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• 🌐 DNS Resolution Flow (Complete Explanation)
  • 1️⃣ What is DNS Resolution?
    • Example:
  • 2️⃣ Why DNS Resolution is Required
  • 3️⃣ High-Level DNS Resolution Flow
  • 4️⃣ Step-by-Step DNS Resolution Flow (Detailed)
    • 🔵 Step 1: Browser Cache Check
    • 🔵 Step 2: Operating System (OS) Cache
    • 🔵 Step 3: Hosts File Check
    • 🔵 Step 4: Recursive DNS Resolver
  • 5️⃣ Recursive Resolver Detailed Flow
    • 🟠 Step 5.1: Query Root Name Server
    • 🟠 Step 5.2: Query TLD Name Server (.com)
    • 🟠 Step 5.3: Query Authoritative Name Server
  • 6️⃣ Step 6: Response Sent Back
  • 7️⃣ Step 7: Caching (Very Important)
  • 8️⃣ Protocols Used in DNS Resolution
    • 🔹 Transport Protocol
    • 🔹 Application Protocol
  • 9️⃣ Types of DNS Queries
    • 1️⃣ Recursive Query
    • 2️⃣ Iterative Query
    • 3️⃣ Non-Recursive Query
  • 🔟 Common DNS Record Types (Exam Important)
  • 1️⃣1️⃣ DNS Resolution in AWS (Route 53)
    • Example:
  • 1️⃣2️⃣ What Happens After DNS Resolution?
  • 1️⃣3️⃣ Common DNS Failures (Troubleshooting)
  • 1️⃣4️⃣ Real-World Analogy
  • 1️⃣5️⃣ One-Line Exam Definition
  • 1️⃣6️⃣ Super Short Summary

🌐 DNS Resolution Flow (Complete Explanation)

1️⃣ What is DNS Resolution?

DNS resolution is the process of converting a human-readable domain name into an IP address.

Example:

www.google.com → 142.250.195.14

Computers understand IP addresses, not domain names.

2️⃣ Why DNS Resolution is Required

• Humans remember names
• Computers route using IPs
• DNS acts like the internet’s phonebook

Without DNS:

• You would need to remember IPs for every website

3️⃣ High-Level DNS Resolution Flow

Browser
 ↓
OS Cache
 ↓
Recursive Resolver (ISP / Public DNS)
 ↓
Root Name Server
 ↓
TLD Name Server (.com)
 ↓
Authoritative Name Server
 ↓
IP Address returned

4️⃣ Step-by-Step DNS Resolution Flow (Detailed)

Let’s assume you type:

www.example.com

🔵 Step 1: Browser Cache Check

Your browser first checks:

• Has this domain been resolved recently?

If yes: ✔ IP returned immediately ✔ DNS process stops

If no: ➡ Move to next step

🔵 Step 2: Operating System (OS) Cache

The OS checks:

• Local DNS cache

Command example (Linux):

nscd

If IP found: ✔ Returned to browser

If not: ➡ Next step

🔵 Step 3: Hosts File Check

The OS checks:

• /etc/hosts (Linux/macOS)
• C:\Windows\System32\drivers\etc\hosts (Windows)

Example entry:

127.0.0.1 mysite.local

If match found: ✔ IP returned

🔵 Step 4: Recursive DNS Resolver

If still unresolved:

• Request goes to Recursive Resolver

Usually:

• ISP DNS

• Or public DNS like:

  • Google (8.8.8.8)
  • Cloudflare (1.1.1.1)

The resolver now takes responsibility to find the IP.

5️⃣ Recursive Resolver Detailed Flow

The recursive resolver performs iterative queries.

🟠 Step 5.1: Query Root Name Server

Resolver asks:

Who knows about .com?

Root server replies:

Ask .com TLD servers

📌 Root servers do NOT know IP addresses They only direct to TLD servers

🟠 Step 5.2: Query TLD Name Server (.com)

Resolver asks:

Who knows example.com?

TLD server replies:

Ask example.com authoritative server

🟠 Step 5.3: Query Authoritative Name Server

Resolver asks:

What is IP of www.example.com?

Authoritative server replies:

142.250.195.14

✔ This server is the final authority

6️⃣ Step 6: Response Sent Back

The IP address flows back:

Authoritative → TLD → Resolver → OS → Browser

7️⃣ Step 7: Caching (Very Important)

At multiple levels:

• Recursive resolver
• OS
• Browser

Caching is based on:

TTL (Time To Live)

Example:

TTL = 300 seconds

After TTL expires:

• DNS resolution happens again

8️⃣ Protocols Used in DNS Resolution

🔹 Transport Protocol

• UDP 53 (default)
• TCP 53 (large responses / DNSSEC)

🔹 Application Protocol

• DNS

9️⃣ Types of DNS Queries

1️⃣ Recursive Query

• Client → Resolver
• Resolver does all the work

2️⃣ Iterative Query

• Resolver → Root / TLD / Authoritative

3️⃣ Non-Recursive Query

• Cached response

🔟 Common DNS Record Types (Exam Important)

1️⃣1️⃣ DNS Resolution in AWS (Route 53)

Example:

example.com hosted in Route 53

Flow:

• Resolver queries Route 53 authoritative servers
• Route 53 returns IP (EC2 / ALB / CloudFront)

Special features:

• Latency-based routing
• Health checks
• Failover

1️⃣2️⃣ What Happens After DNS Resolution?

Once IP is obtained:

1. Browser opens TCP connection
2. HTTP / HTTPS request starts
3. Website loads

DNS is only the first step.

1️⃣3️⃣ Common DNS Failures (Troubleshooting)

1️⃣4️⃣ Real-World Analogy

DNS is like:

• Asking a receptionist:

  “Where does John sit?”

• She checks:

  • Internal list
  • Department
  • Manager

• Finally gives desk number

1️⃣5️⃣ One-Line Exam Definition

DNS resolution is the process of translating a domain name into an IP address using a hierarchical, distributed name server system.

1️⃣6️⃣ Super Short Summary

• Browser checks cache
• Resolver does lookup
• Root → TLD → Authoritative
• IP returned
• Cached for speed