DNS and how domain resolution works in HLD - Architecture Diagram

System Overview - DNS and how domain resolution works

The Domain Name System (DNS) translates human-friendly domain names into IP addresses that computers use to identify each other on the network. It is a distributed system designed to quickly and reliably resolve domain names to IP addresses, enabling users to access websites and services by name instead of numbers.

Architecture Diagram

User
  |
  v
Local DNS Resolver
  |
  v
Root DNS Server
  |
  v
TLD DNS Server
  |
  v
Authoritative DNS Server
  |
  v
Target Website Server

Components

User

client

Initiates domain name lookup requests

Local DNS Resolver

resolver

Caches and forwards DNS queries on behalf of the user

Root DNS Server

dns_server

Directs queries to the appropriate Top-Level Domain (TLD) servers

TLD DNS Server

dns_server

Directs queries to the authoritative DNS server for the domain

Authoritative DNS Server

dns_server

Provides the IP address for the requested domain

Target Website Server

web_server

Hosts the website or service the user wants to access

Request Flow - 9 Hops

User → Local DNS Resolver

Local DNS Resolver → Root DNS Server

Root DNS Server → Local DNS Resolver

Local DNS Resolver → TLD DNS Server

TLD DNS Server → Local DNS Resolver

Local DNS Resolver → Authoritative DNS Server

Authoritative DNS Server → Local DNS Resolver

Local DNS Resolver → User

User → Target Website Server

Failure Scenario

Component Fails:Authoritative DNS Server

Impact:Domain IP address cannot be resolved, users cannot reach the website

Mitigation:Use multiple authoritative servers with failover and DNS caching at resolvers to reduce impact

Architecture Quiz - 3 Questions

Which component directs the resolver to the correct Top-Level Domain server?

A Local DNS Resolver

B Root DNS Server

C Authoritative DNS Server

D Target Website Server

Design Principle

DNS uses a hierarchical and distributed design to efficiently resolve domain names by delegating responsibility across multiple server types, combined with caching to reduce latency and load.

Practice

1. What is the main purpose of DNS in internet communication?

easy

A. To translate domain names into IP addresses

B. To store website content

C. To encrypt internet traffic

D. To manage email delivery

Solution

1. Step 1: Understand DNS role

   DNS stands for Domain Name System and its main job is to convert human-friendly domain names into IP addresses computers use.

2. Step 2: Identify correct function

   Among the options, only translating domain names to IP addresses matches DNS's core function.

3. Final Answer:

   To translate domain names into IP addresses -> Option A

4. Quick Check:

   DNS = domain to IP translation [OK]

Hint: DNS converts names to IPs, not content or encryption [OK]

Common Mistakes:

• Confusing DNS with web hosting
• Thinking DNS encrypts data
• Mixing DNS with email servers

2. Which of the following is the correct order of DNS resolution steps when a user types a domain name?

easy

A. Root server -> TLD server -> Authoritative server

B. Authoritative server -> Root server -> TLD server

C. TLD server -> Root server -> Authoritative server

D. Local cache -> Authoritative server -> Root server

Solution

1. Step 1: Recall DNS resolution hierarchy

   DNS resolution starts at the root server, then goes to the TLD (Top-Level Domain) server, and finally to the authoritative server for the domain.

2. Step 2: Match the correct sequence

   Root server -> TLD server -> Authoritative server correctly lists the order: Root -> TLD -> Authoritative.

3. Final Answer:

   Root server -> TLD server -> Authoritative server -> Option A

4. Quick Check:

   Resolution order = Root, TLD, Authoritative [OK]

Hint: DNS starts at root, then TLD, then authoritative [OK]

Common Mistakes:

• Mixing up the order of servers
• Starting resolution at authoritative server
• Ignoring the root server step

3. Consider this simplified DNS resolution code snippet:

cache = {"example.com": "93.184.216.34"}

query = "example.com"

if query in cache:
    ip = cache[query]
else:
    ip = "198.51.100.1"  # IP from authoritative server

print(ip)

What will be the output when this code runs?

medium

A. 198.51.100.1

B. example.com

C. 93.184.216.34

D. KeyError

Solution

1. Step 1: Check if query is in cache

   The query is "example.com" which exists in the cache dictionary with IP "93.184.216.34".

2. Step 2: Determine printed IP

   Since the query is found in cache, the code assigns ip = cache[query] which is "93.184.216.34" and prints it.

3. Final Answer:

   93.184.216.34 -> Option C

4. Quick Check:

   Cache hit returns cached IP [OK]

Hint: If domain in cache, output cached IP [OK]

Common Mistakes:

• Assuming authoritative IP prints always
• Confusing key and value in dictionary
• Expecting error if key exists

4. A DNS resolver is not returning the correct IP address for a domain. Which of the following is the most likely cause?

medium

A. The domain name is misspelled in the URL

B. The root server is unreachable but TLD server works

C. The authoritative server is down but cache is fresh

D. The local DNS cache has outdated information

Solution

1. Step 1: Analyze DNS caching impact

   If the local DNS cache holds old IP data, the resolver returns outdated IP even if authoritative server changed.

2. Step 2: Evaluate other options

   Misspelled domain would cause failure, not wrong IP. Authoritative down with fresh cache returns correct IP. Root unreachable but TLD working is unlikely scenario causing wrong IP.

3. Final Answer:

   The local DNS cache has outdated information -> Option D

4. Quick Check:

   Outdated cache causes wrong IP [OK]

Hint: Old cache causes wrong IP, refresh fixes it [OK]

Common Mistakes:

• Ignoring cache as source of error
• Assuming authoritative server always reachable
• Confusing root and TLD server roles

5. You want to design a scalable DNS system for a global service. Which approach best improves DNS resolution speed and reduces load on authoritative servers?

hard

A. Use a single global authoritative server for all domains

B. Implement multiple caching layers including local resolver and CDN caches

C. Direct all DNS queries only to the authoritative servers

D. Disable caching to always get fresh IP addresses

Solution

1. Step 1: Understand scalability needs

   To handle many users globally, caching at multiple layers reduces repeated queries to authoritative servers and speeds up responses.

2. Step 2: Evaluate options for load and speed

   Direct queries to authoritative servers increase load and latency. Single global server is a bottleneck and risk. Disabling caching slows resolution and increases load.

3. Final Answer:

   Implement multiple caching layers including local resolver and CDN caches -> Option B

4. Quick Check:

   Multi-layer caching improves speed and scalability [OK]

Hint: Use caching layers to scale DNS efficiently [OK]

Common Mistakes:

• Ignoring caching benefits
• Centralizing authoritative servers causing bottlenecks
• Disabling caching for freshness without tradeoff