Described are computer-based methods and apparatuses, including computer program products, for testing functionality of a firewall. The testing the functionality of the firewall can include a method. The method can include selecting a plurality of valid message types, generating a percentage of valid and invalid messages from the plurality of valid message types, transmitting the plurality of valid and invalid messages to the firewall, receiving an indication of the firewall's handling of valid and invalid messages based on the transmitted message, and determining the functionality of the firewall from the received indication.