Port 3664: ups-engine — A Name Without a Story

What This Port Is

Port 3664 sits in the registered ports range (1024-49151) and carries an official IANA assignment: ups-engine, described as "UPS Engine Port." That registration covers both TCP and UDP.

Beyond the name, public documentation runs dry. The registration exists in the IANA registry¹ but carries no RFC, no contact organization, and no protocol specification linked to it. "UPS" most likely refers to Uninterruptible Power Supply management software, but this port is distinct from the well-known UPS tools: Network UPS Tools (NUT) runs on port 3493², and the original IANA UPS protocol assignment sits at port 401. Whatever "UPS Engine" refers to, it did not become a standard.

The Registered Ports Range

Ports 1024-49151 are registered ports — numbers that organizations can apply to IANA to associate with a specific service.³ Unlike the well-known ports (0-1023), registered ports don't require root or administrator privileges to bind. Any application can listen on 3664 without special system access.

Registration means IANA recorded that someone claimed this number for a purpose. It does not mean the software is widely deployed, actively maintained, or even still exists. The registry contains thousands of ports like this: named, assigned, and largely forgotten.

If You See Traffic on This Port

Port 3664 is obscure enough that unexpected traffic here warrants attention. It could be:

• Legitimate proprietary software that uses this port internally (UPS management, monitoring agents, backup tools)
• A misconfigured or non-standard application that chose this port arbitrarily
• Something you did not authorize

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

macOS / Linux:

# Show which process is listening on port 3664
sudo lsof -i :3664

# Or with ss (Linux)
sudo ss -tlnp | grep 3664

Windows:

# Show listening processes on port 3664
netstat -ano | findstr :3664
# Then look up the PID in Task Manager

To see active connections:

# On macOS / Linux
sudo lsof -i :3664 -n -P

Why Unassigned-in-Practice Ports Matter

The IANA registry was designed to prevent collisions — two services fighting over the same port number. In practice, the registered range is vast enough that many assignments are nominal. A port can be "taken" on paper while sitting effectively unused in the wild.

This matters for two reasons. First, security: a scanner finding activity on port 3664 can't immediately match it to a known service, which makes triage harder. Second, software authors sometimes pick registered-but-dormant ports precisely because they're unlikely to conflict with anything.

If port 3664 appears in your firewall logs or a security scan, the registration to "ups-engine" is a starting point, not an answer. Verify what's actually running.