🌐 Day 9: Domain Name System (DNS)

The Domain Name System (DNS) is a distributed hierarchical system that resolves domain names into IP addresses. 🌐

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Humans remember:
google.com

Machines understand:
142.250.190.14

DNS translates names into numeric IP addresses so computers can communicate.

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🔎 How DNS Resolution Works

Suppose you search for:
google.com

Here’s what happens step by step:

1️⃣ Client Request

• The browser acts as the client.
• It passes the request to the Stub Resolver (part of the OS DNS client service).

Flow:
Browser → OS Stub Resolver

The stub resolver is part of the operating system, not the browser.

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2️⃣ Local Lookup (Before Internet Query)

The stub resolver checks locally in this order:

1. Browser DNS Cache
2. OS DNS Cache
3. Hosts File
   • Windows: C:\Windows\System32\drivers\etc\hosts
   • Linux: /etc/hosts

If the IP is found → resolution stops.
If not found → query is forwarded to a Recursive DNS Server, such as:

• Google Public DNS (8.8.8.8)
• Cloudflare (1.1.1.1)
• Or your ISP DNS

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3️⃣ Recursive DNS Server

The first external DNS server contacted is called the:

Recursive Resolver
Its job is to fully resolve the domain.

It first checks:

• Its own DNS cache

If found → returns IP.
If not → it begins querying the DNS hierarchy.

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4️⃣ Root DNS Servers

There are 13 named root server systems (A–M).
They are globally distributed using Anycast and coordinated by:

• ICANN

When asked:

“Where is google.com?”

The root server replies:

“I don’t know google.com, but here are the name servers for .com.”

It returns NS (Name Server) records for the .com TLD.

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🔹 What is a TLD?

TLD = Top Level Domain

Examples:

• .com
• .org
• .pk
• .io

It is the last portion of a domain name.

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🔹 Manual Root Query Example

You can manually query a root server:

nslookup -type=NS com. m.root-servers.net

This asks:

“Who manages the .com TLD?”

The root server returns TLD name servers like:

a.gtld-servers.net
b.gtld-servers.net
...

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5️⃣ TLD Name Servers (.com Servers)

The recursive resolver contacts one of the .com TLD servers and asks:

“Who is authoritative for google.com?”

The TLD server returns the Authoritative Name Servers for that domain.

Example:

nslookup -type=NS chatgpt.com d.gtld-servers.net

This shows which servers manage chatgpt.com.

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6️⃣ Authoritative Name Servers (Final Step)

Now the recursive resolver asks one of the authoritative servers:

“What is the A record (IP address) for google.com?”

The authoritative server replies with the final IP address.

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7️⃣ Caching and TTL

When the recursive resolver receives the IP:

• It stores it in cache
• For a duration defined by TTL (Time To Live)

Note: DNS TTL is different from packet TTL.

Then:

• Stub resolver caches it
• Browser caches it

Future lookups are faster until TTL expires.

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🧠 Final DNS Flow Summary

1. Browser requests google.com
2. Stub Resolver checks:
   • Browser cache
   • OS cache
   • Hosts file
3. Stub Resolver queries Recursive DNS Server
4. Recursive Server checks its cache
5. If not found:
   • Queries Root Server
   • Gets TLD (.com) servers
6. Queries TLD Server
   • Gets Authoritative Name Servers
7. Queries Authoritative Server
   • Gets IP address
8. IP returned to Browser
9. All layers cache the result using TTL

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