DNX — the abandoned Nagios add-on that farmed check plugins out to a cluster of worker nodes.
IANA registers 3998 as dnx, "Distributed Nagios Executor Service", a port the project specifically asked for. DNX is a Nagios event-broker module plus server and client daemons: the server dequeues checks from Nagios and hands them to dnxClient worker nodes, which run the plugin and return the result, spreading check load across machines. Upstream stalled — the last SourceForge release is 0.20.1 from 2013, still labelled alpha — and Nagios' own library now points people at Mod-Gearman instead. A listener on 3998 is an old monitoring cluster nobody has migrated.
Do not expose
The whole point of the port is dispatching commands for worker nodes to execute, and the code has been unmaintained since 2013 — keep it strictly on the monitoring network.
$ nc -vz TARGET 3998Seen 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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