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 4.2.7p202 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 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 (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 secs
where the
+4.567 +/- 0.089 secs
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),
and the (local) time of
1996-10-15 20:17:25.123
can be converted to UTC time by adding 8 hours and 0 minutes, and
is believed to be correct to within
+/- 0.089
seconds.
This section was generated by AutoGen, the aginfo template and the option descriptions for the sntp program. It documents the sntp usage text and option meanings.
This software is released under a specialized copyright license.
This is the automatically generated usage text for sntp:
sntp - standard Simple Network Time Protocol program - Ver. 4.2.7p202
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 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 -a
-l Fil logfile Log to specified logfile
-M Num steplimit Adjustments less than steplimit msec will be slewed
- It must be in the range:
greater than or equal to 0
-o Num ntpversion Send <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 settimeofday()
-s no slew OK to 'slew' the time with adjtime()
-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 “enable authentication with the key auth-keynumber” option. This option enables 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 “the number of seconds to wait for broadcasts” option. When waiting for a broadcast packet SNTP will wait the number of seconds specified before giving up. Default 68 seconds.
This is the “listen to the address specified for broadcast time sync” option.
This option has some usage constraints. It:
If specified SNTP will listen to the specified address for NTP broadcasts. The default maximum wait time, 68 seconds, can be modified with -B.
This is the “concurrently query all ips returned for host-name” option.
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 “increase debug verbosity level” option.
This option has some usage constraints. It:
This is the “the gap (in milliseconds) between time requests” option. 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. Default 10 milliseconds.
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 “look in this file for the key specified with -a” option. This option specifies the keyfile. SNTP will search for the key specified with -a keyno in this file. Key files follow the following format:
keyid keytype key
Where keyid is a number identifying this key keytype is one of the following: S Key is a 64 Bit hexadecimal number as specified in in the DES specification. N Key is a 64 Bit hexadecimal number as specified in the NTP standard. A Key is a 1-to-8 character ASCII string. M Key is a 1-to-8 character ASCII string using the MD5 authentication scheme.
For more information see ntp.keys(5).
This is the “kod history filename” option. Specifies the filename to be used for the persistent history of KoD responses received from servers. The default is /var/db/ntp-kod .
This is the “log to specified logfile” option. This option causes the client to write log messages to the specified logfile.
This is the “send <int> as our ntp version” option. When sending requests to a remote server, tell them we are running NTP protocol version <ntpversion> .
This is the “set the debug verbosity level” option.
This option has some usage constraints. It:
This is the “ok to 'slew' the time with adjtime()” option.
This is the “ok to 'step' the time with settimeofday()” option.
This is the “adjustments less than steplimit msec will be slewed” option. If the time adjustment is less than steplimit milliseconds, slew the amount using adjtime(). Otherwise, step the correction using settimeofday().
This is the “the number of seconds to wait for unicast responses” option. When waiting for a unicast reply, SNTP will wait the number of seconds specified before giving up. Default 5 seconds.
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 “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.
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