DNS¶

DNS (Domain Name System) translates domain names to IP addresses. It's a hierarchical, distributed system. Lookup flow: browser cache → OS cache → recursive resolver → root server → TLD server → authoritative server. Record types: A (domain → IPv4), AAAA (→ IPv6), CNAME (alias), MX (mail), NS (nameserver), TXT (metadata). TTL controls caching duration.

Key Concepts¶

Deep Dive: DNS Resolution Flow

Browser → "What is the IP of example.com?"

1. Browser cache → miss
2. OS cache → miss
3. Recursive Resolver (ISP)
   → Root Server: "Ask .com TLD server"
   → .com TLD Server: "Ask ns1.example.com"
   → Authoritative Server: "93.184.216.34"
4. Resolver caches result (TTL)
5. Returns IP to browser

Deep Dive: DNS Record Types

Type	Purpose	Example
A	Domain → IPv4	`example.com → 93.184.216.34`
AAAA	Domain → IPv6	`example.com → 2001:db8::1`
CNAME	Alias → another domain	`www.example.com → example.com`
MX	Mail server	`example.com → mail.example.com`
NS	Authoritative nameserver	`example.com → ns1.example.com`
TXT	Text data (SPF, verification)	`"v=spf1 include:..."`

Common Interview Questions

How does DNS work?
What is the difference between A and CNAME records?
What is DNS caching? What is TTL?
What happens if a DNS server is down?
How does DNS load balancing work?