I have the IP and port details of an already established connection between the TCP server and the client. As I am simulating packet injection attack using the packETH tool, I disguise my self as the client(using client's IP and port, but not same seq and ack numbers) and send FIN segment, then I expect connection close or some abnormal behavior, but it isn't happening. My question is, what might exactly be happening? and what happens if I send a repeated SYN segment.
I'd try it with the RST bit set. Your SEQ/ACK shouldn't matter in that case. If that doesn't work it is probably because you messed up your checksum and the packet is getting dropped.