33

Port 33/tcp is registered with IANA as dsp with the description "Display Support Protocol," contact Ed Cain, and a blank reference field (dual-registered on TCP and UDP — the 33/udp row carries the identical registration). The current IANA Service Name and Transport Protocol Port Number Registry lists no RFC or specification document for this assignment, so the reference field stays blank. The name appears in the historical RFC 1340 (Assigned Numbers, July 1992) carrying the reference code [EXC], which is a NIC Ident — a person/contact identifier — rather than a formal protocol-specification RFC; it should not be read as a published DSP standard. No protocol-specification document, no active open-source implementation, and no widely deployed software using port 33 for the Display Support Protocol could be located in publicly accessible primary sources during this research pass: port-database aggregators (GRC Port Authority, AuditMyPC, SpeedGuide) echo only the IANA name with no implementation detail. This is best characterized as a legacy assignment from the early ARPA/Internet era that never saw documented public deployment. For an analyst, port 33 carries no notable scanning profile and no CVEs tied specifically to DSP; it does not appear among commonly targeted ports and would receive only general internet background-noise scanning. A service answering on port 33 is therefore unusual and unexplained by any documented mainstream software — worth investigating as a custom, repurposed, or anomalous listener rather than assuming the registered Display Support Protocol. One caution for downstream sourcing: a portsmaster.org page for port 33 was assessed as AI-generated content with fabricated audit statistics (dated lab/testing figures and a monitoring window, no methodology) and was excluded entirely; do not seed it into any published copy.

IANA assignment

dsp — "Display Support Protocol"; reference blank (no RFC cited in the IANA registry); contact Ed Cain; dual-registered 33/tcp + 33/udp [Confirmed] — IANA Service Name and Transport Protocol Port Number Registry; local registry CSV lines 73–74

Range class

well-known (0–1023) [Confirmed] — port number 33 is below 1024

IANA reference

blank — none listed. RFC 1340 cites [EXC], a NIC Ident (contact identifier), not a specification RFC; blank stays blank [Confirmed] — https://datatracker.ietf.org/doc/html/rfc1340

Prevalence

nmap-services observed open-frequency 33/tcp ≈ 0.001016 — very low (roughly 1 in 1,000 sampled hosts), 205th of 6,486 TCP entries; the UDP sibling 33/udp is ≈ 0.00056 [Likely] — nmap-services dataset. Consistent with the wider picture: port 33 does not appear in lists of commonly scanned/high-traffic ports and is expected to receive only general background-noise scanning

[Likely] — https://data-status.shodan.io/ports.html

Related ports

33/udp (identical registration); the surrounding well-known small-number assignments [Confirmed] — IANA registry lines 73–74

Primary use

Display Support Protocol — a legacy/early-Internet name assignment; no protocol-specification document located and no documented active deployment [Likely] — https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xml ; https://datatracker.ietf.org/doc/html/rfc1340

Other/unofficial uses

Unknown — no specific software product or daemon documented in primary sources as using 33/tcp

Security implications

no CVEs or vulnerability disclosures found specific to port 33/tcp DSP; the original assignment documents no built-in encryption or authentication; risk is largely theoretical absent a documented implementation

Typically seen on

Unknown / anomalous — no documented mainstream software; an open 33 is unexplained by registered DSP [Unknown]

Analyst note

An open port 33 is statistically rare and not explained by any documented Display Support Protocol implementation — treat as a custom/repurposed or anomalous listener and investigate. Source-hygiene caution: the portsmaster.org port-33 page is AI-generated with fabricated stats; exclude it.

Port 33/udp is registered with IANA as dsp with the description "Display Support Protocol," assignee Ed Cain, and a blank reference field. The registration is dual-listed: port 33/tcp carries the identical service name, description, and assignee, so the entry covers both transports with the same minimal metadata. The assignee contact in RFC 1700's "Assigned Numbers" listing is given as Ed Cain at a Defense Communications Agency (DCA, later DISA) address, which places the assignment in the US government/military ARPANET-era port-allocation context rather than in any commercial product line. There is no public specification for Display Support Protocol — no RFC defines it, the IANA reference column is blank, and no protocol document is indexed in public sources — so the registration is effectively a historical name reservation with no documented wire format. No actively maintained software is documented as using port 33/udp, and the assignment is best characterized as dormant legacy. For an analyst, the practical relevance is low: port 33/udp does not appear in commonly referenced bad-port blocklists (for example, Gary Kessler's bad-ports list does not include it), and there are no documented modern scanning campaigns targeting it as of June 2026. Some consumer-oriented port databases flag port 33 generically as having been "used by a trojan in the past," but those claims name no specific trojan, give no date, and cite no source, so they cannot be independently verified and should be treated as low-confidence noise rather than evidence. An open port 33/udp is therefore an anomaly worth a closer look — a misconfiguration, a decoy, or an arbitrary high-port-style listener squatting on a dormant low number — rather than a recognizable standard service.

IANA assignment

dsp — "Display Support Protocol"; reference (blank — no RFC cited in IANA registry); assignee Ed Cain; dual-registered 33/tcp + 33/udp [Confirmed] — IANA Service Name and Transport Protocol Port Number Registry (cached the IANA Service Name and Transport Protocol Port Number Registry line 74; https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.txt)

Range class

well-known (0–1023) [Confirmed]

Registration / modification date

blank in the IANA source row — left null (IEEE/IANA publish no date here; no value invented) [Confirmed] — cached IANA CSV

Related ports

port 33/tcp (identical dual registration); the small-services / low-port cluster

Primary use

Display Support Protocol (DSP), an early ARPA-era protocol with no public specification and no RFC; no widely documented modern software uses this registration

Other/unofficial uses

none documented [Unknown]

Common software

Unknown — no actively maintained software is documented as using port 33/udp; the registration is dormant with no known implementations referenced in public sources [Unknown]

Security implications

not listed in standard bad-port blocklists (Gary Kessler's bad-ports list omits it); some consumer port databases generically flag port 33 as "used by a trojan in the past" with no named trojan, date, or source — unverified and low-confidence; no significant modern scanning campaigns documented as of June 2026; effectively dormant

Typically seen on

not associated with any common host class; an open port 33/udp is an anomaly

Assignee contact

Ed Cain, listed at a DCA (Defense Communications Agency, later DISA) address, indicating a US government/military ARPANET-era origin [Confirmed] — RFC 1700 "Assigned Numbers" listing (https://www.freesoft.org/CIE/RFC/1700/4.htm)

Analyst note

An open port 33/udp is statistically rare and maps to a dormant legacy registration with no public spec — treat it as a misconfiguration, decoy, or squatting listener and investigate rather than assume a standard service.