More review comments.

cmd/outline-ss-server/config.go

@@ -90,9 +90,13 @@ func (c *ListenerConfig) UnmarshalYAML(value *yaml.Node) error {
 	if !ok {
 		return errors.New("`type` field required")
 	}
+	lnTypeStr, ok := rawType.(string)
+	if !ok {
+		return fmt.Errorf("`type` is not a string, but %T", rawType)	
+	}
+	lnType := ListenerType(lnTypeStr)
 	delete(raw, "type")
 
-	lnType := ListenerType(rawType.(string))
 	fieldName, ok := listenerTypeMap[lnType]
 	if !ok {
 		return fmt.Errorf("invalid listener type: %v", lnType)
@@ -123,8 +127,8 @@ func (c *ListenerConfig) validate() error {
 		if field.Kind() == reflect.Ptr && field.IsNil() {
 			continue
 		}
-		if field.Type().Implements(reflect.TypeOf((*Validator)(nil)).Elem()) {
-			if err := field.Interface().(Validator).validate(); err != nil {
+		if validator, ok := field.Interface().(Validator); ok {
+			if err := validator.validate(); err != nil {
 				return fmt.Errorf("invalid config: %v", err)
 			}
 		}

cmd/outline-ss-server/config_test.go

@@ -26,9 +26,24 @@ func TestConfigValidate(t *testing.T) {
 	t.Run("InvalidConfig/InvalidListenerType", func(t *testing.T) {
 		yaml := `
 services:
-	- listeners:
-		- type: foo
-			address: "[::]:9000"
+  - listeners:
+    - type:
+		- tcp
+		- udp
+	  address: "[::]:9000"
+`
+
+		_, err := readConfig([]byte(yaml))
+
+		require.Error(t, err)
+	})
+
+	t.Run("InvalidConfig/UnknownListenerType", func(t *testing.T) {
+		yaml := `
+services:
+  - listeners:
+    - type: foo
+	  address: "[::]:9000"
 `
 
 		_, err := readConfig([]byte(yaml))