enable TLS 1.3 by default

transport/internet/tls/tls.go

@@ -4,6 +4,7 @@ package tls
 
 import (
 	"crypto/tls"
+	"os"
 
 	"v2ray.com/core/common/buf"
 	"v2ray.com/core/common/net"
@@ -65,3 +66,9 @@ func Server(c net.Conn, config *tls.Config) net.Conn {
 	tlsConn := tls.Server(c, config)
 	return &conn{Conn: tlsConn}
 }
+
+func init() {
+	// opt-in TLS 1.3 for Go1.12
+	// TODO: remove this line when Go1.13 is released.
+	_ = os.Setenv("GODEBUG", os.Getenv("GODEBUG")+",tls13=1")
+}