Port 10521 — The Unregistered Logger

What Port 10521 Does

Port 10521 sits empty in the official IANA registry. No registered service bears its name. Yet it shows up in the real world anyway—specifically in rsyslog configurations for MySQL and MariaDB logging. When a database administrator wants to forward logs over TCP, port 10521 appears in some setups as the destination. It works because the Internet allows this: nothing stops you from listening on an unregistered port.

The Port Ranges Explained

The Internet divides port numbers into three ranges, each with different governance:

• Well-known ports (0–1023): Assigned and controlled by IANA. These are the famous ones—port 80 (HTTP), 443 (HTTPS), 22 (SSH). You cannot use these without IANA approval.
• Registered ports (1024–49151): Anyone can request an official assignment from IANA. These ports are registered but not reserved; unauthorized services can still listen on them.
• Dynamic/ephemeral ports (49152–65535): Completely unregulated. Operating systems hand these out automatically to client applications that need temporary connections.

Port 10521 falls squarely into registered territory, but it has never been registered. This makes it available for anyone to use—and some people do.

Known Informal Uses

The primary documented use of port 10521 is in logging pipelines:

• rsyslog forwarding: In some rsyslog configurations, MySQL and MariaDB logs are forwarded to localhost:10521 for collection and processing
• Chronicle Forwarder: Some log aggregation setups use this port specifically for MySQL logging data

These uses are functional—they work—but they're not official. No RFC documents port 10521. No software is required to use it. It simply became convenient for someone, and the convention quietly spread.

How to Check What's Listening

To see if port 10521 is listening on your system:

Linux/macOS:

netstat -tlnp | grep 10521
# or
lsof -i :10521

Windows:

netstat -ano | find "10521"

These commands will show whether anything is bound to port 10521 and what process owns it.

Why Unassigned Ports Matter

The existence of port 10521 in active use—without IANA registration—illustrates a fundamental feature of Internet architecture: the registry is aspirational, not absolute. The IANA list is a map of official names, but the actual Internet is larger than the map. Systems daily use unregistered ports because:

1. Registration takes time (sometimes months)
2. Temporary services don't need formal approval
3. Niche applications serve specific needs without broader adoption
4. Nothing prevents it — the protocol layers don't check IANA's list

Port 10521 is honest about this. It makes no claims. It carries what gets sent to it. The Internet works this way: official channels exist for coordination, but the network remains open enough for improvisation.

If port 10521 ever becomes broadly used, someone might formally register it with IANA. Until then, it remains a port that exists only when someone needs it—and stops existing the moment they turn off their service. That temporary, practical existence is the whole story.