# WHATPORT(5550)

## NAME

port 5550 — CBUS model railway control (formerly RSA sdadmind) — Registered since 2017 to a model-railway control protocol; before that, RSA's remote admin daemon.

Protocol: TCP  
IANA registry

## DESCRIPTION

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

**Do not expose.** Both plausible occupants are control planes — layout control or authentication-server administration — and neither should be reachable from the internet.

## CHECK

```sh
nc -vz -w 3 TARGET 5550
```

## SEE ALSO

- docs — [MERG — CBUS layout control protocol](https://www.merg.org.uk/merg_wiki/doku.php?id=cbus:cbus) (merg.org.uk)
- docs — [RSA — annotated example of the ACE/Server services file (sdadmind 5550)](https://community.rsa.com/t5/securid-knowledge-base/correct-and-annotated-example-of-rsa-ace-server-services-file/ta-p/8600) (community.rsa.com)
- iana — [IANA port registry — 5550 (cbus)](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=5550) (iana.org)

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HTML: <https://whatport.net/5550/> · JSON: <https://whatport.net/5550.json> · full dataset: <https://whatport.net/ports.json>