Apache Reference: http_core, UseCanonicalName

UseCanonicalName

Control the Use of Canonical Server Name
Syntax: UseCanonicalName C
Example: UseCanonicalName off
Since: Apache 1.3

In many situations, the server must construct a self-referential URL - that is, a URL that refers back to the same server. With type set to ``on'', the server will use the ServerName and Port directives to construct a canonical name. This name is then used in all self-referential URLs, as well as for the values of SERVER_NAME and SERVER_PORT in the SSI/==CGI environment.

With type set to ``off'', the server will form self-referential URLs using the host name and port supplied by the client in the HTTP Host header field, if any is supplied (otherwise, it will use the canonical name). These values are the same as those used to implement name-based virtual hosts, and they are available with the same clients. The SSI/CGI variables SERVER_NAME and SERVER_PORT will be constructed from the client-supplied values as well.

There is a third option in setting type: ``dns'', which is intended for use with mass IP-based virtual hosting to support ancient clients that do not provide an HTTP Host header field. With this option, the server performs a reverse DNS lookup on the server IP address to which the client connected to work out self-referential URLs.