The Top DNS Programming Interview Questions for Software Engineers

Domain Name System (DNS) is a fundamental technology that translates human-readable domain names to machine-readable IP addresses. It acts as the directory for the internet by mapping identifiers to locations.

For software engineers having in-depth knowledge of DNS is crucial for building troubleshooting, and optimizing internet-connected applications. Expect DNS-related questions in any interview, especially for network programming roles.

This guide covers the most common and insightful DNS interview questions that assess your conceptual understanding and practical skills Master these questions to confidently ace your next technical interview!

1. What is a DNS server and its role?

A DNS server stores DNS records that map domain names to IP addresses. Its role is to translate requests for domains into the corresponding IPs to route traffic.

There are 2 main types of DNS servers:

• Authoritative – Contains definitive records it’s responsible for
• Recursive – Doesn’t store records but queries other servers

DNS servers provide the directory service that makes the internet usable.

2. What is an authoritative DNS server?

An authoritative DNS server contains the actual DNS records for the domains that it hosts. This makes it the definitive source for mapping those domains to IPs.

Some examples are the root nameservers run by ICANN and organizational nameservers that host company domains.

Authoritative servers directly respond to queries for domains they manage.

3. What is a recursive DNS server?

A recursive DNS server does not contain actual DNS records. Instead, it caches query results and communicates with authoritative servers to resolve domain name mapping on behalf of clients.

Recursive resolvers handle the complex chain of queries needed to fully resolve a name, simplifying the process for end users and clients.

4. What is DNS resolution?

DNS resolution is the process of mapping a hostname like www.example.com to its corresponding IP address using the DNS system.

It starts with a DNS recursor, which queries a chain of authoritative servers to get the final A record containing the IP.

DNS resolution enables accessing resources using human-readable names instead of hard-to-remember IP strings.

5. Explain the DNS lookup/resolution process

The full DNS resolution process is:

1. Client requests domain name translation from recursive resolver
2. Resolver queries root nameserver for top-level domain
3. Root returns registry or authoritative nameserver for TLD
4. Resolver asks the TLD’s authoritative NS for second-level domain info
5. Authoritative server returns registered nameserver for SLD
6. Resolver queries SLD’s nameserver to find A record with IP
7. Final IP is returned to client

Caching reduces real-world lookup time.

6. What is DNS caching and why is it important?

DNS caching stores DNS query results for a fixed time interval known as TTL to improve performance.

It eliminates redundant requests and reduces latency by serving results directly from cache without needing to requery upstream authoritative servers.

Caching is critical because full DNS resolution involves multiple hierarchical queries, so caching drastically speeds up name lookups.

7. What are some common DNS record types?

• A: Maps host to IPv4 address
• AAAA: Maps host to IPv6 address
• CNAME: Alias of one name to another
• MX: Maps domain to mail server
• NS: Delegates zone to nameserver
• PTR: Reverse DNS lookup for an IP
• SOA: Stores admin and zone data

There are over a dozen record types for various purposes.

8. What is DNS TTL and how does it work?

TTL (Time-To-Live) is the length that a DNS record can be cached before it must be re-resolved. Lower TTL means fresher data.

Typical default TTL values:

• Root NS: 2 days
• TLD NS: 2 days
• Authoritative NS: 1-2 days
• A/AAAA records: 1-24 hours

TTL provides a compromise between freshness and reducing latency through caching.

9. How does a web browser interact with DNS?

1. Browser checks cache for hostname
2. If not cached, browser asks OS recursive resolver
3. Resolver performs full DNS query and returns IP
4. Browser initiates TCP handshake with server IP
5. Browser sends HTTP request to download content

Browser trusts OS resolver to handle lookups under the hood.

10. What are the steps of a DNS zone transfer?

Zone transfer is the process of replicating DNS records between servers. Steps are:

1. Secondary server requests zone transfer from Primary
2. Primary lists all DNS record sets in the zone
3. Secondary acknowledges and requests transfer
4. Primary sends records incrementally
5. Secondary acknowledges each chunk
6. Changes get replicated to Secondary

This keeps multiple nameservers in sync.

11. How does DNS load balancing work?

DNS load balancing maps a single hostname to multiple IP addresses. Steps are:

1. Client requests domain that has multiple A records
2. Recursive resolver returns the full set of IPs
3. Client randomly selects an IP
4. Requests are distributed across IPs

This allows a single hostname to scale across multiple servers.

12. What is a DNS sinkhole and how is it used?

A DNS sinkhole provides false information for malicious domains. Instead of resolving to malicious servers, sinkholed domains resolve to legitimate servers controlled by security researchers.

This allows analysis of malware, prevents attacks, and notifies infected victims. Sinkholing is useful for taking down botnets.

13. How does split-horizon DNS work?

Split-horizon DNS returns different IP addresses for the same hostname depending on the source network making the query.

This allows organizations to present internal vs external views of their applications, enabling access restrictions and security policies.

For example, a hostname may resolve to an internal IP for queries from the corporate network but to a public IP for general internet users.

14. What is a reverse DNS lookup?

Reverse DNS lookup determines the domain name associated with an IP address by querying special PTR records.

PTR records map IP blocks to domain names in reverse to enable reverse lookups.

Reverse DNS is commonly used to identify sources of email and combat spam.

15. What are the steps involved in setting up a new zone on a DNS server?

To setup a new DNS zone:

1. Create zone definition on nameserver
2. Add NS, A, and PTR records for the nameserver itself
3. Add SOA record with admin details and default records
4. Add any records for existing hosts like www and mail
5. Sign the zone if using DNSSEC
6. Configure secondary servers
7. Notify secondaries to begin replication

Proper planning avoids misconfiguration and downtime.

16. What are some DNS record types used for security?

Key DNS record types for security:

• TXT: SPF records for anti-spam and DKIM for email security
• CAA: Certification authority authorization records
• DNSKEY: Public key for signed DNSSEC responses
• DS: Delegation signer that chains to parent zone
• NSEC/NSEC3: Proves non-existence of records

These provide authentication, integrity, and proof against exploits.

17. What is DNS hijacking and how is it prevented?

DNS hijacking occurs when attackers take over a domain by modifying DNS records, allowing them to route traffic to malicious servers.

Prevention involves:

• Registry lock – Lock domain from unauthorized transfers
• Registry monitoring – Detect unauthorized changes
• DNSSEC signing – Prevent record tampering
• Access control – Limit users that can modify DNS

Vigilance and multi-factor protections are key.

18. How does DNS over HTTPS enhance privacy and security?

DNS over HTTPS (DoH) encrypts DNS queries in HTTPS requests to prevent eavesdropping and man-in-the-middle attacks.

It hides DNS lookups from potentially malicious local networks and providers.

DoH prevents DNS query logging and protects privacy. It should be enabled alongside DNSSEC for best security.

19. What are some differences between IPv4 and IPv6 DNS records?

Core concepts are similar but addressing details differ.

20. How does a DNS server verify domain ownership during registration?

When registering a new domain, registrants must demonstrate control over the domain by fulfilling one of these verification methods:

• Set DNS records with random values provided by registrar
• Publish domain authorization code on a web page
• Respond to challenge email sent to administrative addresses

This prevents malicious domain squatting and fraud.

21. What tools can be used to query DNS systems and records?

Useful DNS troubleshooting tools:

• nslookup – Classic DNS query tool
• dig – Flexible CLI DNS lookup utility