apcupsd's Network Information Server — UPS status for apcaccess and dashboards, unauthenticated.
apcupsd monitors an APC UPS and shuts the machine down cleanly on a power failure; its Network Information Server listens on TCP 3551 and hands out the status block you see from apcaccess — line voltage, battery charge, time on battery, load percentage. Home servers, NAS boxes, Home Assistant installs, and monitoring agents poll it. The NIS is read-only status by design, and it has no authentication: access control is the NISIP bind address and your firewall.
Do not expose
Unauthenticated and cleartext, and it advertises exactly when a host is running on battery — bind NISIP to 127.0.0.1 or a LAN address and keep it off any public interface.
$ apcaccess status TARGET:3551Seen 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.