Using HttpModules with URL Re-writing to Handle Fake Directory Requests

Introduction

When using HttpModules to re-write URLs, accessing a directory will not work. For example, a request to http://somewebsite/fakedirectory/ will not work unless the fakedirectory actually exists with some default index file in it. This occurs because when a request comes into IIS it first ascertains that the directory and file actually exist before ever instantiating the HttpModule in your ASP.NET application.

Solution

Solving this is actually fairly easy. The first thing to do is to load IIS and change the 404 errors to a URL destination and point it to your site.

Screenshot - pic2.png

This will tell IIS to automatically redirect any unknown request (e.g. to a directory that does not exist) to your application. When you do this your application will receive a request like the following: http://somewebsite/default.aspx?404;http://somewebsite/fakedirectory/

Using the code

Because this is in a standard form, the HttpModule can parse this easily, extract the real website that the user wanted and then perform the standard rewrite.

internal static String GetRequestedURL(String originalRequest)
{
    // Determine the page the user is requesting.
    String url = originalRequest;
 
    // Check for an error URL which IIS will throw when a non-existing 
    // directory is requested and custom errors are thrown back to this site
    if (url.Contains(";") && url.Contains("404"))
    {
        // Split the URL on the semi-colon. Error requests look like:
        // errorpage.aspx?404;http://originalrequest OR
        // errorpage.aspx?404;http://originalrequest:80
        String[] splitUrl = url.Split(';');
 
        // Set the URL to the original request to allow processing of it.
        if (splitUrl.Length >= 1)
        {
            url = splitUrl[1];
 
            // Remove the first http://
            url = url.Replace("http://", "");
 
            // Get only the application path and beyond of the string. This 
            // may be / if it's in the root folder.
            int index = url.IndexOf(
                System.Web.HttpContext.Current.Request.ApplicationPath);
            if (index >= 0)
            {
                url = url.Substring(index);
            }
        }
    }
 
    return url;
}

The GetRequestURL function will take a RawURL parameter and then check if it is a 404 error. If it is, it will parse out the original request and return it. If it is not a 404 error then it will return the untouched RawURL.

Concluding Remarks

While this is a fairly simple answer, I have been looking everywhere for a solution and nobody seems to have an easy answer. Even using the HttpHandler will not solve the problem because there's no extension on the requests for a directory. This is the best solution I have found and it works every time without having to do any extensive changes.