Please Note: The conclusion that the request came via a proxy is based on the presence of at least one of the Via, Forwarded, X-Forwarded-For or Client-ip headers. (see rfc2616 & draft-ietf-http-v10-spec-01 & Squid Configuration Guide & Squid Release Notes 1.1)
Update 08 Feb 2002 by Lagado: Added support for Client-ip header used by some transparent proxies. See more background.
Update 30 Apr 2001 by Lagado: Added Cache-Control: no-cache header to proxy-test responses. This discourages HTTP/1.1 compliant proxies from returning cached test results. Also freshened the document references.