SEO and ASP.NET – An Introduction to URL Rewriting

Lee Zoumas - June 6, 2008

A very important part of implementing an SEO strategy into a website is URL rewriting. Basically, URL rewriting transforms an incoming URL (usually a non-existent one) to a different URL (an existing one). Not only can URL rewriting achieve keyword rich URLs for dynamic, database driven sites, but it can also help the transition from an older site to a newer one, as well as alleviate duplicate content issues (by ensuring that spiders do not index multiple sub-domains of the same domain all with the same content or index the same page under different old URLs).

That being said, ASP.NET provides little native support for URL rewriting. The built-in functionality only boils down to one single method. The good news is that the functionality they do provide is quite extendable and there are several third party products that will help you achieve all of your SEO related goals.

The first and most flexible way to approach URL rewriting is to use an ISAPI (Internet Server Application Program Interface) filter, which does not rely on .NET and its currently-limited rewriting support. The major benefit of using an ISAPI filter is that the URL gets rewritten when the initial page request hits the web server, before the page is even passed along to the ASP.NET extension. Because ISAPI filters work at the web server level, you do not have to add any special code in your ASP.NET application. All your rewriting rules are handled in a separate configuration file. Another advantage of using an ISAPI filter is that you can write requests for any type of resource and not just a webpage, such as an image or PDF.

If your webhost has a URL rewriting ISAPI filter available to use, I strongly suggest taking advantage of it, as they’re easy to implement and perform a lot faster than the alternatives. There are several products that have been created using ISAPI filters, one most notable is Helicon’s ISAPI_Rewrite, which as of now costs $99 for a one server license. In my next post, I will explain other ways to implement URL rewriting into your web applications.