Resource Records:

Every domain whether it is a single host or a top level domain can have a set of resource records associated with it. At any time a resolver (this will be explained later) gives the domain name to DNS it gets the resource record associated with it. As a result DNS can be looked upon as a service which maps domain names to resource records. Every resource record has five fields and looks as below:

• Domain name: It is the domain to which this record applies.
• Class: set to IN for internet information. For other information other codes could be specified.
• Type: It tells what kind of record it is.
• Time to live: Upper Limit on the time to get to the destination
• Value: It could be an IP address, a string or a number depending on the record type.

Resource Record

A Resource Record (RR)  has the following:

• owner which is the domain name and where the RR is found.
• type which is an encoded sixteen bit value that specifies the type of the resource in this resource record. It can be one of the following:

o  A a host address

o  CNAME It identifies the canonical name of an alias o HINFO identifies the CPU and OS used by a host o   MX identifies a mail exchange for the domain.

o  NS It is the authoritative name server for the domain

o  PTR It is a pointer to another part of the domain name space

o  SOA identifies the begining of a zone of authority class which is an encoded 16 bit value which can identifies a protocol family or instance of a protocol.

• class One of: Here the Internet system or CH the Chaos system
• TTL which is the time to live of the RR. This field is a thirty two bit integer in units of seconds, an is mainly used by resolvers when they cache RRs. The TTL describes how lengthily a RR can be cached before it should be discarded.
• RDATA Here the Data in the field depends on the values of the type and class of the RR and a description for each is as follows:

o  for A: For the IN class, a thirty two bit IP address For the CH class, a domain name followed by a sixteen bit octal Chaos address.

o  for CNAME: a domain name.

o  for MX: a 16 bit preference value (lower is better) followed by a host name willing to act as a mail exchange for the owner domain.

o  for NS: a host name.

o  for PTR: a domain name.

o  for SOA: several fields.

Note: While short TTLs can be used to reduce caching, and a zero TTL prohibits caching, the truthiness of Internet performance suggest that these times should be on the order of days for the typical host. If a change could be anticipated, the TTL could be reduced prior to the change to minimize inconsistency during the change, and then return back to its former value  following  the  modifyfication.  The  data  in  the  RDATA  section  of  RRs  is  carried  as  a combination of binary strings and domain names. The domain names are commonly used as "pointers" to other data in the DNS.

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