Networking - Domain Name System

1. What is DNS?

DNS (Domain Name System) is like the phonebook of the internet.
It translates human-friendly domain names (like www.google.com) into IP addresses (like 142.250.190.78) that computers use to identify each other on a network.

  • Humans → Remember domain names.

  • Computers → Communicate using IP addresses.

  • DNS bridges the gap between them.

Example:
When you type www.amazon.com in your browser:

  • DNS translates it into an IP address → 176.32.103.205.

  • Your browser uses that IP to fetch the website.

2. Why DNS is Important

Without DNS, you would need to remember numerical IP addresses for every website.
For example:

  • Instead of typing https://172.217.5.110, you simply type www.google.com.

3. How DNS Works (Step-by-Step Process)

Let’s understand the flow when you open a website like www.example.com:

Step 1 – DNS Query Initiation

You type www.example.com in your browser.

Step 2 – Check Local Cache

Your computer first checks:

  • Browser cache

  • Operating system cache

  • If the IP is found, it connects directly.

Step 3 – Query Recursive DNS Resolver

If not found locally, the request goes to your ISP's DNS resolver.

Step 4 – Contact Root DNS Server

If the resolver doesn’t know the IP, it asks the Root DNS Server where to find it.

Step 5 – TLD DNS Server

The root server directs the resolver to a TLD (Top-Level Domain) DNS Server based on the domain extension (.com, .org, .net, etc.).

Step 6 – Authoritative DNS Server

The TLD server sends the resolver to the Authoritative DNS Server, which holds the actual IP address for www.example.com.

Step 7 – Return the IP Address

The authoritative server replies with the IP → 93.184.216.34.

Step 8 – Connect to the Website

The resolver sends the IP back to your browser, which connects to the web server and loads the website.

4. DNS Hierarchy & Architecture

DNS follows a tree-structured hierarchy:

                [Root DNS Servers]
                       ↓
          [.com] [.org] [.net] [.in] [etc.]
                       ↓
          [Authoritative Name Servers]
                       ↓
          [Domain Names → IP Mapping]

  • Root Servers → At the top, direct queries to TLD servers.

  • TLD Servers → Manage domain extensions (.com, .org, .net, etc.).

  • Authoritative Servers → Store actual IP addresses.

5. Components of DNS

  1. Domain Names → Human-readable website names (e.g., google.com).

  2. IP Addresses → Machine-readable identifiers (e.g., 142.250.190.78).

  3. DNS Zones → Sections of DNS namespace managed by an organization.

  4. DNS Records → Stored in authoritative servers, include:

    • A Record → Maps domain → IPv4 address.

    • AAAA Record → Maps domain → IPv6 address.

    • CNAME Record → Redirects one domain to another.

    • MX Record → Handles mail server info.

    • TXT Record → Stores security and verification data.

  5. Resolvers → Perform lookups to find IP addresses.

  6. Authoritative Servers → Contain final IP mappings.

6. Types of DNS Servers

a) Recursive DNS Resolver

  • Provided by your ISP or Google DNS (8.8.8.8).

  • Finds the IP address on behalf of your browser.

b) Root DNS Servers

  • Top of the DNS hierarchy.

  • 13 main clusters worldwide.

  • Example: A.ROOT-SERVERS.NET.

c) TLD DNS Servers

  • Handle top-level domains like .com, .org, .net.

  • Example: For amazon.com, .com TLD server provides info.

d) Authoritative DNS Servers

  • Final source of truth.

  • Contains the actual A record for a domain.

7. Types of DNS Queries

  1. Recursive Query → The resolver fetches the full answer from start to finish.

  2. Iterative Query → The resolver provides the next best server instead of the final answer.

  3. Non-Recursive Query → The resolver already knows the answer from its cache.

8. DNS Example in Action

Scenario: You open www.flipkart.com

  • Browser asks local DNS cache → Not found.

  • Sends request to ISP DNS Resolver.

  • Resolver contacts Root Server → Directs to .com TLD server.

  • TLD server directs to Flipkart's Authoritative DNS Server.

  • Authoritative server returns IP → 163.53.78.54.

  • Browser connects to Flipkart's server → Website loads.

9. DNS Example Records for google.com

Record Type Host Value Purpose
A google.com 142.250.190.78 IPv4 address mapping
AAAA google.com 2607:f8b0:4005:80a::200e IPv6 address mapping
MX google.com smtp.google.com Mail server
CNAME www.google.com google.com Redirect alias
TXT google.com v=spf1 include:_spf.google.com Email verification

10. Advantages of DNS

  • User-friendly → No need to memorize IPs.

  • Scalable → Supports billions of domain names.

  • Faster browsing → Uses caching for speed.

  • Reliable → Distributed architecture prevents failures.

  • Supports load balancing → Directs users to nearest servers.

11. Disadvantages of DNS

  • Security risks → Vulnerable to DNS spoofing & DDoS attacks.

  • Centralization issues → Root servers are limited.

  • Caching delays → DNS changes take time to propagate.

  • Dependency → If DNS fails, websites become unreachable.

12. Popular Public DNS Providers

Provider Primary DNS Secondary DNS
Google DNS 8.8.8.8 8.8.4.4
Cloudflare DNS 1.1.1.1 1.0.0.1
OpenDNS 208.67.222.222 208.67.220.220
Quad9 9.9.9.9 149.112.112.112

13. Real-Life Example: Netflix DNS

When you open netflix.com:

  • DNS resolves netflix.com → 52.26.14.22.

  • Netflix uses CDN (Content Delivery Network).

  • DNS automatically routes you to the nearest Netflix server for faster streaming.

14. Summary

  • DNS = Internet’s address book → Converts domain names into IP addresses.

  • Uses a hierarchical structure → Root → TLD → Authoritative.

  • Involves DNS records like A, MX, CNAME, TXT.

  • Used in browsing, emails, video streaming, and cloud services.