What This Port Is
Port 3684 is registered with IANA under the service name faxstfx-port, assigned to a piece of software called FAXstfX. The registration was filed by Alec Carlson in January 2003, covering both TCP and UDP. 1
That's about all we know. FAXstfX left almost no trace on the modern Internet. No homepage, no documentation, no user forums. Whatever it was — a fax server, a fax-over-LAN application, something else entirely — it didn't survive into the era of universal broadband and cloud faxing.
Port 3684 is, technically speaking, not unassigned. It's something rarer: a registered port whose software no longer exists.
The Registered Port Range
Port 3684 falls in the registered ports range: 1024 to 49151. IANA manages this range for applications and services that want a stable, named port assignment. Unlike the well-known ports below 1024 (which require elevated system privileges to bind), registered ports are accessible to ordinary software. 2
Anyone can request a registered port assignment from IANA. You submit a request, demonstrate a legitimate use, and if approved, your service name appears in the global registry alongside the port number. The assignment persists indefinitely — there's no expiration, no reclamation process for abandoned software.
This is why ghost entries exist. 2003-era software registered port 3684. The software faded. The registry entry did not.
What You'll Actually Find on Port 3684
In practice, no active software uses this port by default. If something is listening on port 3684 on a machine you control, it's one of:
- Custom software configured to bind there
- A development server that landed on this port arbitrarily
- Something worth investigating
To find out what's listening:
macOS / Linux:
Windows:
The process ID from those commands will tell you exactly what claimed the port.
Why This Matters
The port namespace is finite — 65,535 ports total, split across TCP and UDP. IANA's registry exists to prevent collisions: if two applications both claim port 3684 as their default, users caught in the middle suffer. The registry is the solution.
But the registry doesn't clean house. FAXstfX's entry from 2003 will persist for as long as IANA maintains the document. In a sense, it's a feature: predictability and stability matter more than tidiness. Port 3684 will never be silently reassigned to something else. Whatever legacy FAXstfX deployments exist (probably zero) won't suddenly find their port poached.
A registered ghost is more useful than an unregistered squatter.
Frequently Asked Questions
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