DNS & Domain Resolution

// DNS resolution step by step
//
// 1. Browser cache (Chrome: chrome://net-internals/#dns)
// 2. OS cache (macOS: scutil --dns)
// 3. Recursive resolver (ISP / 8.8.8.8 / 1.1.1.1)
//    └─→ 4. Root server (.) → returns .com TLD server
//    └─→ 5. TLD server (.com) → returns authoritative NS for example.com
//    └─→ 6. Authoritative NS → returns A record: 93.184.216.34
// 7. Response cached at resolver (TTL = 300s)
// 8. Response returned to OS → browser → connection to IP

// dig command to inspect DNS
$ dig api.example.com +trace
// Shows each hop: root → .com → example.com → A record

// Common record types:
// A      → IPv4 address          (api.example.com → 1.2.3.4)
// AAAA   → IPv6 address
// CNAME  → alias to another name (www → api.example.com)
// MX     → mail server
// TXT    → verification, SPF, DKIM
// NS     → nameserver delegation

// Route 53 — latency-based routing with health checks
//
// api.example.com
//   ├── A record → 10.0.1.1 (us-east-1, latency routing, health check)
//   └── A record → 10.0.2.1 (eu-west-1, latency routing, health check)
//
// User in India → resolved to lowest-latency region (likely ap-south-1)
// If us-east-1 health check fails → traffic automatically routed to eu-west-1

// Terraform: Route 53 health check + failover
resource "aws_route53_health_check" "primary" {
  fqdn              = "api-us-east.example.com"
  port              = 443
  type              = "HTTPS"
  request_interval  = 10
  failure_threshold = 3
}

resource "aws_route53_record" "api_primary" {
  zone_id        = var.zone_id
  name           = "api.example.com"
  type           = "A"
  set_identifier = "primary"
  failover_routing_policy { type = "PRIMARY" }
  alias { name = aws_lb.primary.dns_name }
  health_check_id = aws_route53_health_check.primary.id
}