Port 33: Display Support Protocol — The Protocol That Never Arrived

Port 33 is assigned to the Display Support Protocol (DSP), a protocol registered by the U.S. Defense Communications Agency that was never formally defined in an RFC and never saw meaningful adoption. It is one of the Internet's earliest reservations, a name held in the IANA registry for over four decades with nothing behind it.

The Assignment

The IANA Service Name and Transport Protocol Port Number Registry lists port 33 as assigned to DSP, the Display Support Protocol, on both TCP and UDP¹. The contact is listed as Ed Cain, with an email address at edn-unix.dca.mil, placing him within the Defense Communications Agency (DCA).

That is nearly everything that is known.

There is no RFC defining the Display Support Protocol. No specification document has surfaced. No known implementation exists. The protocol appears to have been registered during the early 1980s, when IANA port assignments were being formalized and organizations could claim port numbers with relatively little ceremony. Many early ports were reserved this way, names staked like claims on open land, some of which were built upon and some of which were simply abandoned.

The Defense Communications Agency

The DCA was the U.S. military organization responsible for managing long-haul communications across the Army, Navy, and Air Force². In 1975, it took operational control of the ARPANET. In 1982, the DCA and ARPA established TCP/IP as the standard protocol suite for ARPANET, a decision that shaped the architecture of the modern Internet³.

The DCA was renamed the Defense Information Systems Agency (DISA) in 1991. Whatever the Display Support Protocol was intended to do, it did not survive the transition. The port assignment, however, persists.

What Range Port 33 Belongs To

Port 33 falls within the well-known port range (0 through 1023). These ports are controlled by IANA and on most operating systems require root or system-level privileges to bind to. The well-known range is reserved for established, widely used protocols: HTTP on 80, HTTPS on 443, SSH on 22, DNS on 53.

Port 33 has the pedigree of this range without the traffic.

Observed Uses

Because port 33 sits in the well-known range but carries no active protocol, it occasionally appears in other contexts:

• Non-standard SSH: Some administrators configure SSH to listen on port 33 instead of the default port 22 as a basic obfuscation measure. This is security through obscurity and provides minimal real protection, but it reduces noise from automated brute-force bots scanning for port 22⁴.
• Trojan history: Port 33 has been flagged in some security databases as historically used by malware for command-and-control communication⁵. This is not unique to port 33. Any quiet, unmonitored port becomes attractive to malware authors precisely because no legitimate service is expected there.

How to Check What Is Listening on Port 33

On Linux or macOS:

# Check if anything is listening on port 33
sudo lsof -i :33

# Or using ss (Linux)
sudo ss -tlnp | grep :33

# Or using netstat
sudo netstat -tlnp | grep :33

On Windows:

netstat -an | findstr :33

If something is listening on port 33 and you did not configure it, investigate. This port should be silent on most systems.

Why Unassigned Ports Matter

The port numbering system works because of trust. When your browser connects to port 443, it trusts that HTTPS is on the other side. When your email client reaches for port 25, it expects SMTP. This trust is built on the registry, the shared agreement about what lives where.

Ports like 33, reserved but unused, are the gaps in that agreement. They remind us that the registry is not a natural law. It is a human document, full of intentions that were realized and intentions that were not. Some doors were labeled but never opened.

Port 33 was given a name by someone who worked for an agency that helped build the Internet. The protocol never materialized. The agency changed its name. The port remains in the registry, a placeholder from the earliest days of the network, still waiting.