So can I just connect the problem box to a spare box and run ethereal or
arpwatch on it? I mean usually can you connect two boxes to each other
without a hub or switch or router? Just by using a cat-5 cable?
Going with a single cable between your FC machine and the toshiba device means that you'll not have to wade through other network traffic, so is a good plan.
BTW ethereal will give you a gui-based packet sniffing tool, whereas arpwatch will email root with details of all new or changed machines it sees talking on the network.
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Cheers,
Tony