Registered for remotely piloted aircraft command and control; in practice, VNC display :6.
IANA assigns 5906 to rpas-c2, "Remotely Piloted Vehicle C&C", registered by ICAO for the command-and-control link of remotely piloted aircraft systems. That is aviation infrastructure on private networks, not something a scanner meets. On an ordinary server, 5906 is the RFB port for display :6, typically the sixth VM console on a busy KVM or Proxmox host.
Do not expose
Whichever it is, it is a control plane — a guest's console or an aircraft C2 link. Neither belongs on a public interface.
$ nc -w 3 TARGET 5906 | head -c 12; echoSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.