DNS Message Format

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)