I was looking for a tool to modify the packets in a pcap file for testing purposes that would allow me to change and modify any fields in packets and recompile the checksum to be able to use them for testing and training. I wanted to be able to change and modify some of the content such as IP addresses and where necessary, modify the content of the payload to obfuscate sensitive information. This Packet Editor and Builder by Colasoft is advertise as a freeware tool and very useful to manipulate and send packets over a network interface.
Article Link: https://isc.sans.edu/diary/rss/24682