NAME

       htpdate - Time synchronization (daemon)

SYNOPSIS

       htpdate  [-046acdhlnqstvxDF]  [-f  driftfile]  [-i  pidfile] [-m minpoll] [-M maxpoll] [-p
       precision] [-P <proxyserver>[:port]] [-u user[:group]] <URL> ...

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 using the Greenwich Mean
       Time (GMT) HTTP headers timestamp from web servers. HTTP and HTTPS are both supported.

       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, use a ntp package like ntpd, OpenNTPD or chrony.

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).

       -c     Verify server certificate (default no verification).

       -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. Multiple -d options increase verbosity. The  maximum  is
              3.

       -f     Read/write the systematic drift of the system clock. See also -x.

       -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.

       -n     Don't  use  a  proxy,  even  if  the appropriate http_proxy environment variable is
              defined.

       -p     Precision determines the operating accuracy of  htpdate.  Precision  specifies  the
              number  of  steps  (default  4,  maximum  of 9) for htpdate to determine the second
              boundary.

       -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).

       -v     Show version.

       -x     Let htpdate compensate for the systematisch clock drift by adjusting  system  clock
              frequency.

       -D     Run as daemon. This option requires root privileges.

       -F     Run  daemon  in  foreground.  Daemon  will  not fork or write PID file. This option
              requires root privileges.

       -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   Port number (default 80 and 8080 for proxy server).

       path   Path to resource (e.g. /index.html).

ENVIRONMENT

       Htpdate  supports  proxies  for  HTTP  connections.  The standard way to specify the proxy
       location, which htpdate recognizes, is using the following environment variable:

       http_proxy
           If set, the http_proxy  variable  should  contain  the  URL  of  the  proxy  for  HTTP
           connections.

EXAMPLES

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

       Request time from multiple web servers:
           htpdate www.example.com https://example.com http://example.com:80

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

       Adjust time smoothly and log output to syslog (eg. cron):
           htpdate -al www.example.com:80/htpdate.html

       HTTP/1.0 request in IPv6 literal format (RFC 2732):
           htpdate -0 [2001:db8:1af6::123]:80

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

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

       Read clock drift during start of htpdate and update when a new value has been established:
           htpdate -Dx -f /etc/htpdate.drift www.example.com

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

AUTHOR

       Eddy Vervest <eddy@vervest.org>, http://www.vervest.org/htp

SEE ALSO

       rdate, timed, ntpd, OpenNTPD, chrony, adjtimex(8), ntp_adjtime,