Manual:$wgSquidPurgeUseHostHeader

Set this to false to support Squid configured in forward-proxy mode.

If this is set to true, a Host header will be sent, and only the path component of the URL will appear on the request line, as if the request were a non-proxy HTTP 1.1 request. Varnish only supports this style of request. Squid supports this style of request only if reverse-proxy mode (http_port ... accel) is enabled.

If this is set to false, no Host header will be sent, and the absolute URL will be sent in the request line, as is the standard for an HTTP proxy request in both HTTP 1.0 and 1.1. This style of request is not supported by Varnish, but is supported by Squid in either configuration (forward or reverse).