What is DNS and the DNS Hierarchy?

DNS (Domain Name System) is a hierarchical and decentralized naming system for computers, services, or other resources connected to the internet or a private network. It translates human-readable domain names like www.example.com into IP addresses like 192.0.2.1 that computers use to identify each other on the network. DNS acts as the internet’s phonebook, helping users access websites and services using domain names rather than remembering numeric IP addresses.

Key Functions of DNS

• Name Resolution: Converts domain names into IP addresses so that web browsers can load internet resources.
• Email Routing: Uses DNS records like MX (Mail Exchange) records to route emails to the correct mail servers.
• Load Distribution: Uses CNAME and SRV records for load balancing and distributing traffic to different servers.
• Service Discovery: Helps applications find services like web servers and email servers based on DNS entries.

How DNS Works

When you type a URL into a web browser, the DNS system goes through several steps to resolve the domain name into an IP address:

1. Query Initiation: A user enters a domain name (e.g., www.example.com) in a web browser.
2. Recursive Query: The query is sent to a recursive DNS resolver, often provided by your ISP or set manually (e.g., Google Public DNS).
3. DNS Hierarchy: The recursive resolver queries the DNS hierarchy, starting from the root servers, then TLD (Top-Level Domain) servers, and finally authoritative name servers until it finds the IP address for the requested domain.
4. Caching: The resolved IP address is cached by the resolver and potentially by the user’s browser to speed up future requests.

DNS Hierarchy: Structure and Components

The DNS is organized in a hierarchical tree structure with different levels, each playing a specific role in translating domain names into IP addresses. Here’s a breakdown of the DNS hierarchy:

1. Root Level (Root DNS Servers)

• Description: The root level is the top of the DNS hierarchy. It is represented as a dot (.), and it holds information about the top-level domains (TLDs).
• Root DNS Servers: There are 13 sets of root servers, identified by letters (e.g., A-root, B-root, C-root), which are managed by different organizations worldwide. These servers do not contain the IP addresses for websites but direct queries to the relevant TLD servers.
• Function: When a DNS resolver cannot find an IP address in its cache, it queries the root DNS servers. The root servers respond with the appropriate TLD server’s address for the domain (e.g., .com, .org).

2. Top-Level Domain (TLD) Level

• Description: TLDs are the second level of the DNS hierarchy, and they categorize domains based on their suffix (e.g., .com, .net, .org, .uk, .de). TLD servers hold information about domains under their specific TLD.
• Types of TLDs:
  • gTLD (Generic Top-Level Domains): Includes .com, .org, .net, .edu, and newer TLDs like .app or .tech.
  • ccTLD (Country Code Top-Level Domains): Represents countries or regions, such as .uk (United Kingdom), .us (United States), or .jp (Japan).
• Function: TLD servers direct the DNS resolver to the authoritative name servers responsible for the domain name (e.g., example.com).

3. Second-Level Domain (SLD) and Subdomains

• Description: The second-level domain (SLD) is the part of a domain name that appears directly to the left of the TLD (e.g., example in example.com). SLDs are often managed by organizations or individuals who register them.
• Subdomains: Subdomains are prefixes added to a second-level domain (e.g., www in www.example.com or blog.example.com). Subdomains allow for organizing different sections of a website or service.
• Function: Second-level domains and their subdomains point to authoritative name servers that hold the specific IP address or other DNS records for a domain.

4. Authoritative DNS Servers

• Description: Authoritative DNS servers are the final stop in the DNS query process. They provide the definitive answers to queries about domain names by storing DNS records for domains.
• Types of Records:
  • A Record: Maps a domain name to an IPv4 address.
  • AAAA Record: Maps a domain name to an IPv6 address.
  • CNAME Record: Points a domain or subdomain to another domain.
  • MX Record: Specifies mail servers for email routing.
  • TXT Record: Holds text information for verification and security (e.g., SPF, DKIM).
• Function: When the recursive DNS resolver queries the authoritative DNS server, it returns the corresponding IP address or other DNS records for the requested domain.

5. Recursive DNS Resolvers (Not Part of the Hierarchy but Essential)

• Description: Recursive resolvers are responsible for querying the entire DNS hierarchy on behalf of the client. They are often provided by ISPs or can be set to third-party DNS providers like Google DNS (8.8.8.8) or Cloudflare DNS (1.1.1.1).
• Function: The recursive resolver caches responses to speed up future requests. If a domain’s IP address is cached, the resolver can return the result without querying the DNS hierarchy again.

Example: How a DNS Query Works

Suppose a user types www.example.com into a browser:

1. The recursive DNS resolver checks if www.example.com is in its cache.
2. If not cached, it queries one of the root DNS servers.
3. The root server responds with the address of the .com TLD server.
4. The resolver queries the .com TLD server, which responds with the address of the authoritative name server for example.com.
5. The resolver queries the authoritative name server, which provides the A record (IP address) for www.example.com.
6. The resolver returns the IP address to the browser, which then connects to the web server at that IP address.

Conclusion

DNS is a fundamental part of the internet, allowing users to access resources using human-readable domain names. Understanding the DNS hierarchy—from the root servers down to authoritative DNS servers—helps us appreciate how web traffic is directed and how services like load balancing and failover work. It also aids in troubleshooting issues like domain resolution failures or incorrect DNS configurations.