两种类型的消息：「Query」与「Reply」。
两种报文格式相同，都有：一个头部 + 四个部分
header
A header field contols the content of these four sections.
(16 bit)Identification, used to match responses with queries
(16 bit)Flags,
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.
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
question
NAME Name of the requested resource
TYPE Type of RR (A, AAAA, MX, TXT, etc.)
CLASS Class code
TYPE Type of RR (A, AAAA, MX, TXT, etc.)
CLASS Class code
answer
authority
an additional space