Allow ACME DNS-01 dynamic updates on ns1

Add a dedicated acme_ddns TSIG key (scoped to ns1 only) and an acl_acme rule
that limits it to TXT updates at or under _acme-challenge.<zone>. An external
ACME client can now write challenge records via RFC 2136; Knot signs them and
transfers to ns2, which never holds the key.

machines/ns1/configuration.nix

@@ -1,4 +1,4 @@
-{ ... }:
+{ config, pkgs, ... }:
 let
   domains = import ../../modules/dns/domains.nix;
 in
@@ -10,6 +10,40 @@ in
   time.timeZone = "Etc/GMT-1"; # UTC+1 (fixed offset, no DST)
   services.timesyncd.enable = true;
 
+  # ACME DNS-01 (RFC 2136): a dedicated TSIG key, scoped to ns1 only, that an
+  # external ACME client uses to write _acme-challenge TXT records. acl_acme
+  # (referenced by each zone below) limits the key to TXT updates at or under
+  # _acme-challenge.<zone>; Knot then signs the record and transfers it to ns2,
+  # which never needs this key. Retrieve the secret for the client with:
+  #   clan vars get ns1 dns-acme-tsig/acme.conf
+  clan.core.vars.generators.dns-acme-tsig = {
+    files."acme.conf" = {
+      secret = true;
+      owner = "knot";
+      group = "knot";
+    };
+    runtimeInputs = [ pkgs.knot-dns ];
+    script = ''
+      keymgr -t acme_ddns hmac-sha256 > "$out"/acme.conf
+    '';
+  };
+
+  services.knot.keyFiles = [
+    config.clan.core.vars.generators.dns-acme-tsig.files."acme.conf".path
+  ];
+
+  services.knot.settings.acl = [
+    {
+      id = "acl_acme";
+      key = "acme_ddns";
+      action = [ "update" ];
+      "update-type" = [ "TXT" ];
+      "update-owner" = "name";
+      "update-owner-match" = "sub-or-equal";
+      "update-owner-name" = [ "_acme-challenge" ];
+    }
+  ];
+
   # Automatic DNSSEC signing policy (primary only). ECDSA P-256/SHA-256 with
   # Knot's default key management: the ZSK auto-rolls and the KSK is kept stable,
   # so the DS at the registrar only changes on a manual KSK rollover.
@@ -34,6 +68,6 @@ in
     "dnssec-signing" = true;
     "dnssec-policy" = "cnx";
     notify = [ "ns2" ];
-    acl = [ "acl_ns2" ];
+    acl = [ "acl_ns2" "acl_acme" ]; # ns2 transfers; acme_ddns key does DNS-01 updates
   }) domains;
 }