One of the most popular extensions to the Apache Web server has been mod_rewrite - a filter which rewrites URLs. For example, instead of a URL such as

http://www.xyz.org/MyPage.aspx?id=137

You could provide a filter which accepts URLs such as

http://www.xyz.org/MyPage/137.apsx

And it will silently perform a server-side redirect to the first URL. In this way, the real URL could be hidden, providing an obfuscated facade to the web page. The benefits are easier to remember URLs and increasing the difficulty of hacking a website.

‘mod_rewrite’ became very popular and grew to encompass a couple of other features not related to URL Rewriting, such as caching. This blog demonstrates URL Rewriting with ASP.NET, whereby the requested URL is matched based on a regular expression and the URL mappings are stored in the standard ASP.NET web.config configuration file. ASP.NET includes great caching facilities, so there's no need to duplicate mod_rewrite's caching functionality.

As more and more websites are being rewritten with ASP.NET, the old sites which had been indexed by google and linked from other sites are lost, of course culminating in the dreaded 404 error. I will show how legacy ASP sites can be upgraded to ASP.NET, while maintaining links from search engines.

ASP.NET provides very limited support out of the box. In fact, its support is down to a single method:

void HttpContext.RewritePath(string path)

Which should be called during the Application_BeginRequest() event in the Global.asax file. This is fine as long as the number of URLs to rewrite is a small, finite, manageable number. However, most ASP sites are in some way dynamic, passing parameters in the Query String, so we require a much more configurable approach.

The storage location for all ASP.NET Configuration information is the web.config file, so we'd really like to specify the rewrites in there. Additionally, .Net has a fast regular expression processor, giving free and fast search and replace of URLs.