DNS Records  and Messages

Two types  of records  are used DNS. The  question records  are used in the question  section of the query and response  messages. The resource  records  are used in the  answer  authoritative  and additional  information sections of the  response  message.

Question Record: A questions records in used by the client  to get  information  from a server. This contains the domain  name.

Resource Record: Each domain  name is  associated  with a  record  called  the resource  record. The server database  consists  or resource  returned by the server to the client.

A resource  records is a quadruple that contains the  following  fields.

Name , Value , Type , TTL

TTL  is the time  to live of  the resource records. It determines  when a resource  should be  removed  from a cache form a  cache. In the example  records given below  we ignore the TTL  field.

The  meaning of  name  and value  depend on  type:

If a DNS  server is authoritative for a  particular host name  then the DNS  server will  contain  a type a record for the  hostname. If a server is not authoritative  for a hostname  then the  server will contain a type NS record for the  domain  that includes the hostname. It  will also  contain  a Type  a record that provides the IP address of the DNS  server in the value field of the NS  records. As an example suppose an edu TLD server is not  authoritative for the host gain cs un mass edu. Then this server will contains  a record for a  domain that includes the host cs mass. Edu. For example  ( umass edu dns unmass. Edu  NS) the du TLD server would  also contain a Type a record which maps the DNS. Server dns  umass.edu to an IP address for example  ( dns umass edu 128.119.111A)