Port 772 sits in the well-known ports range, officially assigned by IANA to a protocol called cycleserv2. The problem? Finding anyone who's actually used it is harder than finding the documentation that explains what it does.
What Is Cycleserv2?
According to the sparse information available, cycleserv2 is a protocol for remote management and control of network devices. It operates on both TCP and UDP:1
- TCP port 772 — Connection-oriented management sessions with guaranteed delivery
- UDP port 772 — Connectionless management commands where speed matters more than reliability
But here's the reality: this protocol never achieved widespread adoption. It exists in the IANA registry, shows up in port scanning tools, and occasionally appears in old network configuration files—but active deployments are rare to nonexistent in 2026.
The Well-Known Range
Port 772 lives in the well-known ports range (0-1023), which means it was registered by IANA as an official service. These ports are supposed to be reserved for standardized protocols that anyone can implement.
Getting a well-known port assignment was easier in the early days of the Internet, when the address space seemed infinite and the number of protocols was manageable. Not every assignment became a success story.
Why Obscure Ports Matter
The Internet's port registry is full of entries like cycleserv2—protocols that were designed, assigned a port number, and then quietly faded into obscurity. They matter because:
They occupy valuable space. Well-known ports are finite (only 1,024 of them). Each one assigned to a protocol nobody uses is a number that can't be used for something more active.
They create security confusion. When a port scanner reports port 772 is open, administrators have to research what cycleserv2 is, whether it's legitimate, and whether it's supposed to be running on their network. Obscure ports waste time.
They're archaeological artifacts. Looking at old port assignments tells you what people thought would be important in 1990, 2000, 2010. Most guesses were wrong. The protocols that actually won—HTTP, HTTPS, SSH, DNS—are obvious in hindsight. The rest became digital fossils.
Security Considerations
If you find port 772 open on your network, you should investigate:
- Is cycleserv2 actually running? Unlikely. More likely something else is using the port, either legitimately or maliciously.
- Malware sometimes uses obscure ports because they're less monitored. Port 772 has occasionally been flagged in security reports as a potential indicator of compromise.2
- Default installations rarely open port 772. If it's open, someone (or something) opened it deliberately.
How to Check What's Listening
On Linux or macOS:
On Windows:
If something is listening on port 772 and you don't know why, it's worth investigating. Legitimate cycleserv2 deployments are rare enough that any activity on this port deserves scrutiny.
The Pattern
Port 772 represents a common pattern in Internet infrastructure: the difference between assigned and used. The IANA registry lists thousands of ports with official assignments. Only a fraction see regular use. The rest exist in a liminal state—not quite abandoned, not quite alive, waiting for either a revival that will never come or a cleanup that hasn't happened yet.
When you see cycleserv2 in a port database, you're looking at optimism from decades past. Someone built this protocol. Someone thought it would matter. Someone requested a well-known port assignment and received it. And then the Internet moved on.
The port remains. The protocol is barely a memory.
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