What is DNS?

DNS (Domain Name System) is the phonebook of the internet. It translates human-readable domain names (like google.com) into IP addresses (like 142.250.185.78) that computers use to communicate.

How DNS Works

User types: www.example.com
                │
                ▼
        ┌───────────────┐
        │ DNS Resolver  │ (ISP or public like 8.8.8.8)
        └───────┬───────┘
                │ Cache miss
                ▼
        ┌───────────────┐
        │  Root Server  │ Returns: .com nameserver
        └───────┬───────┘
                │
                ▼
        ┌───────────────┐
        │ TLD Server    │ Returns: example.com nameserver
        │    (.com)     │
        └───────┬───────┘
                │
                ▼
        ┌───────────────┐
        │ Authoritative │ Returns: 93.184.216.34
        │    Server     │
        └───────────────┘

DNS Record Types

Record	Purpose	Example
A	Maps domain to IPv4	`example.com → 93.184.216.34`
AAAA	Maps domain to IPv6	`example.com → 2001:db8::1`
CNAME	Alias to another domain	`www.example.com → example.com`
MX	Mail server	`example.com → mail.example.com`
TXT	Text data (verification, SPF)	`"v=spf1 include:_spf.google.com"`
NS	Nameserver delegation	`example.com → ns1.example.com`

DNS in System Design

Load Distribution

DNS can distribute traffic across multiple servers using Round-Robin:

example.com
    ├──► 192.168.1.1 (Server A)
    ├──► 192.168.1.2 (Server B)
    └──► 192.168.1.3 (Server C)

Geographic Routing

Route users to nearest datacenter based on location.

TTL (Time To Live)

Low TTL (60s): Quick changes, more DNS queries
High TTL (86400s): Fewer queries, slower propagation

Interview Tips

Know the resolution process: Root → TLD → Authoritative
Understand record types: A, AAAA, CNAME, MX
TTL trade-offs: Low for flexibility, high for performance
DNS for load balancing: Round-robin, geographic routing

DNS

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