diff --git a/hosts/falkenstein/modules/dns/default.nix b/hosts/falkenstein/modules/dns/default.nix
index 8fd068a..520990e 100644
--- a/hosts/falkenstein/modules/dns/default.nix
+++ b/hosts/falkenstein/modules/dns/default.nix
@@ -1,12 +1,13 @@
-{ pkgs, lib, config, dns, ... }:
+{ self, pkgs, lib, config, dns, ... }:
 let
+  modulo = number: mod: (number - ((number / mod) * mod));
   secondary = "185.181.104.96";
   zonefile = with dns.lib.combinators; pkgs.writeText "rfive.de.zone.txt" (dns.lib.toString "rfive.de" {
     TTL = 3600;
     SOA = {
       nameServer = "ns.rfive.de.";
       adminEmail = "hostmaster@rfive.de";
-      serial = 2024041709;
+      serial = lib.strings.toInt (builtins.substring 0 8 self.sourceInfo.lastModifiedDate + toString ((modulo self.sourceInfo.lastModified 86400) / 864));
       refresh = 10800;
       retry = 3600;
       expire = 604800;