zones/vhack.eu: Make it obvious, that the serial number must be changed

The comment alone would probably suffice, but having a convenient function
that makes it obvious *what* part of the serial number you are actually
supposed to change seems quite useful, when trying to reduce the
possibilities of forgetting it.

2 files changed, 40 insertions, 4 deletions

diff --git a/zones/default.nix b/zones/default.nix
index cde6def..babb531 100644
--- a/zones/default.nix
+++ b/zones/default.nix
@@ -1,3 +1,3 @@
-{...}: {
-  "vhack.eu" = import ./vhack.eu/zone.nix {};
+{lib, ...}: {
+  "vhack.eu" = import ./vhack.eu/zone.nix {inherit lib;};
 }
diff --git a/zones/vhack.eu/zone.nix b/zones/vhack.eu/zone.nix
index 4897f53..ad4f728 100644
--- a/zones/vhack.eu/zone.nix
+++ b/zones/vhack.eu/zone.nix
@@ -1,8 +1,44 @@
-{...}: {
+{lib, ...}: let
+  /*
+  Computes a reasonable value for the DNS serial number from the date of last change
+  and the iteration of that day.
+
+  # Type
+
+  mkSerial :: Number -> Number -> Number -> Number -> Number
+
+  # Arguments
+
+  year
+  : The year of the last change in the format YYYY.
+
+  month
+  : The month of the last change in the format MM.
+
+  day
+  : The day of the last change in the format DD.
+
+  iteration
+  : The number of change on that day. The format should be CC (assuming there are less
+    than 100 changes happening on a day) .
+
+  # Examples
+
+  mkSerial 2025 04 01 01
+  => 2025040101
+  */
+  mkSerial = year: month: day: iteration: let
+    n2 = lib.strings.fixedWidthNumber 2;
+    n4 = lib.strings.fixedWidthNumber 4;
+  in
+    lib.strings.toIntBase10 "${n4 year}${n2 month}${n2 day}${n2 iteration}";
+in {
   SOA = {
     nameServer = "name-server.foss-syndicate.org.";
     adminEmail = "dns-admin@foss-syndicate.org";
-    serial = 2025031001;
+    # NOTE(@bpeetz): ALWAYS change the serial number, when you change something in the
+    # zone file! <2025-04-01>
+    serial = mkSerial 2025 04 01 01;
   };
   useOrigin = false;