DNS and how domain resolution works in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the first step in DNS resolution.

HLD

When a user types a URL, the browser first sends a request to the [1] to find the IP address.

Drag options to blanks, or click blank then click option'

Aauthoritative DNS server

Broot DNS server

Clocal DNS resolver

Dweb server

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the root DNS server with the local resolver.

The local DNS resolver is the first point of contact that the browser queries to resolve a domain name into an IP address.

2fill in blank

medium

Complete the code to show which server the local resolver queries if it has no cached record.

HLD

If the local DNS resolver does not have the IP cached, it queries the [1] to continue the resolution process.

Drag options to blanks, or click blank then click option'

Aauthoritative DNS server

Bclient browser

Cweb server

Droot DNS server

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing authoritative DNS server too early in the process.

The local resolver queries the root DNS server to find the next step in the domain name resolution hierarchy.

3fill in blank

hard

Fix the error in the DNS resolution step where the resolver queries the wrong server.

HLD

After contacting the root DNS server, the resolver should query the [1] to find the domain's IP address.

Drag options to blanks, or click blank then click option'

Aauthoritative DNS server

Bclient browser

Clocal DNS resolver

Dweb server

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the local resolver with the authoritative server.

The resolver queries the authoritative DNS server for the domain to get the exact IP address.

4fill in blank

hard

Fill both blanks to complete the DNS resolution flow from root to domain.

HLD

The root DNS server directs the resolver to the [1] which then points to the [2] for the domain's IP.

Drag options to blanks, or click blank then click option'

Atop-level domain (TLD) server

Blocal DNS resolver

Cauthoritative DNS server

Dclient browser

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the roles of TLD and authoritative servers.

The root server points to the TLD server, which then directs to the authoritative DNS server that holds the domain's IP.

5fill in blank

hard

Fill all three blanks to complete the DNS caching and resolution process.

HLD

Once the authoritative DNS server returns the IP, the [1] caches it, the [2] uses it to connect, and the [3] displays the website.

Drag options to blanks, or click blank then click option'

Alocal DNS resolver

Bclient browser

Cweb server

Droot DNS server

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the roles of the root server and local resolver.

The local DNS resolver caches the IP for faster future queries, the client browser uses the IP to connect, and the web server displays the website content.

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