Port 377: tnETOS — The NEC Port Nobody Remembers

Port 377 sits in the well-known ports range (0-1023), officially assigned by IANA to a service called "tnETOS" by NEC Corporation.¹ Both TCP and UDP. Registered. Official.

And that's where the story ends, because nobody seems to know what tnETOS actually does.

What We Know

The official registry lists port 377 as assigned to NEC Corporation for a service called "tnETOS."² That's it. No RFC. No documentation. No GitHub repositories. No Stack Overflow questions. No angry forum posts from sysadmins trying to figure out why this port is open.

It's one of those corporate protocol assignments that happened sometime in the past—someone at NEC needed a port number for something they were building, filed the paperwork with IANA, got the assignment, and then the knowledge of what it actually was just evaporated.

The Well-Known Ports Range

Port 377 lives in the well-known ports range (0-1023), the oldest and most controlled section of the port number space. These ports are assigned by IANA through a formal process that requires documentation and justification. They're supposed to be for system-level services and widely-used protocols.

This range includes the ports everyone knows: 80 for HTTP, 443 for HTTPS, 22 for SSH. The assignments are supposed to be meaningful, documented, and stable.

Port 377 is technically one of these assignments, but it's the kind that makes you wonder about all the other mysterious ports in this range—protocols that were important enough to get an official number but not important enough to leave a trace.

Checking What's Actually There

If you want to see if anything is listening on port 377 on your system:

Linux/macOS:

sudo lsof -i :377
netstat -an | grep 377

Windows:

netstat -ano | findstr :377

You'll probably find nothing. Port 377 is one of those assignments that exists in the registry but not in the wild.

Why This Matters

Port 377 represents something interesting about the Internet's infrastructure: not every piece of it is actively used or even remembered. The port registry is full of these ghosts—services that were once important enough to get an official assignment but have since faded into obscurity.

Some ports carry the entire web. Some ports carried something once, and nobody remembers what.

Port 377 is the latter. A NEC protocol that got a number, got registered, and then disappeared into the corporate archives where old network protocols go to be forgotten.

The port number remains. The purpose is gone.

Related Ports

• Port 378 — Also assigned to NEC Corporation for "dsETOS," another mysterious NEC protocol³
• Port 376 — Unassigned, sitting right before the NEC block
• Port 380 — TIA/EIA/IS-99 modem client, another rarely-seen assignment