Hping2 : A network probing utility like ping on steroids
This handy little utility assembles and sends custom ICMP, UDP, or TCP packets and then displays any replies. It was inspired by the ping command, but offers far more control over the probes sent. It also has a handy traceroute mode and supports IP fragmentation. This tool is particularly useful when trying to traceroute/ping/probe hosts behind a firewall that blocks attempts using the standard utilities. This often allows you to map out firewall rulesets. It is also great for learning more about TCP/IP and experimenting with IP protocols.