Port 361: Semantix — The Ghost in the Registry

Port 361 sits in the well-known port range (0-1023), officially assigned to a service called "semantix."¹ The problem? Nobody really knows what semantix was or what it did. The service has vanished, but its port number remains—a permanent reservation for a ghost.

What the Well-Known Range Means

Ports 0-1023 are called "well-known ports" or "system ports." These are assigned by the Internet Assigned Numbers Authority (IANA) through a formal process requiring IETF Review or IESG Approval.² Getting a well-known port number is difficult—you need to demonstrate that your protocol is significant enough to warrant permanent space in this limited range.

Port 361 has that assignment. It's marked as "semantix" in the registry. But the service itself appears to have disappeared sometime in the 1990s or early 2000s, leaving only its port number behind.

The Confusion with OLSR

Some port databases incorrectly list port 361 as being used by OLSR (Optimized Link State Routing protocol), a routing protocol for mobile ad hoc networks.³ This is wrong. OLSR uses port 698, officially assigned by IANA and specified in RFC 3626.⁴

The confusion likely stems from old port scanning results or misconfigured databases. OLSR never officially used port 361.

What Semantix Was

The name "semantix" appears in IANA's official registry with no additional details—no RFC, no description, no contact information beyond the name itself.¹ Historical port lists from the 1990s show the assignment, but provide no context about what the service did.

The name suggests it might have been related to semantic processing or information systems, but that's speculation. Whatever Semantix was, it left no trace beyond its port number.

Why This Matters

Port 361 represents a common pattern in the well-known range: permanent reservations for extinct services. Once IANA assigns a well-known port, that assignment rarely changes. Even when the service dies, the port number stays reserved.

This creates a form of archaeological layering in the port registry. Port 361 is a fossil—evidence that something once existed here, even if we've forgotten what it was.

Checking What's on Port 361

Since port 361 has no active standard service, if you find something listening on this port, it's either:

1. A misconfigured service that chose this port arbitrarily
2. Custom internal software using an "available" port
3. Malware (port 361 has occasionally been used by trojans precisely because it's unmonitored)⁵

To check what's listening on port 361 on your system:

# On Linux or macOS
sudo lsof -i :361
sudo netstat -tulpn | grep :361

# On Windows
netstat -ano | findstr :361

If you find something on port 361, investigate it. It shouldn't be there.

The Ghost Ports

Port 361 isn't alone. The well-known range contains dozens of these phantom assignments—services that were significant enough to get a reserved port number but have since vanished. The port numbers remain, permanently occupied by ghosts.

This is the cost of permanence in a protocol registry. You can't easily reclaim these numbers because you can't be certain the old service is truly extinct. Somewhere, on some forgotten server, semantix might still be running. Probably not. But maybe.

Related Ports

• Port 698 - OLSR (the protocol often confused with port 361)
• Port 360 - Another unassigned/obscure port
• Port 362 - Another ghost in the registry