feat(dns): Validierung für DNS-Settings — upstream IPs, ACL-CIDRs, TTL-Kreuzcheck

Backend: validateSettings() prüft vor dem Reload-Trigger ob upstream_forwards
gültige IPs (inkl. @port), access_acl gültige IPs/CIDRs und listen_addresses
gültige IPs sind; cache_max_ttl ≥ cache_min_ttl.
Frontend: Pattern-Validatoren auf upstream_forwards + access_acl; TTL-
Kreuzvalidierung mit dependencies.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/handlers/dns.go

@@ -3,7 +3,9 @@ package handlers
 import (
 	"context"
 	"errors"
+	"fmt"
 	"log/slog"
+	"net"
 	"os/exec"
 	"strconv"
 	"strings"
@@ -286,6 +288,10 @@ func (h *DNSHandler) UpdateSettings(c *gin.Context) {
 		response.BadRequest(c, err)
 		return
 	}
+	if err := validateSettings(&req); err != nil {
+		response.BadRequest(c, err)
+		return
+	}
 	out, err := h.Repo.UpdateSettings(c.Request.Context(), req)
 	if err != nil {
 		response.Internal(c, err)
@@ -312,6 +318,52 @@ func (h *DNSHandler) FlushCache(c *gin.Context) {
 
 // ── Validation ─────────────────────────────────────────────────
 
+// validateSettings checks user-supplied DNS global settings before they
+// reach unbound. A malformed upstream IP or CIDR would cause unbound to
+// fail on the next reload without any visible error.
+func validateSettings(s *models.DNSSettings) error {
+	if s.ListenPort < 1 || s.ListenPort > 65535 {
+		return fmt.Errorf("listen_port %d out of range (1-65535)", s.ListenPort)
+	}
+	if s.CacheMaxTTL < s.CacheMinTTL {
+		return fmt.Errorf("cache_max_ttl (%d) must be ≥ cache_min_ttl (%d)", s.CacheMaxTTL, s.CacheMinTTL)
+	}
+	for _, raw := range strings.Split(s.UpstreamForwards, ",") {
+		entry := strings.TrimSpace(raw)
+		if entry == "" {
+			continue
+		}
+		// strip optional @port suffix (e.g. 1.1.1.1@853)
+		host, _, _ := strings.Cut(entry, "@")
+		if net.ParseIP(host) == nil {
+			return fmt.Errorf("invalid upstream forwarder IP: %q", host)
+		}
+	}
+	for _, raw := range strings.Split(s.AccessACL, ",") {
+		entry := strings.TrimSpace(raw)
+		if entry == "" {
+			continue
+		}
+		if strings.Contains(entry, "/") {
+			if _, _, err := net.ParseCIDR(entry); err != nil {
+				return fmt.Errorf("invalid access ACL CIDR: %q", entry)
+			}
+		} else if net.ParseIP(entry) == nil {
+			return fmt.Errorf("invalid access ACL IP: %q", entry)
+		}
+	}
+	for _, raw := range strings.Split(s.ListenAddresses, ",") {
+		addr := strings.TrimSpace(raw)
+		if addr == "" {
+			continue
+		}
+		if net.ParseIP(addr) == nil {
+			return fmt.Errorf("invalid listen address: %q", addr)
+		}
+	}
+	return nil
+}
+
 func validateZone(z *models.DNSZone) error {
 	if z.Name == "" {
 		return errors.New("name required")