Domain Name System (DNS)

Scope

The system for translating human-readable domain names into machine-readable IP addresses, and its role in global traffic routing.

Why This Topic Exists

DNS is the phonebook of the internet. It’s a foundational, distributed system that makes the web navigable. Beyond simple name-to-address translation, it is a powerful lever for directing traffic, enabling load balancing, and improving service availability.

Core Tradeoffs

Caching TTL (Time-to-Live): A shorter TTL allows for faster changes but increases the load on DNS servers. A longer TTL reduces load but slows down propagation of updates.
Routing Policy Complexity: Simple round-robin is easy to implement, but more complex strategies like latency-based or geolocation-based routing can improve user experience at the cost of added complexity and potential for misconfiguration.
Centralization vs. Decentralization: Using a single DNS provider is simple, but diversifying providers can increase resilience against provider-specific outages.

Common Failure Modes

DNS Propagation Delays: Changes to DNS records are not instantaneous, leading to periods where traffic is sent to old and new endpoints simultaneously.
DDoS Attacks: Volumetric attacks against DNS servers can render services unreachable even if the services themselves are healthy.
Cache Poisoning: Malicious actors can corrupt DNS caches to redirect users to fraudulent sites.
Mismatched Records: A CNAME record cannot coexist with other records for the same name, a common source of configuration errors.

Interview Signals

A strong candidate can trace the entire lifecycle of a DNS query, from a user’s browser to the authoritative nameserver. They should be able to discuss various record types (A, CNAME, etc.) and explain how to use DNS for load balancing, failover, and CDN integration.

CDN
Load Balancing
Communication
Security