title: HTTP proxy error status codes
body: <p>The application is Node.js based web crawler that does requests to remote servers via <code>request</code> library.</p>
  
  <p>Requests are made through authenticated HTTP proxy servers with 99% and higher uptime, provided by third party vendor.</p>
  
  <p>I'm trying to process HTTP codes for request errors response that can be likely caused by malfunctioning proxy. It's expected that after reaching a threshold of these HTTP codes, a flag that a proxy is unreliable will be triggered, so a proxy will be queued for diagnostics. And I expect some codes to have more weight.</p>
  
  <p>Currently I'm processing 407 (proxy authentication) and 408 (request timeout) HTTP codes for proxies.</p>
  
  <p>Are there other HTTP codes that are commonly caused by malfunctioning HTTP proxies?</p>
  
  <p>I hope the question is specific enough. I assume that possible malfunction (and a set of possible HTTP error status codes) depends on actual proxy and its environment. We should consider these proxies black boxes because they are provided by a third party.</p>
  
  <p><strong>The question isn't about HTTP status codes in general, only error codes can be caused by malfunctioning proxies (forward proxies).</strong></p>