Port 1237: tsdos390 — The mystery registration with a real IoT life

Port 1237 sits in an interesting place—officially registered to a service nobody documents, but quietly used by IoT devices checking in with their networks.

Official Registration

Port 1237 is registered with IANA for a service called tsdos390.¹ Both TCP and UDP are assigned. That's all IANA says. No RFC. No documentation. No explanation of what tsdos390 is or was.

The name suggests something related to IBM's z/OS mainframe operating system (formerly OS/390), but no public documentation confirms this.

Real-World Use: Wi-SUN Networks

While tsdos390 remains a mystery, port 1237 has found actual use in Wi-SUN networks—wireless smart utility networks used for smart grid infrastructure, smart cities, and IoT deployments.

In Silicon Labs' Wi-SUN implementation, devices automatically send UDP status messages to their border router on port 1237:²

• When a device first connects to the network
• Every 30 seconds by default (configurable)
• Contains JSON status information including device ID, connection statistics, and uptime

This is how border routers know which devices are alive and connected. The devices pick port 1237, send their heartbeat, and the border router listens.

The Registered Ports Range

Port 1237 falls in the registered ports range (1024-49151). These ports are registered with IANA for specific services, but unlike well-known ports (0-1023), they don't require special privileges to use.

Organizations and software developers can request registration of a port in this range. Sometimes, like with tsdos390, the original purpose gets lost. Sometimes, like with Wi-SUN, a port gets repurposed for something entirely different.

What's Listening

To check if something is listening on port 1237 on your system:

Linux/Mac:

sudo lsof -i :1237
# or
sudo netstat -tulpn | grep 1237

Windows:

netstat -ano | findstr :1237

If you find something listening on port 1237, it's likely either:

• A Wi-SUN border router receiving device notifications
• Legacy software related to mainframe systems
• Custom software that chose this port because it's registered but rarely used

Why Unassigned Matters

Actually, port 1237 isn't unassigned—it's assigned to tsdos390. But the practical effect is the same: nobody knows what tsdos390 is, so the port is available for other uses.

This happens throughout the registered range. Ports get registered, documentation disappears, original software becomes obsolete, and the ports sit there—officially claimed but effectively abandoned. Then new systems come along and use them anyway.

Port 1237 is registered. But in practice, it belongs to the IoT devices sending their heartbeats through the smart grid.