blog/Automatic_proxy_configuration_with_Debian_Edu___Skolelinux.html

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   6     <title>Petter Reinholdtsen: Automatic proxy configuration with Debian Edu / Skolelinux</title>
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  12  <h1>
  13      <a href="http://people.skolelinux.org/pere/blog/">Petter Reinholdtsen</a>
  14
  15  </h1>
  16
  17 </div>
  18
  19
  20     <div class="entry">
  21       <div class="title">Automatic proxy configuration with Debian Edu / Skolelinux</div>
  22       <div class="date">13th February 2012</div>
  23       <div class="body"><p>New in the Squeeze version of
  24 <a href="http://www.skolelinux.org/">Debian Edu / Skolelinux</a> is the
  25 ability for clients to automatically configure their proxy settings
  26 based on their environment.  We want all systems on the client to use
  27 the WPAD based proxy definition fetched from <tt>http://wpad/wpad.dat</tt>, to
  28 allow sites to control the proxy setting from a central place and make
  29 sure clients do not have hard coded proxy settings.  The schools can
  30 change the global proxy setting by editing
  31 <tt>tjener:/etc/debian-edu/www/wpad.dat</tt> and the change propagate
  32 to all Debian Edu clients in the network.</p>
  33
  34 <p>The problem is that some systems do not understand the WPAD system.
  35 In other words, how do one get from a WPAD file like this (this is a
  36 simple one, they can run arbitrary code):</p>
  37
  38 <blockquote><pre>
  39 function FindProxyForURL(url, host)
  40 {
  41    if (!isResolvable(host) ||
  42        isPlainHostName(host) ||
  43        dnsDomainIs(host, ".intern"))
  44       return "DIRECT";
  45    else
  46       return "PROXY webcache:3128; DIRECT";
  47 }
  48 </pre></blockquote>
  49
  50 <p>to a proxy setting in the process environment looking like this:</p>
  51
  52 <blockquote><pre>
  53 http_proxy=http://webcache:3128/
  54 ftp_proxy=http://webcache:3128/
  55 </pre></blockquote>
  56
  57 <p>To do this conversion I developed a perl script that will execute
  58 the javascript fragment in the WPAD file and return the proxy that
  59 would be used for
  60 <tt><a href="http://www.debian.org/">http://www.debian.org/</a></tt>,
  61 and insert this extracted proxy URL in <tt>/etc/environment</tt> and
  62 <tt>/etc/apt/apt.conf</tt>.  The perl script wpad-extract work just
  63 fine in Squeeze, but in Wheezy the library it need to run the
  64 javascript code is <a href="http://bugs.debian.org/631045">no longer
  65 able to build</a> because the C library it depended on is now a C++
  66 library.  I hope someone find a solution to that problem before Wheezy
  67 is frozen.  An alternative would be for us to rewrite wpad-extract to
  68 use some other javascript library currently working in Wheezy, but no
  69 known alternative is known at the moment.</p>
  70
  71 <p>This automatic proxy system allow the roaming workstation (aka
  72 laptop) setup in Debian Edu/Squeeze to use the proxy when the laptop
  73 is connected to the backbone network in a Debian Edu setup, and to
  74 automatically use any proxy present and announced using the WPAD
  75 feature when it is connected to other networks.  And if no proxy is
  76 announced, direct connections will be used instead.</p>
  77
  78 <p>Silently using a proxy announced on the network might be a privacy
  79 or security problem.  But those controlling DHCP and DNS on a network
  80 could just as easily set up a transparent proxy, and force all HTTP
  81 and FTP connections to use a proxy anyway, so I consider that
  82 distinction to be academic.  If you are afraid of using the wrong
  83 proxy, you should avoid connecting to the network in question in the
  84 first place.  In Debian Edu, the proxy setup is updated using dhcp and
  85 ifupdown hooks, to make sure the configuration is updated every time
  86 the network setup changes.</p>
  87
  88 <p>The WPAD system is documented in a
  89 <a href="http://tools.ietf.org/html/draft-ietf-wrec-wpad-01">IETF
  90 draft</a> and a
  91 <a href="http://en.wikipedia.org/wiki/Web_Proxy_Autodiscovery_Protocol">Wikipedia
  92 page</a> for those that want to learn more.</p>
  93 </div>
  94
  95       <div class="tags">Tags: <a href="http://people.skolelinux.org/pere/blog/tags/debian edu">debian edu</a>, <a href="http://people.skolelinux.org/pere/blog/tags/english">english</a>.</div>
  96
  97
  98     </div>
  99
 100
 101
 102
 103     <div id="sidebar">
 104
 105
 106
 107 <h2>Archive</h2>
 108 <ul>
 109
 110 <li>2013
 111 <ul>
 112
 113 <li><a href="http://people.skolelinux.org/pere/blog/archive/2013/01/">January (4)</a></li>
 114
 115 </ul></li>
 116
 117 <li>2012
 118 <ul>
 119
 120 <li><a href="http://people.skolelinux.org/pere/blog/archive/2012/01/">January (7)</a></li>
 121
 122 <li><a href="http://people.skolelinux.org/pere/blog/archive/2012/02/">February (10)</a></li>
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 143
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 145
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 232
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