Port 3609: CPDI PIDAS Connection Monitor — The Registered Unknown

What This Port Is

Port 3609 is registered with IANA under the name cpdi-pidas-cm — CPDI PIDAS Connection Monitor. It was assigned to PIDAS AG, a Swiss IT services company founded in 1987 and headquartered in Zurich, specializing in customer service management and IT support operations for enterprise clients.¹

The "Connection Monitor" designation suggests it was used for internal monitoring or management traffic within a PIDAS software product. The details of the protocol are not publicly documented. No RFC was published. The implementation lives inside proprietary software that most of the world never encountered.

The Registered Port Range

Port 3609 sits in the registered port range (1024–49151). These ports are assigned by IANA to specific applications and services, but unlike well-known ports (0–1023), they do not require elevated privileges to use. Any process on your system can bind to them.

The registered range is a record of ambition: companies and developers who believed their protocol mattered enough to claim a permanent number. Some of those registrations became the infrastructure of the Internet. Port 1433 (SQL Server), port 3306 (MySQL), port 5432 (PostgreSQL) — all registered ports. Most registrations, though, are like port 3609: technically present in the registry, practically invisible everywhere else.²

What You Will Actually Find on Port 3609

Almost certainly not CPDI PIDAS Connection Monitor.

If port 3609 is open on a system you're examining, it's likely one of:

• An application that chose it arbitrarily — many applications pick ports without consulting the registry
• A development server or tool that bound to a high-numbered port for convenience
• Malware — unusual open ports are worth investigating

Registered-but-obscure ports like 3609 are sometimes chosen by malicious software precisely because they appear generic and raise fewer alarms than well-known ports.

How to Check What Is Listening

On Linux or macOS:

# Show which process is listening on port 3609
sudo ss -tlnp | grep 3609

# Or with lsof:
sudo lsof -i :3609

On Windows:

# Show listening processes with PIDs
netstat -ano | findstr :3609

# Then look up the PID
tasklist | findstr <PID>

Cross-platform with nmap (scan a remote host):

nmap -p 3609 -sV <host>

The -sV flag asks nmap to attempt service version detection — useful for figuring out what is actually running, regardless of what the registry says should be there.

Why Ports Like This Exist

The IANA port registry is not a living document of active protocols. It is a historical record, and much of it reflects the landscape of enterprise software from the 1990s and 2000s — an era when companies routinely registered ports for internal tools that they believed would spread widely.

Most didn't. What remains is a registry full of names like "cpdi-pidas-cm": specific enough to have been assigned, obscure enough that finding documentation requires archaeology.

Port 3609 is not broken. It is not dangerous by nature. It is simply a number that was claimed, used in a context almost no one could observe, and then quietly forgotten by everyone except the database that still holds its name.³