Firewalk is an active reconnaissance network security tool that attempts to determine what layer 4 protocols a given IP forwarding device will pass. It works by sending out TCP or UDP packets with a TTL one greater than the targeted gateway. If the gateway allows the traffic, it will forward the packets to the next hop where they will expire and elicit an ICMP_TIME_EXCEEDED message. The ultimate destination host does not have to be reached – it just needs to be somewhere downstream, on the other side of the gateway, from the scanning host.”]
Source: https://gbhackers.com/firewalk-network-security-tool/