Port 3551: apcupsd — The UPS Watcher

What This Port Does

Port 3551 is registered with IANA for apcupsd — the APC UPS daemon — an open-source service that monitors uninterruptible power supplies made by American Power Conversion (APC). When apcupsd is running, it listens on this port and serves status information: battery charge, load percentage, estimated runtime, input voltage, and whether the UPS is currently on mains power or running on battery.

The port operates on both TCP and UDP.

What apcupsd Actually Does

A UPS is the device between your servers and the wall outlet. When grid power fails, the UPS switches to battery and buys time. The question is: how much time? And what happens when it runs out?

apcupsd answers those questions. It polls the UPS over a serial or USB connection, tracks battery state in real time, and — crucially — initiates a graceful shutdown of connected systems if power doesn't return before the battery hits a configured threshold. Without something like apcupsd, a server simply dies when the battery empties. With it, the server shuts down cleanly, filesystems intact, services stopped properly.

Port 3551 is the network face of this monitoring. Other machines on the same network can query it to check UPS status — useful in environments where multiple servers share a single UPS.

IANA Registration

Despite the "unassigned" appearance in many port lists, IANA formally registered port 3551 for apcupsd. The registration is sparse by design: this is infrastructure software for a specific hardware ecosystem, not a broadly deployed Internet protocol. Most Internet-facing systems will never touch it.

Who Has This Port Open

• Servers in physical data centers or server rooms with APC UPS hardware
• Home labs and small offices with an APC UPS and a Linux/Unix host running apcupsd
• Network-attached storage devices with UPS monitoring enabled

It's rare on cloud instances and containers, which don't typically have physical UPS connections.

Security Considerations

Port 3551 should not be exposed to the public Internet. The apcupsd network protocol has no authentication. Anyone who can reach the port can query UPS status. In a misconfigured deployment, that means battery capacity, load, and timing information — potentially useful for timing an attack around a power event.

The SANS Internet Storm Center has recorded scanning activity against port 3551, which likely represents automated reconnaissance looking for exposed apcupsd instances.¹

If you're running apcupsd, bind it to localhost or an internal interface only. There is no reason for this port to be reachable from outside your local network.

How to Check What's Listening on This Port

Linux/macOS:

# Show what process is listening on port 3551
ss -tlnp | grep 3551
# or
lsof -i :3551

Windows:

netstat -ano | findstr :3551

Query apcupsd directly (if running locally):

apcaccess status

• Port 3551 — SpeedGuide
• IANA Service Name and Transport Protocol Port Number Registry
• apcupsd — Proxmox Support Forum