Disable NGINX caching 304 Responses from Origin Server

[Ryan Barclay]

Thanks for the reply Peter.

I've noticed something interesting and wondered if you could shed some 
light on it.

Simply adding:

proxy_ignore_headers Cache-Control Expires;

Enables 304 responses from the origin server without setting:

proxy_set_header If-Modified-Since $http_if_modified_since;

I'm confused.

On 26/07/2017 09:11, Peter Booth wrote:
> I can’t see an obvious issue, but I can say that there is no such 
> thing as a simple web server setup where caching is involved.
> I have gray hairs that appeared after working with a high traffic 
> retail website that had seven levels of caching
> (browser cache, CDN, hardware load balancer, nginx reverse proxy, 
> servlets that write content, tangosol /oracle coherence, endeca caching)
>
> I’m hoping that you are living in  a saner world than that one but I’m 
> sure that you will have some craziness.
> I would encourage you to add $upstream_cache_status to your log format
> and/or add the directive add_headerX-Cache-Status $upstream_cache_status;
> Instrumenting the cache can be a real life-saver when things go awry.
>
> I’d also strongly encourage you to use redbot.org <http://redbot.org> 
> to check for aberrant behavior and webpagetest.org 
> <http://webpagetest.org>
> to see how different browsers handle your site.
>
> Peter
>
>
>
>> On Jul 26, 2017, at 3:29 AM, Ryan Barclay <ryan at rbftpnetworks.com 
>> <mailto:ryan at rbftpnetworks.com>> wrote:
>>
>> The following config seems to work for the situation I discussed:
>>
>> proxy_cache_valid 200 3M;
>> proxy_cache_valid 304 0;
>> proxy_cache_revalidate on;
>> proxy_set_header If-Modified-Since $http_if_modified_since;
>> proxy_ignore_headers Cache-Control Expires;
>>
>>
>> ... can anybody see any problems with this config or future problems 
>> that may arise?
>>
>>
>> On 24/07/2017 16:20, Ryan Barclay wrote:
>>> We have a pretty simple setup with NGINX sitting on the front and a 
>>> backend server (on a separate physical server) that provides the 
>>> content.
>>>
>>> Nginx then caches content based on the EXPIRES and Cache-Control 
>>> headers set by the origin server.
>>>
>>> We noticed that NGINX was not issuing 304 headers to images that 
>>> were not in the local NGINX cache when the If-Modified-Since header 
>>> was sent by the client. Instead, it would issue a 200 with the full 
>>> data file.
>>>
>>> To fix this, we applied:
>>> proxy_set_header If-Modified-Since $http_if_modified_since
>>>
>>> So then the If-Modified-Since header was passed to the backend and 
>>> of course, it returned correctly with the 304 header - great.
>>>
>>> But what we noticed was that NGINX would cache this 304 response and 
>>> deliver future responses as 304 to clients even without the 
>>> If-Modified-Since header.
>>>
>>> How can we disable caching of 304 responses and fix this issue?
>>>
>>> Thank you for your help, suggestions, and tips in advance.
>>
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>
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