Port 59: Any Private File Service — A Door Left Open on Purpose

What Port 59 Does

Port 59 is registered in the IANA Service Name and Transport Protocol Port Number Registry as "any private file service" for both TCP and UDP¹. That designation means exactly what it says: this port was set aside for any file service that an organization wanted to run privately, without a standardized protocol behind it.

No specific protocol was ever assigned to port 59. No RFC defines its behavior. No widely deployed software claims it as a default. It is one of the quietest doors in the well-known port range.

The "Any Private" Ports

Port 59 belongs to a small family of ports that Jon Postel reserved in the earliest IANA port registries. These "any private" ports were scattered through the well-known range, each earmarked for a different category of private service²:

Postel understood something about infrastructure that most people miss: a system that assigns every number to a specific purpose leaves no room for the people who actually use it. These ports were intentional gaps, spaces where an organization could run its own file server, its own printer service, its own terminal access, without stepping on anyone else's protocol and without needing IANA's permission.

It was thoughtful engineering. It was also, quietly, a statement about autonomy.

The Well-Known Port Range

Port 59 sits in the well-known (or system) port range: ports 0 through 1023. These ports are governed by IANA and carry special significance³:

• On Unix-like systems, binding to a well-known port requires root or superuser privileges
• They are assigned through IETF Review or IESG Approval processes
• They were historically reserved for services that needed to be discoverable by "unknown callers," meaning any client on the network could find them at a predictable address

The fact that port 59 lives in this privileged range but has no assigned protocol makes it unusual. Most well-known ports earned their place by hosting critical infrastructure: HTTP on 80, SMTP on 25, DNS on 53. Port 59 earned its place by being available.

Security Considerations

The absence of an assigned protocol does not mean port 59 goes unnoticed by attackers. Because it lacks a standard service, unexpected activity on port 59 is a red flag.

Known malware associations:

• Backdoor.Sdbot.AJ: A network-aware worm with backdoor capabilities that connects to IRC servers on port 59/TCP to receive remote commands⁴
• DMSetup: A trojan designed to establish persistent backdoors on compromised systems through port 59

If port 59 is open on your system and you did not deliberately configure a service on it, investigate immediately. An open well-known port with no legitimate service is one of the clearest indicators of compromise.

How to Check What Is Listening on Port 59

Linux:

sudo ss -tlnp | grep :59
sudo lsof -i :59

macOS:

sudo lsof -i :59
netstat -an | grep '\.59 '

Windows:

netstat -ano | findstr :59

If any process is bound to port 59 and you do not recognize it, identify the process ID and investigate what placed it there.

Why Unassigned Ports Matter

The Internet's port system works because of agreement. Port 80 means HTTP. Port 443 means HTTPS. Port 22 means SSH. This shared understanding lets machines find each other's services without negotiation.

But Postel's "any private" ports reveal the other side of that coin. A system of pure assignment, where every number has exactly one meaning, is brittle. It cannot accommodate the private, the experimental, the organizational. The "any private" ports are pressure relief valves built into the architecture itself.

Port 59 has no famous protocol. No RFC with its name on it. No billions of daily connections. But it carries something worth noticing: the foresight to leave space for what you might need, even when nobody could predict what that would be.