Differentiating between port scanners and legitimate users

One of my readers asked a very interesting question:
“Is there a way to have a port on a router open for legitimate use while closed to port scanning software and the such. For example. I have SSL VPN configured on my IOS router. Is it possible to have the port seem stealthed to port scanners while still allowing legitimate access to the service? An example being, allowing a web browser to connect using the port but making sure that a port scanner doesn't detect it as open.”
The short answer is no, unless you can differentiate legitimate users by their IP addresses. The port scanners (when using SYN scan) simply open and close a TCP session, and there is no way for a router to differentiate between the legitimate users (who would send valid HTTP GET requests) and port scanners (that would close the session as soon as it's established).

If you can distinguish between legitimate users and everyone else based on their IP address, the task becomes simpler: either you apply inbound access list on Internet-facing interface of the router or configure per-service access-list (for example, ip http access-class acl). When using the inbound access-list, the port appears filtered or stealth, when configuring per-service access-class, it's reported as closed.
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