anchor.diff:      };
   
       networking.defaultGatewayWindowSize = mkOption {
  -      default = "";
  -      example = "524288";
  +      default = null;
  +      example = 524288;
  +      type = types.nullOr types.int;
         description = ''
           The window size of the default gateway. It limits maximal data bursts that TCP peers
           are allowed to send to us.
  
positive.code:                 # Set the default gateway.
                  ${optionalString (cfg.defaultGateway != "") ''
                    # FIXME: get rid of "|| true" (necessary to make it idempotent).
                    ip route add default via "${cfg.defaultGateway}" ${windowSize} || true
                  ''}
  
                  # Turn on forwarding if any interface has enabled proxy_arp.
  
negative.code: let
  
    cfg = config.networking;
    interfaces = attrValues cfg.interfaces;
    hasVirtuals = any (i: i.virtual) interfaces;
  
  