squid-cache.org

	This option defines an external helper for determining whether to
	refresh a stale response. It will be called when Squid receives a
	request for a cached response that is stale; the helper can either
	confirm that the response is stale with a STALE response, or
	extend the freshness of the response (thereby avoiding a refresh
	check) with a FRESH response, along with a freshness=nnn keyword.

	  external_refresh_check [options] FORMAT.. /path/to/helper [helper_args]

	If present, helper_args will be passed to the helper on the command
	line verbatim.

	Options:

	  children=n	Number of processes to spawn to service external
			refresh checks (default 5).
	  concurrency=n	Concurrency level per process. Only used with
			helpers capable of processing more than one query
			at a time.

	When using the concurrency option, the protocol is changed by introducing
	a query channel tag infront of the request/response. The query channel
	tag is a number between 0 and concurrency-1.

	FORMAT specifications:

	  %CACHE_URI	The URI of the cached response
	  %RES{Header}	HTTP response header value
	  %AGE		The age of the cached response

	The request sent to the helper consists of the data in the format
	specification in the order specified.

	The helper receives lines per the above format specification, and
	returns lines starting with OK or ERR indicating the validity of
	the request and optionally followed by additional keywords with
	more details.  URL escaping is used to protect each value in both
	requests and responses.

	General result syntax:

	  FRESH / STALE keyword=value ...

	Defined keywords:

	  freshness=nnn	The number of seconds to extend the freshness of
			the response by.
	  log=string	String to be logged in access.log. Available as
			%ef in logformat specifications.
	  res{Header}=value
			Value to update response headers with. If already
			present, the supplied value completely replaces
			the cached value.

	In the event of a helper-related error (e.g., overload), Squid
	will always default to STALE.