Iis – Configure IIS to rewrite IP Address to Site Name

You don't want rewrite rules at all, you do want to setup a new website configuration. IIS 6 can differentiate between websites via either using a new IP address (so the server has multiple IP addresses), or by using a host header to link a domain to a website configuration.

Try starting here: Hosting Multiple Web Sites on a Single Server (IIS 6.0) and Using Host Headers to host multiple websites on IIS 6.0

You have two options. One is to keep going down the path that you're on. At a high level the issue with the canonical url rewrite issue is with the action. You can use {HTTP_HOST}, so the action should be something like www.{HTTP_HOST}/{URL}. That makes it dynamic based on the domain name entered rather than having it set to a specific name.

However, you may find it easier to continue to use the default docs. The way to do this per site is to use location tags within applicationHost.config. You can't do it the standard way within IIS Manager because, as you pointed out, it shares the same config for all sites. There are 3 fairly easy ways to do it (not counting the programming options). One is with Configuration Editor, another is with appcmd and the 3rd is with notepad or an editor directly.

I'll describe here how to do it the Configuration Manager way. First, make sure that you have it installed: http://www.iis.net/download/AdministrationPack

Then using IIS manager, navigate to the site you want to change the default doc for. Open Configuration Manager. In the top left drop-down, change to system.webServer/defaultDocument. In the top right, make sure to change to the ApplicationHost.config option. In the main window, select the 'files' row and click the (...). The rest is straight forward.