title: What are the options for the gzip_proxied directive for?
body: <p>The <a href="http://nginx.org/en/docs/http/ngx_http_gzip_module.html#gzip_proxied" rel="noreferrer">gzip_proxied</a> directive allows for the following options (non-exhaustive):</p>
  
  <blockquote>
    <ul>
    <li><strong>expired</strong><br>
    enables compression if a response header includes the “Expires” field with a value that disables caching;</li>
    <li><strong>no-cache</strong><br>
    enables compression if a response header includes the “Cache-Control” field with the “no-cache” parameter;</li>
    <li><strong>no-store</strong><br>
    enables compression if a response header includes the “Cache-Control” field with the “no-store” parameter;</li>
    <li><strong>private</strong><br>
    enables compression if a response header includes the “Cache-Control” field with the “private” parameter;</li>
    <li><strong>no_last_modified</strong><br>
    enables compression if a response header does not include the “Last-Modified” field;</li>
    <li><strong>no_etag</strong><br>
    enables compression if a response header does not include the “ETag” field;</li>
    <li><strong>auth</strong><br>
    enables compression if a request header includes the “Authorization” field;</li>
    </ul>
  </blockquote>
  
  <p>I can't see any rational reason to use most of these options. For example, why would whether or not a proxied request contains the <code>Authorization</code> header, or <code>Cache-Control: private</code>, affect whether or not I want to gzip it?</p>
  
  <p>Given that old versions of Nginx <a href="https://thinkingandcomputing.com/2014/09/27/enable-etag-nginx-resources-sent-gzip/" rel="noreferrer">strip ETags from responses when gzipping them</a>, I can see a use case for <strong>no_etag</strong>: if you don't have Nginx configured to generate ETags for your gzipped responses, you may prefer to pass on an uncompressed response <em>with</em> an ETag rather than generate a compressed one without an ETag.</p>
  
  <p>I can't figure out the others, though.</p>
  
  <p>What are the intended use cases of each of these options?</p>