Port 507 sits in the well-known port range (0-1023), officially assigned by the Internet Assigned Numbers Authority to a service called "crs." The contact listed in the registry is Brad Wright. That's all that remains.
What We Don't Know
No one seems to know what "crs" stood for. No RFC describes it. No protocol documentation survives. No community knowledge exists about what this service actually did. The assignment exists in the registry like a name carved on a gravestone in a language no one speaks anymore.
This happens more often than you'd think. The early Internet moved fast. People registered ports for experimental protocols, research projects, or commercial services that never gained traction. Some were documented in RFCs that became obsolete. Others existed only in source code that's been lost. Port 507 is one of these digital fossils.12
The Well-Known Range
Port 507 occupies space in the well-known port range, which IANA originally reserved for system services and widely-deployed protocols. Assignments in this range were meant to be permanent and significant. Getting a well-known port number was supposed to mean something.
But the Internet in the 1980s and 1990s couldn't predict which protocols would matter in 2026. Some assignments became essential infrastructure—DNS on port 53, HTTP on port 80. Others, like port 507, became archaeological artifacts.
What Might Be Listening
Just because the original "crs" service is gone doesn't mean port 507 sits empty. Any application can listen on any port. You might find:
- Custom internal services that picked an obscure port number
- Legacy systems still running forgotten protocols
- Malware that chose this port specifically because it's rarely monitored3
- Nothing at all
Checking Port 507
To see if anything is listening on port 507 on your system:
On Linux/Unix:
On Windows:
The output shows you the process ID (PID) of whatever's listening. Use Task Manager or ps to identify which application owns that process.4
Why Unassigned Ports Matter
Port 507 isn't truly unassigned—it has an official assignment that nobody uses. This creates an interesting situation. The port is "claimed" in the registry but effectively abandoned in practice.
These ghost assignments matter because they represent the archaeology of the Internet. They show us what people thought would be important. They remind us that infrastructure evolves, that protocols die, that the Internet is built on layers of forgotten experiments.
Every active port today might become port 507 tomorrow—officially assigned, technically reserved, practically forgotten. The Internet moves on.
Security Considerations
Obscure ports like 507 can be security risks precisely because they're obscure. System administrators don't monitor them. Firewall rules don't specifically address them. If malware chooses port 507, it might go unnoticed longer than traffic on well-known ports.
If you find unexpected traffic on port 507, investigate it. The official assignment is dead. Anything using this port today is doing something the registry never anticipated.
Related Ports
The well-known range is full of these ghosts:
- Port 505: Assigned to "mailbox-lm" (Mailbox Location Manager)
- Port 506: Assigned to "imink" (another forgotten service)
- Port 508: Assigned to "xvttp" (X.25 Virtual Terminal Transfer Protocol)
Each one represents a moment when someone thought they were building the future.
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