Queries are messages which may be sent to a name server to provoke a
response. In the Internet, queries are carried in UDP datagrams or over
TCP connections. The response by the name server either answers the
question posed in the query, refers the requester to another set of name
servers, or signals some error condition.
In general, the user does not generate queries directly, but instead
makes a request to a resolver which in turn sends one or more queries to
name servers and deals with the error conditions and referrals that may
result. Of course, the possible questions which can be asked in a query
does shape the kind of service a resolver can provide.
DNS queries and responses are carried in a standard message format. The
message format has a header containing a number of fixed fields which
are always present, and four sections which carry query parameters and
RRs.
The most important field in the header is a four bit field called an
opcode which separates different queries. Of the possible 16 values,
one (standard query) is part of the official protocol, two (inverse
query and status query) are options, one (completion) is obsolete, and
the rest are unassigned.
The four sections are:
Question Carries the query name and other query parameters.
Answer Carries RRs which directly answer the query.
Authority Carries RRs which describe other authoritative servers.
May optionally carry the SOA RR for the authoritative
data in the answer section.
Additional Carries RRs which may be helpful in using the RRs in the
other sections.
Note that the content, but not the format, of these sections varies with
header opcode.
