diff --git a/cmd/client.go b/cmd/client.go
index 2bc1f52..5cc6fe3 100644
--- a/cmd/client.go
+++ b/cmd/client.go
@@ -13,7 +13,6 @@ import (
 	"net"
 	"net/http"
 	"os"
-	"strings"
 	"time"
 
 	"github.com/lucas-clemente/quic-go"
@@ -28,7 +27,6 @@ import (
 	"github.com/tobyxdd/hysteria/pkg/socks5"
 	"github.com/tobyxdd/hysteria/pkg/tproxy"
 	"github.com/tobyxdd/hysteria/pkg/transport"
-	"github.com/tobyxdd/hysteria/pkg/tun"
 )
 
 func client(config *clientConfig) {
@@ -248,51 +246,7 @@ func client(config *clientConfig) {
 	}
 
 	if len(config.TUN.Name) != 0 {
-		go func() {
-			timeout := time.Duration(config.TUN.Timeout) * time.Second
-			if timeout == 0 {
-				timeout = 300 * time.Second
-			}
-			tunServer, err := tun.NewServer(client, time.Duration(config.TUN.Timeout)*time.Second,
-				config.TUN.Name, config.TUN.MTU)
-			if err != nil {
-				logrus.WithField("error", err).Fatal("Failed to initialize TUN server")
-			}
-			tunServer.RequestFunc = func(addr net.Addr, reqAddr string) {
-				logrus.WithFields(logrus.Fields{
-					"src": addr.String(),
-					"dst": reqAddr,
-				}).Debugf("TUN %s request", strings.ToUpper(addr.Network()))
-			}
-			tunServer.ErrorFunc = func(addr net.Addr, reqAddr string, err error) {
-				if err != nil {
-					if err == io.EOF {
-						logrus.WithFields(logrus.Fields{
-							"src": addr.String(),
-							"dst": reqAddr,
-						}).Debugf("TUN %s EOF", strings.ToUpper(addr.Network()))
-					} else if err == core.ErrClosed && strings.HasPrefix(addr.Network(), "udp") {
-						logrus.WithFields(logrus.Fields{
-							"src": addr.String(),
-							"dst": reqAddr,
-						}).Debugf("TUN %s closed for timeout", strings.ToUpper(addr.Network()))
-					} else if nErr, ok := err.(net.Error); ok && nErr.Timeout() && strings.HasPrefix(addr.Network(), "tcp") {
-						logrus.WithFields(logrus.Fields{
-							"src": addr.String(),
-							"dst": reqAddr,
-						}).Debugf("TUN %s closed for timeout", strings.ToUpper(addr.Network()))
-					} else {
-						logrus.WithFields(logrus.Fields{
-							"error": err,
-							"src":   addr.String(),
-							"dst":   reqAddr,
-						}).Infof("TUN %s error", strings.ToUpper(addr.Network()))
-					}
-				}
-			}
-			logrus.WithField("interface", config.TUN.Name).Info("TUN up and running")
-			errChan <- tunServer.ListenAndServe()
-		}()
+		go startTUN(config, client, errChan)
 	}
 
 	if len(config.TCPRelay.Listen) > 0 {
diff --git a/cmd/client_gpl.go b/cmd/client_gpl.go
new file mode 100644
index 0000000..5b633ff
--- /dev/null
+++ b/cmd/client_gpl.go
@@ -0,0 +1,77 @@
+//go:build gpl
+// +build gpl
+
+package main
+
+import (
+	"github.com/sirupsen/logrus"
+	"github.com/tobyxdd/hysteria/pkg/core"
+	"github.com/tobyxdd/hysteria/pkg/tun"
+	"io"
+	"net"
+	"strings"
+	"time"
+)
+
+const license = `Hysteria is a feature-packed proxy & relay utility optimized for lossy, unstable connections.
+Copyright (C) 2022  Toby
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <https://www.gnu.org/licenses/>.
+`
+
+func startTUN(config *clientConfig, client *core.Client, errChan chan error) {
+	timeout := time.Duration(config.TUN.Timeout) * time.Second
+	if timeout == 0 {
+		timeout = 300 * time.Second
+	}
+	tunServer, err := tun.NewServer(client, time.Duration(config.TUN.Timeout)*time.Second,
+		config.TUN.Name, config.TUN.MTU)
+	if err != nil {
+		logrus.WithField("error", err).Fatal("Failed to initialize TUN server")
+	}
+	tunServer.RequestFunc = func(addr net.Addr, reqAddr string) {
+		logrus.WithFields(logrus.Fields{
+			"src": addr.String(),
+			"dst": reqAddr,
+		}).Debugf("TUN %s request", strings.ToUpper(addr.Network()))
+	}
+	tunServer.ErrorFunc = func(addr net.Addr, reqAddr string, err error) {
+		if err != nil {
+			if err == io.EOF {
+				logrus.WithFields(logrus.Fields{
+					"src": addr.String(),
+					"dst": reqAddr,
+				}).Debugf("TUN %s EOF", strings.ToUpper(addr.Network()))
+			} else if err == core.ErrClosed && strings.HasPrefix(addr.Network(), "udp") {
+				logrus.WithFields(logrus.Fields{
+					"src": addr.String(),
+					"dst": reqAddr,
+				}).Debugf("TUN %s closed for timeout", strings.ToUpper(addr.Network()))
+			} else if nErr, ok := err.(net.Error); ok && nErr.Timeout() && strings.HasPrefix(addr.Network(), "tcp") {
+				logrus.WithFields(logrus.Fields{
+					"src": addr.String(),
+					"dst": reqAddr,
+				}).Debugf("TUN %s closed for timeout", strings.ToUpper(addr.Network()))
+			} else {
+				logrus.WithFields(logrus.Fields{
+					"error": err,
+					"src":   addr.String(),
+					"dst":   reqAddr,
+				}).Infof("TUN %s error", strings.ToUpper(addr.Network()))
+			}
+		}
+	}
+	logrus.WithField("interface", config.TUN.Name).Info("TUN up and running")
+	errChan <- tunServer.ListenAndServe()
+}
diff --git a/cmd/client_nongpl.go b/cmd/client_nongpl.go
new file mode 100644
index 0000000..ca1efb5
--- /dev/null
+++ b/cmd/client_nongpl.go
@@ -0,0 +1,36 @@
+//go:build !gpl
+// +build !gpl
+
+package main
+
+import (
+	"github.com/sirupsen/logrus"
+	"github.com/tobyxdd/hysteria/pkg/core"
+)
+
+const license = `The MIT License (MIT)
+
+Copyright (c) 2021 Toby
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+`
+
+func startTUN(config *clientConfig, client *core.Client, errChan chan error) {
+	logrus.Fatalln("TUN mode is only available in the GPL build, please rebuild hysteria with -tags gpl")
+}
diff --git a/cmd/main.go b/cmd/main.go
index b5d815b..abc3e4f 100644
--- a/cmd/main.go
+++ b/cmd/main.go
@@ -65,6 +65,12 @@ var rootCmd = &cobra.Command{
 			})
 		}
 
+		// license
+		if viper.GetBool("license") {
+			fmt.Printf("%s\n", license)
+			os.Exit(0)
+		}
+
 		// check update
 		if !viper.GetBool("no-check") {
 			go checkUpdate()
@@ -154,6 +160,7 @@ func init() {
 	rootCmd.PersistentFlags().String("log-timestamp", time.RFC3339, "log timestamp format")
 	rootCmd.PersistentFlags().String("log-format", "txt", "log output format(txt/json)")
 	rootCmd.PersistentFlags().Bool("no-check", false, "disable update check")
+	rootCmd.PersistentFlags().Bool("license", false, "show license and exit")
 
 	// add to root cmd
 	rootCmd.AddCommand(clientCmd, serverCmd, completionCmd)
@@ -165,6 +172,7 @@ func init() {
 	_ = viper.BindPFlag("log-timestamp", rootCmd.PersistentFlags().Lookup("log-timestamp"))
 	_ = viper.BindPFlag("log-format", rootCmd.PersistentFlags().Lookup("log-format"))
 	_ = viper.BindPFlag("no-check", rootCmd.PersistentFlags().Lookup("no-check"))
+	_ = viper.BindPFlag("license", rootCmd.PersistentFlags().Lookup("license"))
 
 	// bind env
 	_ = viper.BindEnv("config", "HYSTERIA_CONFIG")
diff --git a/pkg/tun/server.go b/pkg/tun/server.go
index 67a9fab..b3ed73d 100644
--- a/pkg/tun/server.go
+++ b/pkg/tun/server.go
@@ -1,3 +1,6 @@
+//go:build gpl
+// +build gpl
+
 package tun
 
 import (
diff --git a/pkg/tun/tcp.go b/pkg/tun/tcp.go
index 25e3b11..df61dcc 100644
--- a/pkg/tun/tcp.go
+++ b/pkg/tun/tcp.go
@@ -1,3 +1,6 @@
+//go:build gpl
+// +build gpl
+
 package tun
 
 import (
diff --git a/pkg/tun/udp.go b/pkg/tun/udp.go
index 7dfb532..167fbf3 100644
--- a/pkg/tun/udp.go
+++ b/pkg/tun/udp.go
@@ -1,3 +1,6 @@
+//go:build gpl
+// +build gpl
+
 package tun
 
 import (