fix: defaultNS not working in system dns

Author: wwqgtxx

dns/system.go

@@ -31,12 +31,12 @@ type systemClient struct {
 
 func (c *systemClient) ExchangeContext(ctx context.Context, m *D.Msg) (msg *D.Msg, err error) {
 	dnsClients, err := c.getDnsClients()
+	if len(dnsClients) == 0 && len(c.defaultNS) > 0 {
+		dnsClients = c.defaultNS
+		err = nil
+	}
 	if err != nil {
-		if len(c.defaultNS) > 0 {
-			dnsClients = c.defaultNS
-		} else {
-			return
-		}
+		return
 	}
 	msg, _, err = batchExchange(ctx, dnsClients, m)
 	return
@@ -45,11 +45,16 @@ func (c *systemClient) ExchangeContext(ctx context.Context, m *D.Msg) (msg *D.Ms
 // Address implements dnsClient
 func (c *systemClient) Address() string {
 	dnsClients, _ := c.getDnsClients()
+	isDefault := ""
+	if len(dnsClients) == 0 && len(c.defaultNS) > 0 {
+		dnsClients = c.defaultNS
+		isDefault = "[defaultNS]"
+	}
 	addrs := make([]string, 0, len(dnsClients))
 	for _, c := range dnsClients {
 		addrs = append(addrs, c.Address())
 	}
-	return fmt.Sprintf("system(%s)", strings.Join(addrs, ","))
+	return fmt.Sprintf("system%s(%s)", isDefault, strings.Join(addrs, ","))
 }
 
 var _ dnsClient = (*systemClient)(nil)