ASP.NET offers several ways of mapping nice public URLs to cryptic internal URLs matching the actual implementation.

The simplest way is to match each public URL to its internal value. You can do this by either creating dummy pages which do the redirecting or by adding them to the urlMappings section of the web.config file (brought by framework 2.0). The latter solution is preferable because of simpler maintenance but both of them have the downside of being completely static (an entry is necessary for each mapped page).

A better solution is to implement an IHttpHandler or IHttpModule and do the mapping by calling HttpContext.RewritePath(). You could also change the requested page by using Server.Transfer() or maybe even Response.Redirect() but they have their disadvantages when used for URL mapping.

To avoid having to recompile your IHttpHandler implementation every time the mapping logic changes or extends you could configure it through a custom section in the web.config file. Regular expressions are a nice tool for defining such mappings as demonstrated by here (also check the comments) or implemented in dasBlog sources (check newtelligence.DasBlog.Web.Core.UrlMapperModule). You could even use UrlRewritingNet.UrlRewrite if you don’t like reinventing the wheel.

Be careful when hosting your web site based on DasBlog from a Windows XP machine. While IIS 6 in Windows 2003 prevents the download of files with unknown extensions by default, the IIS 5.1 in Windows XP allows downloading such files. In the case of DasBlog all *.blogtemplate files are at risk. There are a few sites out there where these files can be downloaded. Although this probably isn’t a big security risk it might be something you want to prevent. Probably the easiest way to do that is by modifying the web.config file. You should add the following line at the end of the <httpHandlers> section:

<add verb="*" path="*.snippet" type="System.Web.HttpForbiddenHandler" />

The problem is pretty straightforward: the web site redesign causes the structure to change, thus the old addresses become invalid. Since you don’t want the users to get the dreaded error 404: Object not found, there are a couple of options available to you (if you’re using IIS – Internet Information Services, that is).

You could just change the error page to match the style of your web site and inform the visitor about the now missing page or just make the redirection to your new starting page. This is a bit unfriendly to the visitors if you kept the old content since they have to find it themselves. It would certainly be better to redirect them directly to the new address of the old content. But still it’s not a bad idea to do this. It’s an easy way to keep the users on your website even when they encounter invalid URLs by whatever reason. Just open up the Custom Errors tab of the virtual directory or web site properties and set the desired URL for the error 404. But don’t forget that you have to enter the complete path starting from the root of the site, for example: "/mydirectory/myurl.html".

If you want to make a different redirection for each page you could just keep the old pages but instead of having any actual content they would just make a redirection to the correct new address. This solution has two problems:

• It’s difficult to maintain if you have many pages.
• You’re stuck with the client side redirection, i.e. meta refresh tag.

To make the redirection server side you could use the redirect options on the Home Directory tab of the virtual directory or web site properties. But they have some serious limitations and tend not to work as expected, even more so because the documentation doesn’t explain them very well. But there’s no reason to worry, I have a better solution for you. Setup a special 404 URL on the Custom Errors tab as already suggested. But this time use an asp or aspx page for it. The supplied query string (Request.QueryString) contains the missing URL which you can parse out and use to determine the correct new address corresponding to it. For a few pages a simple select or switch clause will do but nothing prevents you from having the mappings stored externally, in a special file or a database table for example. All that’s left is to make a Response.Redirect to the new address.

There’s one more thing to take care of. If you moved your site to a new subdirectory and chose the last suggested solution, don’t forget to setup a similar simple starting page which just redirects the visitors to the new starting page. Trying to open the site without this page will namely cause an error 403: Forbidden, because a directory listing will be attempted which you have (hopefully) prevented.

Thanks go to Peter Forret for some of the ideas I used to make this work when redesigning my page.