1. A method of modifying a network message for use with a message generator testing a device-under-analysis (DUA), comprising: receiving at the message generator including a processor a network message conforming to a network message protocol sent by a source system that was intended for the DUA, the message being received at the message generator before the network message was received by the DUA; after receiving the message, determining a packet structure of the network message based, at least in part, on the content of the network message; after determining the packet structure, creating a description of the packet structure of the message; modifying the network message based on the description to create a modified message that is at least one of invalid or malformed with respect to the protocol or inappropriate based on the protocol's current state; and sending the modified message from the message generator toward the DUA in place of the network message.