Ubuntu Manpage: htpdate - Time synchronization (daemon)

Provided by: htpdate_1.2.6+git20210903.a3c428a-2_amd64

NAME

       htpdate - Time synchronization (daemon)

SYNOPSIS

       htpdate  [-046abdhlqstxDF]  [-i  pid  file]  [-m  minpoll] [-M maxpoll] [-p precision] [-P
       <proxyserver>[:port]] [-u user[:group]] <host[:port]> ...

DESCRIPTION

       The HTTP Time Protocol (HTP) is used to synchronize a computer's time with web servers  as
       reference  time  source.  Htp will synchronize your computer's time to Greenwich Mean Time
       (GMT) via HTTP headers from web servers.  The  htpdate  package  includes  a  program  for
       retrieving  the  date  and  time from remote machines via a network. Htpdate works through
       proxy servers. Accuracy of htpdate  will  be  usually  within  0.5  seconds  (better  with
       multiple servers). If this is not good enough for you, try the ntpd package.

OPTIONS

-0 HTTP/1.0 request (default is HTTP/1.1).

-4 Force IPv4 name resolution only. Default behaviour is to try IPv6 first and fall
back to IPv4.

-6 Force IPv6 name resolution only.

-a Adjust time smoothly (default in daemon mode).

-b Burst mode uses multiple polls for each web server to enhance accuracy.

-d Turn debug on. Shows the "raw" timestamp, round trip time, time delta and and basic
statistics of web server responses. Useful to determining the quality of a specific
web server as time source.

-h Show help.

-i Set the pid file (default /var/run/htpdate.pid).

-l Use syslog for output (levels LOG_WARNING and LOG_INFO). Convenient if you use
htpdate from cron.

-m -M These options specify the minimum (-m) and maximum (-M) polling intervals for HTP
requests, in seconds. The default range is between 30 minutes and 32 hours. Htpdate
calculates the optimal polling frequency between minimum and maximum values. Only
applicable when running in daemon mode.

-p Precision (in milliseconds) specifies the operating accuracy of htpdate. Internally
htpdate uses a different algorithm to detect a time offset, when precision is
specified. Precision only has effect in daemon mode. Use with caution.

-q Query web server and display time, but do not change time (default in interactive
mode).

-s Set time immediate. In daemon mode -s only applies the first poll.

-t Turn off sanity time check. By default a time offset larger than a year, compared
to current localtime, is rejected. With -t set, any time stamp will be accepted.

-u Set the user and group that the server normally runs at (default is root).

-x Let htpdate compensate for the systematisch clock drift.

-D Run as daemon (requires root privileges).

-F Run in the foreground (requires root privileges). This is the same as -D but will
not fork or write a PID file.

-P Proxy server hostname or ip-address.

host Web server hostname or ip-address. Up to 16 hosts may be specified, but in general
3 to 5 hosts should be enough for a redundant and accurate setup.

port Portnumber (default 80 and 8080 for proxy server), accept prefix like http:// or
https://

EXAMPLES

       Request time from web server (don't update local clock):
              htpdate -q www.example.com

       Request time from https web server:
              htpdate -q https://www.example.com

       Verbose output (don't update local clock):
              htpdate -d -q www.example.com

       Adjust time smoothly and log output to syslog (eg. cron):
              htpdate -a -l www.example.com

       HTTP/1.0 request in IPv6 literal format (RFC 2732):
              htpdate -0 [2001:DB8:1af6::123]:80

       Run htpdate as daemon:
              htpdate -D www.example.com

       Run htpdate in the foreground with all output going to the terminal:
              htpdate -F www.example.com

       Daemon mode for the security minded:
              htpdate -D -u nobody:nogroup www.example.com

AUTHOR