ddns-route53

#!/usr/bin/env bash

declare -r DATE_FORMAT="%Y-%m-%d %H:%M:%S"
declare -r VERSION="2.1.0"
declare ttl=${DDNS_ROUTE53_TTL:-300}
declare record_type="${DDNS_ROUTE53_TYPE:-A}"
declare comment="${DDNS_ROUTE53_COMMENT:-Updated at $(date +"$DATE_FORMAT")}"
declare zone_id="$DDNS_ROUTE53_ZONE_ID"
declare record_set="$DDNS_ROUTE53_RECORD_SET"
declare handler_script="$DDNS_ROUTE53_SCRIPT"
declare old_ip ip

function errlog() {
  echo "$@" >&2
}

function log() {
  echo "$@"
}

function usage() {
  cat <<HELP
Usage: $(basename "$0") [OPTIONS]

Dynamic DNS updater using Amazon Route53.

Options:
  -z, --zone-id <ZONE_ID>         Amazon Route53 hosted zone ID (required)
  -r, --record-set <RECORD_SET>   Amazon Route53 record set name (required)
  -t, --ttl <SECONDS>             TTL for DNS record
  -y, --type <TYPE>               DNS record type
  -i, --ip <IP_ADDRESS>           Force usage of IP address
  -s, --script <SCRIPT_PATH>      Path to script to execute on change
  -h, --help                      You're looking at it
  -v, --version                   Print version and exit
HELP
}

function valid-ip() {
  local ip=$1 octets
  if [[ "$ip" =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
    IFS='.' read -r -a octets <<< "$ip"
    for octet in "${octets[@]}"; do
      (( octet <= 255 )) || return 1
    done
  elif [[ "$ip" =~ ^[0-9a-fA-F:]\+$ ]]; then # Let's take the easy way out with validation of IPv6 addresses
      return 1
  fi
  return 0
}

function fetch-current-ip() {
  aws route53 list-resource-record-sets \
    --hosted-zone-id "$zone_id" \
    --output text \
    --query "ResourceRecordSets[?Name == '$record_set.' && Type == '$record_type'].ResourceRecords[0].Value"
}

function init-args() {
  while (( $# > 0 )); do
    case "$1" in
      -z|--zone-id) zone_id="$2"; shift ;;
      -r|--record-set) record_set="$2"; shift ;;
      -i|--ip) ip="$2"; shift ;;
      -t|--ttl) ttl="$2"; shift ;;
      -y|--type) record_type="$2"; shift ;;
      -s|--script) handler_script="$2"; shift ;;
      -h|--help) usage; exit 0 ;;
      -v|--version) echo "$VERSION"; exit 0 ;;
      *) errlog "Unknown option: $1"; usage; exit 1 ;;
    esac
    shift
  done
  if [[ -z "$ip" ]]; then
    if [[ $record_type = "AAAA" ]]; then
      ip="$(dig +short -6 myip.opendns.com aaaa @resolver1.opendns.com)"
    else
      ip="$(dig +short -4 myip.opendns.com @resolver1.opendns.com)"
    fi
    # shellcheck disable=SC2181 # this is intentional
    (( $? != 0 )) && ip=""
  fi
  old_ip="$(fetch-current-ip 2> /dev/null)"
  return 0
}

function validate-args() {
  if [[ -z "$zone_id" ]]; then
    errlog "Missing -z or --zone-id or \$DDNS_ROUTE53_ZONE_ID"
    return 1
  elif [[ -z "$record_set" ]]; then
    errlog "Missing -r or --record-set or \$DDNS_ROUTE53_RECORD_SET"
    return 1
  elif [[ -n "$handler_script" && ! -x "$handler_script" ]]; then
    errlog "Script at '$handler_script' is not executable"
    return 1
  elif [[ -z "$ip" ]]; then
    errlog "Unable to determine current IP address"
    return 1
  fi
  return 0
}

function update-dns-entry() {
  local ip=$1 output success batch
  read -r -d '' batch << EOF
{
  "Comment": "$comment",
  "Changes": [
    {
      "Action": "UPSERT",
      "ResourceRecordSet": {
        "ResourceRecords": [{"Value": "$ip"}],
        "Name": "$record_set",
        "Type": "$record_type",
        "TTL": $ttl
      }
    }
  ]
}
EOF
  output="$(aws route53 change-resource-record-sets --hosted-zone-id "$zone_id" --change-batch "$batch" 2>&1)"
  success=$?
  if (( success == 0 )); then
    log "$output"
  else
    errlog "Failed to update DNS entry:"
    errlog "$output"
  fi
  return $success
}

function run-handler-script() {
  local old_ip=$1 new_ip=$2
  [[ -z "$handler_script" ]] || "$handler_script" "$old_ip" "$new_ip"
}

function main() {
  init-args "$@"
  validate-args || exit 1
  if ! valid-ip "$ip"; then
    log "Invalid IP address: '$ip'"
    exit 1
  fi
  if [[ "$old_ip" != "$ip" ]]; then
    log "IP changed from '$old_ip' to '$ip', updating entry"
    update-dns-entry "$ip" && run-handler-script "$old_ip" "$ip"
  else
    log "Current IP == $ip"
  fi
  exit $?
}

main "$@"