dual_stack_dns_preference_check.sh

#!/usr/bin/env bash

set -euo pipefail

# dual_stack_dns_preference_check.sh
# Compares A/AAAA availability and resolver answer-order bias for dual-stack hostnames.

usage() {
  cat <<'EOF'
Usage: ./dual_stack_dns_preference_check.sh [options]

Options:
  --host NAME            Hostname to check (repeatable)
  --resolver IP          Resolver to query with dig (optional)
  --timeout SEC          dig timeout (default: 2)
  --tries N              dig tries (default: 1)
  --strict               Exit non-zero if host lacks one family or shows mismatch
  -h, --help             Show help

Exit codes:
  0 = balanced dual-stack posture (or warnings in non-strict mode)
  2 = strict mode and warnings detected
EOF
}

HOSTS=("cloudflare.com" "google.com")
RESOLVER=""
TIMEOUT=2
TRIES=1
STRICT=false

while [[ $# -gt 0 ]]; do
  case "$1" in
    --host) HOSTS+=("$2"); shift 2 ;;
    --resolver) RESOLVER="$2"; shift 2 ;;
    --timeout) TIMEOUT="$2"; shift 2 ;;
    --tries) TRIES="$2"; shift 2 ;;
    --strict) STRICT=true; shift ;;
    -h|--help) usage; exit 0 ;;
    *) echo "Unknown option: $1"; usage; exit 1 ;;
  esac
done

if [[ "${#HOSTS[@]}" -gt 2 ]]; then
  HOSTS=("${HOSTS[@]:2}")
fi

if ! command -v dig >/dev/null 2>&1; then
  echo "dig command not found"
  exit 1
fi

warn_count=0

echo "Dual-stack DNS preference check timestamp: $(date -Iseconds)"
echo "hosts=${HOSTS[*]} resolver=${RESOLVER:-system-default} timeout=${TIMEOUT}s tries=$TRIES strict_mode=$STRICT"
echo

for host in "${HOSTS[@]}"; do
  echo "=== host=$host ==="

  if [[ -n "$RESOLVER" ]]; then
    a=$(dig @"$RESOLVER" "$host" A +short +time="$TIMEOUT" +tries="$TRIES" 2>/dev/null | sed '/^$/d' | sort -u || true)
    aaaa=$(dig @"$RESOLVER" "$host" AAAA +short +time="$TIMEOUT" +tries="$TRIES" 2>/dev/null | sed '/^$/d' | sort -u || true)
  else
    a=$(dig "$host" A +short +time="$TIMEOUT" +tries="$TRIES" 2>/dev/null | sed '/^$/d' | sort -u || true)
    aaaa=$(dig "$host" AAAA +short +time="$TIMEOUT" +tries="$TRIES" 2>/dev/null | sed '/^$/d' | sort -u || true)
  fi

  acount=$(printf '%s\n' "$a" | sed '/^$/d' | wc -l)
  aaaacount=$(printf '%s\n' "$aaaa" | sed '/^$/d' | wc -l)

  answer_order="unknown"
  if command -v getent >/dev/null 2>&1; then
    first_ip=$(getent ahosts "$host" 2>/dev/null | awk 'NR==1 {print $1}')
    if [[ "$first_ip" == *:* ]]; then
      answer_order="ipv6-first"
    elif [[ -n "$first_ip" ]]; then
      answer_order="ipv4-first"
    fi
  fi

  status="OK"
  if [[ "$acount" -eq 0 || "$aaaacount" -eq 0 ]]; then
    status="WARN"
    ((warn_count+=1))
  fi

  echo "A_count=$acount AAAA_count=$aaaacount resolver_order=$answer_order status=$status"
  [[ "$acount" -gt 0 ]] && echo "A: $(printf '%s' "$a" | paste -sd ';' -)"
  [[ "$aaaacount" -gt 0 ]] && echo "AAAA: $(printf '%s' "$aaaa" | paste -sd ';' -)"
  echo
done

echo "Summary: warn_count=$warn_count strict_mode=$STRICT"
if [[ "$STRICT" == true && "$warn_count" -gt 0 ]]; then
  exit 2
fi

exit 0