[client] Replace WG interface monitor polling with netlink subscription on Linux

client/internal/wg_iface_monitor.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"net"
 	"runtime"
-	"time"
 
 	log "github.com/sirupsen/logrus"
 
@@ -28,6 +27,10 @@ func NewWGIfaceMonitor() *WGIfaceMonitor {
 
 // Start begins monitoring the WireGuard interface.
 // It relies on the provided context cancellation to stop.
+//
+// On Linux the watcher is event-driven (RTNLGRP_LINK netlink subscription)
+// to avoid the allocation churn of repeatedly dumping the kernel link
+// table; on other platforms it falls back to a low-frequency poll.
 func (m *WGIfaceMonitor) Start(ctx context.Context, ifaceName string) (shouldRestart bool, err error) {
 	defer close(m.done)
 
@@ -56,31 +59,7 @@ func (m *WGIfaceMonitor) Start(ctx context.Context, ifaceName string) (shouldRes
 
 	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
-			}
-		}
-	}
-
+	return watchInterface(ctx, ifaceName, expectedIndex)
 }
 
 // getInterfaceIndex returns the index of a network interface by name.

client/internal/wg_iface_monitor_linux.go

@@ -0,0 +1,134 @@
+//go:build linux
+
+package internal
+
+import (
+	"context"
+	"fmt"
+	"syscall"
+
+	log "github.com/sirupsen/logrus"
+	"github.com/vishvananda/netlink"
+)
+
+// watchInterface uses an RTNLGRP_LINK netlink subscription to detect
+// deletion or recreation of the WireGuard interface.
+//
+// The previous implementation polled net.InterfaceByName every 2 s, which
+// on Linux issues syscall.NetlinkRIB(RTM_GETLINK, ...) and dumps the
+// entire kernel link table on every call. On hosts with many veth
+// interfaces (containers, bridges) the resulting allocation churn was on
+// the order of ~1 GB/day from this single ticker, which on small ARM
+// hosts manifested as a slow RSS climb (see netbirdio/netbird#3678).
+//
+// The event-driven version below allocates only when the kernel actually
+// publishes a link event for the tracked interface — typically zero
+// allocations between events.
+func watchInterface(ctx context.Context, ifaceName string, expectedIndex int) (bool, error) {
+	done := make(chan struct{})
+	defer close(done)
+
+	// Buffer the channel to absorb event bursts (e.g. when many veth
+	// pairs are created/destroyed at once by container runtimes).
+	linkChan := make(chan netlink.LinkUpdate, 32)
+	if err := netlink.LinkSubscribe(linkChan, done); err != nil {
+		// Return shouldRestart=true so the engine recovers monitoring
+		// via triggerClientRestart instead of silently losing it for
+		// the rest of the process lifetime.
+		return true, fmt.Errorf("subscribe to link updates: %w", err)
+	}
+
+	// Race window: the interface could have been deleted (or recreated)
+	// between the initial getInterfaceIndex() in Start and LinkSubscribe
+	// completing its handshake with the kernel. Re-check explicitly so we
+	// do not block forever waiting for an event that already fired.
+	if currentIndex, err := getInterfaceIndex(ifaceName); err != nil {
+		log.Infof("Interface monitor: %s deleted before subscription completed", ifaceName)
+		return true, fmt.Errorf("interface %s deleted: %w", ifaceName, err)
+	} else if currentIndex != expectedIndex {
+		log.Infof("Interface monitor: %s recreated (index changed from %d to %d) before subscription completed",
+			ifaceName, expectedIndex, currentIndex)
+		return true, nil
+	}
+
+	for {
+		select {
+		case <-ctx.Done():
+			log.Infof("Interface monitor: stopped for %s", ifaceName)
+			return false, fmt.Errorf("wg interface monitor stopped: %w", ctx.Err())
+
+		case update, ok := <-linkChan:
+			if !ok {
+				// The vishvananda/netlink subscription goroutine closes
+				// the channel on receive errors. Signal the engine to
+				// restart so monitoring is re-established instead of
+				// silently ending.
+				log.Warnf("Interface monitor: link subscription channel closed unexpectedly for %s", ifaceName)
+				return true, fmt.Errorf("link subscription channel closed unexpectedly")
+			}
+			if restart, err := inspectLinkEvent(update, ifaceName, expectedIndex); restart {
+				return true, err
+			}
+		}
+	}
+}
+
+// inspectLinkEvent classifies a single netlink link update against the
+// tracked WireGuard interface. It returns (true, err) when the engine
+// should restart monitoring; (false, nil) means the event is unrelated
+// and the caller should keep waiting.
+//
+// The error component, when non-nil, describes the kernel-side reason
+// (deletion or rename); the recreation case returns (true, nil) since
+// no error condition is reported.
+func inspectLinkEvent(update netlink.LinkUpdate, ifaceName string, expectedIndex int) (bool, error) {
+	eventIndex := int(update.Index)
+	eventName := ""
+	if attrs := update.Attrs(); attrs != nil {
+		eventName = attrs.Name
+	}
+
+	switch update.Header.Type {
+	case syscall.RTM_DELLINK:
+		return inspectDelLink(eventIndex, ifaceName, expectedIndex)
+	case syscall.RTM_NEWLINK:
+		return inspectNewLink(eventIndex, eventName, ifaceName, expectedIndex)
+	}
+	return false, nil
+}
+
+// inspectDelLink reports a restart when an RTM_DELLINK arrives for the
+// tracked interface index.
+func inspectDelLink(eventIndex int, ifaceName string, expectedIndex int) (bool, error) {
+	if eventIndex != expectedIndex {
+		return false, nil
+	}
+	log.Infof("Interface monitor: %s deleted", ifaceName)
+	return true, fmt.Errorf("interface %s deleted", ifaceName)
+}
+
+// inspectNewLink reports a restart when an RTM_NEWLINK either:
+//
+//  1. Introduces a link with our name at a different index (recreation
+//     after a delete), or
+//
+//  2. Reports a link still at our index but with a different name
+//     (in-place rename). The previous polling implementation caught
+//     this implicitly because net.InterfaceByName(ifaceName) would
+//     start failing; the event-driven version has to test it.
+//
+// Same name + same index is just a flag/state change on the existing
+// interface and is ignored.
+func inspectNewLink(eventIndex int, eventName, ifaceName string, expectedIndex int) (bool, error) {
+	if eventName == ifaceName && eventIndex != expectedIndex {
+		log.Infof("Interface monitor: %s recreated (index changed from %d to %d), restarting engine",
+			ifaceName, expectedIndex, eventIndex)
+		return true, nil
+	}
+	if eventIndex == expectedIndex && eventName != "" && eventName != ifaceName {
+		log.Infof("Interface monitor: %s renamed to %s (index %d), restarting engine",
+			ifaceName, eventName, expectedIndex)
+		return true, fmt.Errorf("interface %s renamed to %s", ifaceName, eventName)
+	}
+	return false, nil
+}

client/internal/wg_iface_monitor_other.go

@@ -0,0 +1,56 @@
+//go:build !linux
+
+package internal
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	log "github.com/sirupsen/logrus"
+)
+
+// watchInterface polls net.InterfaceByName at a fixed interval to detect
+// deletion or recreation of the WireGuard interface.
+//
+// This is the fallback used on non-Linux desktop and server platforms
+// (darwin, windows, freebsd). It is also compiled on android and ios so
+// the package builds on every supported GOOS, but it is never reached
+// at runtime there because Start() in wg_iface_monitor.go exits early
+// on mobile platforms.
+//
+// The Linux build (see wg_iface_monitor_linux.go) uses an event-driven
+// RTNLGRP_LINK netlink subscription instead, because on Linux
+// net.InterfaceByName issues syscall.NetlinkRIB(RTM_GETLINK, ...) which
+// dumps the entire kernel link table on every call and produces
+// significant allocation churn (netbirdio/netbird#3678).
+//
+// Windows is also reported in #3678 as affected by RSS climb. A future
+// follow-up could implement an event-driven watcher there using
+// NotifyIpInterfaceChange from iphlpapi.
+func watchInterface(ctx context.Context, ifaceName string, expectedIndex int) (bool, error) {
+	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: %w", 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
+			}
+		}
+	}
+}