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

[Alexander Lazarev]

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