[client] Add WireGuard interface lifecycle monitoring (#4370)

* [client] Add WireGuard interface lifecycle monitoring

client/internal/engine.go

@@ -198,6 +198,10 @@ type Engine struct {
 	latestSyncResponse  *mgmProto.SyncResponse
 	connSemaphore       *semaphoregroup.SemaphoreGroup
 	flowManager         nftypes.FlowManager
+
+	// WireGuard interface monitor
+	wgIfaceMonitor   *WGIfaceMonitor
+	wgIfaceMonitorWg sync.WaitGroup
 }
 
 // Peer is an instance of the Connection Peer
@@ -341,6 +345,9 @@ func (e *Engine) Stop() error {
 		log.Errorf("failed to persist state: %v", err)
 	}
 
+	// Stop WireGuard interface monitor and wait for it to exit
+	e.wgIfaceMonitorWg.Wait()
+
 	return nil
 }
 
@@ -479,6 +486,22 @@ func (e *Engine) Start(netbirdConfig *mgmProto.NetbirdConfig, mgmtURL *url.URL)
 
 	// starting network monitor at the very last to avoid disruptions
 	e.startNetworkMonitor()
+
+	// monitor WireGuard interface lifecycle and restart engine on changes
+	e.wgIfaceMonitor = NewWGIfaceMonitor()
+	e.wgIfaceMonitorWg.Add(1)
+
+	go func() {
+		defer e.wgIfaceMonitorWg.Done()
+
+		if shouldRestart, err := e.wgIfaceMonitor.Start(e.ctx, e.wgInterface.Name()); shouldRestart {
+			log.Infof("WireGuard interface monitor: %s, restarting engine", err)
+			e.restartEngine()
+		} else if err != nil {
+			log.Warnf("WireGuard interface monitor: %s", err)
+		}
+	}()
+
 	return nil
 }
 

client/internal/wg_iface_monitor.go

@@ -0,0 +1,98 @@
+package internal
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"runtime"
+	"time"
+
+	log "github.com/sirupsen/logrus"
+)
+
+// WGIfaceMonitor monitors the WireGuard interface lifecycle and restarts the engine
+// if the interface is deleted externally while the engine is running.
+type WGIfaceMonitor struct {
+	done chan struct{}
+}
+
+// NewWGIfaceMonitor creates a new WGIfaceMonitor instance.
+func NewWGIfaceMonitor() *WGIfaceMonitor {
+	return &WGIfaceMonitor{
+		done: make(chan struct{}),
+	}
+}
+
+// Start begins monitoring the WireGuard interface.
+// It relies on the provided context cancellation to stop.
+func (m *WGIfaceMonitor) Start(ctx context.Context, ifaceName string) (shouldRestart bool, err error) {
+	defer close(m.done)
+
+	// Skip on mobile platforms as they handle interface lifecycle differently
+	if runtime.GOOS == "android" || runtime.GOOS == "ios" {
+		log.Debugf("Interface monitor: skipped on %s platform", runtime.GOOS)
+		return false, errors.New("not supported on mobile platforms")
+	}
+
+	if ifaceName == "" {
+		log.Debugf("Interface monitor: empty interface name, skipping monitor")
+		return false, errors.New("empty interface name")
+	}
+
+	// Get initial interface index to track the specific interface instance
+	expectedIndex, err := getInterfaceIndex(ifaceName)
+	if err != nil {
+		log.Debugf("Interface monitor: interface %s not found, skipping monitor", ifaceName)
+		return false, fmt.Errorf("interface %s not found: %w", ifaceName, err)
+	}
+
+	log.Infof("Interface monitor: watching %s (index: %d)", ifaceName, expectedIndex)
+
+	ticker := time.NewTicker(2 * time.Second)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			log.Infof("Interface monitor: stopped for %s", ifaceName)
+			return false, fmt.Errorf("wg interface monitor stopped: %v", ctx.Err())
+		case <-ticker.C:
+			currentIndex, err := getInterfaceIndex(ifaceName)
+			if err != nil {
+				// Interface was deleted
+				log.Infof("Interface monitor: %s deleted", ifaceName)
+				return true, fmt.Errorf("interface %s deleted: %w", ifaceName, err)
+			}
+
+			// Check if interface index changed (interface was recreated)
+			if currentIndex != expectedIndex {
+				log.Infof("Interface monitor: %s recreated (index changed from %d to %d), restarting engine",
+					ifaceName, expectedIndex, currentIndex)
+				return true, nil
+			}
+		}
+	}
+
+}
+
+// getInterfaceIndex returns the index of a network interface by name.
+// Returns an error if the interface is not found.
+func getInterfaceIndex(name string) (int, error) {
+	if name == "" {
+		return 0, fmt.Errorf("empty interface name")
+	}
+	ifi, err := net.InterfaceByName(name)
+	if err != nil {
+		// Check if it's specifically a "not found" error
+		if errors.Is(err, &net.OpError{}) {
+			// On some systems, this might be a "not found" error
+			return 0, fmt.Errorf("interface not found: %w", err)
+		}
+		return 0, fmt.Errorf("failed to lookup interface: %w", err)
+	}
+	if ifi == nil {
+		return 0, fmt.Errorf("interface not found")
+	}
+	return ifi.Index, nil
+}