SNTP(1) Programmer's Manual SNTP(1)NAMEsntp - standard SNTP program
SYNOPSISsntp [-flag [value]]... [--opt-name [[=| ]value]]...
All arguments must be options.
DESCRIPTION
This manual page documents, briefly, the sntp command. sntp can be
used as a SNTP client to query a NTP or SNTP server and either display
the time or set the local system's time (given suitable privilege). It
can be run as an interactive command or in a cron job. NTP is the Net‐
work Time Protocol (RFC 1305) and SNTP is the Simple Network Time Pro‐
tocol (RFC 2030, which supersedes RFC 1769).
Options
sntp recognizes the following options:
-v indicates that diagnostic messages for non-fatal errors and a
limited amount of tracing should be written to standard error.
Fatal ones always produce a diagnostic. This option should be
set when there is a suspected problem with the server, network
or the source.
-V requests more and less comprehensible output, mainly for inves‐
tigating problems with apparently inconsistent timestamps. This
option should be set when the program fails with a message indi‐
cating that is the trouble.
-W requests very verbose debugging output, and will interfere with
the timing when writing to the terminal (because of line
buffered output from C). Note that the times produced by this
are the corrections needed, and not the error in the local
clock. This option should be set only when debugging the
source.
-q indicates that it should query a daemon save file being main‐
tained by it. This needs no privilege and will change neither
the save file nor the clock.
The default is that it should behave as a client, and the following
options are then relevant:
-r indicates that the system clock should be reset by settimeofday.
Naturally, this will work only if the user has enough privilege.
-a indicates that the system clock should be reset by adjtime.
Naturally, this will work only if the user has enough privilege.
The default is to write the estimated correct local date and time (i.e.
not UTC) to the standard output in a format like '1996 Oct 15
20:17:25.123 + 4.567 +/- 0.089 secs', where the '+ 4.567 +/- 0.089
secs' indicates the estimated error in the time on the local system.
-l lockfile
sets the name of the lock file to ensure that there is only one
copy of sntp running at once. The default is installation-
dependent, but will usually be /etc/sntp.pid.
-e minerr
sets the maximum ignorable variation between the clocks to min‐
err. Acceptable values are from 0.001 to 1, and the default is
0.1 if a NTP host is is specified and 0.5 otherwise.
-E maxerr
sets the maximum value of various delays that are deemed accept‐
able to maxerr. Acceptable values are from 1 to 60, and the
default is 5. It should sometimes be increased if there are
problems with the network, NTP server or system clock, but take
care.
-P prompt
sets the maximum clock change that will be made automatically to
maxerr. Acceptable values are from 1 to 3600 or no, and the
default is 30. If the program is being run interactively in
ordinary client mode, and the system clock is to be changed,
larger corrections will prompt the user for confirmation. Spec‐
ifying no will disable this and the correction will be made
regardless.
-c count
sets the maximum number of NTP packets required to count.
Acceptable values are from 1 to 25 if a NTP host is specified
and from 5 to 25 otherwise, and the default is 5. If the maxi‐
mum isn't enough, the system needs a better consistency algo‐
rithm than this program uses.
-d delay
sets a rough limit on the total running time to delay seconds.
Acceptable values are from 1 to 3600, and the default is 15 if a
NTP host is specified and 300 otherwise.
-4 force IPv4 DNS resolution.
-6 force IPv6 DNS resolution.
address(es) are the DNS names or IP numbers of hosts to use for the
challenge and response protocol; if no names are given, the program
waits for broadcasts. Polling a server is vastly more reliable than
listening to broadcasts. Note that a single component numeric address
is not allowed, to avoid ambiguities. If more than one name is give,
they will be used in a round-robin fashion.
Constraints:
minerr must be less than maxerr which must be less than delay
(or, if a NTP host is not specified delay/count), and count must
be less than half of delay.
In update mode, maxerr must be less than prompt.
Note that none of the above values are closely linked to the limits
described in the NTP protocol (RFC 1305).
USAGE
The simplest use of this program is as an unprivileged command to check
the current time and error in the local clock. For example:
sntp ntpserver.somewhere
With suitable privilege, it can be run as a command or in a cron job to
reset the local clock from a reliable server, like the ntpdate and
rdate commands. For example:
sntp-a ntpserver.somewhere
More information on how to use this utility is given in the README file
in the distribution. In particular, this man page does not describe
how to set it up as a server, which needs special care to avoid propa‐
gating misinformation.
RETURN VALUE
When used as a client in non-daemon mode, the program returns a zero
exit status for success, and a non-zero one otherwise. When used as a
daemon (either client or server), it does not return except after a
serious error.
BUGS
The program implements the SNTP protocol, and does not provide all NTP
facilities. In particular, it contains no checks against any form of
spoofing. If this is a serious concern, some network security mecha‐
nism (like a firewall or even just tcpwrappers) should be installed.
There are some errors, ambiguities and inconsistencies in the RFCs, and
this code may not interwork with all other NTP implementations. Any
unreasonable restrictions should be reported as bugs to whoever is
responsible. It may be difficult to find out who that is.
The program will stop as soon as it feels that things have got out of
control. In client daemon mode, it will usually fail during an
extended period of network or server inaccessibility or excessively
slow performance, or when the local clock is reset by another process.
It will then need restarting manually. Experienced system administra‐
tors can write a shell script, a cron job or put it in inittab, to do
this automatically.
The error cannot be estimated reliably with broadcast packets or for
the drift in daemon mode (even with client-server packets), and the
guess made by the program may be wrong (possibly even very wrong). If
this is a problem, then setting the -c option to a larger value may
help. Or it may not.
AUTHORsntp was developed by N.M. Maclaren of the University of Cambridge Com‐
puting Service.
OPTIONS-4, --ipv4
Force IPv4 DNS name resolution. This option is a member of the
ipv4 class of options.
Force DNS resolution of following host names on the command line
to the IPv4 namespace.
-6, --ipv6
Force IPv6 DNS name resolution. This option is a member of the
ipv4 class of options.
Force DNS resolution of following host names on the command line
to the IPv6 namespace.
-u, --unprivport
Use an unprivileged port.
Use an unprivilegded UDP port for our queries.
-v, --normalverbose
Slightly verbose. This option must not appear in combination
with any of the following options: extraverbose, megaverbose.
Diagnostic messages for non-fatal errors and a limited amount of
tracing should be written to standard error. Fatal ones always
produce a diagnostic. This option should be set when there is a
suspected problem with the server, network or the source.
-V, --extraverbose
Extra verbose. This option must not appear in combination with
any of the following options: normalverbose, megaverbose.
Produce more and less comprehensible output, mainly for investi‐
gating problems with apparently inconsistent timestamps. This
option should be set when the program fails with a message indi‐
cating that is the trouble.
-W, --megaverbose
Mega verbose. This option must not appear in combination with
any of the following options: normalverbose, extraverbose.
Very verbose debugging output that will interfere with the tim‐
ing when writing to the terminal (because of line buffered out‐
put from C). Note that the times produced by this are the cor‐
rections needed, and not the error in the local clock. This
option should be set only when debugging the source.
-r, --settimeofday
Set (step) the time with settimeofday(). This option must not
appear in combination with any of the following options: adj‐
time.
-a, --adjtime
Set (slew) the time with adjtime(). This option must not appear
in combination with any of the following options: settimeofday.
-?, --help
Display usage information and exit.
-!, --more-help
Extended usage information passed thru pager.
-> [rcfile], --save-opts[=rcfile]
Save the option state to rcfile. The default is the last con‐
figuration file listed in the OPTION PRESETS section, below.
-< rcfile, --load-opts=rcfile, --no-load-opts
Load options from rcfile. The no-load-opts form will disable
the loading of earlier RC/INI files. --no-load-opts is handled
early, out of order.
-v [{v|c|n}], --version[={v|c|n}]
Output version of program and exit. The default mode is `v', a
simple version. The `c' mode will print copyright information
and `n' will print the full copyright notice.
OPTION PRESETS
Any option that is not marked as not presettable may be preset by load‐
ing values from configuration ("RC" or ".INI") file(s) and values from
environment variables named:
SNTP_<option-name> or SNTP
The environmental presets take precedence (are processed later than)
the configuration files. The homerc files are "$HOME", and ".". If
any of these are directories, then the file .ntprc is searched for
within those directories.
AUTHOR
ntp.org
Please send bug reports to: http://bugs.ntp.org, bugs@ntp.org
General Public Licence for the software known as MSNTP
------------------------------------------------------
(c) Copyright, N.M. Maclaren, 1996, 1997, 2000
(c) Copyright, University of Cambridge, 1996, 1997, 2000
Free use of MSNTP in source and binary forms is permitted, provided that this
entire licence is duplicated in all copies, and that any documentation,
announcements, and other materials related to use acknowledge that the software
was developed by N.M. Maclaren (hereafter refered to as the Author) at the
University of Cambridge. Neither the name of the Author nor the University of
Cambridge may be used to endorse or promote products derived from this material
without specific prior written permission.
The Author and the University of Cambridge retain the copyright and all other
legal rights to the software and make it available non-exclusively. All users
must ensure that the software in all its derivations carries a copyright notice
in the form:
(c) Copyright N.M. Maclaren,
(c) Copyright University of Cambridge.
NO WARRANTY
Because the MSNTP software is licensed free of charge, the Author and the
University of Cambridge provide absolutely no warranty, either expressed or
implied, including, but not limited to, the implied warranties of
merchantability and fitness for a particular purpose. The entire risk as to
the quality and performance of the MSNTP software is with you. Should MSNTP
prove defective, you assume the cost of all necessary servicing or repair.
In no event, unless required by law, will the Author or the University of
Cambridge, or any other party who may modify and redistribute this software as
permitted in accordance with the provisions below, be liable for damages for
any losses whatsoever, including but not limited to lost profits, lost monies,
lost or corrupted data, or other special, incidental or consequential losses
that may arise out of the use or inability to use the MSNTP software.
COPYING POLICY
Permission is hereby granted for copying and distribution of copies of the
MSNTP source and binary files, and of any part thereof, subject to the
following licence conditions:
1. You may distribute MSNTP or components of MSNTP, with or without additions
developed by you or by others. No charge, other than an "at-cost" distribution
fee, may be charged for copies, derivations, or distributions of this material
without the express written consent of the copyright holders.
2. You may also distribute MSNTP along with any other product for sale,
provided that the cost of the bundled package is the same regardless of whether
MSNTP is included or not, and provided that those interested only in MSNTP must
be notified that it is a product freely available from the University of
Cambridge.
3. If you distribute MSNTP software or parts of MSNTP, with or without
additions developed by you or others, then you must either make available the
source to all portions of the MSNTP system (exclusive of any additions made by
you or by others) upon request, or instead you may notify anyone requesting
source that it is freely available from the University of Cambridge.
4. You may not omit any of the copyright notices on either the source files,
the executable files, or the documentation.
5. You may not omit transmission of this License agreement with whatever
portions of MSNTP that are distributed.
6. Any users of this software must be notified that it is without warranty or
guarantee of any nature, express or implied, nor is there any fitness for use
represented.
October 1996
April 1997
October 2000
This manual page was AutoGen-erated from the sntp option definitions.
( 4.2.4p8) 2009-12-08 SNTP(1)
