Port 536 is officially assigned to opalis-rdv (Opalis RendezVous), a communication protocol for OpalisRobot—an IT automation platform that changed how enterprises thought about system management in the early 2000s.
The port registration remains. The service that needed it does not.
What Opalis-rdv Was
OpalisRobot, first released in the late 1990s, was a comprehensive system management and automation solution. It delivered real-time monitoring, notification, corrective action, and event-driven job scheduling.1
The "rdv" in opalis-rdv stands for "RendezVous"—Opalis's term for its communication and coordination protocol. This is what port 536 carried: the messages that let OpalisRobot components find each other, coordinate actions, and respond to events across distributed systems.
What made Opalis different was this: it didn't just schedule jobs. It watched for events and responded to them. A backup fails at 3am? OpalisRobot notices, tries to fix it, and alerts someone if it can't. A server's disk fills up? OpalisRobot sees it happen and clears space before anything breaks.
This was novel in 2002. Today it's how every automation platform works.
The Evolution
By the early 2000s, OpalisRobot had outgrown its architecture. Opalis retired the RendezVous and Robot product lines and shifted focus to building a better integration platform—Opalis Integration Server (OIS).2
In 2009, Microsoft acquired Opalis and killed the RendezVous name entirely. The technology became System Center Opalis Integration Server 6.3, then System Center Orchestrator when it joined the System Center 2012 suite in December 2012.3
The underlying engine remained the same. Policies built in Opalis Integration Server could import directly into Orchestrator. But the RendezVous protocol—the thing that needed port 536—was gone.
What Runs on Port 536 Today
Probably nothing.
The IANA registry still lists opalis-rdv on port 536, assigned to Laurent Domenech (likely an Opalis engineer who requested the allocation). But the service that needed this port was discontinued 15 years ago.
Modern Microsoft System Center Orchestrator uses different protocols and ports. It doesn't speak RendezVous anymore.
If you see traffic on port 536, it's either:
- A very old Opalis installation still running somewhere
- Something else using an abandoned port number
- Malicious traffic (some sources note trojans have exploited unused well-known ports)4
Checking What's Listening
On Linux or macOS:
On Windows:
If nothing appears, nothing is listening. Which is the expected result.
Why Unassigned Ports Matter
Port 536 isn't truly unassigned—it has an official reservation. But it's effectively unassigned because the service that needed it no longer exists.
The Internet Assigned Numbers Authority (IANA) maintains the official registry of port assignments. Once assigned, port numbers rarely get reclaimed, even when the service dies. This is intentional: reclaiming ports risks confusion if old software still expects them.
The well-known port range (0-1023) is littered with these ghosts. Services that mattered in 1985 or 2002, assigned a port number, then faded into history. The reservations remain as archaeological markers—evidence of what the Internet used to need.
Port 536 is one of those markers. It tells the story of when IT automation first learned to listen instead of just execute. That's worth remembering, even if nothing listens on port 536 anymore.
Related Ports
- Port 135 — Microsoft RPC Endpoint Mapper (used by modern Windows automation)
- Port 445 — SMB/CIFS (used by Windows remote management)
- Port 5985-5986 — WinRM HTTP/HTTPS (PowerShell remoting, modern Windows automation)
Frequently Asked Questions
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