A server MUST respond with a 400 (Bad Request) status code to any
HTTP/1.1 request message that lacks a Host header field and to any
request message that contains more than one Host header field or a
Host header field with an invalid field-value.
Even worse, the last Host header field appears to "win" and be the selected Host header field in the request forwarded to the origin.
Posts about the Web and the software that powers it. Light on content right now but will hopefully change that soon.