Squid: The Definitive Guide

When certain HTTP/1.1 origin servers receive an HTTP/1.0 request (e.g., from Squid), and the response contains a Vary header, they also add an Expires header set to the current time. This is to prevent HTTP/1.0 caches, which may not understand the Vary header, from incorrectly reusing a cached response.

Squid understands and implements the Vary header but still sends the string "HTTP/1.0" in its requests . You'll need to enable this directive if you want to get cache hits from responses with Vary and with Expires equal to Date . This directive is somewhat dangerous because the origin server may have its own reasons (other than maintaining backward compatibility) for setting the Expires header.