HTTP Request Header Host

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External

https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.23

Internal

Overview

The "Host" request header specifies the Internet host and optionally port of the resource being requested, as obtained from the original URI given by the user to the user agent. If "port" is not included, it is assumed 80.

The "Host" header must be present, even if it has an empty value, otherwise the request is considered invalid and the proxies handling it are required to return a 400 Bad Request. Host is required to allow the server to differentiate between URLs when it servers multiple host names on a single IP address.