556

Port 556/tcp is registered with IANA as remotefs with the description "rfs server" and blank assignee, contact, and reference fields (dual-registered on TCP and UDP); the classic /etc/services file expands the comment as "rfs_server rfs # Brunhoff remote filesystem." It denotes an early remote-filesystem server protocol that is distinct from NFS. The lineage is genuinely uncertain: the BSD /etc/services annotation credits a "Brunhoff remote filesystem," while the name RFS is also widely associated with AT&T Remote File Sharing, a System V Release 3 (1980s) distributed filesystem that was connection-oriented and preserved full UNIX file semantics (stateful, unlike early NFS), built on the STREAMS Transport Provider Interface and shipping in SVR3/SVR4 before being eclipsed by NFS and removed by Sun from Solaris 2.4. Either way, RFS here is an obscure, historic remote filesystem with a minimal modern footprint. The IANA reference field is blank, and confidence is Medium because the precise origin of the 556 assignment predates clear public documentation. Security-wise there is effectively no modern exposure — the service is legacy and almost never seen — so an open 556 most likely reflects a stale /etc/services label or a niche or embedded implementation rather than active RFS. For an analyst, an open 556/tcp is rare and should be treated as legacy or anomalous, with investigation of what is actually listening rather than an assumption of RFS.

IANA assignment

remotefs — "rfs server"; reference (blank — no RFC cited in IANA registry); assignee/contact blank; dual-registered 556/tcp + 556/udp [IANA-assigned] — IANA Service Name and Transport Protocol Port Number Registry

Range class

well-known (0–1023)

Prevalence

nmap-services open-frequency ~0.000125 (de-facto; rare) [Well-established] — nmap-services file

Related ports

2049 (NFS), 111 (sunrpc/portmapper)

Primary use

early remote-filesystem server protocol (RFS), distinct from NFS

Other/unofficial uses

none today [Well-established]

Security implications

effectively none in modern environments; legacy and almost never seen [Unverified]

Typically seen on

legacy Unix RFS (rare)

Analyst note

An open 556/tcp is rare; treat as legacy/anomalous and investigate what is actually listening rather than assuming RFS.