SendIP is a commandline tool to allow sending arbitrary IP packets.
SendIP has a large number of command line options to specify the content of every header of a NTP, BGP, RIP, RIPng, TCP, UDP, ICMP or raw IPv4 and IPv6 packet. It also allows any data to be added to the packet.
SendIP now comes with a manpage, as well as the README file. The source is also quite readable... ;)
SendIP is distributed according to the GNU General Public Licence and was written by Mike Ricketts.