Internet-Security.com UDP port 517 details

UDP port 517 is assigned to the BSD “talk” service, used for real-time text chat on Unix systems.
The talk server daemon (talkd/in.talkd) listens on UDP/517 on classic Unix platforms such as 4.3BSD, SunOS/Solaris, HP-UX, and AIX.
On Linux, implementations include the netkit-talk/bsd-talk packages (often managed by inetd/xinetd), with clients like talk, otalk, and ytalk.
ntalk (a newer variant) commonly uses UDP/518, but the original talk control traffic uses UDP/517.
The service is largely obsolete today; historically, vulnerabilities in talkd existed on some systems, so exposed UDP/517 on legacy hosts may be probed by attackers.

UDP 517