TCPDUMP:  How do I filter an arp message by checking  for equality with arp's  hexadecimal value of 0806?

Posted on 2009-02-18
Last Modified: 2013-12-23
I've spent the last hour trying to figure this challenge out.  I know I can filter out by name but would like to also know how to by hexadecimal value.

I have a capture file and would like to filter arp by its hexadecimal value 0806?  Any additional explanation will be appreciated.  Thanks.
Question by:jeffw22
    LVL 3

    Accepted Solution

    The pattern to capture specific bytes from a protocol header in tcpdump is
    proto [ offset : length ]

    You know that the ethernet header has the 2-byte protocol field after the first 12 bytes (destination and source). So match them with

    Then apply the bitwise or comparison operations you need; in your case the whole field must have a specific value.

    But watch out the presence of LLC when filtering some protocols, in those cases better using the protocol name.

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