两种报文格式相同，都有：一个头部 + 四个部分
A header field contols the content of these four sections.
(16 bit)Identification, used to match responses with queries
QR, Indicates if the message is a query (0) or a reply (1) OPCODE, The type can be QUERY (standard query, 0), IQUERY (inverse query, 1), or STATUS (server status request, 2) AA, Authoritative Answer, in a response, indicates if the DNS server is authoritative for the queried hostname TC, TrunCation, indicates that this message was truncated due to excessive length RD, Recursion Desired , indicates if the client means a recursive query RA, Recursion Available, in a response, indicates if the replying DNS server supports recursion Z, Zero, reserved for future use RCODE, Response code, can be NOERROR (0), FORMERR(1, Format error), SERVFAIL (2), NXDOMAIN (3, Non existent domain), etc.
(16 bit)Number of questions,
(16 bit)Number of answers,
(16 bit)Number of authority resource records (RRs),
(16 bit)Number of additional RRs
# question, answer, authority, an additional space
NAME Name of the requested resource TYPE Type of RR (A, AAAA, MX, TXT, etc.) CLASS Class code
an additional space