Jpcap is a Java library for capturing and sending network packets.  Using Jpcap, you can develop applications to capture packets from a network interface and visualize/analyze them in Java. You can also develop Java applications to send arbitrary packets through a network interface.  Jpcap has been tested on Microsoft Windows (98/2000/XP/Vista), Linux (Fedora, Mandriva, Ubuntu), Mac OS X (Darwin), FreeBSD, and Solaris.  Jpcap can capture Ethernet, IPv4, IPv6, ARP/RARP, TCP, UDP, and ICMPv4 packets.  Jpcap is open source, and is licensed under GNU LGPL.