icinga2-checks/check_external_ip_dynamic.sh

#!/bin/bash

usage() {
  echo "Usage: $0 -i <IP checker service> -d <domain to resolve> -r <DNS server to query>"
  exit 3
}

IP_CHECKER=""
DOMAIN=""
DNS_SERVER=""

while getopts "i:d:r:" opt; do
  case ${opt} in
    i )
      IP_CHECKER=$OPTARG
      ;;
    d )
      DOMAIN=$OPTARG
      ;;
    r )
      DNS_SERVER=$OPTARG
      ;;
    \? )
      echo "Invalid option: $OPTARG" 1>&2
      usage
      exit 1
      ;;
    : )
      echo "Invalid option: $OPTARG requires an argument" 1>&2
      usage
      exit 1
      ;;
  esac
done
shift $((OPTIND -1))

if [ -z "$IP_CHECKER" ] || [ -z "$DOMAIN" ] || [ -z "$DNS_SERVER" ]; then
    echo "All parameters are required"
    usage
    exit 1
fi

CURRENT_IP=$(curl -s "$IP_CHECKER")
RESOLVED_IP=$(dig +short @$DNS_SERVER $DOMAIN | head -n 1)

if [ "$CURRENT_IP" == "$RESOLVED_IP" ]; then
    echo "OK - $DOMAIN resolves to $CURRENT_IP"
    exit 0
else
    echo "CRITICAL - $DOMAIN does not resolve to $CURRENT_IP. Resolved: $RESOLVED_IP"
    exit 2
fi