Well-Known Port (0-1023) | Status: Officially assigned, effectively obsolete | Protocols: TCP/UDP
What This Port Was For
Port 561 is officially assigned by IANA to the MONITOR protocol1, an early network monitoring and diagnostic tool that let administrators query remote systems for status information.
Think of it as a primitive ancestor of SNMP (Simple Network Management Protocol). Devices would listen on port 561, waiting for probe packets asking "how are you doing?" They'd respond with basic performance metrics and status information.
That was the theory, anyway.
Why You'll Never See It
The MONITOR protocol is obsolete. Not deprecated, not legacy—obsolete. Modern networks use:
- SNMP (ports 161/162) for device monitoring
- Prometheus for metrics collection
- Syslog for centralized logging
- Custom telemetry over HTTP/HTTPS
These protocols are more secure, more capable, and actually maintained. MONITOR is none of those things.
If you scan a modern network, port 561 will be closed. Always. You're more likely to find a running Gopher server than an active MONITOR listener.
What Range This Port Belongs To
Port 561 sits in the well-known ports range (0-1023), which means it was assigned by IANA through formal IETF procedures2. These assignments are permanent, even when the protocol dies.
This is why port 561 still officially "belongs" to MONITOR, even though nothing uses it. The Internet doesn't delete history—it just moves on.
How to Check This Port
If you're auditing your network and want to verify nothing is listening on port 561:
On Linux/macOS:
On Windows:
Using nmap:
You'll almost certainly find nothing. If you do find something listening on port 561, it's either:
- A misconfigured service that grabbed the wrong port
- A very, very old system that somehow still runs
- Something pretending to be MONITOR (unusual, but possible)
Why Unassigned and Obsolete Ports Matter
Ports like 561 teach us something important: the Internet has a long memory. Protocols die, technologies evolve, but the port assignments remain.
This matters for:
Security: Attackers sometimes hide services on "dead" ports, assuming nobody monitors them. They're wrong—you should still audit these ranges.
Compatibility: Old port assignments prevent conflicts. Even though MONITOR is dead, port 561 won't be reassigned to something else.
History: Every assigned port represents a problem someone tried to solve. Port 561 reminds us that network monitoring isn't new—we've been trying to answer "is this machine okay?" since the 1980s. We just have better tools now.
The Honest Truth
Port 561 is assigned but unused. It has no active community, no modern implementations, no security advisories (because there's nothing to secure), and no reason to exist beyond historical record-keeping.
And that's completely fine. Not every port needs to carry the Internet. Some just need to be remembered.
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