{
  "port": 5550,
  "protocol": "tcp",
  "service": "CBUS model railway control (formerly RSA sdadmind)",
  "oneLiner": "Registered since 2017 to a model-railway control protocol; before that, RSA's remote admin daemon.",
  "whatItIs": "IANA reassigned 5550/tcp in 2017 to CBUS, the MERG (Model Electronic Railway Group) message protocol for controlling model railway layouts over a network. The earlier occupant, still present in old services files and firewall rules, is sdadmind, RSA ACE/Server's remote administration daemon. UDP 5550 is reserved. Which one you are looking at depends entirely on the host: a hobby layout controller or a legacy SecurID server.",
  "exposure": {
    "verdict": "never",
    "note": "Both plausible occupants are control planes — layout control or authentication-server administration — and neither should be reachable from the internet."
  },
  "checkCommand": "nc -vz -w 3 TARGET 5550",
  "links": [
    {
      "title": "MERG — CBUS layout control protocol",
      "url": "https://www.merg.org.uk/merg_wiki/doku.php?id=cbus:cbus",
      "kind": "official-docs"
    },
    {
      "title": "RSA — annotated example of the ACE/Server services file (sdadmind 5550)",
      "url": "https://community.rsa.com/t5/securid-knowledge-base/correct-and-annotated-example-of-rsa-ace-server-services-file/ta-p/8600",
      "kind": "official-docs"
    },
    {
      "title": "IANA port registry — 5550 (cbus)",
      "url": "https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=5550",
      "kind": "iana"
    }
  ]
}