DNS and how domain resolution works in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary role of a DNS resolver in domain name resolution?

In the process of resolving a domain name to an IP address, what does the DNS resolver do?

A It stores the website content and serves it to the user after resolution.

B It translates the domain name into an IP address by querying DNS servers on behalf of the client.

C It assigns IP addresses to devices within a local network.

D It encrypts the domain name to secure the connection between client and server.

💡 Hint

Think about the component that acts as a middleman between your device and DNS servers.

The DNS resolver acts as an intermediary that takes the domain name from the client and queries DNS servers to find the corresponding IP address.

❓ Architecture

intermediate

Which component is responsible for storing authoritative DNS records for a domain?

In the DNS hierarchy, which server holds the definitive information about a domain's IP address?

A Authoritative DNS server

B Root DNS server

C Recursive resolver

D Caching DNS server

💡 Hint

This server provides the final answer for a domain's IP address.

The authoritative DNS server holds the official DNS records for a domain and provides the final IP address answer.

❓ scaling

intermediate

How does DNS caching improve the scalability of domain resolution?

Consider a large number of users requesting the same domain name. How does DNS caching help handle this load efficiently?

A By increasing the number of root DNS servers to handle more requests.

B By assigning new IP addresses dynamically to reduce server load.

C By encrypting DNS queries to prevent unauthorized access.

D By storing previously resolved IP addresses locally to reduce repeated queries to authoritative servers.

💡 Hint

Think about how repeated requests for the same domain can be answered faster.

DNS caching stores resolved IP addresses temporarily so that repeated requests do not need to query authoritative servers again, reducing load and latency.

❓ tradeoff

advanced

What is a tradeoff when setting a very long TTL (Time To Live) for DNS records?

If a DNS record has a very long TTL, what is a potential downside?

A Changes to the domain's IP address take longer to propagate, causing outdated information to be used.

B It increases the number of DNS queries to authoritative servers.

C It causes DNS resolvers to ignore the record completely.

D It reduces the security of DNS queries by exposing them longer.

💡 Hint

Think about how long cached information stays valid before updating.

A long TTL means DNS records are cached longer, so if the IP address changes, clients may still use the old IP until the cache expires.

❓ estimation

expert

Estimate the number of DNS queries a recursive resolver handles per second if it serves 1 million users daily, each making 100 domain requests, with a cache hit rate of 90%.

Calculate the approximate number of DNS queries sent to upstream servers per second by the recursive resolver.

A Approximately 11,574 queries per second

B Approximately 100,000 queries per second

C Approximately 116 queries per second

D Approximately 10,000 queries per second

💡 Hint

Calculate total daily requests, apply cache hit rate, then divide by seconds in a day.

1 million users * 100 requests = 100 million requests daily. With 90% cache hit, 10% go upstream = 10 million queries daily. Divide by 86,400 seconds/day ≈ 116 queries per second. Check options carefully.

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