Thread: ARP requests

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    ARP requests


    I'm hoping that someone can help me. When a host issues an ARP request broadcast across the local network do all hosts on the network put the broadcasting hosts information into their ARP cache or do they completely ignore it and only the host to which the broadcast is directed puts the information in it's ARP cache?

    I don't know if that makes any sense to any of you but I would appreciate any insight.
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    No. An ARP request (on ethernet for example) does no contain the requestor's IP address. The requestor includes it's ethernet address, addresses the packet to the *ethernet* broadcast addr, and the question in the ARP request is "which ethernet address out there handles IP 1.2.3.4?"

    So a host listening to ARP request broadcasts cannot learn the *requestor's* IP address.

    -c
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    ...ok, the little voices in my head made me look in the book. An ARP request does in fact contain the requestor's IP address. So the information is there for any host on the network to detect. (ie, you could learn the requesting host's IP [or at least one of its IPs] from it's ARP requests.)

    However, I'm abosolutely sure that in the many protocol stacks I've worked with, I've never seen ARP entries present except for those that correspond to network traffic the host itself was originating.

    -c

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