Port 3491: SWR Port — A name with no story

Port 3491 has an IANA registration. It has a name — "swr-port," formally titled "SWR Port." It was registered in May 2002 by Ian Manning. Beyond that: nothing. No RFC. No protocol documentation. No known software. No traffic signatures. Just an entry in the registry that has sat untouched for over two decades.¹

The Registered Port Range

Port 3491 lives in the registered ports range, which spans 1024 to 49151. This middle tier sits between the well-known ports (0–1023, where HTTP, SSH, and DNS live) and the ephemeral ports (49152–65535, used for temporary outbound connections).

Registered ports have a looser bargain than well-known ports. IANA accepts applications from anyone with a protocol to register. The bar is paperwork, not necessity. As a result, the registered range contains a mix of the actively used (PostgreSQL on 5432, Redis on 6379) and the forgotten — ports claimed once and never deployed.²

Port 3491 belongs to the second category.

What Is SWR Port?

Unknown. "SWR" could stand for many things depending on context. In radio engineering, SWR means Standing Wave Ratio. In software, it could be an acronym for any number of internal project names. The IANA registry entry provides only the service name, the transport protocols (TCP and UDP), and the registration date. No description. No contact information for the registrant. No public-facing documentation has surfaced in the two decades since registration.¹

This happens more often than you'd expect. Someone builds a system, claims a port number to avoid collisions with other software, and then — the project ships internally, gets cancelled, or quietly evolves into something else. The IANA registration stays forever.

Is Anyone Using It?

There is no documented unofficial use of port 3491 in security databases, application logs, or network scan data. It does not appear in common malware signatures or in port scanning blacklists. It is simply quiet.

If you're seeing traffic on port 3491 on your network, it's almost certainly application-specific — an internal tool, a game server, or some piece of software that chose this number because it happened to be available.

How to Check What's Listening on This Port

macOS / Linux:

# Show process listening on port 3491
sudo lsof -i :3491

# Alternative with ss (Linux)
ss -tlnp | grep 3491

Windows:

netstat -ano | findstr :3491

From outside your machine:

# Check if port 3491 is open on a remote host
nmap -p 3491 <hostname>

If something is listening, the process name will tell you everything the IANA registry won't.

Why Ghost Registrations Matter

The IANA port registry isn't a directory of active services — it's a historical ledger of claims. Port 3491 is registered, which means no other party should formally register it for a different service. But registration doesn't mean activity, and it doesn't prevent unofficial use.

The registered range has tens of thousands of ports. Many are like this one: named, dated, and silent. They're the archaeological record of software projects that almost were.