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copied!<p>This <a href="http://edn.embarcadero.com/article/38123" rel="nofollow noreferrer">blog post</a> helped me a lot in order to tame the "conditional get" beast. </p> <p>An interesting excerpt (which partially contradicts Ben's answer) states that:</p> <blockquote> <p>If a normal response would have included an ETag header, that header must also be included in the 304 response.</p> <p>Cache headers (Expires, Cache-Control, and/or Vary), if their values might differ from those sent in a previous response.</p> </blockquote> <p><strong>This is in complete accordance with the <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5" rel="nofollow noreferrer">RFC 2616 sec 10.3.5</a>.</strong></p> <hr> <p>Below a 200 request...</p> <pre><code>HTTP/1.1 200 OK Server: nginx/0.8.52 Date: Thu, 18 Nov 2010 16:04:38 GMT Content-Type: image/png Last-Modified: Thu, 15 Oct 2009 02:04:11 GMT Expires: Thu, 31 Dec 2010 02:04:11 GMT Cache-Control: max-age=315360000 Accept-Ranges: bytes Content-Length: 6394 Via: 1.1 proxyIR.my.corporate.proxy.name:8080 (IronPort-WSA/6.3.3-015) Connection: keep-alive Proxy-Connection: keep-alive X-Junk: xxxxxxxxxxxxxxxx </code></pre> <p>...And its optimal valid 304 counterpart.</p> <pre><code>HTTP/1.1 304 Not Modified Server: nginx/0.8.52 Date: Thu, 18 Nov 2010 16:10:35 GMT Expires: Thu, 31 Dec 2011 16:10:35 GMT Cache-Control: max-age=315360000 Via: 1.1 proxyIR.my.corporate.proxy.name:8080 (IronPort-WSA/6.3.3-015) Connection: keep-alive Proxy-Connection: keep-alive X-Junk: xxxxxxxxxxx </code></pre> <p>Notice that the <code>Expires</code> header is at most <code>Current Date + One Year</code> as per <a href="https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.21" rel="nofollow noreferrer">RFC-2616 14.21</a>.</p>

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