Supporting HTTP Access

Whether generated by a browser or a specialized client, HTTP requests from the client computer are sent to the front-end server. The front-end server uses Active Directory to determine which back-end server to proxy the request to.
After determining the appropriate back-end server, the front-end server forwards the request to the back-end server. Apart from specific header information that indicates the request was passed through a front-end server, the request is almost the same as the original request sent from the client. In particular, the HTTP host header, which matches the name of the front-end server to which the request was sent (meaning the hostname or fully qualified domain name that the user entered in the browser), remains unchanged. The front-end server contacts the back-end server using the hostname of the back-end server (for example, backend1), but in the HTTP headers of the request, the front-end server sends the host header used by the client, for example, www.adatum.com. The host header setting ensures that the appropriate back-end Exchange virtual server handles the request. For more information about configuring virtual servers on a back-end server, see Configuring a Back-End Server.
For HTTP requests, the front-end server always contacts the back-end server over TCP port 80 (the default HTTP port), regardless of whether the client contacted the front-end server through port 80 or 443 (the SSL port). This means that:
• HTTP virtual servers on the Exchange front-end server can listen only on port 80 (HTTP) or 443 (HTTPS).
Note:
No other ports other than port 80 and port 443 can be used for HTTP virtual servers on the Exchange front-end servers.
• SSL encryption is never used between the front-end and back-end servers, although the client should use it to communicate with the front-end server.
• HTTP virtual servers that differentiate themselves from other servers only by port number are not supported in a front-end and back-end topology. For example, if a back-end server has an HTTP virtual server listening on port 8080, a client can access that back-end server only if the client is pointed directly to the back-end server (for example, http://backend1:8080/data). A client connecting to the front-end server cannot access this data.
The back-end server processes the HTTP request from the front-end normally, and the response is sent unchanged through the front-end server back to the client. This whole process is not visible to the client, which just interacts with the front-end server. The client is unaware of how the request was handled internally.