external auth from request header values

[Bob T. Terminal]

Former squid user looking at trying to transition to Varnish. Trying  
to setup an auth check for incoming requests that would look at the  
request headers cookie and pass values to a mysql server for  
authentication. Currently have a external application which can handle  
this, what is the best way to go about calling this external app  
within Varnish for the authentication check? (Squid refereed to this  
as an external acl helper)  if there is a better solution please point  
me in the right direction

Thanks
Bob