Tcptraceroute : A traceroute implementation using TCP packets
The problem is that with the widespread use of firewalls on the modern Internet, many of the packets that the conventional traceroute(8) sends out (ICMP echo or UDP) end up being filtered, making it impossible to completely trace the path to the destination. However, in many cases, these firewalls will permit inbound TCP packets to specific ports that hosts sitting behind the firewall are listening for connections on. By sending out TCP SYN packets instead of UDP or ICMP ECHO packets, tcptraceroute is able to bypass the most common firewall filters.