Why does Request["host"] == "dev.testhost.com:1234" whereas Request.Url.Host == "localhost"

Question

Hi all, I seem to have found a discrepancy when testing ASP.NET applications locally on the built-in web server with Visual Studio 2008 (Cassini).

I've set up a host on my local machine associating dev.testhost.com with 127.0.0.1, since I have an application that needs to change its appearance depending on the host header used to call it.

However, when I request my test application using http://dev.testhost.com:1234/index.aspx, the value of Request.Url.Host is always "localhost". Whereas the value of Request.Headers["host"] is "dev.testhost.com:1234" (as I would expect them both to be).

I'm NOT concerned that the second value includes the port number, but I am mighty confused as to why the HOST NAMES are completely different! Does anyone know if this is a known issue, or by design? Or am I being an idiot?!

I'd rather use Request.Url.Host, since that avoids having to strip out the port number when testing... - Removed due to possibly causing confusion! - Sam

Answer 1

Request.Headers["host"] is the value received from the application that connects to the server, while the other value is the one the server gets when it tries to get the domain name.

The browser uses in the request the domain name entered because that is used in the case of virtual domains. The server reports the one set in the server preferences, or the first one it finds.

EDIT: Looking at the code of Cassini to see if it uses some particular settings, I noticed the following code:

public string RootUrl {
  get {
    if (_port != 80) {
      return "http://localhost:" + _port + _virtualPath;
    }
    else {
      return "http://localhost" + _virtualPath;
    }
  }
}

//
// Socket listening
//

public void Start() {
  try {
    _socket = CreateSocketBindAndListen(AddressFamily.InterNetwork, IPAddress.Loopback, _port);
  }
  catch {
    _socket = CreateSocketBindAndListen(AddressFamily.InterNetworkV6, IPAddress.IPv6Loopback, _port);
  }
  // …
}

The explanation seems to be that Cassini makes explicit reference to localhost, and doesn't try to make a reverse DNS lookup. Differently, it would not use return "http://localhost" + _virtualPath;.

Answer 2

The Request.Headers["host"] is the host as specified in the http header from the browser. (e.g. this is what you'd see if you examined the traffic with Fiddler or HttpWatch)

However, ASP.NET loasds this (and other request info) into a System.Uri instance, which parses the request string into its constituent parts. In this case, "Host" refers to literally the host machine part of the original request (e.g. with the tcp port being in the Port) property.

This System.Uri class is a very useful helper class that takes all the pain out of splitting your request into it's parts, whereas the "Host:" (and for that matter the "GET") from the http header are just raw request data.

Although they both have the same name, they are not meant to be the same thing.