Port 3599: Quasar Accounting Server — The bookkeeper's port

What Port 3599 Is

Port 3599 sits in the registered ports range (1024–49151) — the middle tier of the port number space, where applications request a dedicated home from IANA so they don't collide with each other.

In September 2002, Quasar Accounting claimed this port. IANA recorded it. The registration has been there ever since.

The Registered Service: Quasar Accounting Server

Quasar is an open-source accounting and point-of-sale system built for small businesses. It runs as a client-server application: the quasard daemon listens on the network, clients connect to retrieve company data, and the server handles all database operations on their behalf.

Port 3599 is Quasar's default. From the admin documentation:

"If left blank or set to 0, the Quasar server will run on its default port, 3599."

The server communicates over TCP socket connections, with optional NoDelay settings for latency-sensitive operations and KeepAlive for monitoring connection health. It's a straightforward architecture: one server, multiple clients, one port.¹

What You'll Actually Find Here

Quasar Accounting has a modest user base. The chance that a device you discover listening on port 3599 is actually running Quasar's accounting daemon is low.

More likely candidates:

• A development server assigned a port that happened to be available
• A custom application that picked this number for no particular reason
• A proxy or tunnel using it as a passthrough

The registered service is a formality. The actual traffic on any given machine is almost certainly something else entirely.

How to Check What's Listening

On Linux or macOS:

# Show process listening on port 3599
ss -tlnp sport = :3599

# Or with lsof
lsof -i :3599

On Windows:

netstat -ano | findstr :3599

Then take the PID from the output and look it up:

tasklist /fi "pid eq <PID>"

Why Unassigned — or Lightly Used — Ports Matter

The registered ports range exists to prevent chaos. Without it, two applications competing for the same port number would mean broken software and confused administrators.

But registration isn't enforcement. Nothing stops an application from using port 3599 whether or not it's running Quasar Accounting. The IANA registry is a coordination system, not a lock. When you find unexpected traffic on a registered port, the registry tells you what should be there — not what is.

That gap between should and is is where network security lives.