Internet-Security.com UDP port 138 details

UDP port 138 is used by the NetBIOS Datagram Service (NBT) in Microsoft Windows networking.
Windows File and Printer Sharing over SMBv1/NetBIOS (e.g., on Windows XP/7/10 and Windows Server 2003/2008 when “NetBIOS over TCP/IP” is enabled) uses UDP 138 for browser announcements, workgroup discovery, and Messenger alerts.
The legacy Windows Messenger service (net send) received messages via UDP 138 on LANs.
Samba’s nmbd on Linux/Unix (including NAS devices like Synology and QNAP when NetBIOS/SMB1 is enabled) listens on UDP 138 for NetBIOS datagrams such as master‑browser elections and browse lists.
Older embedded systems and printers implementing SMB1/NBT (e.g., Windows CE devices) also use UDP 138 for these datagram functions.
Security: attackers often probe UDP 137/138 to enumerate NetBIOS names and browse/share info; exposing 138 to the internet can aid SMB-targeted attacks, so it’s commonly blocked by firewalls.

UDP 138