posts/ntp-over-http.en.html

   1 <!-- subject: {NTP} over {HTTP} -->
   2 <!-- date: 2010-01-16 01:27:45 -->
   3 <!-- tags: ntp over http, ntp http proxy, time synchronisation, ntp, http -->
   4 <!-- categories: Articles, Techblog -->
   5
   6 <p>Sitting in a dark office, after swearing for hours at ATI video cards
   7   I noticed time on my computer was incorrect.  ‘No problem,’ I thought as
   8   I started typing <code>ntpdate</code>.  That’s where it struck me that our
   9   <em>beloved</em> IT department had blocked most of the Internet.  Checking the
  10   time on a watch or a mobile phone was not an option — I had neither — nor was
  11   looking at GKrellM on another PC — that’s lame.
  12
  13 <p>‘I wish there was a NTP-over-HTTP protocol’ I sighed.  And then I realised
  14   there was…
  15
  16 <!-- FULL -->
  17
  18 <p>If you recall that web servers include their time in response headers
  19 solution becomes clear.
  20
  21 <pre>
  22 httpdate="$(wget --no-cache -S -O /dev/null google.com 2&gt;&amp;1 |
  23         sed -n -e 's/  *Date: *//p' -eT -eq)"
  24 [ -n "$httpdate" ] && date -s "$httpdate"</pre>
  25
  26 <p>Did the trick!
  27
  28 <p>It won’t guarantee sub-second accuracy, but will usually work to withing
  29   a few seconds.  It also requites GNU coreutils
  30   (for <code>date</code>’s <code>-s</code> switch) (but
  31   implementing <a href="https://gist.github.com/278534">simple RFC1123 date
  32   parser</a> is not that hard), wget (which can however be replaced by other
  33   tools including telnet) and sed (which is POSIX utility and also can be
  34   replaced by <a href="https://gist.github.com/278539">a 13-line C program</a>).
  35
  36 <p>Implementing a complete tool in C, C++, Rust or similar is left as a simple
  37 exercise for the reader.  (Less ambitious reader may limit themselves to some
  38 scripting language with regexes).
  39
  40 <!-- COMMENT -->
  41 <!-- date: 2012-07-05 15:05:17 -->
  42 <!-- nick: Anonim -->
  43
  44 <p>Improved version to ignore proxy cache servers:
  45
  46 <pre>
  47 date -s "$(wget --no-cache -S -O /dev/null google.com 2&gt;&amp;1 | \
  48     sed -n -e '/  *Date: */ {' -e s///p -e q -e '}')"</pre>
  49
  50 <p>And set http_proxy env. variable if you have a proxy server in your way, as:
  51
  52 <pre>http_proxy=http://youruser:yourpassword@proxyip:proxyport;export http_proxy</pre>
  53
  54 <p>For instance the whole thing could be:
  55
  56 <pre>
  57 http_proxy=http://user:password@192.168.1.5:3128
  58 export http_proxy
  59 date -s "$(wget --no-cache -S -O /dev/null google.com 2&gt;&amp;1 | \
  60     sed -n -e '/  *Date: */ {' -e s///p -e q -e '}')"</pre>
  61
  62 <!-- COMMENT -->
  63 <!-- date: 2012-07-05 22:34:06 -->
  64 <!-- nick: mina86 -->
  65 <!-- nick_url: http://mina86.com -->
  66
  67 <p>Looks good, thanks, even thought I'd get rid of the export not to pollute the environment as so:
  68 <pre>
  69 date -s "$(
  70     http_proxy=http://youruser:yourpassword@proxyip:proxyport \
  71     wget --no-cache -S -O /dev/null google.com 2&gt;&amp;1 | \
  72     sed -n -e '/  *Date: */ {' -ep -eq -e '}')"</pre>
  73
  74 <!-- COMMENT -->
  75 <!-- date: 2012-08-17 11:23:52 -->
  76 <!-- nick: almaz -->
  77
  78 <p>If no connection is reset time at 00:00:00
  79
  80 <!-- COMMENT -->
  81 <!-- date: 2013-05-14 21:21:08 -->
  82 <!-- nick: mina86 -->
  83 <!-- nick_url: http://mina86.com -->
  84
  85 <blockquote>
  86   <p>If no connection is reset time at 00:00:00.
  87 </blockquote>
  88
  89 <p>True. To solve that one would have to check the output. I’ll update
  90 the entry.