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DNS Message Format


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The Header Section is always present



  • ID is a 16 bit identifier assigned by the resolver and is used to match replies with outstanding queries
  • QR is a 1 bit quantity thay identifies the message as

    • a query (0)
    • a response (1)
  • OPCODE is a 4 bit quantity which identifies whether the query is

    • 0 - a standard query
    • 1 - an inverse query
    • 2 - a server status request
    • 5 - an update
  • AA - Authoritative Answer
  • TC - Truncated response
  • RD - Recursion Desired (set by the client)
  • RA - Recursion Available (set by the server)
  • Zero - For further use
  • RCODE - response code (4 bits)

    Message Reason
    NOERROR no error condition
    FORMERR name server was unable to interpret the request due to a format error
    SERVFAIL name server encountered an internal error while processing the request
    NXDOMAIN name does not exist
    NOTIMP the Opcode requested is not implemented on this server
    REFUSED name server refuses to perform the operation for security or policy reasons
    YXDOMAIN a name that should not exist, does exist(DYNDNS)
    YXRRSET a RR set that should not exist, does exist (DYNDNS)
    NXRRSET RR set that should exist, does not exist (DYNDNS)

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