anchor.diff:        '';
       };
       firewallFilter = mkOption {
  -      type = types.str;
  -      default = "-p tcp -m tcp --dport ${toString cfg.${name}.port}";
  -      defaultText = "-p tcp -m tcp --dport ${toString port}";
  +      type = types.nullOr types.str;
  +      default = null;
         example = literalExample ''
           "-i eth0 -p tcp -m tcp --dport ${toString port}"
         '';
  
positive.code: let
    inherit (lib) concatStrings foldl foldl' genAttrs literalExample maintainers
                  mapAttrsToList mkDefault mkEnableOption mkIf mkMerge mkOption
                  optional types;
  
    cfg = config.services.prometheus.exporters;
  
  
negative.code: 
    mkSubModule = { name, port, extraOpts, imports }: {
      ${name} = mkOption {
        type = types.submodule [{
          inherit imports;
          options = (mkExporterOpts {
            inherit name port;
          } // extraOpts);
        } ({ config, ... }: mkIf config.openFirewall {
          firewallFilter = mkOptionDefault "-p tcp -m tcp --dport ${toString config.port}";
        })];
        internal = true;
        default = {};
      };
  