Add initial installer

nixos/modules/dynamic-motd.nix

@@ -0,0 +1,27 @@
+{ lib, pkgs, config, ... }:
+let
+  inherit (lib) optionalAttrs filterAttrs genAttrs mkIf mkDefault;
+  inherit (lib.my) mkOpt' mkBoolOpt';
+
+  cfg = config.my.dynamic-motd;
+
+  scriptBin = pkgs.writeShellScript "dynamic-motd-script" cfg.script;
+in
+{
+  options.my.dynamic-motd = with lib.types; {
+    enable = mkBoolOpt' true "Whether to enable the dynamic message of the day PAM module.";
+    services = mkOpt' (listOf str) [ "login" "ssh" ] "PAM services to enable the dynamic message of the day module for.";
+    script = mkOpt' (nullOr lines) null "Script that generates message of the day.";
+  };
+
+  config = mkIf (cfg.enable && cfg.script != null) {
+    security.pam.services = genAttrs cfg.services (s: {
+      text = mkDefault
+        ''
+          session optional ${pkgs.pam}/lib/security/pam_exec.so stdout quiet ${scriptBin}
+        '';
+    });
+  };
+
+  meta.buildDocsInSandbox = false;
+}