Port 3543: qftest Lookup — The Registered Port You've Never Heard Of

What Port 3543 Is

Port 3543 is registered to the qftest Lookup Port — the discovery and coordination port for QF-Test, a GUI test automation tool made by Quality First Software GmbH, a German software company. The registration was filed with IANA in July 2002 and applies to both TCP and UDP.

QF-Test automates testing of Java desktop applications — Swing, AWT, JavaFX, SWT — as well as web interfaces. When a test suite communicates with a running application under test, port 3543 serves as the lookup mechanism: a way for the testing framework to find and connect to the target process.

If you see port 3543 active on a machine, someone is probably running QF-Test.

The Registered Port Range

Port 3543 lives in the registered ports range: 1024 through 49151.

The registered range sits between the well-known ports (0–1023, reserved for foundational protocols like HTTP, SSH, and DNS) and the ephemeral ports (49152–65535, used temporarily by operating systems for outbound connections).

Registered ports are open for any software to claim with IANA. The registration is voluntary — it's more like staking a flag than acquiring a deed. No one enforces exclusivity. Another application could use port 3543 without knowing or caring that QF-Test registered it first. On your network, port 3543 could be anything.

Checking What's Listening

If port 3543 shows up in a scan or a firewall log, find out what's actually using it:

On Linux/macOS:

# Show which process is listening on port 3543
ss -tlnp | grep 3543

# Or with lsof
lsof -i :3543

On Windows:

# Show process using port 3543
netstat -ano | findstr :3543

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

If nothing is listening, you'll get no output — which is the expected state on most machines that don't run QF-Test.

Why Unassigned-Looking Ports Matter

Most registrations in the 1024–49151 range are for niche tools, legacy enterprise software, and internal protocols that never became household names. Port 3543 is typical: legitimately registered, rarely encountered, easy to mistake for unassigned.

This matters for two practical reasons:

Security scanning: An unfamiliar open port isn't automatically suspicious. Before escalating, check the registry. "Unknown" in a port scanner means "not in my database," not "malicious."

Firewall rules: If you're writing rules to block unused ports, verify first. Some developer tools, testing frameworks, and build systems quietly use registered ports that look obscure but are doing legitimate work.

The registered port space is large. Most of it is quiet. Quiet doesn't mean empty.