Coupling device, crankcase ventilation device and fresh air system

The coupling device (1) has a connecting piece (2), which has an external resting contour (7) at its exterior side. A connecting piece is connected with a fluid line (5). Another connecting piece (3) has another external resting contour (8) at its external side. The latter connecting piece is connected to another fluid line (6). A case (4) is coated by a protective cover. Independent claims are also included for the following: (1) a crank case ventilation system with a blow-by-gas line; and (2) a fresh air plant with a filter.