DNS Queries
The major task carried out by a DNS server is to respond to queries (questions) from a local or remote resolver or other DNS acting on behalf of a resolver - a query would be somthing like 'what is the IP address of host=fred in domain=mydomain.com'. There are three types of queries that DNS support:
- A recursive query - the real answer to the question is always returned. DNS servers are not required to support recursive queries.
- An Iterative (or non-recursive) query - where the real answer MAY be returned. All DNS servers must support Iterative queries.
- A Inverse query - where the user wants to know the domain name given a resource record. Inverse queries were not mandatory, rarely useful and consequently not widely implemented. They were finally obsoleted by RFC 3425.
Note: The process called Reverse Mapping does not use Inverse queries but instead uses recursive and non-recursive queries with the special domain name IN-ADDR.ARPA.
Historically reverse IPv4 mapping was not mandatory. Many systems however now use reverse mapping for security and simple authentication schemes and proper implementation and maintenance is now essential. IPv6 made reverse mapping mandatory from its earliest specification.
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