Why does Apache send 200 OK while Last-modified matches If-modified-since

apache-2.4cachelast-modified

I'm trying to have a basic behavior regarding my caching strategy: files should be cached, and revalidated with server each time. So I would like Apache to send a 304 back.

Here is the dialog that happens for each browser refresh:

Status Code:200 OK

Request Headers

Accept:text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Encoding:gzip,deflate,sdch
Accept-Language:fr-FR,fr;q=0.8,en-US;q=0.6,en;q=0.4
Cache-Control:max-age=0
Connection:keep-alive
Cookie: ...
Host:...
If-Modified-Since:Tue, 14 Oct 2014 15:10:37 GMT
If-None-Match:"1461-505636af08fcd-gzip"
User-Agent:Mozilla/5.0 (X11; Linux i686) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/34.0.1847.116 Safari/537.36

Response Headers

Accept-Ranges:bytes
Cache-Control:No-cache
Connection:Keep-Alive
Content-Encoding:gzip
Content-Length:1412
Content-Type:text/html
Date:Tue, 14 Oct 2014 16:58:05 GMT
ETag:"1461-505636af08fcd-gzip"
Keep-Alive:timeout=5, max=99
Last-Modified:Tue, 14 Oct 2014 15:10:37 GMT
Server:Apache/2.4.6 (Ubuntu)
Vary:Accept-Encoding

(this is from chrome devtools, with Disable cache unchecked)

You can see that the response contains the Cache-Control:No-cache Header, and that the If-modified-since header matches the Last-modified. The ETag matches too.

Shouldn't Apache sends a 304 in that case ?

EDIT

Disabling ETags in apache with

 Header  unset ETag

makes the caching behavior more predictible…

Best Answer

This seems to be an old bug, explaining why Header unset ETag makes a difference.

Apache 2.4.0+ automatically appends the compression method name to the ETag (as seen in your headers), and prevents a 304 response.

Newest versions of mod_deflate support a DeflateAlterETag that can be used to control this behavior:

DeflateAlterETag NoChange
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