Firewalk

Firewalk is an active reconnaissance network security tool that attempts to determine what layer 4 protocols a  given IP forwarding device will pass. Firewalk  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.  If the gateway host does not allow the traffic, it will likely drop the packets on  the floor and we will see no response.