Designing a DNS System

DNS (Domain Name System) translates human-readable domain names into IP addresses.

DNS Hierarchy

Root DNS Servers (.)
    ↓
TLD Servers (.com, .org, .net)
    ↓
Authoritative DNS Servers (example.com)
    ↓
Your IP Address

DNS Resolution Flow

1. Browser checks local cache
2. OS checks /etc/hosts
3. Recursive Resolver (ISP/8.8.8.8)
4. Root Server → TLD Server
5. Authoritative Server → IP returned
6. Cached at each layer

Record Types

A     → IPv4 address
AAAA  → IPv6 address
CNAME → Alias to another domain
MX    → Mail server
TXT   → Verification, SPF records
NS    → Nameserver delegation
SOA   → Zone authority record

TTL (Time To Live)

high TTL (86400s): Stable IPs, reduces resolver load
low TTL (60s):     Frequent changes, faster propagation

DNS Caching

• Browser cache: 1 min
• OS cache: TTL-based
• Recursive resolver: TTL-based
• Negative cache: NXDOMAIN responses also cached

DNS Load Balancing

# Round-Robin DNS
example.com → 1.2.3.4
example.com → 5.6.7.8
example.com → 9.10.11.12

# GeoDNS: Return closest IP based on resolver location

Designing a Custom DNS Resolver

Incoming query
    ↓
Cache lookup (Redis)
    ↓ (miss)
Forward to upstream resolver (8.8.8.8)
    ↓
Cache result with TTL
    ↓
Return IP

DNSSEC

Adds cryptographic signatures to DNS records to prevent spoofing.

DNS Amplification Attack

Small query → Large response → DDoS vector. Mitigation: Rate limiting, response rate limiting (RRL).

Conclusion

DNS is the internet's phonebook. TTL tuning, GeoDNS, and caching are key levers for performance and reliability.