# WHATPORT(5904)

## NAME

port 5904 — ag-swim (ICAO) / VNC display :4 in practice — IANA calls it Air-Ground SWIM; in the field it is VNC display :4.

Protocol: TCP  
IANA registry

## DESCRIPTION

The registered service on 5904 is ag-swim, "Air-Ground SWIM", part of ICAO's block of aeronautical registrations (5903 through 5914) for System Wide Information Management traffic between aircraft and ground systems. Those run on dedicated aviation networks. On a general-purpose host, 5904 is 5900 plus display 4 — a fourth VNC desktop or VM console, common on hypervisors that allocate one console port per running guest.

## EXPOSURE

**Do not expose.** Hypervisor VNC consoles hand whoever connects a keyboard and screen attached to the guest, often with no password at all. Restrict them to the management network.

## CHECK

```sh
nc -w 3 TARGET 5904 | head -c 12; echo
```

## SEE ALSO

- iana — [IANA port registry — 5904 (ag-swim)](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=5904) (iana.org)
- spec — [RFC 6143 — The Remote Framebuffer Protocol](https://datatracker.ietf.org/doc/html/rfc6143) (datatracker.ietf.org)

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HTML: <https://whatport.net/5904/> · JSON: <https://whatport.net/5904.json> · full dataset: <https://whatport.net/ports.json>