DNS Deep Dive — Beginner Friendly System Design Guide

What Happens When You Type Google.com?

When you type:

google.com

your browser does NOT understand:

• google.com
• swiggy.com
• youtube.com

Computers communicate using:

IP Addresses

like:

142.250.xxx.xxx

So before opening website, internet must first answer:

"What is the IP address of this domain?"

This entire process is called:

DNS Resolution

---

What Is DNS?

DNS stands for:

Domain Name System

It works like:

• internet phonebook
• contact list for websites

---

Real-Life Analogy

Imagine you want to call friend.

You search:

Rahul

inside contacts.

Phone internally uses:

• actual phone number

Similarly:

• humans use domain names
• computers use IP addresses

DNS connects both worlds.

---

Why DNS Exists

Without DNS, users would need to remember:

172.217.160.142

instead of:

google.com

Impossible at internet scale.

---

Real Example — Opening Swiggy

Suppose you open:

swiggy.com

Before page loads:

• browser performs DNS lookup

to find Swiggy server IP.

---

High-Level DNS Flow

User Types swiggy.com
        │
        ▼
Browser Checks Local Cache
        │
        ▼
DNS Resolver Asked
        │
        ▼
Root DNS Server
        │
        ▼
TLD DNS Server (.com)
        │
        ▼
Authoritative DNS Server
        │
        ▼
Returns IP Address
        │
        ▼
Browser Connects To Server

---

Step 1 — Browser Cache Check

Browser first checks:

"Do I already know this IP?"

If recently visited:

• cached result used
• much faster

---

Browser Cache Flow

User Opens Website
       │
       ▼
Browser Cache Check
   │             │
Found          Not Found
   │              │
   ▼              ▼
Use Cached    Ask DNS Server
IP

---

Step 2 — Operating System Cache

If browser doesn't know:

• OS checks its own DNS cache

Modern operating systems cache DNS responses.

---

Step 3 — DNS Resolver

If cache miss happens:

request goes to:

DNS Resolver

Usually provided by:

• ISP
• Google DNS
• Cloudflare DNS

---

Popular DNS Providers

Provider	DNS IP
Google DNS	8.8.8.8
Cloudflare	1.1.1.1
OpenDNS	208.67.222.222

---

Step 4 — Root DNS Server

Resolver asks:

"Who handles .com domains?"

Root server responds:

• ask .com TLD server

---

DNS Hierarchy

Root DNS
   │
   ▼
TLD DNS (.com)
   │
   ▼
Authoritative DNS
   │
   ▼
Website IP Returned

---

Step 5 — TLD Server

TLD means:

Top Level Domain

Examples:

• .com
• .in
• .org

TLD server says:

"swiggy.com is managed by this DNS server"

---

Step 6 — Authoritative DNS Server

Final DNS server contains actual records.

Example:

swiggy.com → 13.xxx.xxx.xxx

IP returned to browser.

---

Step 7 — Browser Connects To Server

Now browser knows:

• exact server IP

Browser can:

• establish TCP connection
• send HTTP request

---

Complete Real DNS Resolution Flow

User Types swiggy.com
        │
        ▼
Browser Cache
        │
        ▼
OS Cache
        │
        ▼
DNS Resolver
        │
        ▼
Root DNS Server
        │
        ▼
.com TLD Server
        │
        ▼
Authoritative DNS Server
        │
        ▼
Returns IP Address
        │
        ▼
Browser Connects To Swiggy Server

---

What Is An IP Address?

IP address identifies device/server on internet.

Like:

• home address for computers

---

Example

142.250.190.46

---

IPv4 vs IPv6

---

IPv4

Traditional format:

192.168.1.1

Limited addresses.

---

IPv6

New format:

2409:40f0:1021:...

Supports huge internet growth.

---

DNS Records

DNS stores multiple record types.

---

Common DNS Records

Record	Purpose
A	Domain → IPv4
AAAA	Domain → IPv6
CNAME	Alias
MX	Mail server
TXT	Verification/security

---

Example DNS Record

swiggy.com → 13.xxx.xxx.xxx

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What Is TTL?

TTL means:

Time To Live

It defines:

• how long DNS result stays cached

---

Example

TTL = 300 seconds

DNS cached for:

• 5 minutes

---

Why Caching Matters

Without caching:

• every website visit requires full DNS lookup

Internet would become slower.

Caching improves:

• speed
• scalability
• latency

---

DNS Cache Flow

First Request
     │
     ▼
DNS Lookup Performed
     │
     ▼
Result Cached
     │
     ▼
Future Requests Faster

---

Real Example — Hotstar During IPL

During IPL:

• crores of users open Hotstar simultaneously

DNS helps:

• distribute traffic
• route users
• reduce latency

Without DNS scalability:

• platform may fail.

---

DNS And Load Balancing

DNS can distribute traffic across regions.

---

Example

Indian users:

• routed to Mumbai servers

US users:

• routed to US servers

---

Geo Routing Flow

User Location
      │
      ▼
DNS Detects Region
      │
 ┌────┼────┐
 ▼         ▼
India     USA
Server    Server

---

DNS And CDN

CDNs heavily depend on DNS.

DNS routes users to:

• nearest CDN edge server

---

CDN Routing Example

Jaipur User
     │
     ▼
DNS Resolution
     │
     ▼
Nearest CDN Server Selected

---

Why DNS Sometimes Feels Slow

Slow DNS can delay:

• page loading
• API requests
• app startup

---

Possible DNS Problems

Problem	Impact
Slow resolver	High latency
DNS outage	Website inaccessible
Cache miss	Extra lookup time
Wrong records	Traffic routing failure

---

Real Example — Internet Outages

Large outages sometimes happen because:

• DNS misconfiguration
• DNS provider failure

Even if servers are healthy:

• users cannot find them

---

DNS And Security

DNS attacks are common.

---

Examples

Attack	Meaning
DNS Spoofing	Fake DNS response
DDoS	Overload DNS servers
Cache Poisoning	Corrupt cached records

---

Why Cloudflare Became Popular

Cloudflare provides:

• fast DNS
• DDoS protection
• CDN
• security

---

DNS Over UDP

Most DNS requests use:

UDP

because:

• lightweight
• faster
• tiny packets

---

DNS Protocol Flow

Browser
   │
   ▼
DNS Request (UDP)
   │
   ▼
DNS Resolver
   │
   ▼
IP Address Returned

---

Real Internet Journey Example

User Opens Swiggy
        │
        ▼
DNS Resolves Domain
        │
        ▼
Browser Gets IP Address
        │
        ▼
TCP Connection Established
        │
        ▼
HTTPS Handshake
        │
        ▼
HTTP Request Sent
        │
        ▼
Swiggy Server Responds

---

Common Beginner Misconceptions

---

"DNS Is Just Small Internet Feature"

No.

Without DNS:

• internet usability collapses

---

"DNS Lookup Happens Every Time"

Not always.

Caching avoids repeated lookups.

---

"Fast Internet Means Fast DNS"

Not necessarily.

Slow DNS can make:

• fast internet feel slow

---

Final Mental Model

DNS =
Internet's phonebook
that converts domain names into IP addresses

---

Final Architecture Diagram

User
  │
  ▼
Browser
  │
  ▼
DNS Resolver
  │
  ▼
Root DNS
  │
  ▼
TLD DNS
  │
  ▼
Authoritative DNS
  │
  ▼
IP Address Returned
  │
  ▼
Website Server

---

One-Line Interview Definition

DNS (Domain Name System) is a distributed hierarchical system that translates human-readable domain names into IP addresses so clients can locate servers across the internet.