Recommendation for TCP/IP message modifier

Question

I would like a recommendation for a tool(sniffer that able to modify packets maybe?) that would be able to mess up the TCP packets sent/recieved between the 2 apps in various ways. The main purpose is to test the behavior of the apps when bad/invalid messages arrive. An invalid message is a valid TCP packet but the application level format is bad in some way (so this tool would operate on levels 6,7 of the OSI model and screw up messages sent by the apps)

Answer 1

I've played with Fiddler2 and found it quite good for http(s) packets.

Answer 2

This is only half an answer, but if you just want a network sniffer (no ability to modify packets) here is a review of 11 packet sniffers.

Answer 3

Setup proxy with netcat which will pipe input from transmitting application through your custom mangler and then to receiving application. You will need to write mangler, however.