DNS Message

There are  the only  two kinds  of DNS message DNS query and reply messages. Further mote both query  and reply message have  the same  format  as shown in figure. The  semantics of the  various fields in a DNS message  are as follows.

                                                            Figure  DNS message  format

The first 12 bytes is the header section which  has a number of fields. The first  field is a 16 bit numbers  that identifies the query. This identifier is copied into the reply message to a query allowing the client  to match received replies with sent  queries. There are a  number of flags in the flag field. A 1 bit query reply flag indicates whether the message  is a query or a reply a 1 bit  authoritative flag is set in a reply message  when  a DNS server is  an authoritative server for a queried name. A 1 bit  recursion desired flag  is set  when  a client  host  or DNS  server desired that the DNs  server perform recursion  when it doesn't have  the records. A 1 bit  recursion available field  is set in a reply  if the  DNS  server  supports  recursion. In  the header there are also   four number of fields. These fields  indicate  the number  of occurrences of the four  type  of the  data section that  follow the header.

a.The question  section  carries  one or more  question that is  queries for information  being  sent to a DNS  name  server. This  section  includes a name field  that containing the name  that is  being queried and a type field that indicates the type  of question being  asked  about  the name  for example   a host  address  associated with  a name  or the mail server for a name .

b.Answer carries  one or more  resources records  that answer  the questions  indicated in the question  section above.

c.Recall that  in each  resource  records there is  the type  ( for example A , NS, MNAME, and MX), the  value  and the TTL. A reply can return multiple RRS in  the answer since  a hostname   can have multiple  IP address.

d.The authority  contains  one  more  resource  records that paid  to authoritative name  servers  that can  be used  to continue the resolution process.

e.The additional  section  conveys one or more  resource records that contain additional information  related to the query that is not  strictly necessary to answer  the queries.