This machine is A target machine is B
AB coexistence in A local area network (LAN), assuming A now by arp cheat, make all return packets sent to the A, B A forward again after processing to B

Goal is to replace the B to download file to the specified file,

My idea is:
Suppose now downloaded from host C B 1. Rar, captures A of the first package, and then send C RST and then give A fake their documents prepared TCP flow,

Please help to see if there is any problem in theory,
If no problem I the idea is to send A file yourself prepared TCP flow to his judgment and maintain the TCP state it is not too much trouble,
You have any better ideas,
Hope your comments! Thank you!