firewalk

Firewalk is a network auditing tool that attempts to
determine what transport protocols a given gateway 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.