karldns.de/scripts/karldns-update.sh

#!/bin/sh
set -eu

CONFIG_FILE="${KARLDNS_CONFIG:-${XDG_CONFIG_HOME:-$HOME/.config}/karldns/updater.env}"

if [ -r "$CONFIG_FILE" ]; then
  # shellcheck disable=SC1090
  . "$CONFIG_FILE"
fi

: "${KARLDNS_UPDATE_URL:?Set KARLDNS_UPDATE_URL in $CONFIG_FILE or the environment}"
KARLDNS_BASE_URL="${KARLDNS_BASE_URL:-https://karldns.de}"
KARLDNS_LOCK_FILE="${KARLDNS_LOCK_FILE:-/tmp/karldns-updater.lock}"
KARLDNS_CACHE_FILE="${KARLDNS_CACHE_FILE:-${XDG_CACHE_HOME:-$HOME/.cache}/karldns/last-ip}"
KARLDNS_CURL="${KARLDNS_CURL:-curl}"
KARLDNS_IPV4="${KARLDNS_IPV4:-auto}"
KARLDNS_IPV6="${KARLDNS_IPV6:-auto}"
case "$KARLDNS_UPDATE_URL" in
  *"<ipaddr>"*|*"<ip6addr>"*) KARLDNS_UPDATE_URL="${KARLDNS_UPDATE_URL%%\?*}" ;;
esac

mkdir -p "$(dirname "$KARLDNS_CACHE_FILE")"
mkdir -p "$(dirname "$KARLDNS_LOCK_FILE")" 2>/dev/null || true

if command -v flock >/dev/null 2>&1; then
  exec 9>"$KARLDNS_LOCK_FILE"
  if ! flock -n 9; then
    exit 0
  fi
else
  lock_dir="${KARLDNS_LOCK_FILE}.d"
  if ! mkdir "$lock_dir" 2>/dev/null; then
    exit 0
  fi
  trap 'rmdir "$lock_dir"' EXIT INT TERM
fi

ipv4=""
ipv6=""
if [ "$KARLDNS_IPV4" = "auto" ]; then
  ipv4=""
elif [ "$KARLDNS_IPV4" != "off" ]; then
  ipv4="$KARLDNS_IPV4"
fi
if [ "$KARLDNS_IPV6" = "auto" ]; then
  ipv6=""
elif [ "$KARLDNS_IPV6" != "off" ]; then
  ipv6="$KARLDNS_IPV6"
fi

case "$ipv4" in *" "*) echo "KarlDNS: could not read a valid IPv4 address" >&2; exit 1 ;; esac
case "$ipv6" in *" "*) echo "KarlDNS: could not read a valid IPv6 address" >&2; exit 1 ;; esac
public_ip="${ipv4}|${ipv6}"
has_discovered_ip=0
if [ -n "$ipv4" ] || [ -n "$ipv6" ]; then
  has_discovered_ip=1
fi

last_ip=""
if [ -r "$KARLDNS_CACHE_FILE" ]; then
  last_ip="$(cat "$KARLDNS_CACHE_FILE")"
fi

if [ "$has_discovered_ip" = "1" ] && [ "$public_ip" = "$last_ip" ] && [ "${KARLDNS_FORCE:-0}" != "1" ]; then
  echo "KarlDNS: unchanged ${ipv4:-no-ipv4} ${ipv6:-no-ipv6}"
  exit 0
fi

if [ "$has_discovered_ip" = "1" ]; then
  set -- --get "$KARLDNS_UPDATE_URL"
  if [ -n "$ipv4" ]; then
    set -- "$@" --data-urlencode "myip=$ipv4"
  fi
  if [ -n "$ipv6" ]; then
    set -- "$@" --data-urlencode "myipv6=$ipv6"
  fi
  response="$("$KARLDNS_CURL" -fsS --max-time 20 "$@")"
else
  response="$("$KARLDNS_CURL" -fsS --max-time 20 "$KARLDNS_UPDATE_URL")"
fi
case "$response" in
  good*|nochg*)
    if [ "$has_discovered_ip" = "1" ]; then
      printf '%s\n' "$public_ip" > "$KARLDNS_CACHE_FILE"
    fi
    echo "KarlDNS: $response"
    ;;
  *)
    echo "KarlDNS: update failed. Server said: $response" >&2
    exit 1
    ;;
esac