Added support for cert-only login without user and password

Signed-off-by: Mihai Todor <todormihai@gmail.com>

auth.go

@@ -54,8 +54,23 @@ func (auth *AMQPlainAuth) Response() string {
 	return buf.String()[4:]
 }
 
+// ExternalAuth for RabbitMQ-auth-mechanism-ssl.
+type ExternalAuth struct {
+}
+
+// Mechanism returns "EXTERNAL"
+func (*ExternalAuth) Mechanism() string {
+	return "EXTERNAL"
+}
+
+// Response returns an AMQP encoded credentials table, without the field table size.
+func (*ExternalAuth) Response() string {
+	return "\000*\000*"
+}
+
 // Finds the first mechanism preferred by the client that the server supports.
 func pickSASLMechanism(client []Authentication, serverMechanisms []string) (auth Authentication, ok bool) {
+
 	for _, auth = range client {
 		for _, mech := range serverMechanisms {
 			if auth.Mechanism() == mech {

connection.go

@@ -157,6 +157,23 @@ func DialTLS(url string, amqps *tls.Config) (*Connection, error) {
 	})
 }
 
+// DialTLS_ExternalAuth accepts a string in the AMQP URI format and returns a
+// new Connection over TCP using EXTERNAL auth. Defaults to a server heartbeat
+// interval of 10 seconds and sets the initial read deadline to 30 seconds.
+//
+// This mechanism is used, when RabbitMQ is configured for EXTERNAL auth with
+// ssl_cert_login plugin for userless/passwordless logons
+//
+// DialTLS_ExternalAuth uses the provided tls.Config when encountering an
+// amqps:// scheme.
+func DialTLS_ExternalAuth(url string, amqps *tls.Config) (*Connection, error) {
+	return DialConfig(url, Config{
+		Heartbeat:       defaultHeartbeat,
+		TLSClientConfig: amqps,
+		SASL:            []Authentication{&ExternalAuth{}},
+	})
+}
+
 // DialConfig accepts a string in the AMQP URI format and a configuration for
 // the transport and connection setup, returning a new Connection.  Defaults to
 // a server heartbeat interval of 10 seconds and sets the initial read deadline