Order of HTTP headers change cache behaviour

[Maxim Dounin]

Hello!

On Wed, Feb 22, 2012 at 05:05:32PM +0100, Simone Fumagalli wrote:

> Hello. 
> 
> Looking at my log I've noticed that some pages were not cached. After a bit of debugging I've noticed that the order in which the HTTP header are returned from Apache change how NGINX this save the page in the cache 
> 
> So a page with this code is *NOT* cached
> 
> <?php
> 
> header('Content-Type: text/html; charset=UTF-8');
> header('Last-Modified: Wed, 22 Feb 2012 14:44:11 GMT');
> header('Expires: Wed, 11 Jan 1984 05:00:00 GMT');
> header("X-Accel-Expires: 600");
> header('Cache-Control: no-cache, must-revalidate, max-age=0');
> header('Pragma: no-cache');
> 
> Echo "Hello World !";
> 
> ?>
> 
> if I comment/delete the line header('Expires: Wed, 11 Jan 1984 05:00:00 GMT'); the page is cached.
> 
> Even if I move header("X-Accel-Expires: 600") before header('Expires: Wed, 11 Jan 1984 05:00:00 GMT'); the page is cached
> Same thing happen for the header Cache-Control. If is before X-Accel-Expires the page is not cached if is after it is !
> 
> The conf for NGINX is very standard and proxy_cache_valid is not specified.
> 
> Is this correct ? Where am I wrong ?

This probably should be counted as a bug (or at least 
misfeature).

Here are what happens: if the Expires header comes first, it 
disables caching due to being set to date in the past.  The 
X-Accel-Expires header which comes later can't re-enable caching.

On the other hand, if X-Accel-Expires comes first, it will set 
cache expiration time.  The Expires and Cache-Control headers 
later will be just ignored as cache expiration time is already 
set.

As a workaround you may want to always sent X-Accel-Expires first, 
or explicitly ignore other headers with proxy_ignore_headers.

Maxim Dounin