A tunnel interface is a logical network interface that represents encapsulated traffic between two endpoints. Instead of carrying packets directly on a physical link, the device wraps them inside another protocol, such as GRE or IPsec, and forwards them through a tunnel.
In cyber security, tunnel interfaces matter because they can create hidden or hard-to-see traffic paths. They are common in VPNs, overlays, and site-to-site connectivity, but they can also be abused to bypass segmentation, move data off a network, or establish an unauthorized management path. Defenders should treat every tunnel interface as an asset to inventory, document, and validate against approved routing and configuration records. An unexpected tunnel is not proof of compromise by itself, but it is a strong signal to investigate state drift, access history, and traffic flow.