Implicitly allow requests in IIS from valid hostname

Solution 1:

I don't think there is a global setting to disallow any requests with a "valid" hostname, but you can try the following

Create a new fallback site on your IIS server, while all other sites require hostnames this site is bound to all Unassigned IP addresses without any hostnames.

So any http requests to the server without an otherwise bound hostname end up on this site which just has a simple static html page.

For this site edit the IP Address and Domain Restrictions, don't add a new entry, just go to Edit Feature Settings... :

Set the default access to Deny and then the Action Type to Abort

This way no response is send to the client. You also have the options to send a 401, 403 or 404 instead, but if you want no response, use Abort

There may be other ways to accomplish this, but this should work pretty well.