A packet preprocessing device is used in conjunction with a non-programmable packet forwarding processor (NLFP) to apply a different system function to received data packets than the function normally applied by the NLFP on the packets. Received data packets are pre-processed (e.g., modifying, manipulating, altering, spoofing, etc.) in order to enable, or cause, NLFPs that process the data packets to provide, in effect, a system-level behavior on the packets that is different from the system-level behavior for which the NLFP is/was conventionally designed. The data packet is “pre-processed” to change or manipulate the data packet, and then the NLFP processes the pre-processed data packet in accordance with its conventional function(s) which alters the overall function applied to the data packet.