Lecture 9: The Domain Name System (DNS)

Recursive Queries

A host within a specified domain (eg a machine at Bendigo, in domain bendigo.latrobe.edu.au) is configured to "know" the IP address of its local nameserver. What happens when it sends a query for a non-local name, (eg amazon.com)? The sequence of events is something like:

The local nameserver will forward the query to its "parent" nameserver -- in our case, the nameserver for domain latrobe.edu.au.
This nameserver, in turn, (usually) forwards the query recursively up the "tree", to where a root nameserver will pass it to a nameserver for the .com domain, which will have the desired name-to-address mapping.
The result of the query is then passed back through the chain of nameservers (each of whom will normally cache the information), finally arriving at the originating host. This process is called a recursive query.

It's obvious that recursive queries could be quite slow. The DNS provides a way of "short-circuiting" the whole process. If (for example) the local nameserver already knows (due to caching) the IP address of a nameserver for the .com domain in the above example, it can contact it directly, thus avoiding many recursive stages. This is called an iterative query. In practice, most (all?) queries to root nameservers are iterative. Every nameserver is configured to know the IP address of at least one root nameserver.

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