Port 93: DCP — The Protocol That Left Only Its Name

Port 93 is assigned to the Device Control Protocol, or DCP. It has held this assignment since at least 1987, when it appeared in RFC 1010.¹ It is registered for both TCP and UDP. The contact on file is Daniel Tappan.²

That is nearly everything the public record tells us.

What We Know

The Device Control Protocol has no published RFC. No open specification exists. No widely known implementation has surfaced. The protocol appears in every revision of the IANA Assigned Numbers document, from RFC 1010 in 1987¹ through RFC 1060 in 1990³, RFC 1340 in 1992⁴, and RFC 1700 in 1994², always with the same entry:

dcp    93/tcp    Device Control Protocol
dcp    93/udp    Device Control Protocol

The name suggests a protocol for controlling devices over a network, something that talks to hardware, tells it what to do, reads its state. But beyond the name and the port number, the original protocol left no public trail.

The Ghost and the Builder

Daniel Tappan, the engineer who registered port 93, went on to work at Cisco Systems in Chelmsford, Massachusetts. There he co-authored the MPLS Label Stack Encoding specification alongside Eric Rosen, Yakov Rekhter, and Dino Farinacci.⁵ MPLS became one of the foundational technologies of modern Internet routing, the system that lets service providers move traffic efficiently across massive networks.

Whatever DCP was meant to be, its creator moved on to build something that reshaped how packets travel across the Internet. Port 93 is the early sketch by someone who later painted on a much larger canvas.

DCP: The Name That Lived On

The abbreviation "DCP" has been reused several times in networking, which can cause confusion:

• The ATM Forum published a "Device Control Protocol" specification (AF-PHY-0138) in February 2000 for controlling devices within ATM network elements.⁶ This is a different protocol from the one registered on port 93.
• PROFINET DCP (Discovery and Configuration Protocol) is used in industrial automation networks for configuring station names and IP addresses.⁷ Also unrelated.
• Various IoT protocols have adopted the DCP name over the years.

None of these are the DCP that lives on port 93. The original sits alone.

The Port's Neighborhood

Port 93 sits among other well-known ports from the early assignment era:

These ports are neighbors in a registry curated by Jon Postel and Joyce Reynolds at the Information Sciences Institute. Some of these protocols thrived. Some, like DCP, remained quiet.

Security

Port 93 has been flagged in some security databases as having been used by trojans or malware for communication in the past.⁸ This is not unusual for obscure, rarely-used well-known ports. Attackers sometimes choose quiet ports precisely because no legitimate service is expected to be listening on them, making the traffic less likely to attract attention.

If you see unexpected traffic on port 93, investigate. There is no common legitimate reason for a modern system to have this port open.

How to Check What's Listening

# Linux
sudo ss -tlnp | grep :93
sudo netstat -tlnp | grep :93

# macOS
sudo lsof -iTCP:93 -sTCP:LISTEN

# Windows
netstat -an | findstr :93

Why Unassigned and Quiet Ports Matter

Port 93 is technically assigned, not unassigned. But it functions like an unassigned port in practice: no widely deployed service uses it, no modern software defaults to it, and most administrators have never seen legitimate traffic on it.

Ports like this reveal something about how the Internet was built. In the 1980s, engineers reserved port numbers for protocols they were developing or planning to develop. Some of those protocols became the infrastructure we depend on. Others never materialized, or stayed internal, or were superseded before they could spread. The IANA registry honored every reservation regardless.

Port 93 belongs to a well-known range (0 to 1023) that requires elevated privileges to bind on Unix-like systems. This range was meant to be a curated space, a set of ports reserved for protocols important enough to earn a permanent address. DCP earned its address. It just never moved in.