(fīr´wâk-ing) (n.) Developed by Mike Schiffman and David Goldsmith, a technique for testing the vulnerability of a firewall and mapping the routers of a network that sits behind a firewall. Firewalking is a method of disguising port scans. In practical applications, firewalking is similar to tracerouting and works by sending into the firewall TCP or UDP packets that have a TTL set at one hop greater than the targeted firewall. If the packet makes it through the gateway, it is forwarded to the next hop where the TTL equals zero and elicits a TTL "exceeded in transit" message, at which point the packet is discarded. Using this method, access information on the firewall can be determined if successive probe packets are sent.
Other forms: firewalk (v.)