code: func NetworkOptionInternalNetwork() NetworkOption {
  	return func(n *network) {
  		if n.generic == nil {
  			n.generic = make(map[string]interface{})
  		}
  		n.internal = true
  		n.generic[netlabel.Internal] = true
  	}
  }
docs: // NetworkOptionInternalNetwork returns an option setter to config the network
  // to be internal which disables default gateway service