Host Header 

Before the overview of the individual scenarios and the affects on the configuration table is a description of how the browser sets the host header.

When a request is fetched from the server, the full URL is written to the browser, including the protocol (HTTP), the host name, port, and path. The HTTP protocol however still sends a slightly different request.

GET / HTTP/1.1

Accept: */*

Accept-Encoding: gzip, deflate

Accept-Language: de,en-us;q=0.5

Host: intranet.sap.com

User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0)

The request line contains the HTTP specification and the necessary path (in the example here this is GET /). A host header is also set containing the full name of the server that the browser sees as its communication partner. If the URL points to a non-standard port (HTTP uses by default port 80 and HTTPS Port 443), then the port will be included in the host header. So the host header reflects the name that the browser uses to reach the server. The browser assumes this name is valid. It does not have to be the actual name of the server.

If a proxy is used it must contain the host header and must not convert it to another name. Note that in particular with apache proxies this feature is available only as of version 2 (configuration option ProxyPreserveHost).