Registered as the ARPANET Host Name Server; in practice this is Microsoft WINS replication traffic.
IANA registers 42 under the service names 'name' and 'nameserver' for the Host Name Server, a pre-DNS lookup service that no longer exists in the wild. What actually uses the port today is Windows Internet Name Service replication: WINS servers push and pull NetBIOS name-to-address mappings between each other over TCP 42, and Microsoft's network-port requirements documentation lists the port under WINS. Seeing it open means a Windows Server with the WINS role, which is a deprecated feature kept alive for legacy NetBIOS applications.
Do not expose
Internal Windows name-infrastructure replication with no place on a public interface; keep it inside the network boundary between WINS partners only.
$ nmap -Pn -sV -p 42 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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