Configure Varnish to cache every GET-Request and fetch in background

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http://serverfault.com – I'm trying to configure varnish to cache every GET-Request to our CMS based website (running on apache) without considering the ttl given by the CMS for dynamic or apache for static content. Invalidation should take place after 15 minutes or if we send a purge/delete request from our CMS. I thought the configuration parameter req.grace defines the time period where varnish is allowed to provide ttl invalidated objects to the client but with the following configuration this is not working: Actual Result If valid cache object is present object is returned very fast -> this is ok although I t (HowTos)