Abstract: A system for testing a data processing network and diagnosing malfunctions therein is disclosed. It is assumed that the data processing network includes a plurality of processing units coupled by communication paths in which each processing unit communicates with one or more other processing units in the data processing network by sending and receiving data packets over one or more of the communication paths. The system includes probes and a local processor for creating modified data packets from the data packets present on the network communication paths. The modified data packets allow a central processing unit which is communication with the local processor to diagnose network errors while maintaining the security of the data contained in the original data packets.