feat: add resilient updates and access controls

internal/config/config.go

@@ -1,8 +1,10 @@
 package config
 
 import (
+	"errors"
 	"fmt"
 	"net"
+	"net/url"
 	"os"
 	"path/filepath"
 	"strconv"
@@ -27,6 +29,9 @@ type Config struct {
 	HCloudBaseURL   string
 	Mode            string
 	ShutdownTimeout time.Duration
+	UpdateAPIURL    string
+	UpdateAgentSock string
+	UpdateInterval  time.Duration
 }
 
 func Load() (Config, error) {
@@ -51,6 +56,14 @@ func Load() (Config, error) {
 	}
 
 	keyFile := value("APP_MASTER_KEY_FILE", "/secrets/master_key")
+	updateInterval, err := duration("APP_SOFTWARE_UPDATE_CHECK_INTERVAL", 6*time.Hour)
+	if err != nil || updateInterval < 15*time.Minute || updateInterval > 7*24*time.Hour {
+		return Config{}, fmt.Errorf("APP_SOFTWARE_UPDATE_CHECK_INTERVAL must be between 15m and 168h")
+	}
+	updateAPIURL := value("APP_SOFTWARE_UPDATE_API_URL", "https://git.j-mahler.de/api/v1/repos/admin/hetzner-api-dyndns-records-webinterface/releases/latest")
+	if err := validateUpdateURL(updateAPIURL); err != nil {
+		return Config{}, fmt.Errorf("APP_SOFTWARE_UPDATE_API_URL: %w", err)
+	}
 
 	cookieSecure := mode == "https"
 	if raw := os.Getenv("APP_COOKIE_SECURE"); raw != "" {
@@ -71,9 +84,28 @@ func Load() (Config, error) {
 		HCloudBaseURL:   strings.TrimRight(value("APP_HCLOUD_BASE_URL", "https://api.hetzner.cloud/v1"), "/"),
 		Mode:            mode,
 		ShutdownTimeout: shutdownTimeout,
+		UpdateAPIURL:    updateAPIURL,
+		UpdateAgentSock: value("APP_SOFTWARE_UPDATE_AGENT_SOCKET", "/run/domain-control-update/agent.sock"),
+		UpdateInterval:  updateInterval,
 	}, nil
 }
 
+func validateUpdateURL(raw string) error {
+	parsed, err := url.Parse(raw)
+	if err != nil || parsed.Host == "" || parsed.User != nil || (parsed.Scheme != "https" && parsed.Scheme != "http") {
+		return errors.New("must be an absolute HTTPS URL without embedded credentials")
+	}
+	if parsed.Scheme == "https" {
+		return nil
+	}
+	host := parsed.Hostname()
+	address := net.ParseIP(host)
+	if host == "localhost" || (address != nil && address.IsLoopback()) {
+		return nil
+	}
+	return errors.New("HTTP is allowed only for loopback test endpoints")
+}
+
 func value(name, fallback string) string {
 	if v := strings.TrimSpace(os.Getenv(name)); v != "" {
 		return v