) to the
114	system syslog facility. This is designed primarily for
115	convenience of cron scripts.
116	.TP
117	.BI \-t \ timeout
118	Specify the maximum time waiting for a server response as the
119	value timeout, in seconds and fraction. The value is is rounded
120	to a multiple of 0.2 seconds. The default is 1 second, a value
121	suitable for polling across a LAN.
122	.TP
123	.B \-u
124	Direct ntpdate to use an unprivileged port or outgoing packets.
125	This is most useful when behind a firewall that blocks incoming
126	traffic to privileged ports, and you want to synchronise with
127	hosts beyond the firewall. Note that the -d option always uses
128	unprivileged ports.
129	.TP
130	.B \-v
131	Be verbose. This option will cause ntpdate's version
132	identification string to be logged.
133	.SH FILES
134	.TP
135	.I /etc/ntp.keys
136	\- encryption keys used by ntpdate.
137	.SH BUGS
138	The slew adjustment is actually 50% larger than the measured offset,
139	since this (it is argued) will tend to keep a badly drifting clock
140	more accurate. This is probably not a good idea and may cause a
141	troubling hunt for some values of the kernel variables tick and
142	tickadj.
143	.SH AUTHOR
144	David L. Mills (mills@udel.edu)
145	.br
146	This manpage converted from html to roff by
147	Fabrizio Polacco <fpolacco@debian.org>
148