nixos/timesyncd: allow NTP servers advertised by DHCP to be used

- add option `fallbackServers` with default to `networking.timeServers`
- option `servers` now default to null

Fix #335050

nixos/doc/manual/release-notes/rl-2411.section.md

@@ -448,6 +448,10 @@
 
 - The hooks `yarnConfigHook` and `yarnBuildHook` were added. These should replace `yarn2nix.mkYarnPackage` and other `yarn2nix` related tools. The motivation to get rid of `yarn2nix` tools is the fact that they are too complex and hard to maintain, and they rely upon too much Nix evaluation which is problematic if import-from-derivation is not allowed (see more details at [#296856](https://github.com/NixOS/nixpkgs/issues/296856). The transition from `mkYarnPackage` to `yarn{Config,Build}Hook` is tracked at [#324246](https://github.com/NixOS/nixpkgs/issues/324246).
 
+- `services.timesyncd.servers` now defaults to `null`, allowing systemd-timesyncd to use NTP servers advertised by DHCP.
+
+- `services.timesyncd.fallbackServers` was added and defaults to `networking.timeServers`.
+
 - Cinnamon has been updated to 6.2, please check [upstream announcement](https://www.linuxmint.com/rel_wilma_whatsnew.php) for more details.
   Following Mint 22 defaults, the Cinnamon module no longer ships geary and hexchat by default.
 

nixos/modules/system/boot/timesyncd.nix

@@ -19,8 +19,7 @@ in
         '';
       };
       servers = mkOption {
-        default = config.networking.timeServers;
-        defaultText = literalExpression "config.networking.timeServers";
+        default = null;
         type = nullOr (listOf str);
         description = ''
           The set of NTP servers from which to synchronise.
@@ -32,6 +31,20 @@ in
           See man:timesyncd.conf(5) for details.
         '';
       };
+      fallbackServers = mkOption {
+        default = config.networking.timeServers;
+        defaultText = literalExpression "config.networking.timeServers";
+        type = nullOr (listOf str);
+        description = ''
+          The set of fallback NTP servers from which to synchronise.
+
+          Setting this option to an empty list will write `FallbackNTP=` to the
+          `timesyncd.conf` file as opposed to setting this option to null which
+          will remove `FallbackNTP=` entirely.
+
+          See man:timesyncd.conf(5) for details.
+        '';
+      };
       extraConfig = mkOption {
         default = "";
         type = lines;
@@ -92,6 +105,9 @@ in
     + optionalString (cfg.servers != null) ''
       NTP=${concatStringsSep " " cfg.servers}
     ''
+    + optionalString (cfg.fallbackServers != null) ''
+      FallbackNTP=${concatStringsSep " " cfg.fallbackServers}
+    ''
     + cfg.extraConfig;
 
     users.users.systemd-timesync = {