Squid stale-while-revalidate not working when max-age=0

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http://serverfault.com – Squid 2.7 always reaches backend, expected is to reach backend using stale-while-revalidate only when cache expires, not when client triggers max-age=0. Script: <?php header('Cache-Control: public, max-age=10, stale-if-error=200, stale-while-revalidate=500'); header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT"); sleep(2); die("OK"); And squid config: # http_port public_ip:port accel defaultsite= default hostname, if not provided http_port 80 accel defaultsite=mydomain.com # IP and port of your main application server (or multiple) cache_peer parent 8000 0 no-query (HowTos)