Try to make the tun replacable

device/middle_device.go

@@ -1,9 +1,12 @@
 package device
 
 import (
+	"io"
 	"net/netip"
 	"os"
 	"sync"
+	"sync/atomic"
+	"time"
 
 	"github.com/fosrl/newt/logger"
 	"golang.zx2c4.com/wireguard/tun"
@@ -18,14 +21,68 @@ type FilterRule struct {
 	Handler PacketHandler
 }
 
-// MiddleDevice wraps a TUN device with packet filtering capabilities
-type MiddleDevice struct {
+// closeAwareDevice wraps a tun.Device along with a flag
+// indicating whether its Close method was called.
+type closeAwareDevice struct {
+	isClosed     atomic.Bool
 	tun.Device
-	rules    []FilterRule
-	mutex    sync.RWMutex
-	readCh   chan readResult
-	injectCh chan []byte
-	closed   chan struct{}
+	closeEventCh chan struct{}
+	wg           sync.WaitGroup
+	closeOnce    sync.Once
+}
+
+func newCloseAwareDevice(tunDevice tun.Device) *closeAwareDevice {
+	return &closeAwareDevice{
+		Device:       tunDevice,
+		isClosed:     atomic.Bool{},
+		closeEventCh: make(chan struct{}),
+	}
+}
+
+// redirectEvents redirects the Events() method of the underlying tun.Device
+// to the given channel.
+func (c *closeAwareDevice) redirectEvents(out chan tun.Event) {
+	c.wg.Add(1)
+	go func() {
+		defer c.wg.Done()
+		for {
+			select {
+			case ev, ok := <-c.Device.Events():
+				if !ok {
+					return
+				}
+
+				if ev == tun.EventDown {
+					continue
+				}
+
+				select {
+				case out <- ev:
+				case <-c.closeEventCh:
+					return
+				}
+			case <-c.closeEventCh:
+				return
+			}
+		}
+	}()
+}
+
+// Close calls the underlying Device's Close method
+// after setting isClosed to true.
+func (c *closeAwareDevice) Close() (err error) {
+	c.closeOnce.Do(func() {
+		c.isClosed.Store(true)
+		close(c.closeEventCh)
+		err = c.Device.Close()
+		c.wg.Wait()
+	})
+
+	return err
+}
+
+func (c *closeAwareDevice) IsClosed() bool {
+	return c.isClosed.Load()
 }
 
 type readResult struct {
@@ -36,58 +93,124 @@ type readResult struct {
 	err    error
 }
 
+// MiddleDevice wraps a TUN device with packet filtering capabilities
+// and supports swapping the underlying device.
+type MiddleDevice struct {
+	devices    []*closeAwareDevice
+	mu         sync.Mutex
+	cond       *sync.Cond
+	rules      []FilterRule
+	rulesMutex sync.RWMutex
+	readCh     chan readResult
+	injectCh   chan []byte
+	closed     atomic.Bool
+	events     chan tun.Event
+}
+
 // NewMiddleDevice creates a new filtered TUN device wrapper
 func NewMiddleDevice(device tun.Device) *MiddleDevice {
 	d := &MiddleDevice{
-		Device:   device,
+		devices:  make([]*closeAwareDevice, 0),
 		rules:    make([]FilterRule, 0),
-		readCh:   make(chan readResult),
+		readCh:   make(chan readResult, 16),
 		injectCh: make(chan []byte, 100),
-		closed:   make(chan struct{}),
+		events:   make(chan tun.Event, 16),
 	}
-	go d.pump()
+	d.cond = sync.NewCond(&d.mu)
+
+	if device != nil {
+		d.AddDevice(device)
+	}
+
 	return d
 }
 
-func (d *MiddleDevice) pump() {
+// AddDevice adds a new underlying TUN device, closing any previous one
+func (d *MiddleDevice) AddDevice(device tun.Device) {
+	d.mu.Lock()
+	if d.closed.Load() {
+		d.mu.Unlock()
+		_ = device.Close()
+		return
+	}
+
+	var toClose *closeAwareDevice
+	if len(d.devices) > 0 {
+		toClose = d.devices[len(d.devices)-1]
+	}
+
+	cad := newCloseAwareDevice(device)
+	cad.redirectEvents(d.events)
+
+	d.devices = []*closeAwareDevice{cad}
+
+	// Start pump for the new device
+	go d.pump(cad)
+
+	d.cond.Broadcast()
+	d.mu.Unlock()
+
+	if toClose != nil {
+		logger.Debug("MiddleDevice: Closing previous device")
+		if err := toClose.Close(); err != nil {
+			logger.Debug("MiddleDevice: Error closing previous device: %v", err)
+		}
+	}
+}
+
+func (d *MiddleDevice) pump(dev *closeAwareDevice) {
 	const defaultOffset = 16
-	batchSize := d.Device.BatchSize()
-	logger.Debug("MiddleDevice: pump started")
+	batchSize := dev.BatchSize()
+	logger.Debug("MiddleDevice: pump started for device")
 
 	for {
-		// Check closed first with priority
-		select {
-		case <-d.closed:
-			logger.Debug("MiddleDevice: pump exiting due to closed channel")
+		// Check if this device is closed
+		if dev.IsClosed() {
+			logger.Debug("MiddleDevice: pump exiting, device is closed")
+			return
+		}
+
+		// Check if MiddleDevice itself is closed
+		if d.closed.Load() {
+			logger.Debug("MiddleDevice: pump exiting, MiddleDevice is closed")
 			return
-		default:
 		}
 
 		// Allocate buffers for reading
-		// We allocate new buffers for each read to avoid race conditions
-		// since we pass them to the channel
 		bufs := make([][]byte, batchSize)
 		sizes := make([]int, batchSize)
 		for i := range bufs {
 			bufs[i] = make([]byte, 2048) // Standard MTU + headroom
 		}
 
-		n, err := d.Device.Read(bufs, sizes, defaultOffset)
+		n, err := dev.Read(bufs, sizes, defaultOffset)
 
-		// Check closed again after read returns
-		select {
-		case <-d.closed:
-			logger.Debug("MiddleDevice: pump exiting due to closed channel (after read)")
+		// Check if device was closed during read
+		if dev.IsClosed() {
+			logger.Debug("MiddleDevice: pump exiting, device closed during read")
 			return
-		default:
 		}
 
-		// Now try to send the result
+		// Check if MiddleDevice was closed during read
+		if d.closed.Load() {
+			logger.Debug("MiddleDevice: pump exiting, MiddleDevice closed during read")
+			return
+		}
+
+		// Try to send the result
 		select {
 		case d.readCh <- readResult{bufs: bufs, sizes: sizes, offset: defaultOffset, n: n, err: err}:
-		case <-d.closed:
-			logger.Debug("MiddleDevice: pump exiting due to closed channel (during send)")
-			return
+		default:
+			// Channel full, check if we should exit
+			if dev.IsClosed() || d.closed.Load() {
+				return
+			}
+			// Try again with blocking
+			select {
+			case d.readCh <- readResult{bufs: bufs, sizes: sizes, offset: defaultOffset, n: n, err: err}:
+			case <-dev.closeEventCh:
+				return
+			}
 		}
 
 		if err != nil {
@@ -99,16 +222,21 @@ func (d *MiddleDevice) pump() {
 
 // InjectOutbound injects a packet to be read by WireGuard (as if it came from TUN)
 func (d *MiddleDevice) InjectOutbound(packet []byte) {
+	if d.closed.Load() {
+		return
+	}
 	select {
 	case d.injectCh <- packet:
-	case <-d.closed:
+	default:
+		// Channel full, drop packet
+		logger.Debug("MiddleDevice: InjectOutbound dropping packet, channel full")
 	}
 }
 
 // AddRule adds a packet filtering rule
 func (d *MiddleDevice) AddRule(destIP netip.Addr, handler PacketHandler) {
-	d.mutex.Lock()
-	defer d.mutex.Unlock()
+	d.rulesMutex.Lock()
+	defer d.rulesMutex.Unlock()
 	d.rules = append(d.rules, FilterRule{
 		DestIP:  destIP,
 		Handler: handler,
@@ -117,8 +245,8 @@ func (d *MiddleDevice) AddRule(destIP netip.Addr, handler PacketHandler) {
 
 // RemoveRule removes all rules for a given destination IP
 func (d *MiddleDevice) RemoveRule(destIP netip.Addr) {
-	d.mutex.Lock()
-	defer d.mutex.Unlock()
+	d.rulesMutex.Lock()
+	defer d.rulesMutex.Unlock()
 	newRules := make([]FilterRule, 0, len(d.rules))
 	for _, rule := range d.rules {
 		if rule.DestIP != destIP {
@@ -130,18 +258,113 @@ func (d *MiddleDevice) RemoveRule(destIP netip.Addr) {
 
 // Close stops the device
 func (d *MiddleDevice) Close() error {
-	select {
-	case <-d.closed:
-		// Already closed
-		return nil
-	default:
-		logger.Debug("MiddleDevice: Closing, signaling closed channel")
-		close(d.closed)
+	if !d.closed.CompareAndSwap(false, true) {
+		return nil // already closed
 	}
-	logger.Debug("MiddleDevice: Closing underlying TUN device")
-	err := d.Device.Close()
-	logger.Debug("MiddleDevice: Underlying TUN device closed, err=%v", err)
-	return err
+
+	d.mu.Lock()
+	devices := d.devices
+	d.devices = nil
+	d.cond.Broadcast()
+	d.mu.Unlock()
+
+	var lastErr error
+	logger.Debug("MiddleDevice: Closing %d devices", len(devices))
+	for _, device := range devices {
+		if err := device.Close(); err != nil {
+			logger.Debug("MiddleDevice: Error closing device: %v", err)
+			lastErr = err
+		}
+	}
+
+	close(d.events)
+	return lastErr
+}
+
+// Events returns the events channel
+func (d *MiddleDevice) Events() <-chan tun.Event {
+	return d.events
+}
+
+// File returns the underlying file descriptor
+func (d *MiddleDevice) File() *os.File {
+	for {
+		dev := d.peekLast()
+		if dev == nil {
+			if !d.waitForDevice() {
+				return nil
+			}
+			continue
+		}
+
+		file := dev.File()
+
+		if dev.IsClosed() {
+			time.Sleep(1 * time.Millisecond)
+			continue
+		}
+
+		return file
+	}
+}
+
+// MTU returns the MTU of the underlying device
+func (d *MiddleDevice) MTU() (int, error) {
+	for {
+		dev := d.peekLast()
+		if dev == nil {
+			if !d.waitForDevice() {
+				return 0, io.EOF
+			}
+			continue
+		}
+
+		mtu, err := dev.MTU()
+		if err == nil {
+			return mtu, nil
+		}
+
+		if dev.IsClosed() {
+			time.Sleep(1 * time.Millisecond)
+			continue
+		}
+
+		return 0, err
+	}
+}
+
+// Name returns the name of the underlying device
+func (d *MiddleDevice) Name() (string, error) {
+	for {
+		dev := d.peekLast()
+		if dev == nil {
+			if !d.waitForDevice() {
+				return "", io.EOF
+			}
+			continue
+		}
+
+		name, err := dev.Name()
+		if err == nil {
+			return name, nil
+		}
+
+		if dev.IsClosed() {
+			time.Sleep(1 * time.Millisecond)
+			continue
+		}
+
+		return "", err
+	}
+}
+
+// BatchSize returns the batch size
+func (d *MiddleDevice) BatchSize() int {
+	dev := d.peekLast()
+	if dev == nil {
+		return 1
+	}
+	return dev.BatchSize()
 }
 
 // extractDestIP extracts destination IP from packet (fast path)
@@ -176,156 +399,231 @@ func extractDestIP(packet []byte) (netip.Addr, bool) {
 
 // Read intercepts packets going UP from the TUN device (towards WireGuard)
 func (d *MiddleDevice) Read(bufs [][]byte, sizes []int, offset int) (n int, err error) {
-	// Check if already closed first (non-blocking)
-	select {
-	case <-d.closed:
-		logger.Debug("MiddleDevice: Read returning os.ErrClosed (pre-check)")
-		return 0, os.ErrClosed
-	default:
-	}
-
-	// Now block waiting for data
-	select {
-	case res := <-d.readCh:
-		if res.err != nil {
-			logger.Debug("MiddleDevice: Read returning error from pump: %v", res.err)
-			return 0, res.err
+	for {
+		if d.closed.Load() {
+			logger.Debug("MiddleDevice: Read returning io.EOF, device closed")
+			return 0, io.EOF
 		}
 
-		// Copy packets from result to provided buffers
-		count := 0
-		for i := 0; i < res.n && i < len(bufs); i++ {
-			// Handle offset mismatch if necessary
-			// We assume the pump used defaultOffset (16)
-			// If caller asks for different offset, we need to shift
-			src := res.bufs[i]
-			srcOffset := res.offset
-			srcSize := res.sizes[i]
-
-			// Calculate where the packet data starts and ends in src
-			pktData := src[srcOffset : srcOffset+srcSize]
-
-			// Ensure dest buffer is large enough
-			if len(bufs[i]) < offset+len(pktData) {
-				continue // Skip if buffer too small
+		// Wait for a device to be available
+		dev := d.peekLast()
+		if dev == nil {
+			if !d.waitForDevice() {
+				return 0, io.EOF
 			}
-
-			copy(bufs[i][offset:], pktData)
-			sizes[i] = len(pktData)
-			count++
-		}
-		n = count
-
-	case pkt := <-d.injectCh:
-		if len(bufs) == 0 {
-			return 0, nil
-		}
-		if len(bufs[0]) < offset+len(pkt) {
-			return 0, nil // Buffer too small
-		}
-		copy(bufs[0][offset:], pkt)
-		sizes[0] = len(pkt)
-		n = 1
-
-	case <-d.closed:
-		logger.Debug("MiddleDevice: Read returning os.ErrClosed")
-		return 0, os.ErrClosed // Signal that device is closed
-	}
-
-	d.mutex.RLock()
-	rules := d.rules
-	d.mutex.RUnlock()
-
-	if len(rules) == 0 {
-		return n, nil
-	}
-
-	// Process packets and filter out handled ones
-	writeIdx := 0
-	for readIdx := 0; readIdx < n; readIdx++ {
-		packet := bufs[readIdx][offset : offset+sizes[readIdx]]
-
-		destIP, ok := extractDestIP(packet)
-		if !ok {
-			// Can't parse, keep packet
-			if writeIdx != readIdx {
-				bufs[writeIdx] = bufs[readIdx]
-				sizes[writeIdx] = sizes[readIdx]
-			}
-			writeIdx++
 			continue
 		}
 
-		// Check if packet matches any rule
-		handled := false
-		for _, rule := range rules {
-			if rule.DestIP == destIP {
-				if rule.Handler(packet) {
-					// Packet was handled and should be dropped
-					handled = true
-					break
+		// Now block waiting for data from readCh or injectCh
+		select {
+		case res := <-d.readCh:
+			if res.err != nil {
+				// Check if device was swapped
+				if dev.IsClosed() {
+					time.Sleep(1 * time.Millisecond)
+					continue
+				}
+				logger.Debug("MiddleDevice: Read returning error from pump: %v", res.err)
+				return 0, res.err
+			}
+
+			// Copy packets from result to provided buffers
+			count := 0
+			for i := 0; i < res.n && i < len(bufs); i++ {
+				src := res.bufs[i]
+				srcOffset := res.offset
+				srcSize := res.sizes[i]
+
+				pktData := src[srcOffset : srcOffset+srcSize]
+
+				if len(bufs[i]) < offset+len(pktData) {
+					continue
+				}
+
+				copy(bufs[i][offset:], pktData)
+				sizes[i] = len(pktData)
+				count++
+			}
+			n = count
+
+		case pkt := <-d.injectCh:
+			if len(bufs) == 0 {
+				return 0, nil
+			}
+			if len(bufs[0]) < offset+len(pkt) {
+				return 0, nil
+			}
+			copy(bufs[0][offset:], pkt)
+			sizes[0] = len(pkt)
+			n = 1
+		}
+
+		// Apply filtering rules
+		d.rulesMutex.RLock()
+		rules := d.rules
+		d.rulesMutex.RUnlock()
+
+		if len(rules) == 0 {
+			return n, nil
+		}
+
+		// Process packets and filter out handled ones
+		writeIdx := 0
+		for readIdx := 0; readIdx < n; readIdx++ {
+			packet := bufs[readIdx][offset : offset+sizes[readIdx]]
+
+			destIP, ok := extractDestIP(packet)
+			if !ok {
+				if writeIdx != readIdx {
+					bufs[writeIdx] = bufs[readIdx]
+					sizes[writeIdx] = sizes[readIdx]
+				}
+				writeIdx++
+				continue
+			}
+
+			handled := false
+			for _, rule := range rules {
+				if rule.DestIP == destIP {
+					if rule.Handler(packet) {
+						handled = true
+						break
+					}
 				}
 			}
-		}
 
-		if !handled {
-			// Keep packet
-			if writeIdx != readIdx {
-				bufs[writeIdx] = bufs[readIdx]
-				sizes[writeIdx] = sizes[readIdx]
+			if !handled {
+				if writeIdx != readIdx {
+					bufs[writeIdx] = bufs[readIdx]
+					sizes[writeIdx] = sizes[readIdx]
+				}
+				writeIdx++
 			}
-			writeIdx++
 		}
-	}
 
-	return writeIdx, err
+		return writeIdx, nil
+	}
 }
 
 // Write intercepts packets going DOWN to the TUN device (from WireGuard)
 func (d *MiddleDevice) Write(bufs [][]byte, offset int) (int, error) {
-	d.mutex.RLock()
-	rules := d.rules
-	d.mutex.RUnlock()
+	for {
+		if d.closed.Load() {
+			return 0, io.EOF
+		}
 
-	if len(rules) == 0 {
-		return d.Device.Write(bufs, offset)
-	}
-
-	// Filter packets going down
-	filteredBufs := make([][]byte, 0, len(bufs))
-	for _, buf := range bufs {
-		if len(buf) <= offset {
+		dev := d.peekLast()
+		if dev == nil {
+			if !d.waitForDevice() {
+				return 0, io.EOF
+			}
 			continue
 		}
 
-		packet := buf[offset:]
-		destIP, ok := extractDestIP(packet)
-		if !ok {
-			// Can't parse, keep packet
-			filteredBufs = append(filteredBufs, buf)
-			continue
-		}
+		d.rulesMutex.RLock()
+		rules := d.rules
+		d.rulesMutex.RUnlock()
 
-		// Check if packet matches any rule
-		handled := false
-		for _, rule := range rules {
-			if rule.DestIP == destIP {
-				if rule.Handler(packet) {
-					// Packet was handled and should be dropped
-					handled = true
-					break
+		var filteredBufs [][]byte
+		if len(rules) == 0 {
+			filteredBufs = bufs
+		} else {
+			filteredBufs = make([][]byte, 0, len(bufs))
+			for _, buf := range bufs {
+				if len(buf) <= offset {
+					continue
+				}
+
+				packet := buf[offset:]
+				destIP, ok := extractDestIP(packet)
+				if !ok {
+					filteredBufs = append(filteredBufs, buf)
+					continue
+				}
+
+				handled := false
+				for _, rule := range rules {
+					if rule.DestIP == destIP {
+						if rule.Handler(packet) {
+							handled = true
+							break
+						}
+					}
+				}
+
+				if !handled {
+					filteredBufs = append(filteredBufs, buf)
 				}
 			}
 		}
 
-		if !handled {
-			filteredBufs = append(filteredBufs, buf)
+		if len(filteredBufs) == 0 {
+			return len(bufs), nil
 		}
-	}
 
-	if len(filteredBufs) == 0 {
-		return len(bufs), nil // All packets were handled
-	}
+		n, err := dev.Write(filteredBufs, offset)
+		if err == nil {
+			return n, nil
+		}
 
-	return d.Device.Write(filteredBufs, offset)
+		if dev.IsClosed() {
+			time.Sleep(1 * time.Millisecond)
+			continue
+		}
+
+		return n, err
+	}
 }
+
+func (d *MiddleDevice) waitForDevice() bool {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+
+	for len(d.devices) == 0 && !d.closed.Load() {
+		d.cond.Wait()
+	}
+	return !d.closed.Load()
+}
+
+func (d *MiddleDevice) peekLast() *closeAwareDevice {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+
+	if len(d.devices) == 0 {
+		return nil
+	}
+
+	return d.devices[len(d.devices)-1]
+}
+
+// WriteToTun writes packets directly to the underlying TUN device,
+// bypassing WireGuard. This is useful for sending packets that should
+// appear to come from the TUN interface (e.g., DNS responses from a proxy).
+// Unlike Write(), this does not go through packet filtering rules.
+func (d *MiddleDevice) WriteToTun(bufs [][]byte, offset int) (int, error) {
+	for {
+		if d.closed.Load() {
+			return 0, io.EOF
+		}
+
+		dev := d.peekLast()
+		if dev == nil {
+			if !d.waitForDevice() {
+				return 0, io.EOF
+			}
+			continue
+		}
+
+		n, err := dev.Write(bufs, offset)
+		if err == nil {
+			return n, nil
+		}
+
+		if dev.IsClosed() {
+			time.Sleep(1 * time.Millisecond)
+			continue
+		}
+
+		return n, err
+	}
+}

device/tun_unix.go

@@ -5,6 +5,7 @@ package device
 import (
 	"net"
 	"os"
+	"runtime"
 
 	"github.com/fosrl/newt/logger"
 	"golang.org/x/sys/unix"
@@ -13,6 +14,11 @@ import (
 )
 
 func CreateTUNFromFD(tunFd uint32, mtuInt int) (tun.Device, error) {
+	if runtime.GOOS == "android" { // otherwise we get a permission denied
+		theTun, _, err := tun.CreateUnmonitoredTUNFromFD(int(tunFd))
+		return theTun, err
+	} 
+	
 	dupTunFd, err := unix.Dup(int(tunFd))
 	if err != nil {
 		logger.Error("Unable to dup tun fd: %v", err)

dns/dns_proxy.go

@@ -12,7 +12,6 @@ import (
 	"github.com/fosrl/newt/util"
 	"github.com/fosrl/olm/device"
 	"github.com/miekg/dns"
-	"golang.zx2c4.com/wireguard/tun"
 	"gvisor.dev/gvisor/pkg/buffer"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/adapters/gonet"
@@ -36,8 +35,7 @@ type DNSProxy struct {
 	upstreamDNS  []string
 	tunnelDNS    bool                 // Whether to tunnel DNS queries over WireGuard or to spit them out locally
 	mtu          int
-	tunDevice    tun.Device           // Direct reference to underlying TUN device for responses
-	middleDevice *device.MiddleDevice // Reference to MiddleDevice for packet filtering
+	middleDevice *device.MiddleDevice // Reference to MiddleDevice for packet filtering and TUN writes
 	recordStore  *DNSRecordStore      // Local DNS records
 
 	// Tunnel DNS fields - for sending queries over WireGuard
@@ -53,7 +51,7 @@ type DNSProxy struct {
 }
 
 // NewDNSProxy creates a new DNS proxy
-func NewDNSProxy(tunDevice tun.Device, middleDevice *device.MiddleDevice, mtu int, utilitySubnet string, upstreamDns []string, tunnelDns bool, tunnelIP string) (*DNSProxy, error) {
+func NewDNSProxy(middleDevice *device.MiddleDevice, mtu int, utilitySubnet string, upstreamDns []string, tunnelDns bool, tunnelIP string) (*DNSProxy, error) {
 	proxyIP, err := PickIPFromSubnet(utilitySubnet)
 	if err != nil {
 		return nil, fmt.Errorf("failed to pick DNS proxy IP from subnet: %v", err)
@@ -68,7 +66,6 @@ func NewDNSProxy(tunDevice tun.Device, middleDevice *device.MiddleDevice, mtu in
 	proxy := &DNSProxy{
 		proxyIP:           proxyIP,
 		mtu:               mtu,
-		tunDevice:         tunDevice,
 		middleDevice:      middleDevice,
 		upstreamDNS:       upstreamDns,
 		tunnelDNS:         tunnelDns,
@@ -694,9 +691,9 @@ func (p *DNSProxy) runPacketSender() {
 				pos += len(slice)
 			}
 
-			// Write packet to TUN device
+			// Write packet to TUN device via MiddleDevice
 			// offset=16 indicates packet data starts at position 16 in the buffer
-			_, err := p.tunDevice.Write([][]byte{buf}, offset)
+			_, err := p.middleDevice.WriteToTun([][]byte{buf}, offset)
 			if err != nil {
 				logger.Error("Failed to write DNS response to TUN: %v", err)
 			}

olm/olm.go

@@ -35,6 +35,7 @@ var (
 	uapiListener     net.Listener
 	tdev             tun.Device
 	middleDev        *olmDevice.MiddleDevice
+	interfaceName    string
 	dnsProxy         *dns.DNSProxy
 	apiServer        *api.API
 	olmClient        *websocket.Client
@@ -237,11 +238,11 @@ func StartTunnel(config TunnelConfig) {
 	stopPing = make(chan struct{})
 
 	var (
-		interfaceName = config.InterfaceName
-		id            = config.ID
-		secret        = config.Secret
-		userToken     = config.UserToken
+		id        = config.ID
+		secret    = config.Secret
+		userToken = config.UserToken
 	)
+	interfaceName = config.InterfaceName
 
 	apiServer.SetOrgID(config.OrgID)
 
@@ -307,12 +308,7 @@ func StartTunnel(config TunnelConfig) {
 
 		tdev, err = func() (tun.Device, error) {
 			if config.FileDescriptorTun != 0 {
-				if runtime.GOOS == "android" { // otherwise we get a permission denied
-					theTun, _, err := tun.CreateUnmonitoredTUNFromFD(int(config.FileDescriptorTun))
-					return theTun, err
-				} else {
-					return olmDevice.CreateTUNFromFD(config.FileDescriptorTun, config.MTU)
-				}
+				return olmDevice.CreateTUNFromFD(config.FileDescriptorTun, config.MTU)
 			}
 			var ifName = interfaceName
 			if runtime.GOOS == "darwin" { // this is if we dont pass a fd
@@ -329,11 +325,11 @@ func StartTunnel(config TunnelConfig) {
 			return
 		}
 
-		if config.FileDescriptorTun == 0 {
-			if realInterfaceName, err2 := tdev.Name(); err2 == nil {
-				interfaceName = realInterfaceName
-			}
+		// if config.FileDescriptorTun == 0 {
+		if realInterfaceName, err2 := tdev.Name(); err2 == nil { // if the interface is defined then this should not really do anything?
+			interfaceName = realInterfaceName
 		}
+		// }
 
 		// Wrap TUN device with packet filter for DNS proxy
 		middleDev = olmDevice.NewMiddleDevice(tdev)
@@ -389,7 +385,7 @@ func StartTunnel(config TunnelConfig) {
 		}
 
 		// Create and start DNS proxy
-		dnsProxy, err = dns.NewDNSProxy(tdev, middleDev, config.MTU, wgData.UtilitySubnet, config.UpstreamDNS, config.TunnelDNS, interfaceIP)
+		dnsProxy, err = dns.NewDNSProxy(middleDev, config.MTU, wgData.UtilitySubnet, config.UpstreamDNS, config.TunnelDNS, interfaceIP)
 		if err != nil {
 			logger.Error("Failed to create DNS proxy: %v", err)
 		}
@@ -956,6 +952,33 @@ func StartTunnel(config TunnelConfig) {
 	logger.Info("Tunnel process context cancelled, cleaning up")
 }
 
+func AddDevice(fd uint32) {
+	if middleDev == nil {
+		logger.Error("MiddleDevice is nil, cannot add device")
+		return
+	}
+
+	if tunnelConfig.MTU == 0 {
+		logger.Error("No MTU configured, cannot create device")
+		return
+	}
+
+	tdev, err := olmDevice.CreateTUNFromFD(fd, tunnelConfig.MTU)
+
+	if err != nil {
+		logger.Error("Failed to create TUN device: %v", err)
+		return
+	}
+
+	// if config.FileDescriptorTun == 0 {
+	if realInterfaceName, err2 := tdev.Name(); err2 == nil { // if the interface is defined then this should not really do anything?
+		interfaceName = realInterfaceName
+	}
+
+	// Here we replace the existing TUN device in the middle device with the new one
+	middleDev.AddDevice(tdev)
+}
+
 func Close() {
 	// Restore original DNS configuration
 	// we do this first to avoid any DNS issues if something else gets stuck