75

Port 75/tcp is registered in the IANA Service Name and Transport Protocol Port Number Registry with a blank service-name field, the description "any private dial out service," and Jon Postel listed as both assignee and contact; the reference column is blank, so no RFC or other document is cited (dual-registered identically on 75/tcp and 75/udp). This is a legacy placeholder rather than a defined protocol: the "any private dial out" wording — like the matching "any private dial in" entries elsewhere in the low ports — was a way to reserve a number for an organization's own dial-out use without naming a public service, and it does not correspond to any standardized, documented wire protocol. Some third-party port databases attach the informal label priv-dial to this entry, but that name is not present in the authoritative IANA CSV cache, which leaves the service-name field empty. No widely deployed application or daemon is documented as binding TCP/75 under this designation, and because the port sits in the well-known/system range (0–1023) any legitimate server here would need superuser privileges to bind. The analyst-relevant facts are that it is rarely seen open — nmap-services records an open-frequency of roughly 0.000063 — and that its obscurity plus privileged status make an unexpected open port 75 worth investigating as misconfiguration, legacy software, or a deliberately obscure rogue/backdoor listener rather than a normal service. AuditMyPC historically flagged the port in connection with trojan activity, but no specific malware family or CVE could be confirmed against it, and no current (2024–2026) threat reporting targets it specifically; that history is noted but unverified.

IANA assignment

blank service name — "any private dial out service"; reference (blank — no RFC cited in IANA registry); assignee Jon Postel; dual-registered 75/tcp + 75/udp [Confirmed] — IANA Service Name and Transport Protocol Port Number Registry (local cache line 174–175)

Range class

well-known (0–1023) [Confirmed]

Prevalence

nmap-services open-frequency 75/tcp ≈ 0.000063 (very rarely seen open) [Likely] — https://svn.nmap.org/nmap/nmap-services

Registration date

Unknown (blank in IANA registry — not fabricated) [Confirmed] — IANA registry (local cache line 174)

Related ports

NetRJS Remote Job Service cluster on 71–74 (distinct — port 75 has no RJE association); matching "private dial out/in" placeholder reservations

Primary use

legacy "any private dial out" reservation; not a standardized/documented protocol; informal third-party label priv-dial (not in IANA CSV) [Likely] — https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.txt , https://isc.sans.edu/services.html

Other/unofficial uses

none documented; no widely deployed daemon binds TCP/75 under this designation

Security implications

low inherent risk (no active protocol to exploit); obscurity + privileged-range status make it a candidate for rogue/backdoor listeners avoiding common-port blocklists; AuditMyPC historically flagged trojan use but no named family/CVE verified; no current CVEs tied to 75/tcp as of June 2026

Typically seen on

not associated with any standard host profile; an open 75/tcp is an anomaly worth investigating

Analyst note

An open port 75 is statistically rare with no legitimate standard service behind it — treat as misconfiguration, legacy, or a possible rogue/backdoor listener and investigate.

Port 75/udp is registered in the IANA Service Name and Transport Protocol Port Number Registry under the description "any private dial out service," assigned and contacted by Jon Postel, with a blank reference field. Two angles agree on a small naming wrinkle worth recording: the registry CSV row carries no machine-readable service name (the service-name column is empty), while the human-readable IANA text registry and downstream port databases surface the placeholder token priv-dial for the same entry. The assignment is dual-registered on TCP and UDP at port 75 with identical fields, placing it in the well-known range (0–1023). This is not a protocol in any meaningful sense — it is one of the early "any private … service" catch-all reservations Postel set aside in the 70–79 block so that vendors or sites could run their own dial-out signaling under a reserved number rather than colliding with a standardized service. No RFC specifies a wire format, no canonical application claims it, and the IANA reference column is genuinely blank (no RFC should be fabricated for it). For an analyst, the practical meaning is that 75/udp has no legitimate modern use case: there is no named software that is supposed to answer here, so a host responding on UDP port 75 is an anomaly to inventory and firewall rather than a recognizable service. No malware or trojan family is specifically tied to this port in current threat databases, and public scan data (e.g., Shodan) shows no notable exposure pattern for it — but UDP's connectionless nature means obscure UDP ports in the well-known range are historically under-scanned and easy to overlook in firewall reviews, so an unexpected listener here deserves investigation rather than assumption.

IANA assignment

description "any private dial out service"; assignee Jon Postel; contact Jon Postel; reference blank (no RFC cited in the registry); service-name column empty in the CSV [Confirmed] — IANA Service Name and Transport Protocol Port Number Registry (the IANA Service Name and Transport Protocol Port Number Registry, line 175)

Service-name token

priv-dial surfaced by the IANA text registry / downstream port databases for this entry, though the CSV service-name column is blank [Likely] — https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.txt , https://www.auditmypc.com/udp-port-75.asp

Range class

well-known (0–1023) [Confirmed]

Dual registration

75/tcp carries identical fields (line 174) [Confirmed] — IANA registry

Registration / modification date

Unknown — genuinely blank in the registry; not fabricated [Confirmed] — IANA registry

Primary use

catch-all IANA reservation for site-local / vendor-specific private dial-out signaling; no standardized protocol defined and no RFC

Common software

Unknown — no named application or protocol standard is documented as the canonical user of UDP port 75

Security implications

no malware/trojan family specifically associated; no notable public exposure per available scan data; UDP ports in this range are easily overlooked in firewall reviews, so any listener should be inventoried and firewalled absent an explicit need

Historical context

the "any private … service" assignments in the 70–79 block are early Postel placeholders for vendor/site-local use, predating modern internet standardization and ephemeral/registered-port conventions [Likely] — IANA registry

Analyst note

75/udp has no legitimate modern use case; treat a responsive host as an anomaly to investigate, not a known service.