Port 557 belongs to the well-known ports range (0-1023), assigned by IANA to openvms-sysipc—OpenVMS System Inter-Process Communication.1
If you've never heard of OpenVMS, you're not alone. But somewhere, in a basement server room or a mission-critical system that can't be turned off, it's still running.
What Is OpenVMS?
OpenVMS is an operating system created by Digital Equipment Corporation (DEC) in the 1970s for their VAX minicomputers.2 It was known for extraordinary reliability and uptime—systems that ran for years without rebooting. Banks used it. Hospitals used it. The kind of infrastructure where downtime meant people died or lost money.
DEC eventually failed as a company. VAX became history. But OpenVMS survived. It was ported to Alpha processors, then Itanium, and recently to x86-64. The operating system outlived the hardware it was designed for, and the company that built it.
What Is SYSIPC?
SYSIPC is OpenVMS's mechanism for inter-process communication over a network. When processes on different OpenVMS machines need to talk to each other—sharing data, coordinating work, synchronizing state—they use SYSIPC.
Port 557 is where that conversation happens.
The protocol details are largely undocumented outside of OpenVMS technical manuals. This isn't a protocol with an RFC. This is internal plumbing for an operating system that predates the modern Internet.
Why This Port Still Exists
Most well-known ports serve protocols everyone depends on: HTTP, HTTPS, DNS, SSH. Port 557 serves a protocol almost nobody uses anymore.
But "almost nobody" isn't "nobody."
There are still OpenVMS systems running in production. Financial systems. Manufacturing control systems. Government infrastructure. Systems so old and so critical that migrating them is riskier than just keeping them alive.
And those systems need port 557.
The port stays reserved because the Internet doesn't forget. Once a port is assigned, it remains assigned. Even if the company that requested it no longer exists. Even if most people have never heard of the protocol. Someone, somewhere, depends on it.
Security Considerations
Port 557 should not be exposed to the public Internet. If you see it open on a system, one of two things is happening:
- You're looking at a legacy OpenVMS system that actually needs this port
- You're looking at a misconfiguration or something pretending to be OpenVMS
Because OpenVMS is so rare, attackers sometimes use uncommon ports like 557 for command-and-control traffic or backdoors, betting that security tools won't scrutinize them as closely as common ports.
If port 557 is open and you're not running OpenVMS, investigate immediately.
How to Check What's Using Port 557
On Linux or macOS:
On Windows:
If something is listening and you don't know why, find out what it is before assuming it's legitimate.
Why Unassigned Ports Matter
Port 557 isn't unassigned—it has a specific owner, even if that owner is a ghost. But its story illustrates why the port registry matters.
Ports are namespaces. Without assignments, protocols would collide. Two services would try to use the same port, and neither would work. The IANA registry prevents this by giving each protocol a permanent home.
Even obscure protocols. Even ones most people will never use. Because the Internet is bigger than what's popular today. It includes everything that was ever built on it, still running somewhere, still depending on the infrastructure to stay stable.
Port 557 is a monument to that principle. A reserved seat for a protocol from another era, still honored, still protected, still there for the systems that need it.
Related Ports
- Port 135: Microsoft RPC Endpoint Mapper—another vendor-specific RPC protocol
- Port 111: ONC RPC Port Mapper (Sun RPC)—the more common RPC protocol
- Port 512-514: BSD r-commands—other legacy Unix protocols from the same era
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