The most-typed port number there is: registered to italk, famous as the NetBus backdoor, and used daily as a throwaway test port.
IANA registers 12345 to italk, the Italk Chat System, which nobody runs. The number's reputation comes from NetBus, a Windows remote-control backdoor from 1998 that listened on 12345 with a second channel on 12346, and which is well enough documented that Nmap ships a netbus-info NSE script and Nmap's service table names the port netbus. Trend Micro's OfficeScan agent also used 12345, which produced a long history of false-positive backdoor alerts on corporate desktops. In modern practice most sightings are neither: 12345 is the reflexive choice for a scratch listener, a debug server, or a container port mapping.
Do not expose
Either it is a remote-control backdoor, or it is a hastily configured test service that nobody hardened. Neither belongs on a public IP.
$ nmap -Pn -p 12345 --script netbus-info 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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