anchor.diff:  let
   
     cfg = config.networking;
  +
  +  windowSize = if cfg.defaultGatewayWindowSize != "" then 
  +    "window ${cfg.defaultGatewayWindowSize}" else "";
  +
     interfaces = attrValues cfg.interfaces;
     hasVirtuals = any (i: i.virtual) interfaces;
   
  
positive.code:       example = "131.211.84.1";
        description = ''
          The default gateway.  It can be left empty if it is auto-detected through DHCP.
        '';
      };
  
  
negative.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.
  