Port 3356: upnotifyps — Registered, But Unexplained

What This Port Is

Port 3356 sits in the registered port range (1024–49151) — the middle tier of the port system, between the well-known ports (0–1023) that belong to foundational Internet protocols, and the ephemeral ports (49152–65535) that operating systems hand out temporarily for outbound connections.

Registered ports are supposed to be claimed by specific services, with IANA acting as the registry. Port 3356 is registered. It has a name: upnotifyps.

Beyond that, the trail goes cold.

The Ghost Registration

The IANA Service Name and Transport Protocol Port Number Registry lists port 3356 (both TCP and UDP) as assigned to upnotifyps, with the description field containing only "UPNOTIFYPS" — the name, repeated. There is no RFC, no specification document, no organization, and no real-world documentation about what this service does or who uses it.¹

This happens more than you'd expect. IANA registered ports require minimal bureaucracy: a name, a contact person, a brief description. Some registrations were made decades ago for internal tools that never shipped publicly, or for software that quietly disappeared. The registry entry remains; the software doesn't.

Port 3356 may be a notification service of some kind — "notify" appears in the name — but that's reading tea leaves, not documentation.

What's Actually Listening on Port 3356

In practice, any software can bind to port 3356. The IANA registration carries no enforcement power. You might see it used by:

• Custom application servers that picked a registered-but-obscure port to avoid collisions
• Development environments running a local service on an arbitrary port
• Malware and remote access tools that favor obscure registered ports to blend into normal-looking traffic

The only way to know what's actually running on port 3356 on any given machine is to ask the machine.

How to Check What's Listening

On Linux or macOS:

# Show what process is listening on port 3356
sudo ss -tlnp sport = :3356

# Or with lsof
sudo lsof -i :3356

On Windows:

netstat -ano | findstr :3356

Then cross-reference the PID with Task Manager, or:

tasklist | findstr <PID>

If nothing is listening, that's the most common answer. Most machines have no service on this port.

Why Unassigned Ports Matter

The port system works because of shared expectations. When you connect to port 443, you expect HTTPS. When you connect to port 22, you expect SSH. That predictability is load-bearing for the entire Internet.

Unassigned and poorly-documented ports are the edges of that system — places where the social contract of port numbering breaks down. They matter for a few reasons:

For defenders: An unexpected process listening on port 3356 is worth investigating. You probably didn't put it there.

For developers: Picking an obscure registered port for a new service has an advantage (unlikely to conflict) and a risk (no documentation means anyone who sees it in a packet capture has no idea what they're looking at).

For the port system itself: Ghost registrations like upnotifyps occupy namespace. They're not blocking anything meaningful — the registered range has 48,128 ports — but they're a reminder that the registry is a historical artifact as much as a living standard.