This document describes the use of the NTP Project's sntp program,
that can be used to query a Network Time Protocol (NTP) server and
display the time offset of the system clock relative to the server
clock. Run as root, it can correct the system clock to this offset as
well. It can be run as an interactive command or from a cron job.
This document applies to version 4.2.8p11 of sntp.
The program implements the SNTP protocol as defined by RFC 5905, the NTPv4 IETF specification.
By default, sntp writes the local data and time (i.e., not UTC) to the
standard output in the format:
1996-10-15 20:17:25.123 (+0800) +4.567 +/- 0.089 secs
where YYYY-MM-DD HH:MM:SS.SUBSEC is the local date and time, (+0800) is the local timezone adjustment (so we would add 8 hours and 0 minutes to convert the reported local time to UTC), and the +4.567 +/- 0.089 secs indicates the time offset and error bound of the system clock relative to the server clock.
sntp
can be used as an 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 from a
cron
job.
NTP (the Network Time Protocol) and SNTP (the Simple Network Time Protocol) are defined and described by RFC 5905.
The default is to write the estimated correct local date and time (i.e. not UTC) to the standard output in a format like:
'1996-10-15 20:17:25.123 (+0800) +4.567 +/- 0.089 [host] IP sN'
where the
'(+0800)'
means that to get to UTC from the reported local time one must
add 8 hours and 0 minutes,
the
'+4.567'
indicates the local clock is 4.567 seconds behind the correct time
(so 4.567 seconds must be added to the local clock to get it to be correct).
Note that the number of decimals printed for this value will change
based on the reported precision of the server.
'+/- 0.089'
is the reported
synchronization distance
(in seconds), which represents the maximum error due to all causes.
If the server does not report valid data needed to calculate the
synchronization distance, this will be reported as
'+/- ?'.
If the
host
is different from the
IP,
both will be displayed.
Otherwise, only the
IP
is displayed.
Finally, the
stratum
of the host is reported
and the leap indicator is decoded and displayed.
This section was generated by AutoGen,
using the agtexi-cmd template and the option descriptions for the sntp program.
This software is released under the NTP license, <http://ntp.org/license>.
This is the automatically generated usage text for sntp.
The text printed is the same whether selected with the help option
(--help) or the more-help option (--more-help). more-help will print
the usage text by passing it through a pager program.
more-help is disabled on platforms without a working
fork(2) function. The PAGER environment variable is
used to select the program, defaulting to more. Both will exit
with a status code of 0.
sntp - standard Simple Network Time Protocol client program - Ver. 4.2.7p245
USAGE: sntp [ -<flag> [<val>] | --<name>[{=| }<val>] ]... \
[ hostname-or-IP ...]
Flg Arg Option-Name Description
-4 no ipv4 Force IPv4 DNS name resolution
- prohibits these options:
ipv6
-6 no ipv6 Force IPv6 DNS name resolution
- prohibits these options:
ipv4
-a Num authentication Enable authentication with the key @var{auth-keynumber}
-B Num bctimeout The number of seconds to wait for broadcasts
-b Str broadcast Listen to the address specified for broadcast time sync
- may appear multiple times
-c Str concurrent Concurrently query all IPs returned for host-name
- may appear multiple times
-d no debug-level Increase debug verbosity level
- may appear multiple times
-D Str set-debug-level Set the debug verbosity level
- may appear multiple times
-g Num gap The gap (in milliseconds) between time requests
-K Fil kod KoD history filename
-k Fil keyfile Look in this file for the key specified with @option{-a}
-l Fil logfile Log to specified logfile
-M Num steplimit Adjustments less than @var{steplimit} msec will be slewed
- It must be in the range:
greater than or equal to 0
-o Num ntpversion Send @var{int} as our NTP version
- It must be in the range:
0 to 7
-r no usereservedport Use the NTP Reserved Port (port 123)
-S no step OK to 'step' the time with @command{settimeofday(2)}
-s no slew OK to 'slew' the time with @command{adjtime(2)}
-u Num uctimeout The number of seconds to wait for unicast responses
no wait Wait for pending replies (if not setting the time)
- disabled as --no-wait
- enabled by default
opt version Output version information and exit
-? no help Display extended usage information and exit
-! no more-help Extended usage information passed thru pager
-> opt save-opts Save the option state to a config file
-< Str load-opts Load options from a config file
- disabled as --no-load-opts
- may appear multiple times
Options are specified by doubled hyphens and their name or by a single
hyphen and the flag character.
The following option preset mechanisms are supported:
- reading file $HOME/.ntprc
- reading file ./.ntprc
- examining environment variables named SNTP_*
please send bug reports to: http://bugs.ntp.org, bugs@ntp.org
This is the “force ipv4 dns name resolution” option.
This option has some usage constraints. It:
Force DNS resolution of the following host names on the command line to the IPv4 namespace.
This is the “force ipv6 dns name resolution” option.
This option has some usage constraints. It:
Force DNS resolution of the following host names on the command line to the IPv6 namespace.
This is the “enable authentication with the key auth-keynumber” option. This option takes a number argument auth-keynumber. Enable authentication using the key specified in this option's argument. The argument of this option is the keyid, a number specified in the keyfile as this key's identifier. See the keyfile option (-k) for more details.
This is the “listen to the address specified for broadcast time sync” option. This option takes a string argument broadcast-address.
This option has some usage constraints. It:
If specified sntp will listen to the specified address
for NTP broadcasts. The default maximum wait time
can (and probably should) be modified with -t.
This is the “concurrently query all ips returned for host-name” option. This option takes a string argument host-name.
This option has some usage constraints. It:
Requests from an NTP "client" to a "server" should never be sent
more rapidly than one every 2 seconds. By default, any IPs returned
as part of a DNS lookup are assumed to be for a single instance of
ntpd, and therefore sntp will send queries to these IPs
one after another, with a 2-second gap in between each query.
The -c or --concurrent flag says that any IPs returned for the DNS lookup of the supplied host-name are on different machines, so we can send concurrent queries.
This is the “the gap (in milliseconds) between time requests” option. This option takes a number argument milliseconds. Since we're only going to use the first valid response we get and there is benefit to specifying a good number of servers to query, separate the queries we send out by the specified number of milliseconds.
This is the “kod history filename” option. This option takes a file argument file-name. Specifies the filename to be used for the persistent history of KoD responses received from servers. If the file does not exist, a warning message will be displayed. The file will not be created.
This is the “look in this file for the key specified with -a” option.
This option takes a file argument file-name.
This option specifies the keyfile.
sntp will search for the key specified with -a
keyno in this file. See ntp.keys(5) for more
information.
This is the “log to specified logfile” option. This option takes a file argument file-name. This option causes the client to write log messages to the specified logfile.
This is the “adjustments less than steplimit msec will be slewed” option. This option takes a number argument. If the time adjustment is less than steplimit milliseconds, slew the amount using adjtime(2). Otherwise, step the correction using settimeofday(2). The default value is 0, which means all adjustments will be stepped. This is a feature, as different situations demand different values.
This is the “send int as our ntp protocol version” option. This option takes a number argument. When sending requests to a remote server, tell them we are running NTP protocol version ntpversion .
This is the “use the ntp reserved port (port 123)” option. Use port 123, which is reserved for NTP, for our network communications.
This is the “the number of seconds to wait for responses” option.
This option takes a number argument seconds.
When waiting for a reply, sntp will wait the number
of seconds specified before giving up. The default should be
more than enough for a unicast response. If sntp is
only waiting for a broadcast response a longer timeout is
likely needed.
This is the “wait for pending replies (if not setting the time)” option.
This option has some usage constraints. It:
If we are not setting the time, wait for all pending responses.
Any option that is not marked as not presettable may be preset by
loading values from configuration ("rc" or "ini") files, and values from environment variables named SNTP and SNTP_<OPTION_NAME>. <OPTION_NAME> must be one of
the options listed above in upper case and segmented with underscores.
The SNTP variable will be tokenized and parsed like
the command line. The remaining variables are tested for existence and their
values are treated like option arguments.
libopts will search in 2 places for configuration files:
HOME, and PWD
are expanded and replaced when sntp runs.
For any of these that are plain files, they are simply processed.
For any that are directories, then a file named .ntprc is searched for
within that directory and processed.
Configuration files may be in a wide variety of formats. The basic format is an option name followed by a value (argument) on the same line. Values may be separated from the option name with a colon, equal sign or simply white space. Values may be continued across multiple lines by escaping the newline with a backslash.
Multiple programs may also share the same initialization file. Common options are collected at the top, followed by program specific segments. The segments are separated by lines like:
[SNTP]
or by
<?program sntp>
Do not mix these styles within one configuration file.
Compound values and carefully constructed string values may also be specified using XML syntax:
<option-name>
<sub-opt>...<...>...</sub-opt>
</option-name>
yielding an option-name.sub-opt string value of
"...<...>..."
AutoOpts does not track suboptions. You simply note that it is a
hierarchicly valued option. AutoOpts does provide a means for searching
the associated name/value pair list (see: optionFindValue).
The command line options relating to configuration and/or usage help are:
Print the program version to standard out, optionally with licensing information, then exit 0. The optional argument specifies how much licensing detail to provide. The default is to print just the version. The licensing infomation may be selected with an option argument. Only the first letter of the argument is examined:
One of the following exit values will be returned:
The simplest use of this program is as an unprivileged command to check the current time, offset, and error in the local clock. For example:
sntp ntpserver.somewhere
With suitable privilege, it can be run as a command or in a
crom job to reset the local clock from a reliable server, like
the ntpdate and rdate commands.
For example:
sntp -a ntpserver.somewhere