Waterfall Unidirectional Security Gateways replace firewalls in industrial network environments, providing absolute protection to control systems and industrial control networks from attacks emanating from external lesstrusted networks. Waterfall Gateways contain both hardware and software components. The hardware is physically able to send information in only one direction. The software replicates servers and emulates devices. At Hyundai Steel, the gateway software produces an accurate, timely replica of a production OPC-UA server. Enterprise and cloud-based IIoT applications and users use the OPC-UA replica exclusively. Unidirectional Gateways enable control-system intrusion detection, vendor monitoring, industrial cloud services, and visibility into operations for modern enterprises and customers. Unidirectional Gateways replicate servers, emulate industrial devices and translate industrial data to cloud formats. Unidirectional Gateway technology represents a plug-and-play replacement for firewalls, without the vulnerabilities and maintenance issues that accompany firewall deployments. Replacing at least one layer of firewalls in a defense-in-depth architecture breaks the attack path from the Internet to critical systems.