Port 1925: Surrogate Discovery Port — Registered but undocumented

Port 1925 sits in the registered ports range (1024–49151) and holds an official IANA assignment: Surrogate Discovery Port, service name discovery-port, registered to a Keith Thompson for both TCP and UDP.

That's the entirety of what's publicly known.

No RFC defines this protocol. No technical specification describes what "surrogate discovery" means in this context or how the protocol is supposed to work. No major software package is known to use it. It's a name in a registry, assigned at some point, and then apparently never built or deployed publicly.

The Registered Ports Range

The registered ports (1024–49151) are where applications live. Below 1024, you need root privileges to bind — those are reserved for core protocols like HTTP, SSH, and DNS. Above 49151, ports are ephemeral, handed out temporarily by operating systems for outbound connections.

The middle range is open territory: anyone can apply to IANA to reserve a port number for their service. The barrier is low. You submit a form, describe your intended use, and IANA records the assignment. What IANA doesn't require is that you actually build the thing.

This is how ports like 1925 end up in the registry: reserved with intent, then abandoned or built privately without public documentation. There are hundreds like it.

What Could "Surrogate Discovery" Mean?

The phrase "surrogate" appears in networking contexts involving content delivery — a surrogate is a server that acts on behalf of an origin, serving cached or proxied content. Discovery protocols help systems find surrogates automatically rather than requiring manual configuration.

This is speculative. Without documentation, "Surrogate Discovery Port" is just a name. Whatever problem it was meant to solve, the solution was never shared.

What to Do If You See Port 1925 Active

If something on your network is listening on port 1925, it wasn't put there by a well-known application. Check what's bound to it:

On Linux/macOS:

# Show what process is listening on port 1925
sudo ss -tlnp | grep 1925
# or
sudo lsof -i :1925

On Windows:

netstat -ano | findstr :1925

The process ID in the output will tell you what's running. If you don't recognize it, investigate before assuming it's benign. An unassigned-in-practice port with active traffic is worth a second look.