IIS URL Rewriting and ASP.NET Routing

With the release of the URL Rewrite Module for IIS and the inclusion of ASP.NET routing into the .NET Framework 4, there have been a lot of questions from ASP.NET developers about how these two features relate to each other and when you should use one or the other. This document describes the differences between these two technologies and provides guidance for Web developers about when to use IIS URL rewriting and when to use ASP.NET routing.

From a high-level perspective, it seems like these technologies provide very similar functionality--both allow your Web applications to have user-friendly and search-engine-friendly URLs.

However, there are fundamental differences between these two technologies that are important to understand to make the right decision about what to use for your Web application.

To help you understand those differences, we will first explain how IIS URL rewriting and ASP.NET routing work.

Differences Between IIS URL Rewriting and ASP.NET Routing

Based on the above explanation, there are the following main conceptual differences between IIS URL rewriting and ASP.NET routing:

1. URL rewriting is used to manipulate URL paths before the request is handled by the Web server. The URL rewriting module does not know which handler will eventually process the rewritten URL. In addition, the actual request handler might not know that the URL has been rewritten.
2. ASP.NET routing is used to dispatch a request to a handler based on the requested URL path. As opposed to URL rewriting, the routing module knows about the handlers and selects the handler that should generate a response for the requested URL. You can think of ASP.NET routing as an advanced handler-mapping mechanism.

In addition to these conceptual differences, there are the following functional differences between IIS URL rewriting and ASP.NET routing:

1. The IIS URL Rewrite module can be used with any type of Web application, which includes ASP.NET, PHP, ASP, and static files. ASP.NET routing can be used only with .NET Framework-based Web applications.
2. The IIS URL Rewrite module works the same way regardless of whether integrated or classic IIS pipeline mode is used for the application pool. For ASP.NET routing, it is preferable to use integrated pipeline mode. ASP.NET routing can work in classic mode, but in that case the application URLs must include file name extensions or the application must be configured to use "*" handler mapping in IIS.
3. The IIS URL Rewrite module can make rewriting decisions based on domain names, HTTP headers, and server variables. By default, ASP.NET routing works only with URL paths and with the HTTP-Method header.
4. In addition to rewriting, the URL Rewrite module can perform HTTP redirection, issue custom status codes, and abort requests. ASP.NET routing does not perform these tasks.
5. The URL Rewrite module is not extensible in its current version. ASP.NET routing is fully extensible and customizable.

Which Option Should You Use?

What does all this information mean if you need to choose a technology to enable clean URLs for your Web applications? In this section, we explain how to make this choice.

If your Web application is built by using anything except ASP.NET, use the IIS URL Rewrite module. Otherwise, the rules are:

1. If you are developing a new ASP.NET Web application that uses either ASP.NET MVC or ASP.NET Dynamic Data technologies, use ASP.NET routing. Your application will benefit from native support for clean URLs, including generation of clean URLs for the links in your Web pages. Note that ASP.NET routing does not support standard Web Forms applications yet, although there are plans to support it in the future.
2. If you already have a legacy ASP.NET Web application and do not want to change it, use the URL Rewrite module. The URL Rewrite module lets you translate search engine-friendly URLs into a format that your application currently uses. Also, it lets you create redirect rules that can be used to redirect search engine crawlers to clean URLs.

In practice, however, the choice does not have to be either/or. The technologies can be used together and can complement each other. In the following sections, we outline some scenarios where you can use ASP.NET routing and IIS URL rewriting together.

Enforcing canonical URLs for your application.
You should force the use of http://www.mysite.com/home/about instead of http://mysite.com/Home/About. When a Web client requests a URL that does not conform to the format that you want, the client is redirected to a canonical URL. In this scenario, you can use the URL Rewrite module to enforce canonical URLs and perform redirection, and use ASP.NET routing to select a handler that would process the requested URL path.

Conclusion

Either IIS URL rewriting or ASP.NET routing can be used to implement URL manipulation scenarios for your Web application.

ASP.NET routing is a solution that is optimized for ASP.NET, thus it may be preferable for Web developers who design their ASP.NET applications from the ground up and want to have a clean URL structure.

IIS URL rewriting is a generic URL manipulation mechanism that addresses a multitude of scenarios.

In particular, it can be used by Web developers as well as Web server/site administrators to enable clean URLs for existing Web applications without modifying the application code.