Squid: The Definitive Guide

This directive works in conjunction with header_access . If you use header_replace , Squid replaces HTTP headers that are denied (removed) by an header_access rule. In other words, an HTTP header must be filtered out by header_access before it can be replaced by header_replace .

header_replace isn't especially flexible. You can only define one replacement value for each header. You can't, for example, use one value for some requests and a different value for others.