diff --git a/cmd/outline-ss-server/config.go b/cmd/outline-ss-server/config.go
index 8d8cd37e..112ba58f 100644
--- a/cmd/outline-ss-server/config.go
+++ b/cmd/outline-ss-server/config.go
@@ -30,8 +30,12 @@ type Service struct {
 	Keys      []Key
 }
 
+type ListenerType string
+
+const listenerTypeDirect ListenerType = "direct"
+
 type Listener struct {
-	Type    string
+	Type    ListenerType
 	Address string
 }
 
@@ -79,8 +83,8 @@ func ReadConfig(filename string) (*Config, error) {
 	for port, keys := range ports {
 		s := Service{
 			Listeners: []Listener{
-				Listener{Type: "direct", Address: fmt.Sprintf("tcp://[::]:%d", port)},
-				Listener{Type: "direct", Address: fmt.Sprintf("udp://[::]:%d", port)},
+				Listener{Type: listenerTypeDirect, Address: fmt.Sprintf("tcp://[::]:%d", port)},
+				Listener{Type: listenerTypeDirect, Address: fmt.Sprintf("udp://[::]:%d", port)},
 			},
 			Keys: keys,
 		}
@@ -111,7 +115,7 @@ func validateListener(u *url.URL) error {
 	return nil
 }
 
-func NewListener(addr string) (io.Closer, error) {
+func newListener(addr string) (io.Closer, error) {
 	u, err := url.Parse(addr)
 	if err != nil {
 		return nil, err
diff --git a/cmd/outline-ss-server/config_test.go b/cmd/outline-ss-server/config_test.go
index 329c517f..88660388 100644
--- a/cmd/outline-ss-server/config_test.go
+++ b/cmd/outline-ss-server/config_test.go
@@ -29,8 +29,8 @@ func TestReadConfig(t *testing.T) {
 		Services: []Service{
 			Service{
 				Listeners: []Listener{
-					Listener{Type: "direct", Address: "tcp://[::]:9000"},
-					Listener{Type: "direct", Address: "udp://[::]:9000"},
+					Listener{Type: listenerTypeDirect, Address: "tcp://[::]:9000"},
+					Listener{Type: listenerTypeDirect, Address: "udp://[::]:9000"},
 				},
 				Keys: []Key{
 					Key{"user-0", "chacha20-ietf-poly1305", "Secret0"},
@@ -39,8 +39,8 @@ func TestReadConfig(t *testing.T) {
 			},
 			Service{
 				Listeners: []Listener{
-					Listener{Type: "direct", Address: "tcp://[::]:9001"},
-					Listener{Type: "direct", Address: "udp://[::]:9001"},
+					Listener{Type: listenerTypeDirect, Address: "tcp://[::]:9001"},
+					Listener{Type: listenerTypeDirect, Address: "udp://[::]:9001"},
 				},
 				Keys: []Key{
 					Key{"user-2", "chacha20-ietf-poly1305", "Secret2"},
@@ -59,8 +59,8 @@ func TestReadConfigParsesDeprecatedFormat(t *testing.T) {
 		Services: []Service{
 			Service{
 				Listeners: []Listener{
-					Listener{Type: "direct", Address: "tcp://[::]:9000"},
-					Listener{Type: "direct", Address: "udp://[::]:9000"},
+					Listener{Type: listenerTypeDirect, Address: "tcp://[::]:9000"},
+					Listener{Type: listenerTypeDirect, Address: "udp://[::]:9000"},
 				},
 				Keys: []Key{
 					Key{"user-0", "chacha20-ietf-poly1305", "Secret0"},
@@ -69,8 +69,8 @@ func TestReadConfigParsesDeprecatedFormat(t *testing.T) {
 			},
 			Service{
 				Listeners: []Listener{
-					Listener{Type: "direct", Address: "tcp://[::]:9001"},
-					Listener{Type: "direct", Address: "udp://[::]:9001"},
+					Listener{Type: listenerTypeDirect, Address: "tcp://[::]:9001"},
+					Listener{Type: listenerTypeDirect, Address: "udp://[::]:9001"},
 				},
 				Keys: []Key{
 					Key{"user-2", "chacha20-ietf-poly1305", "Secret2"},
diff --git a/cmd/outline-ss-server/main.go b/cmd/outline-ss-server/main.go
index 323bcda7..2af841cc 100644
--- a/cmd/outline-ss-server/main.go
+++ b/cmd/outline-ss-server/main.go
@@ -49,8 +49,6 @@ const tcpReadTimeout time.Duration = 59 * time.Second
 // A UDP NAT timeout of at least 5 minutes is recommended in RFC 4787 Section 4.3.
 const defaultNatTimeout time.Duration = 5 * time.Minute
 
-var directListenerType = "direct"
-
 func init() {
 	var prefix = "%{level:.1s}%{time:2006-01-02T15:04:05.000Z07:00} %{pid} %{shortfile}]"
 	if term.IsTerminal(int(os.Stderr.Fd())) {
@@ -98,7 +96,7 @@ func (s *SSServer) serve(listener io.Closer, cipherList service.CipherList) erro
 }
 
 func (s *SSServer) start(addr string, cipherList service.CipherList) (io.Closer, error) {
-	listener, err := NewListener(addr)
+	listener, err := newListener(addr)
 	if err != nil {
 		//lint:ignore ST1005 Shadowsocks is capitalized.
 		return nil, fmt.Errorf("Shadowsocks service failed to start on address %v: %w", addr, err)
@@ -107,7 +105,7 @@ func (s *SSServer) start(addr string, cipherList service.CipherList) (io.Closer,
 
 	err = s.serve(listener, cipherList)
 	if err != nil {
-		return nil, fmt.Errorf("failed to serve on listener %w: %w", listener, err)
+		return nil, fmt.Errorf("failed to serve on listener %v: %w", listener, err)
 	}
 
 	return listener, nil
@@ -168,7 +166,7 @@ func (s *SSServer) loadConfig(filename string) error {
 		for _, listener := range serviceConfig.Listeners {
 			switch t := listener.Type; t {
 			// TODO: Support more listener types.
-			case directListenerType:
+			case listenerTypeDirect:
 				addrChanges[listener.Address] = 1
 				addrCiphers[listener.Address] = ciphers
 			default: