nixos/dnsmasq: Use attrs instead of plain text config

This should make it easier to configure in multiple places, override
defaults, etc.

nixos/doc/manual/from_md/release-notes/rl-2305.section.xml

···
       </listitem>
       <listitem>
         <para>
+          The <literal>dnsmasq</literal> service now takes configuration
+          via the <literal>services.dnsmasq.settings</literal> attribute
+          set. The option
+          <literal>services.dnsmasq.extraConfig</literal> will be
+          deprecated when NixOS 22.11 reaches end of life.
+        </para>
+      </listitem>
+      <listitem>
+        <para>
           A new <literal>virtualisation.rosetta</literal> module was
           added to allow running <literal>x86_64</literal> binaries
           through

nixos/doc/manual/release-notes/rl-2305.section.md

···
 
 - `services.mastodon` gained a tootctl wrapped named `mastodon-tootctl` similar to `nextcloud-occ` which can be executed from any user and switches to the configured mastodon user with sudo and sources the environment variables.
 
+- The `dnsmasq` service now takes configuration via the
+  `services.dnsmasq.settings` attribute set. The option
+  `services.dnsmasq.extraConfig` will be deprecated when NixOS 22.11 reaches
+  end of life.
+
 - A new `virtualisation.rosetta` module was added to allow running `x86_64` binaries through [Rosetta](https://developer.apple.com/documentation/apple-silicon/about-the-rosetta-translation-environment) inside virtualised NixOS guests on Apple silicon. This feature works by default with the [UTM](https://docs.getutm.app/) virtualisation [package](https://search.nixos.org/packages?channel=unstable&show=utm&from=0&size=1&sort=relevance&type=packages&query=utm).
 
 - Resilio sync secret keys can now be provided using a secrets file at runtime, preventing these secrets from ending up in the Nix store.

nixos/modules/services/networking/dnsmasq.nix

···
   dnsmasq = pkgs.dnsmasq;
   stateDir = "/var/lib/dnsmasq";
 
+  # True values are just put as `name` instead of `name=true`, and false values
+  # are turned to comments (false values are expected to be overrides e.g.
+  # mkForce)
+  formatKeyValue =
+    name: value:
+    if value == true
+    then name
+    else if value == false
+    then "# setting `${name}` explicitly set to false"
+    else generators.mkKeyValueDefault { } "=" name value;
+
+  settingsFormat = pkgs.formats.keyValue {
+    mkKeyValue = formatKeyValue;
+    listsAsDuplicateKeys = true;
+  };
+
+  # Because formats.generate is outputting a file, we use of conf-file. Once
+  # `extraConfig` is deprecated we can just use
+  # `dnsmasqConf = format.generate "dnsmasq.conf" cfg.settings`
   dnsmasqConf = pkgs.writeText "dnsmasq.conf" ''
-    dhcp-leasefile=${stateDir}/dnsmasq.leases
-    ${optionalString cfg.resolveLocalQueries ''
-      conf-file=/etc/dnsmasq-conf.conf
-      resolv-file=/etc/dnsmasq-resolv.conf
-    ''}
-    ${flip concatMapStrings cfg.servers (server: ''
-      server=${server}
-    '')}
+    conf-file=${settingsFormat.generate "dnsmasq.conf" cfg.settings}
     ${cfg.extraConfig}
   '';
 
 in
 
 {
+
+  imports = [
+    (mkRenamedOptionModule [ "services" "dnsmasq" "servers" ] [ "services" "dnsmasq" "settings" "server" ])
+  ];
 
   ###### interface
 
···
         '';
       };
 
-      servers = mkOption {
-        type = types.listOf types.str;
-        default = [];
-        example = [ "8.8.8.8" "8.8.4.4" ];
-        description = lib.mdDoc ''
-          The DNS servers which dnsmasq should query.
-        '';
-      };
-
       alwaysKeepRunning = mkOption {
         type = types.bool;
         default = false;
···
         '';
       };
 
+      settings = mkOption {
+        type = types.submodule {
+
+          freeformType = settingsFormat.type;
+
+          options.server = mkOption {
+            type = types.listOf types.str;
+            default = [ ];
+            example = [ "8.8.8.8" "8.8.4.4" ];
+            description = lib.mdDoc ''
+              The DNS servers which dnsmasq should query.
+            '';
+          };
+
+        };
+        default = { };
+        description = lib.mdDoc ''
+          Configuration of dnsmasq. Lists get added one value per line (empty
+          lists and false values don't get added, though false values get
+          turned to comments). Gets merged with
+
+              {
+                dhcp-leasefile = "${stateDir}/dnsmasq.leases";
+                conf-file = optional cfg.resolveLocalQueries "/etc/dnsmasq-conf.conf";
+                resolv-file = optional cfg.resolveLocalQueries "/etc/dnsmasq-resolv.conf";
+              }
+        '';
+        example = literalExpression ''
+          {
+            domain-needed = true;
+            dhcp-range = [ "192.168.0.2,192.168.0.254" ];
+          }
+        '';
+      };
+
       extraConfig = mkOption {
         type = types.lines;
         default = "";
         description = lib.mdDoc ''
           Extra configuration directives that should be added to
           `dnsmasq.conf`.
+
+          This option is deprecated, please use {option}`settings` instead.
         '';
       };
 
···
   ###### implementation
 
   config = mkIf cfg.enable {
+
+    warnings = lib.optional (cfg.extraConfig != "") "Text based config is deprecated, dnsmasq now supports `services.dnsmasq.settings` for an attribute-set based config";
+
+    services.dnsmasq.settings = {
+      dhcp-leasefile = mkDefault "${stateDir}/dnsmasq.leases";
+      conf-file = mkDefault (optional cfg.resolveLocalQueries "/etc/dnsmasq-conf.conf");
+      resolv-file = mkDefault (optional cfg.resolveLocalQueries "/etc/dnsmasq-resolv.conf");
+    };
 
     networking.nameservers =
       optional cfg.resolveLocalQueries "127.0.0.1";

nixos/tests/dnscrypt-proxy2.nix

···
       };
 
       services.dnsmasq.enable = true;
-      services.dnsmasq.servers = [ "127.0.0.1#${toString localProxyPort}" ];
+      services.dnsmasq.settings.server = [ "127.0.0.1#${toString localProxyPort}" ];
     };
   };
 

nixos/tests/kubernetes/dns.nix

···
   extraConfiguration = { config, pkgs, lib, ... }: {
     environment.systemPackages = [ pkgs.bind.host ];
     services.dnsmasq.enable = true;
-    services.dnsmasq.servers = [
+    services.dnsmasq.settings.server = [
       "/cluster.local/${config.services.kubernetes.addons.dns.clusterIp}#53"
     ];
   };

nixos/tests/schleuder.nix

···
     # Since we don't have internet here, use dnsmasq to provide MX records from /etc/hosts
     services.dnsmasq = {
       enable = true;
-      extraConfig = ''
-        selfmx
-      '';
+      settings.selfmx = true;
     };
 
     networking.extraHosts = ''