title: How to avoid nginx "upstream sent too big header" errors?
body: <p>I'm running nginx, Phusion Passenger and Rails.</p>
  
  <p>I am running up against the following error:</p>
  
  <pre><code>upstream sent too big header while reading response header from upstream, client: 87.194.2.18, server: xyz.com, request: "POST /user_session HTTP/1.1", upstream: "passenger://unix:/tmp/passenger.3322/master/helper_server.sock
  </code></pre>
  
  <p>It is occuring on the callback from an authentication call to Facebook Connect.</p>
  
  <p>After googling, and trying to change nginx settings including proxy_buffer_size and large_client_header_buffers is having no effect.</p>
  
  <p>How can I debug this?</p>