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SNTP(8)								       SNTP(8)

NAME
       sntp - a SNTP utility (command and daemon)

SYNOPSIS
       sntp  [ -h | --help | -?	 ] [ -v | -V | -W ] [ -q [ -f savefile ] | [ {
       -r | -a } [ -P prompt ] [ -l lockfile ] ] [ -e minerr ] [ -E maxerr ] [
       -c  count ] [ -d delay | -x [ separation ] ] [ -f savefile ] ] [ -4 ] |
       [ -6 ] [ address(es) ] ]

DESCRIPTION
       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, in a cron job  or
       as  a  daemon.	It can be run as a daemon to provide a SNTP server for
       other clients.  NTP is the Network Time Protocol (RFC 1305) and SNTP is
       the Simple Network Time Protocol (RFC 2030, which supersedes RFC 1769).

   Options
       sntp recognizes the following options:

       -h     displays	the syntax error message.  If there are no other argu‐
	      ments, it then stops; otherwise it then does what was requested.
	      --help and -?  are synonyms.

       -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.
       In daemon mode, it will add drift information in a format like ' +  1.3
       +/-  0.1	 ppm',	and  display this at roughly separation intervals (see
       under the -x option for details).

       -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.

       -x separation
	      causes the program to run as a daemon  (i.e.  forever),  and  to
	      estimate	and  correct for the clock drift.  separation sets the
	      minimum time between calls to the server in  minutes  if	a  NTP
	      host  is	specified,  and	 between  broadcast  packets  if  not.
	      Acceptable values are from 1 to 1440 (a day),  and  the  default
	      (if -x is specified but separation is omitted) is 300.

       -f savefile
	      may  be  used  with  the -x option to store a record of previous
	      packets, which speeds up recalculating the drift after sntp  has
	      to be restarted (e.g. because of network or server outages).  In
	      order to restart the data, sntp  must  be	 restarted  reasonably
	      soon after it died (within a few times the value of separation),
	      with the same value of the -c option, the same value of  separa‐
	      tion,  in	 the  same mode (i.e. broadcast or client), though the
	      NTP servers need not be the same for client mode, and with  com‐
	      patible  values  of  other settings.  Note that the file will be
	      created with the default ownerships and permissions, using stan‐
	      dard  C  facilities.  The default is installation-dependent, but
	      will usually be /etc/sntp.state.

       -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.

	      In  daemon mode (i.e. when -x is specified), minerr must be less
	      than maxerr which must be less than separation (note  that  this
	      is in minutes, not seconds, but the numeric value is compared).

       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

       It  can	be  run	 as  a unprivileged background process to check on the
       clock drift as well as the current error; this will  probably  fail  if
       the local clock is reset while it is running.  For example:

	      sntp -x ntpserver.somewhere > output 2>1

       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

       It  can	also  be run as a daemon to keep the local clock in step.  For
       example:

	      sntp -a -x ntpserver.somewhere > output 2>1

       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.

AUTHOR
       sntp was developed by N.M. Maclaren of the University of Cambridge Com‐
       puting Service.

								       SNTP(8)
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