Port 200: IBM System Resource Controller — The Command Channel

Port 200 is officially assigned by IANA to IBM System Resource Controller (SRC), a system management daemon that runs on IBM's AIX operating system.¹²

What It Does

The System Resource Controller is AIX's central command dispatcher for subsystems. When you need to start a database daemon, stop a network service, or check the status of any managed process, SRC is what makes it happen. Port 200 (UDP) is how SRC listens for remote control requests.³

Think of it as the stage manager for all the processes running on an AIX server. The srcmstr daemon sits there, listening on port 200, waiting for commands like startsrc, stopsrc, or lssrc to arrive from administrators who might not be sitting at the console.⁴

How It Works

SRC operates through a master daemon called srcmstr that starts during system initialization. This daemon can be configured to accept remote requests using the -r flag. When started with remote access enabled, it listens on UDP port 200 for incoming control commands.⁵

The communication is straightforward: an administrator issues a command to control a subsystem (like NFS daemons, database services, or any program registered with SRC), and if that command comes from a remote machine, it travels over port 200 to reach the srcmstr daemon.⁶

SRC supports three communication types: signals, sockets, and IPC message queues. Port 200 specifically handles the socket-based remote requests.⁷

The AIX Context

This port only matters if you're running IBM AIX. It's not a protocol you'll find on Linux, Windows, or other Unix variants. It's IBM's solution to a problem every Unix system has: how do you consistently manage the dozens of background services that keep a server running?

AIX chose to build a unified control system. Instead of having scattered init scripts and inconsistent management tools, SRC provides a single interface. And port 200 is how that interface extends across the network.

Security Considerations

The srcmstr daemon can be started with or without the -r flag. Without it, remote requests are ignored, and port 200 effectively does nothing. With the -r flag enabled, the daemon accepts remote subsystem control commands.⁸

This is a powerful capability—remote control of system daemons—which means it should be protected. If port 200 is accepting connections, an attacker with access could potentially start or stop critical services on your AIX system.

In practice, this port should only be accessible from trusted management networks. Firewalls should block port 200 from untrusted sources.

How to Check What's Listening

On AIX, you can check if srcmstr is listening on port 200:

netstat -an | grep 200
lssrc -a  # List all subsystems and their status
ps -ef | grep srcmstr  # Check if the SRC master daemon is running

On other systems, port 200 shouldn't be in use unless something is masquerading as an AIX service (which would be unusual).

Why This Port Matters

Port 200 represents a design philosophy: centralized system management. While modern systems have moved toward tools like systemd (Linux) or launchd (macOS), AIX has maintained its System Resource Controller for decades.

Every time an AIX administrator remotely restarts a daemon, checks the status of a database service, or coordinates a controlled shutdown of subsystems, port 200 is how the command reaches its destination.

It's infrastructure for managing infrastructure—a meta-layer that keeps the machine healthy by controlling what runs on it.

Related Ports

• Port 111 — Portmapper (can be managed by SRC on AIX)
• Port 22 — SSH (the more common way to remotely manage systems)
• Port 514 — Syslog (another system management protocol)