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 then 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.

WWW: http://www.packetfactory.net/firewalk/