URL Rewriting with ASP.NET
In one of my recent applications I encountered a situations like this:
The client was having his web site hosted with one of the web hosts. He was
also having multiple sub domain pointers i.e. the main domain was something like
somedomain.com and there were sub domains like products.somedomain.com and
support.somedomain.com pointing to the same location. Now he wanted that
whenever somebody requests any page like support.somedomain.com/helpdesk.aspx
then the page from somedomain.com/support/helpdesk.aspx be served. In other
words he wanted to "hide" the actual URL http://www.somedomain.com/support/helpdesk.aspx
from the end user. The end user instead always sees http://support.somedomain.com/helpdesk.aspx
in the browser. This process of changing the URL from what was originally
requested is called as URL rewriting. In this article we will develop one
possible solution to the said problem.
What options are available for URL rewriting?
URL rewriting can be achieved with many ways. Following are more popular:
- Use Application_BeginRequest event and HttpContext class to rewrite the
URL. This is the simplest and quickest approach.
- Develop an ASP.NET http module that serves the same purpose.
- Develop an ISAPI filter that intercepts the requests and does the
In this article we will examine the first method. The reason I used this
method in above described scenario is because of its simplicity and quick
implementation. In advanced and more generic scenarios you may opt for the other
The Application_BeginRequest Event
The Application_BeginRequest event is the first event to be fired in the HTTP
pipeline chain of processing. This event is the right place to place URL
The HttpContext class contains HTTP-specific information about a specific
HTTP request. This class also gives you access to other ASP.NET objects such as
Request and Response. The class has a static property called Current that holds
reference to the current application. The RewritePath() method of this class is
the key to developing our URL rewriter. The method has two signatures in .NET
public void RewritePath(string);
public void RewritePath(string, string, string);
The main role of RewritePath() method is to update the Request object's Path
and QueryString properties as per our requirements. In other words it allows the
requested URL to be different than the internal path of the page.
Example - Step-By-Step
- Create a new ASP.NET Web Application project in C#
- Open the web.config file and add following markup to it.
<add key="productsSite" value="products"></add>
<add key="servicesSite" value="services"></add>
<add key="supportSite" value="support"></add>
We will store the physical folder names that contains the actual pages in <appSettings>
section. We will read these settings in our code latter.
- Create three folders in the application - Products, Services and
- Add one webform in each of the folders called MainPage.aspx
- Open Global.asax and have a look at following event handler
protected void Application_BeginRequest
(Object sender, EventArgs e)
- Add following code to the event handler:
Let's examine the code in bit details.
We first obtained the host address of the requested URL via Request.Url.Host
property. In this case we will get it something like support.somedomain.com or
We also obtained the path and query string of the requested page via
Request.PathAndQuery property. In our scenario we will get it something like /helpdesk.aspx
The switch statement then tests the host and depending on the host we read
the physical folder from the configuration file. We form the new URL from root
to this folder (as indicated by ~ character).
Finally, we called RewritePath() method of HttpContext class and pass this
new URL to it. As a result even though user requested support.somedomain.com we
serve the page that actually resides in somedomain.com/support folder.
In this article we learnt to write URL rewriting code using
Application_BeginRequest and HttpContext.RewritePath() method. This is a quick
and easy method of supplying contents from URLs different than what were