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If you are familiar with IIS 7, you will notice an important feature added named Request Filtering.
Then you must know the following KB article,
However, a latest query from a user reveals a new question. This guy wants to block all unknown extensions (by setting allowUnlisted to false), but he noticed that he received a 404.7 error when accessing http://(server_name)/ which has no extension specified.
So how to make it work? Well, a simple trick is to add a new Allow rule whose extension is “.” (yes, a simple DOT). This extra rule will allow you to access “/” and redirect you to the default document configured.
It is a pity that such trick is not included in the KB article, but now you know how to do it.