# WHATPORT(666)

## NAME

port 666 — mdqs / doom — Registered twice — as mdqs, and to id Software for Doom — but neither is what you will find running.

Protocol: TCP/UDP  
IANA registry

## DESCRIPTION

IANA carries two registrations for 666 on both TCP and UDP: "mdqs", with no description, and "doom", assigned to id Software. The Doom entry is a 1990s artefact from the era when games registered ports; Doom's own multiplayer ran over IPX, and modern source ports such as Chocolate Doom and GZDoom use their own UDP ports rather than 666. Because the number is memorable and unclaimed in practice, it gets picked deliberately by hobby projects and by scanner and malware authors alike, so treat a listener here as unidentified until you fingerprint it.

## EXPOSURE

**Do not expose.** Nothing standard listens here, so an open 666 is an unknown service you have not accounted for — identify it before deciding anything else about it.

## CHECK

```sh
nc -vz -w 3 TARGET 666
```

## SEE ALSO

- iana — [IANA Service Name and Transport Protocol Port Number Registry — 666](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=666) (iana.org)

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HTML: <https://whatport.net/666/> · JSON: <https://whatport.net/666.json> · full dataset: <https://whatport.net/ports.json>