An assembly checks at least one firewall device and a method protects at least one data receiver. In the method, permissible and non-permissible data traffic is differentiated in data traffic in the direction of the data receiver using specific rules. Non-permissible data traffic is blocked, and permissible data traffic is allowed through. In order to check the function of the firewall device, the data traffic which has been allowed through is interrupted if the data traffic which has been allowed through has non-permissible data traffic.