I don't see what the ultimate goal is here. Are you trying to set up a DoS or an ARP hijack? If the latter, you probably want to enable ip_forward also. You haven't elaborated on the network topology, either. Protocol/flow description with the "usual suspects" Alice, Bob, Celia would no doubt be enlightening.
If you want to work a trick in the classroom, you've plenty of alternatives. For example: scripted login to plug the holes and harden all hosts. In any case, please refrain from posting script-kiddie tools.
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