You can download the tool from here:
Packit is a network auditing tool. Its value is derived from its ability to customize, inject, monitor, and manipulate IP traffic. By allowing you to define (spoof) nearly all TCP, UDP, ICMP, IP, ARP, RARP, and Ethernet header options, Packit can be useful in testing firewalls, intrusion detection systems, port scanning, simulating network traffic, and general TCP/IP auditing. Packit is also an excellent tool for learning TCP/IP.
Packit requires libnet 1.1 or greater as well as libpcap. It has been successfully compiled and tested to run on FreeBSD, NetBSD, OpenBSD, MacOS X and Linux.