GRE, or Generic Routing Encapsulation, is a network protocol that carries one packet inside another. By wrapping traffic in an outer IP packet, GRE creates a tunnel between two endpoints, letting different types of network traffic cross infrastructure that would not normally forward it directly. It is a simple encapsulation method, not a security feature by itself.
In cyber security, GRE matters because tunnels can be useful for legitimate site-to-site connectivity, but they also create paths that may hide traffic from routine monitoring. Defenders look for GRE interfaces, peer addresses, and configuration records to confirm that a tunnel is approved and expected. Attackers may abuse tunnels to move data, maintain covert access, or create routing paths that are hard to notice if live state is not compared with the intended configuration.