nuauth, the authentication daemon of the NuFW identity-aware Linux firewall.
NuFW extends Netfilter with per-user authentication: the packet-filtering side queues a connection and asks nuauth whether the user behind it is allowed, and clients authenticate to nuauth over TCP 4129. IANA assigned 4128 to nufw and 4129 to nuauth in 2007. The project has seen no significant development in over a decade, so this is a legacy sighting on older Linux gateways.
Do not expose
It is the decision point of a firewall's authorisation policy on unmaintained code — reachable only from the client networks it authenticates, never the public internet.
$ nc -vz -w 3 TARGET 4129Seen 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.