date -s "`curl --head -s https://example.com | grep -i "Date: " | cut -d' ' -f2-`"
The above one-liner might result in unexpected behavior,
- when the website is not reachable
- when the website has the wrong time
- when time steps/jumps (backwards!) in time
- because precision is only 1 second
Htpdate addresses these issues by using multiple time sources, filtering out 'false tickers', gradually adjusts time, enhancing precision and capable of running as a daemon.
The HTTP Time Protocol (HTP) is used to synchronize a computer's time with web servers as reference time source. Htpdate will synchronize your computer's time by extracting timestamps from HTTP headers found in web server responses. Htpdate can be used as a daemon, to keep your computer synchronized. The accuracy of htpdate is at least -+0.5 seconds, but can be in the range of ~10 ms (see -p option). If this is not good enough for you, use a ntp package.
Install the htpdate package if you need tools for keeping your system's time synchronized via the HTP protocol. Htpdate works also through proxy servers.
build:
make
or for HTTPS support (OpenSSL is required)
make https
install:
make install
Many Linux distributions and FreeBSD system provide a htpdate package, which should be preferred over manual compilation and installation from downloaded source code as the packages are likely integrated with the rest of the system (e.g. service files and networking scripts).
Htpdate can be used to query the time of one or multiple web servers,
htpdate www.example.com http://www.example.com https://example.com
Htpdate can run as daemon,
htpdate -D http://www.example.com
Another option is to run htpdate periodically from cron. For a daily time synchronization,
5 3 * * * /usr/sbin/htpdate -a www.example.com
All htpdate options,
Usage: htpdate [-046acdhlnqstvxDF] [-f driftfile] [-i pidfile] [-m minpoll]
[-M maxpoll] [-p precision] [-P <proxyserver>[:port]]
[-u user[:group]] <URL> ...
See man page for more details.
- https://www.vervest.org/htp, home of HTTP Time Protocol
- https://www.ntppool.org/scores/ntp6.vervest.org, htpdate as part of NTP pool
- https://github.com/twekkel/httpdate, non daemon version using libcurl
- https://github.com/angeloc/htpdate, forked from htpdate 1.2.2
