automatically set the tls.Config.ServerName if unset (#4032)

client.go

@@ -55,11 +55,11 @@ func DialAddr(ctx context.Context, addr string, tlsConf *tls.Config, conf *Confi
 	if err != nil {
 		return nil, err
 	}
-	dl, err := setupTransport(udpConn, tlsConf, true)
+	tr, err := setupTransport(udpConn, tlsConf, true)
 	if err != nil {
 		return nil, err
 	}
-	return dl.Dial(ctx, udpAddr, tlsConf, conf)
+	return tr.dial(ctx, udpAddr, addr, tlsConf, conf, false)
 }
 
 // DialAddrEarly establishes a new 0-RTT QUIC connection to a server.
@@ -73,13 +73,13 @@ func DialAddrEarly(ctx context.Context, addr string, tlsConf *tls.Config, conf *
 	if err != nil {
 		return nil, err
 	}
-	dl, err := setupTransport(udpConn, tlsConf, true)
+	tr, err := setupTransport(udpConn, tlsConf, true)
 	if err != nil {
 		return nil, err
 	}
-	conn, err := dl.DialEarly(ctx, udpAddr, tlsConf, conf)
+	conn, err := tr.dial(ctx, udpAddr, addr, tlsConf, conf, true)
 	if err != nil {
-		dl.Close()
+		tr.Close()
 		return nil, err
 	}
 	return conn, nil
@@ -163,12 +163,6 @@ func dial(
 }
 
 func newClient(sendConn sendConn, connIDGenerator ConnectionIDGenerator, config *Config, tlsConf *tls.Config, onClose func(), use0RTT bool) (*client, error) {
-	if tlsConf == nil {
-		tlsConf = &tls.Config{}
-	} else {
-		tlsConf = tlsConf.Clone()
-	}
-
 	srcConnID, err := connIDGenerator.GenerateConnectionID()
 	if err != nil {
 		return nil, err

transport.go

@@ -148,24 +148,15 @@ func (t *Transport) ListenEarly(tlsConf *tls.Config, conf *Config) (*EarlyListen
 
 // Dial dials a new connection to a remote host (not using 0-RTT).
 func (t *Transport) Dial(ctx context.Context, addr net.Addr, tlsConf *tls.Config, conf *Config) (Connection, error) {
-	if err := validateConfig(conf); err != nil {
-		return nil, err
-	}
-	conf = populateConfig(conf)
-	if err := t.init(t.isSingleUse); err != nil {
-		return nil, err
-	}
-	var onClose func()
-	if t.isSingleUse {
-		onClose = func() { t.Close() }
-	}
-	tlsConf = tlsConf.Clone()
-	tlsConf.MinVersion = tls.VersionTLS13
-	return dial(ctx, newSendConn(t.conn, addr), t.connIDGenerator, t.handlerMap, tlsConf, conf, onClose, false)
+	return t.dial(ctx, addr, "", tlsConf, conf, false)
 }
 
 // DialEarly dials a new connection, attempting to use 0-RTT if possible.
 func (t *Transport) DialEarly(ctx context.Context, addr net.Addr, tlsConf *tls.Config, conf *Config) (EarlyConnection, error) {
+	return t.dial(ctx, addr, "", tlsConf, conf, true)
+}
+
+func (t *Transport) dial(ctx context.Context, addr net.Addr, hostname string, tlsConf *tls.Config, conf *Config, use0RTT bool) (EarlyConnection, error) {
 	if err := validateConfig(conf); err != nil {
 		return nil, err
 	}
@@ -179,7 +170,16 @@ func (t *Transport) DialEarly(ctx context.Context, addr net.Addr, tlsConf *tls.C
 	}
 	tlsConf = tlsConf.Clone()
 	tlsConf.MinVersion = tls.VersionTLS13
-	return dial(ctx, newSendConn(t.conn, addr), t.connIDGenerator, t.handlerMap, tlsConf, conf, onClose, true)
+	// If no ServerName is set, infer the ServerName from the hostname we're connecting to.
+	if tlsConf.ServerName == "" {
+		if hostname == "" {
+			if udpAddr, ok := addr.(*net.UDPAddr); ok {
+				hostname = udpAddr.IP.String()
+			}
+		}
+		tlsConf.ServerName = hostname
+	}
+	return dial(ctx, newSendConn(t.conn, addr), t.connIDGenerator, t.handlerMap, tlsConf, conf, onClose, use0RTT)
 }
 
 func (t *Transport) init(allowZeroLengthConnIDs bool) error {