Port 283: Rescap — The Protocol That Never Was

Port 283 is officially assigned to rescap (Resource Capabilities Discovery Protocol), a protocol that was designed, drafted, and abandoned before anyone ever deployed it. It sits in IANA's registry as a monument to well-intentioned Internet engineering that never shipped.

What Range This Port Belongs To

Port 283 is part of the well-known ports range (0-1023), also called system ports. These ports are assigned by IANA through formal IETF review processes and are supposed to be reserved for widely-used standard services.

Port 283's assignment is technically legitimate — it was properly registered through IETF channels. The protocol just never made it past the proposal stage.

What Rescap Was Supposed to Do

The rescap protocol was designed around 1999-2000 by Paul Hoffman at the Internet Mail Consortium.¹ The idea was simple: create a lightweight client-server resolution protocol that could translate resource identifiers into lists of attributes.

The canonical use case was email. Before sending someone a message with an attachment, your mail client could query a rescap server to ask: "Can this recipient's mail client display PDF files natively? What about HTML? What about video?" The server would respond with a list of capabilities.²

Rescap was designed to be minimal — just a resolution protocol, not a full directory service. Clients would connect to port 283 via TCP or UDP, send a query, and receive back a structured response describing what the remote resource could handle.

The protocol had two forms: basic (no authentication, no privacy) and secure (with encryption and mutual authentication).³

Why It Never Shipped

The IETF rescap working group concluded in October 2003.⁴ The protocol never progressed beyond Internet-Draft status. The last known draft expired around August 2000, and no RFC was ever published.

There's no public record of exactly why rescap died. But the pattern is familiar: the problem it solved wasn't urgent enough, the deployment model was too complex, or simpler alternatives emerged. Email clients never standardized on capability negotiation. The web moved on.

Current Status

Port 283 remains officially assigned to rescap in IANA's registry.⁵ That assignment is permanent unless someone explicitly requests its removal or reassignment.

But the protocol itself is completely unused. There are no rescap servers running on the Internet. There are no rescap clients. The codebase, if it ever existed, is gone.

What remains is the port number — listed in every firewall configuration tool, every network scanning utility, every port reference database. A permanent placeholder for a protocol that never was.

The Honest Reality

If you see port 283 open on a modern system, it's not running rescap. It might be:

• A misconfigured service listening on the wrong port
• Malware (some sources note port 283 has been used by trojans in the past)⁶
• A developer testing something and picking an "unused" port without realizing it's technically reserved

To check what's actually listening on port 283:

# On Linux/macOS
sudo lsof -i :283
sudo netstat -tulpn | grep :283

# On Windows
netstat -ano | findstr :283

What This Port Teaches Us

Port 283 is a reminder that the Internet's infrastructure includes not just what we built, but what we planned to build. IANA's registry is full of ports assigned to protocols that never deployed, standards that never shipped, good ideas that couldn't find adoption.

The port number is still there. The protocol is gone. And somewhere in IANA's database, rescap remains officially assigned to a service that never existed outside a few expired documents from the year 2000.