I have done some similar url rewriting that parses out some params from the url and constructs the destination url. It was not a DNN site, but it is a fairly simple asp.net technique. I saw some nice url rewriting rule engines on codeproject if you want something more in depth, but since I only had one case to handle for the time being I just did a very small solution using Context.RewritePath().

In global.asax do something like:

protected void Application_BeginRequest(Object sender, EventArgs e)
{
    HttpApplication Appl = (HttpApplication)sender;
    string requestedUrl = Appl.Request.Path.ToLower();

    //avoid any further processing in the common case:
    if (!requestedUrl.StartsWith("/user/"))
    {
        return;
    }
    else
    {
        //parse out whatever value you're looking for from requestedUrl
        string redirectedUrl = ...;
        
        //send the client where they should go
        Appl.Context.RewritePath(redirectedUrl);
    }
}


-Adrian