A network interface has an input port, which is designed to accept messages from a first device or first network, and an output port, which is designed to forward the messages to a second device or second network. A memory is provided for a timetable. The network interface is designed to forward messages arriving at the input port during open times defined by the timetable to the output port and to discard messages arriving at the input port during closed times defined by the timetable. A configuration unit is designed to accept and store in the memory a timetable defined by a monitoring unit as a shared secret for the network interface and at least one sender of messages, and/or to negotiate a timetable with at least one sender of messages as a shared secret.