The upsd daemon from Network UPS Tools — battery status, and the commands that can shut machines down.
Network UPS Tools splits UPS monitoring into drivers, a network daemon (upsd) on TCP 3493, and clients such as upsc, upsmon, and the many NAS and home-automation integrations that poll it. The protocol is a simple line-oriented text protocol — LIST UPS, GET VAR ups.status, USERNAME/PASSWORD, INSTCMD — documented in RFC 9271. It shows up on Synology and QNAP boxes, Proxmox and TrueNAS hosts, and Raspberry Pis wired to an APC or Eaton UPS. Privileged clients can issue FSD, the forced-shutdown flag that tells every subscribed machine to power down.
Do not expose
Credentials and commands are plaintext unless you have explicitly configured NUT's optional TLS, and an authenticated peer can trigger a coordinated shutdown — keep upsd on the LAN and restrict it with LISTEN plus per-host ACLs.
$ upsc -l TARGETSeen 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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