SendIP has a large number of command line options to specify the content of
every header of a RIP, TCP, UDP, ICMP or raw IPv4 and IPv6 packet. It also
allows any data to be added to the packet. Checksums can be calculated
automatically, but if you wish to send out wrong checksums, that is
supported too.

WWW: http://www.earth.li/projectpurple/progs/sendip.html