vnstat service: init (#19809)

nixos/modules/module-list.nix

@@ -332,6 +332,7 @@
   ./services/monitoring/telegraf.nix
   ./services/monitoring/ups.nix
   ./services/monitoring/uptime.nix
+  ./services/monitoring/vnstat.nix
   ./services/monitoring/zabbix-agent.nix
   ./services/monitoring/zabbix-server.nix
   ./services/network-filesystems/cachefilesd.nix

nixos/modules/services/monitoring/vnstat.nix

@@ -0,0 +1,43 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.vnstat;
+in {
+  options.services.vnstat = {
+    enable = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to enable update of network usage statistics via vnstatd.
+      '';
+    };
+  };
+
+  config = mkIf cfg.enable {
+    users.extraUsers.vnstatd = {
+      isSystemUser = true;
+      description = "vnstat daemon user";
+      home = "/var/lib/vnstat";
+      createHome = true;
+    };
+
+    systemd.services.vnstat = {
+      description = "vnStat network traffic monitor";
+      path = [ pkgs.coreutils ];
+      after = [ "network.target" ];
+      wantedBy = [ "multi-user.target" ];
+      unitConfig.documentation = "man:vnstatd(1) man:vnstat(1) man:vnstat.conf(5)";
+      preStart = "chmod 755 /var/lib/vnstat";
+      serviceConfig = {
+        ExecStart = "${pkgs.vnstat}/bin/vnstatd -n";
+        ExecReload = "kill -HUP $MAINPID";
+        ProtectHome = true;
+        PrivateDevices = true;
+        PrivateTmp = true;
+        User = "vnstatd";
+      };
+    };
+  };
+}