corrupted cache file: proxy_cache_valid ignored

[Maxim Dounin]

Hello!

On Mon, Feb 07, 2022 at 11:39:08PM -0800, Roger Fischer wrote:

> we have observed a case where it seems that the 
> proxy_cache_valid directive is ignored.
> 
> nginx version: 1.19.9
> 
> Config: proxy_cache_valid 200 206 30d;
> 
> Scenario:
> * A cache file was corrupted (a file system issue). A part of 
> the section that contains the headers had been overwritten with 
> binary data.
> * The resource represented by the corrupted cache file is 
> requested.
> * NGINX detects the corrupted cache file, and proxies the 
> request upstream.
> * The request is rejected by the upstream, with the upstream 
> returning a 403 status.
> * The 403 is returned to the client.
> This is all good, but
> * The request is repeated, and a cached 403 is returned, despite 
> only caching 200 and 206.
> * Upon examination, the cache file contains the 403 response 
> from upstream.
> 
> Has anyone else seen something like this? Could this possibly be 
> a bug?

As long as you don't have corresponding proxy_ignore_headers in 
your configuration, cache validity can be also set by the response 
itself: via Cache-Control, Expires, and X-Accel-Expires headers.  
Check the response headers in the cache file to see if there are 
any of these.

-- 
Maxim Dounin
http://mdounin.ru/