A stealth firewall. The stealth firewall can include a first network interface to an external network; a second network interface to an internal network; a packet filter for restricting access to the internal network, the packet filter ignoring requests from the external network to access the internal network; and, a state machine. Importantly, the state machine can be pre-configured to transition across one or more internal states conditioned upon receiving particular requests to access the internal network. The state machine further can include at least one state transition reachable through a pre-specified sequence of states which causes the packet filter to permit access to the internal network.