nixos: add htpdate module

nixos/modules/module-list.nix

@@ -354,6 +354,7 @@
   ./services/networking/haproxy.nix
   ./services/networking/heyefi.nix
   ./services/networking/hostapd.nix
+  ./services/networking/htpdate.nix
   ./services/networking/i2pd.nix
   ./services/networking/i2p.nix
   ./services/networking/iodine.nix

nixos/modules/services/networking/htpdate.nix

@@ -0,0 +1,80 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  inherit (pkgs) htpdate;
+
+  cfg = config.services.htpdate;
+in
+
+{
+
+  ###### interface
+
+  options = {
+
+    services.htpdate = {
+
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Enable htpdate daemon.
+        '';
+      };
+
+      extraOptions = mkOption {
+        type = types.str;
+        default = "";
+        description = ''
+          Additional command line arguments to pass to htpdate.
+        '';
+      };
+
+      servers = mkOption {
+        type = types.listOf types.str;
+        default = [ "www.google.com" ];
+        description = ''
+          HTTP servers to use for time synchronization.
+        '';
+      };
+
+      proxy = mkOption {
+        type = types.str;
+        default = "";
+        example = "127.0.0.1:8118";
+        description = ''
+          HTTP proxy used for requests.
+        '';
+      };
+
+    };
+
+  };
+
+  ###### implementation
+
+  config = mkIf cfg.enable {
+
+    systemd.services.htpdate = {
+      description = "htpdate daemon";
+      wantedBy = [ "multi-user.target" ];
+      serviceConfig = {
+        Type = "forking";
+        PIDFile = "/var/run/htpdate.pid";
+        ExecStart = concatStringsSep " " [
+          "${htpdate}/bin/htpdate"
+          "-D -u nobody"
+          "-a -s"
+          "-l"
+          "${optionalString (cfg.proxy != "") "-P ${cfg.proxy}"}"
+          "${cfg.extraOptions}"
+          "${concatStringsSep " " cfg.servers}"
+        ];
+      };
+    };
+
+  };
+
+}