add resolver for cmd

pkg/resolver/exchanger/exchanger.go

@@ -71,6 +71,8 @@ type exchanger struct {
 }
 
 // NewExchanger create an Exchanger.
+// The addr should be URL-like format,
+// e.g. udp://1.1.1.1:53, tls://1.1.1.1:853, https://1.0.0.1/dns-query
 func NewExchanger(addr string, opts ...Option) (Exchanger, error) {
 	var options Options
 	for _, opt := range opts {
@@ -85,6 +87,10 @@ func NewExchanger(addr string, opts ...Option) (Exchanger, error) {
 		return nil, err
 	}
 
+	if options.timeout <= 0 {
+		options.timeout = 5 * time.Second
+	}
+
 	ex := &exchanger{
 		network: u.Scheme,
 		addr:    u.Host,