Some of you may be aware of the technique of banner grabbing to find out what software is running on a remote machine. It is done by reading out transmitted data that contains a product name and/or version, such as HTTP's "Server:" response header. Like virtually every content sent by a server, the server is totally free what to send, i.e. this can be easily faked by an HTTP server's admin, and is sometimes even done.
web server usage statistics
presentation on varnish cache
http reverse cache
