Port 3522: NSSocketPort — Apple's Distributed Objects, Frozen in 2002

What Port 3522 Is

Port 3522 is a registered port — assigned by IANA in June 2002 under the name nssocketport, short for Distributed Objects over NSSocketPort.

Unlike truly unassigned ports, this one has a documented owner. It just happens to own something nobody uses anymore.

The Technology Behind It

In the early 2000s, Apple's Cocoa framework included a system called Distributed Objects (DO). The idea was elegant: Objective-C objects could call methods on remote objects — running on another process or another machine entirely — as if they were local. No explicit serialization, no manually defined message formats. You just called a method, and the framework handled the network.

NSSocketPort was the network-capable variant of this system. Unlike Mach ports and message ports (which are restricted to a single machine), NSSocketPort used BSD sockets and could communicate across the Internet. The NSSocketPortNameServer let you register your socket port by name so other machines could find it without hard-coding port numbers.¹

Port 3522 was the default port for this name server — the rendezvous point where Distributed Objects over NSSocketPort could find each other.

Why You've Never Seen It

Distributed Objects was deprecated. Modern Apple development uses XPC for local inter-process communication, and REST, gRPC, or similar protocols for network communication. The elegant idea of transparent remote method calls ran into the reality that networks are unreliable, latency matters, and transparency can become a liability when remote calls fail in ways local calls never do.²

Port 3522 isn't in active use on modern systems. You might see it on legacy macOS machines running old Cocoa applications from the early-to-mid 2000s, but don't expect to find it on anything built in the last decade.

What Range This Port Lives In

Port 3522 falls in the registered port range (1024–49151). These ports are registered with IANA for specific services — they're not reserved for the operating system like well-known ports (0–1023), but they're not ephemeral or dynamically assigned either. A registered port means someone asked IANA for it, documented a purpose, and got it assigned. Whether the technology survives is another matter.

How to Check What's Listening

If you see activity on port 3522 and want to know what's using it:

macOS / Linux:

# Show what process is listening on port 3522
sudo lsof -i :3522

# Alternative with ss (Linux)
ss -tlnp | grep 3522

# Or with netstat
netstat -an | grep 3522

Windows:

netstat -ano | findstr :3522

The PID in the output maps to a process — cross-reference with Task Manager or ps aux | grep <PID> on Unix systems.

On most modern machines, you'll find nothing. That's expected.

Why Unassigned (and Dormant-Registered) Ports Matter

The port registry is a coordination mechanism, not a guarantee. Ports get registered for technologies that seemed important at the time. Some of those technologies won. Others lost. The registry keeps their numbers reserved either way — partly to avoid confusion, partly because nothing else should squash that space without a good reason.

Port 3522's existence in the IANA registry is honest documentation of where networking was heading in 2002. The Internet is littered with ports like it: registered, documented, and nearly invisible.