title: apt-get fails with raspberry pi although ping works
body: <p>I'm trying to update and install stuff with my raspberry pi.  </p>
  
  <p>Unfortunately it's failing, and I get these errors when doing <code>sudo apt-get update</code> :</p>
  
  <pre><code>Err http://raspberrypi.collabora.com wheezy Release.gpg
   Could not resolve 'host'
  Err http://mirrordirector.raspbian.org wheezy Release.gpg
   Could not resolve 'host'
  Err http://archive.raspberrypi.org wheezy Release.gpg
   Could not resolve 'host'
  Err http://repository.wolfram.com wheezy Release.gpg
   Could not resolve 'host'
  </code></pre>
  
  <p>And a few more lines like that...<br>
  The weird thing is that if I ping those addresses I get a response, so my DNS is probably ok:</p>
  
  <pre><code>ping mirrordirector.raspbian.org
  </code></pre>
  
  <p>Any other ideas why this would fail ? and of course, how can i fix this?</p>