What headers do I want to send together with a 304 response?

Question

When I send a 304 response. How will the browser interpret other headers which I send together with the 304?

E.g.

header("HTTP/1.1 304 Not Modified");
header("Expires: " . gmdate("D, d M Y H:i:s", time() + $offset) . " GMT");

Will this make sure the browser will not send another conditional GET request (nor any request) until $offset time has "run out"?

Also, what about other headers?

Should I send headers like this together with the 304:

header('Content-Type: text/html');

Do I have to send:

header("Last-Modified:" . $modified);
header('Etag: ' . $etag);

To make sure the browser sends a conditional GET request the next time the $offset has "run out" or does it simply save the old Last Modified and Etag values?

Are there other things I should be aware about when sending a 304 response header?

Answer 1

This blog post helped me a lot in order to tame the "conditional get" beast.

An interesting excerpt (which partially contradicts Ben's answer) states that:

If a normal response would have included an ETag header, that header must also be included in the 304 response.

Cache headers (Expires, Cache-Control, and/or Vary), if their values might differ from those sent in a previous response.

This is in complete accordance with the RFC 2616 sec 10.3.5.

---

Below a 200 request...

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

...And its optimal valid 304 counterpart.

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

Notice that the Expires header is at most Current Date + One Year as per RFC-2616 14.21.

Answer 2

The Content-Type header only applies to responses which contain a body. A 304 response does not contain a body, so that header does not apply. Similarly, you don't want to send Last-Modified or ETag because a 304 response means that the document hasn't changed (and so neither have the values of those two headers).

For an example, see this blog post by Anne van Kesteren examining WordPress' http_modified function. Note that it returns either Last-Modified and ETag or a 304 response.