title: Message "X-Accel-Mapping header missing" in Nginx error log
body: <p>I am running a Rails 3 site on Ubuntu 8.04 with Nginx 1.0.0 and Passenger 3.0.7.</p>
  
  <p>In my Nginx error.log I started seeing the message <code>X-Accel-Mapping header missing</code> quite a lot. Googling lead me to the docs of <a href="http://rack.rubyforge.org/doc/Rack/Sendfile.html" rel="noreferrer"><code>Rack::Sendfile</code></a> and to the <a href="http://wiki.nginx.org/NginxXSendfile" rel="noreferrer">Nginx docs</a>.</p>
  
  <p>Now, my app can be accessed through several domains and I am using <code>send_file</code> in my app to deliver some files specific to the domain they are requested from, e.g., if you come to <code>domain1.com/favicon.ico</code> I look up the favicon in at <code>public/websites/domain1/favicon.ico</code>.
  This works fine and I don't think I need/want to get Nginx involved and create some private area where I store those files, as the samples in the <a href="http://rack.rubyforge.org/doc/Rack/Sendfile.html" rel="noreferrer"><code>Rack::Sendfile</code> docs</a> suggest.</p>
  
  <p>How can I get rid of the error message?</p>