Packit (Packet toolkit) 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/prevention systems, port scanning, simulating network traffic, and
general TCP/IP auditing. Packit is also an excellent tool for learning TCP/IP.

This requires libnet.