I am using squid as proxy. But when the website is down and the cache is exceed the max time config in squid.conf, can I just let squid send the old to client before squid revaliadate it. If squid can do this, how to config?
You can use several options - refresh_pattern, max_stale, refresh_stale_hit, icp_hit_stale
If You dont need to serve stale content long time, then you can use only refresh_stale_hit option. i.e.:
refresh_stale_hit 1 minute
This allows concurrent requests while object is being refreshed to be processed as cache hits only if object expired less than 1 minute.