This directive provides a way to customize Squid’s algorithm for validating cached responses. HTTP has a relatively complex procedure for determining whether or not a cached response is fresh or stale. In some cases, origin servers provide an explicit expiration time. However, the majority of responses don’t have this information. For these, Squid applies some heuristics to the response. See Section 7.7 for more information.
refresh_pattern . 0 20% 4320
refresh_pattern \.jpg$ 0 75 7200