Domain Forwarding with IHttpHandler

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Introduction

In this article I describe how to use a custom HttpHandler as a domain-forwarder.

Background

With the Hostheader feature in IIS you can map many hostnames to one IP-Address. For example your IIS Server has the IP-Address 195.243.118.165. You or your ISP has some A-Records for this IP-Address.

Now, you have configured a website in IIS which responds to all these domains. The following image show a sample IIS configuration.

All requests to www.mydomain.com, www.virtualdomain1.com and www.virtualdomain2.com target the same website. Often you have to do such configurations because the amount of public IP-addresses is limited.

At this point you want to provide the user a different webpage depending on the used domain name. This sample application will provide a simple solution for this problem.

Using the code

Create a class in your ASP.NET application which implements the IHttpHandler. You have to implement one method and one property:

• public void ProcessRequest (HttpContext)
• public void IsReusable

Here is the sample code:

public class DomainHandler : IHttpHandler
{
    /// Implementing IHttpHandler
    public void ProcessRequest(HttpContext context) 
    {
        // Extract the host name from the request url
        // an look up the redirect url in the Web.config
        String redir = ConfigurationSettings.AppSettings
                              [context.Request.Url.Host];
        if (redir != null)
        {
            // check for valid redirect url, preventing loops
            if (RedirectIsValid(redir, context.Request.Url))
            {
                // redirect the user to the url found in web.config
                context.Response.Redirect(redir, true);
            }
            else
            {
                // display an error.
                context.Response.Write("<h1><font 
                   color=red>Error invalid DomainHandler 
                   configuration</font></h1><br>
                   <b>Please check your Web.config 
                   file.</b>");
            }
        }
    }

    // prevents possible redirect loops
    // it is not allowed to have an redirect url targeting its self
    private bool RedirectIsValid(String redir, Uri currentUri)
    {
        String val1 = redir.ToLower();
        String url = currentUri.AbsoluteUri.ToLower();
        String host = currentUri.Host.ToLower();

        if (val1 == url) { return false; } 
        if (val1 == ( "http://" + host)) { return false; }
        if (val1 == ("http://" + host + "/")) { return false; }
        if (val1 == host) { return false; }
        if (val1 == (host + "/")) { return false; }

        return true;
    }

    /// Implementing IHttpHandler
    public bool IsReusable
    {
        get
        {
            return true;
        }
    }
}
//

Points of interest

I decided to store my URL mappings in the web.config file as key-value pairs in the appSettings Section. Additionally you must register the new HttpHandler in your web.config file.

Sample web.config file is provided here:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<!--
Use as keys your full qualified host names.
As Value you provide a absolute url, you want to redirect to.
--> 
<add key="localhost" value="http://localhost/DomainForward/target1.htm" />
<add key="www.virualdomain1.com" value="http://www.myDomain.com/domain1" />
<add key="www.virualdomain2.com" value="http://www.myDomain.com/domain2" />
</appSettings>

<system.web>

<compilation 
defaultLanguage="c#"
debug="true"
/>

<httpHandlers>
<add verb="*" path="Default.aspx" 
    type="DomainFilter.DomainHandler, DomainFilter" />
</httpHandlers> 

</system.web>

</configuration>

In the path attribute in the httpHandlers section you specify a really existing filename which matches your "IIS-Defaultpage-Configuration".

In my sample application for simplicity I used a subweb, the default in Visual Studio. In a real world example, you would install the application in the root-web.