nixos/ntp: use upstream default restrictions to avoid DDoS (#50762)

Fixes #50732

nixos/doc/manual/release-notes/rl-1903.xml

@@ -111,6 +111,16 @@
       without Syncthing resetting the permission on every start.
     </para>
    </listitem>
+   <listitem>
+    <para>
+      The <literal>ntp</literal> module now has sane default restrictions.
+      If you're relying on the previous defaults, which permitted all queries
+      and commands from all firewall-permitted sources, you can set
+      <varname>services.ntp.restrictDefault</varname> and
+      <varname>services.ntp.restrictSource</varname> to
+      <literal>[]</literal>.
+    </para>
+   </listitem>
    <listitem>
     <para>
      Package <varname>rabbitmq_server</varname> is renamed to

nixos/modules/services/networking/ntpd.nix

@@ -15,6 +15,10 @@ let
   configFile = pkgs.writeText "ntp.conf" ''
     driftfile ${stateDir}/ntp.drift
 
+    restrict default ${toString cfg.restrictDefault}
+    restrict -6 default ${toString cfg.restrictDefault}
+    restrict source ${toString cfg.restrictSource}
+
     restrict 127.0.0.1
     restrict -6 ::1
 
@@ -36,11 +40,40 @@ in
       enable = mkOption {
         default = false;
         description = ''
-          Whether to synchronise your machine's time using the NTP
+          Whether to synchronise your machine's time using ntpd, as a peer in
-          protocol.
+          the NTP network.
+          </para>
+          <para>
+          Disables <literal>systemd.timesyncd</literal> if enabled.
         '';
       };
 
+      restrictDefault = mkOption {
+        type = types.listOf types.str;
+        description = ''
+          The restriction flags to be set by default.
+          </para>
+          <para>
+          The default flags prevent external hosts from using ntpd as a DDoS
+          reflector, setting system time, and querying OS/ntpd version. As
+          recommended in section 6.5.1.1.3, answer "No" of
+          http://support.ntp.org/bin/view/Support/AccessRestrictions
+        '';
+        default = [ "limited" "kod" "nomodify" "notrap" "noquery" "nopeer" ];
+      };
+
+      restrictSource = mkOption {
+        type = types.listOf types.str;
+        description = ''
+          The restriction flags to be set on source.
+          </para>
+          <para>
+          The default flags allow peers to be added by ntpd from configured
+          pool(s), but not by other means.
+        '';
+        default = [ "limited" "kod" "nomodify" "notrap" "noquery" ];
+      };
+
       servers = mkOption {
         default = config.networking.timeServers;
         description = ''
@@ -51,6 +84,7 @@ in
       extraFlags = mkOption {
         type = types.listOf types.str;
         description = "Extra flags passed to the ntpd command.";
+        example = literalExample ''[ "--interface=eth0" ]'';
         default = [];
       };