The extract-source function extracts the specified values from the given sources and adds them to the resource description.
Lists source names. You can use the -> operator to define a new name for the RD attribute. For example, type->content-type would take the value of the source named type and save it in the RD under the attribute named content-type.
Generate fn=extract-source src="md5,depth,rd-expires,rd-last-modified"