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httpdate - set local date and time from a web server


While ntp may be a great protocol, I find it quite bloated and slow for the simple purpose of just setting a local date and time to a reference clock. I do not need 20ms accuracy on a notebook's clock :-). Thus I use(d) rdate for a decade now but the public rdate servers are slowly dying out. So I'm replacing it more and more with htpdate which works quite nicely. It's written in C and a perl alternative is available on the author's site. There is also a forked windows version of it available.

Developing a bit larger bash script (which syncs a few servers), I wondered whether I could realize the time sync part in bash as well.

It's quite possible:

  1. # open a tcp connection to
  2. exec 3<>/dev/tcp/
  3. # say hello HTTP-style
  4. echo -e "GET / HTTP/1.0\n\n">&3
  5. # parse for a Date: line and with a bit of magic throw the date-string at the date command
  6. LC_ALL=C LANG=en date --rfc-2822 --utc -s "$(head <&3 | grep -i "Date: " | sed -e s/Date\:\ //I)"
  7. # close the tcp connection
  8. exec 3<&-

Simple, eh?

A stand-alone version is available as httpdate and looks like this in v0.1:

  1. #!/bin/bash
  2. #
  3. # Purpose: display or set date from a host via http
  4. #
  5. # Requires: bash, ping, head, date, grep & sed
  6. # Autor:  Daniel Lange,
  7. # License: GPL v3 or later,
  8. #
  9. # Version: v0.1, 08-10-22, initial release
  10. #
  12. if [ "$1" = "-h" -o "$1" = "--help" ] ; then
  13.  echo "$0 v0.1 (c) Daniel Lange, 2008. Released under the GNU GPL v3 or later."
  14.  echo "Usage: $0 <-d|-s> <HOST>"
  15.  echo "       -d = display date (default)"
  16.  echo "       -s = set date (may require root privileges)"
  17.  echo "      HOST = FQDN of the server to retrieve the Date: header from"
  18.  echo "            (default:"
  19.  exit 0
  20. fi
  22. if [ "$1" != "-d" -a "$1" != "-s" ] ; then
  23.  echo "Error: either -d or -s expected as first argument".
  24.  exit 1
  25. fi
  27. if [ "$#" -gt 2 ] ; then
  28.  echo "Error: too many arguments, use -h for help"
  29.  exit 2
  30. fi
  32. DATE_OPTION="$1"
  33. DATE_HOST="${}"
  35. ping -q -c 1 -w 10 -W 10 $DATE_HOST > /dev/null 2>&1
  37. if [ $? -ne 0 ] ; then
  38.  echo "Error: Host $DATE_HOST not found (or ping missing)."
  39.  exit 3
  40. fi
  42. exec 3<>/dev/tcp/$DATE_HOST/80
  43. echo -e "GET / HTTP/1.0\n\n">&3
  44. LC_ALL=C LANG=en_EN date --rfc-2822 --utc $DATE_OPTION "$(head <&3 | grep -i "Date: " | sed -e s/Date\:\ //I)"
  45. exec 3<&-


27.09.09: Just FYI: The Debian/Ubuntu (before 09.10, launchpad bug entry) bash does not have the capability to use /dev/tcp. It's been compiled with –disable-net-redirections.


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Jorge Ventura on :

Hi Daniel, I am trying to use your script httpdate but I don't have /dev/tcp in my Debian...

To tell you the truth I never heard about /dev/tcp. Do you have any pointer to me???

Thank you, Ventura

P.S: Your script is very useful. I have a situation where the ntp port is not open in the firewall.

Daniel Lange on :

Dear Ventura,

Debian has deliberately compiled bash to disable the /dev/tcp functionality (--disable-net-redirections). See for the corresponding bug. So you'd need to re-compile bash manually. Or use Ubuntu. Or most other distros. Easier is probably using the C or perl version of htpdate linked at the beginning of the blog entry.

Philipp Gühring on :

I could need a httpdate that works with Proxies (using the http_proxy or https_proxy environment variable ...)

Daniel Lange on :

If the http proxy doesn't require authentication a simple amendment like

exec 3/dev/tcp/$PROXY_IP/$PROXY_PORT
echo -e "GET HTTP/1.0\n\n">&3

will do.

If the proxy requires authentication you need to construct an "Authorization:" Header and send that after the GET line.
For details see e.g. .

Misel on :

I almost coded this myself but figured someone must have done this before.

My reasoning wasn't the dislike for NTP, though. I operate a RaspberryPi behind a rather strict firewall that only allows HTTP(S) to the outside. So this is an easy way to set the time, when NTP is not available.

Thank you very much. :-)

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