[問題] ARP

[leslieha (哈)]

        問題一

        請問一下

        ARP

        是讓主機可由已知的 protocol address (IP address)詢問

        來得到 hardware address (MAC address)

        如果在一台 end host 與 switch 間架一個 bridge

        把 ARP 全都 filter 掉

        那 end host 有其它辦法可以得到 hardware address 嗎?


        問題二

        看了一下 RFC826 http://www.faqs.org/rfcs/rfc826.html

        在處理 ARP 的封包時的 algorithm 如下

?Do I have the hardware type in ar$hrd?
Yes: (almost definitely)
  [optionally check the hardware length ar$hln]
  ?Do I speak the protocol in ar$pro?
  Yes:
    [optionally check the protocol length ar$pln]
    Merge_flag := false
    If the pair <protocol type, sender protocol address> is
        already in my translation table, update the sender
        hardware address field of the entry with the new
        information in the packet and set Merge_flag to true.
    ?Am I the target protocol address?
    Yes:
      If Merge_flag is false, add the triplet <protocol type,
          sender protocol address, sender hardware address> to
          the translation table.
      ?Is the opcode ares_op$REQUEST?  (NOW look at the opcode!!)
      Yes:
        Swap hardware and protocol fields, putting the local
            hardware and protocol addresses in the sender fields.
        Set the ar$op field to ares_op$REPLY
        Send the packet to the (new) target hardware address on
            the same hardware on which the request was received.

        按上面標顏色的部份

        應該會先 add sender information 至 ARP table內

        才會去判斷該封包的 opcode

        試了一下發現

        在 ARP table 沒有該 sender 的記錄情況下

        收到 ARP reply 卻不會新增 sender 的記錄

        是因為 ARP reply 在更前面就處理結束了嗎?

        還是 RFC826 有被其它 RFC 所更新? (因為我並沒有找到)

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