[squid-users] Squid reverse-proxy. How it decides when to refresh?

[Amos Jeffries]

On 26/08/17 00:37, Alexander Lazarev wrote:
> Hello guys!
> I'm using squid as a reverse-proxy. And I can't understand how squid 
> decides when to check for fresh version of file from origin server.
> It looks like for some documents it sends 'If-Modified-Since' or similar 
> headers and if it gets 304, it serves file from cache. And for some 
> documents it doesn't check for fresh version and always serves from cache. > I was testing that with curl without any additional headers.
> Can some explain how that works or where I can read about that in 
> detail?

The HTTP specification RFC 723x series was re-written to be a lot more 
easily understood, so those are probably the best place to read up about it.

The features you are asking about are covered in:

Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests
  <https://tools.ietf.org/html/rfc7232>

Hypertext Transfer Protocol (HTTP/1.1): Caching
  <https://tools.ietf.org/html/rfc7234>


> And is it possible to make squid always check for fresh version 
> before serving from cache?

It does when needed. The situation may be clearer after reading the above.

Amos