Internet-Security.com UDP port 13 details

UDP port 13 is used by the Daytime Protocol (RFC 867), which returns a human-readable timestamp.
Unix/Linux systems often expose this via inetd/xinetd; for example, the openbsd-inetd or xinetd packages provide a “daytime” service that can listen on UDP/13 when enabled.
Microsoft Windows historically offered Daytime over UDP/13 through the “Simple TCP/IP Services” feature (e.g., on Windows Server 2003/2008/2012), though it’s deprecated and off by default.
Cisco IOS can enable Daytime on UDP/13 as part of its diagnostic “small servers” using the command “service udp-small-servers.”
Embedded Linux distributions using BusyBox inetd (such as OpenWrt when configured) can also run a Daytime UDP service for testing.
Security: exposed Daytime services are occasionally abused for UDP reflection DDoS or reconnaissance, so best practice is to keep them disabled on Internet-facing systems.

UDP 13