IIS Stopped Logging – How to Fix

This is by design. A URL Reservation is not the same as a URL Registration, which happens dynamically when an application registers with HTTP.SYS to being listening. In the case of a URL Reservation, HTTP.SYS does not know whether the URL in a given Reservation is simply unavailable temporarily or does not exist at all. All it can know in this case is that there is a valid host name match (because of your strong/weak wildcard), but the URL path does not match for any currently registered listeners.

If you want a 400 or 503 response, then either do not use any URL Reservation, or use explicit URL Reservations that do not include wildcards. In short, if you configure HTTP.SYS such that it can match the prefix of the URL to any Reservation (including dynamic ones created via Registration), then it will either deliver the request to listening application (if the rest matches), or return a 404 if it can't find an active registered listener.

Another solution that may be simpler is to use the IIS service itself to manage sending a 503. To do this don't stop the site, but instead stop the Application Pool(s) associated with it. This will cause a fully qualified match to occur in HTTP.SYS (even with wlidcard URL Reservations), but will result in a 503 because there is no AppPool to deliver the request to.

Check if either the IIS log file has rolled over due to a restart of an instance (or if there is a new date/time entry line) and do the same for the HTTPERR log file (located roughly here C:\Windows\System32\LogFiles\HTTPERR) This will just give you a rought estimate of when the site went down.