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DNS Server (DNS Resolver - Root Server - Top-Level Server - Authoritative Server)

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DNS Servers contains DNS Records

DNS Server Types

  • DNS Resolver - answers DNS queries by querying the High-Level Domain Servers and Authoritative Name Servers itself
  • High-Level Domain Servers - define which DNS Server is the Authoritative Name Server for a specific domain name
    • Root Domain Servers -
    • Top-Level Domain Servers -
  • Authoritative Name Servers (Name Server) - holds the up-to-date information for that hostname/domain name

DNS Server Type

Description

DNS Resolver
  • answers DNS queries by querying the High-Level Domain Servers and Authoritative Name Servers itself for the appropriate IP Address
  • thus, a DNS Resolver is both a client and server
  • types:

Root Domain (RD) Server
  • the Root Server is the first step in the journey from hostname to IP address
  • the Root Server is a nameless domain (i.e ”.”)
  • the Root Server extracts the top-level domain from the DNS query’s FQDN — for example, the top-level domain of www.example.com is .com — and the Root Server provides details of the .com TLD Server to the DNS Resolver. In turn, that TLD Server will provide details of the domains with the .com DNS zone, such as “example.com” to the DNS Resolver
  • there are 13 root servers worldwide, indicated by the letters A through M

Top-Level Domain (TLD) Server
  • the top-level server sits between the Root Server and Authoritative Server
  • top-level domain types:
    • Generic Top-Level Domains (gTLD) - e.g. com, info, net, edu, org, etc
    • Country Code Top-Level Domains (ccTLD) - e.g. au, de, fi, fr, jp, kr, nl, se, etc

Authoritative Name Server

Name Server (NS)
  • the Authoritative Server is the last stop in the name server query—it takes the hostname and returns the correct IP address to the DNS Resolver (or if it cannot find the domain, returns the message NXDOMAIN for non-existent domain)

How a DNS Query Resolves

The DNS Resolver is pre-configured with a set of 13 root server IP addresses which are permanently assigned. The resolving process for “example.com” would be as follows:

  1. DNS Resolver randomly selects one root server from the pre-configured root servers list and queries it.
  2. the Root Servers only maintain list of top level domain (TLD) with NS records. So, the response for the query will list out NS records for “com” TLD.
  3. the Resolver will read the NS records from the response and randomly pick one and repeat the same query.
  4. The “com” TLD server maintains list of NS records for the next level i.e. “example.com” and returns a new set of NS records which host the “example.com” zone.
  5. The resolver will then select a NS record and repeat the same query.
  6. The “example.com” DNS server which is hosting the zone will have the IP address configured for the zone as A record and will return the record as the final response.

Server Implementations

DNS Domain Servers Hierarchy

DNS Query Types

Recursive DNS Query
Iterative DNS Query

for more details of DNS queries