tun: import mobile particularities

tun/tun_darwin.go

@@ -171,7 +171,7 @@ func CreateTUNFromFile(file *os.File, mtu int) (TUNDevice, error) {
 	tun := &NativeTun{
 		tunFile: file,
 		events:  make(chan TUNEvent, 10),
-		errors:  make(chan error, 1),
+		errors:  make(chan error, 5),
 	}
 
 	name, err := tun.Name()
@@ -200,10 +200,12 @@ func CreateTUNFromFile(file *os.File, mtu int) (TUNDevice, error) {
 
 	go tun.routineRouteListener(tunIfindex)
 
-	err = tun.setMTU(mtu)
-	if err != nil {
-		tun.Close()
-		return nil, err
+	if mtu > 0 {
+		err = tun.setMTU(mtu)
+		if err != nil {
+			tun.Close()
+			return nil, err
+		}
 	}
 
 	return tun, nil

tun/tun_linux.go

@@ -443,3 +443,25 @@ func CreateTUNFromFile(file *os.File, mtu int) (TUNDevice, error) {
 
 	return tun, nil
 }
+
+func CreateUnmonitoredTUNFromFD(tunFd int) (TUNDevice, string, error) {
+	file := os.NewFile(uintptr(tunFd), "/dev/tun")
+	tun := &NativeTun{
+		tunFile: file,
+		fd:      file.Fd(),
+		events:  make(chan TUNEvent, 5),
+		errors:  make(chan error, 5),
+		nopi:    true,
+	}
+	var err error
+	tun.fdCancel, err = rwcancel.NewRWCancel(int(tun.fd))
+	if err != nil {
+		return nil, "", err
+	}
+	name, err := tun.Name()
+	if err != nil {
+		tun.fdCancel.Cancel()
+		return nil, "", err
+	}
+	return tun, name, nil
+}