Overview

The Domain Name System (DNS) is essential because everything connected to the internet is based on IP addresses.

Without DNS you wouldn’t be able to browse to www.google.com or www.wikipedia.com. Instead, you would have to browse
to 216.239.59.104 or 145.97.39.155. Doesn’t exactly role off the tongue, does it?

The role of DNS is, at its most basic form, that of a database which stores records of domain names and IP addresses to
facilitate the lookup between the two. This is all done behind the scenes so that the user need not intervene. There are two basic forms of DNS server on the internet, Recursive and Iterative (aka non-recursive).

Iterative - Iterative DNS queries are queries in which a DNS server is queried and returns an answer without querying other DNS servers, even if it cannot provide a definitive answer. These are usually the sort of DNS servers which are usually associated with Web Hosting as they are there to provide authoritive DNS results which are usually managed by that organisation.

Recursive - Recursive DNS queries occur when a DNS client requests information from a DNS server that is set to query subsequent DNS servers until a definitive answer is returned to the client. The queries made to subsequent DNS servers from the first DNS server are iterative queries. Recursive DNS Servers are usually found within businesses for their own internal use only or by ISP's for their end users to use.

The domain name.

A domain name usually consists of two or more parts, separated by dots, ie  whatsmydns.com

• The last part of a domain name is the top-level domain in this case it has the top-level domain of com.
• Anything to the left of the top-level domain is considered a subdomain. In this example whatsmydns is a subdomain
  of the com domain.
• If the domain name was hello.whatsmydns.com then hello would be a subdomain of the whatsmydns.com domain

Figure 1: Basic Structure of DNS Servers on the internet.

Figure 2: Steps a recursive DNS server takes for finding out an IP address.

Below are details of the steps which are outlined in Figure 2 above.

1. Recursive query for www.whatsmydns.com (A resource record) from Requesting client to DNS server.

2. Iterative query for www.whatsmydns.com (A resource record) from DNS Server to one of the internet's 'Root' DNS servers.

3. Referral to the .com name server (NS resource records, for .com); for simplicity, iterative A queries by the DNS server (on the left) to resolve the IP addresses of the Host names of the name server’s returned by other DNS servers have been omitted.

4. Iterative query for www.whatsmydns.com (A resource record) from DNS Server to one of the .com DNS servers.

5. Referral to the whatsmydns.com name server (NS resource record, for whatsmydns.com)

6. Iterative query for www.whatsmydns.com (A resource record) from DNS Server to one of the authoritative DNS servers for the whatsmydns.com domain name.

7. Answer to the iterative query from whatsmydns.com server (www.whatsmydns.com’s IP address)

8. Answer to the original recursive query from local DNS server to Revolver (www.whatsmydns.com’s IP address)