Replace exclusion routes with scoped default + IP_BOUND_IF on macOS

client/internal/routemanager/systemops/systemops_bsd_other.go

@@ -0,0 +1,10 @@
+//go:build (dragonfly || freebsd || netbsd || openbsd) && !darwin
+
+package systemops
+
+// Non-darwin BSDs don't support the IP_BOUND_IF + scoped default model. They
+// always fall through to the ref-counter exclusion-route path; these stubs
+// exist only so systemops_unix.go compiles.
+func (r *SysOps) setupAdvancedRouting() error   { return nil }
+func (r *SysOps) cleanupAdvancedRouting() error { return nil }
+func (r *SysOps) flushPlatformExtras() error    { return nil }

client/internal/routemanager/systemops/systemops_darwin.go

@@ -0,0 +1,217 @@
+//go:build darwin && !ios
+
+package systemops
+
+import (
+	"errors"
+	"fmt"
+	"net/netip"
+
+	"github.com/hashicorp/go-multierror"
+	log "github.com/sirupsen/logrus"
+	"golang.org/x/net/route"
+	"golang.org/x/sys/unix"
+
+	nberrors "github.com/netbirdio/netbird/client/errors"
+	"github.com/netbirdio/netbird/client/internal/routemanager/vars"
+	nbnet "github.com/netbirdio/netbird/client/net"
+)
+
+// setupAdvancedRouting installs an RTF_IFSCOPE default route per address family
+// pinned to the current physical egress, so IP_BOUND_IF scoped lookups can
+// resolve gateway'd destinations while the VPN's split default owns the
+// unscoped table.
+//
+// Timing note: this runs during routeManager.Init, which happens before the
+// VPN interface is created and before any peer routes propagate. The initial
+// mgmt / signal / relay TCP dials always fire before this runs, so those
+// sockets miss the IP_BOUND_IF binding and rely on the kernel's normal route
+// lookup, which at that point correctly picks the physical default. Those
+// already-established TCP flows keep their originally-selected interface for
+// their lifetime on Darwin because the kernel caches the egress route
+// per-socket at connect time; adding the VPN's 0/1 + 128/1 split default
+// afterwards does not migrate them since the original en0 default stays in
+// the table. Any subsequent reconnect via nbnet.NewDialer picks up the
+// populated bound-iface cache and gets IP_BOUND_IF set cleanly.
+func (r *SysOps) setupAdvancedRouting() error {
+	if err := r.flushScopedDefaults(); err != nil {
+		log.Warnf("flush residual scoped defaults: %v", err)
+	}
+
+	var merr *multierror.Error
+	installed := 0
+
+	for _, unspec := range []netip.Addr{netip.IPv4Unspecified(), netip.IPv6Unspecified()} {
+		ok, err := r.installScopedDefaultFor(unspec)
+		if err != nil {
+			merr = multierror.Append(merr, err)
+			continue
+		}
+		if ok {
+			installed++
+		}
+	}
+
+	if installed == 0 && merr != nil {
+		return nberrors.FormatErrorOrNil(merr)
+	}
+	if merr != nil {
+		log.Warnf("advanced routing setup partially succeeded: %v", nberrors.FormatErrorOrNil(merr))
+	}
+	return nil
+}
+
+// installScopedDefaultFor resolves the physical default nexthop for the given
+// address family, installs a scoped default via it, and caches the iface for
+// subsequent IP_BOUND_IF / IPV6_BOUND_IF socket binds.
+func (r *SysOps) installScopedDefaultFor(unspec netip.Addr) (bool, error) {
+	nexthop, err := GetNextHop(unspec)
+	if err != nil {
+		if errors.Is(err, vars.ErrRouteNotFound) {
+			return false, nil
+		}
+		return false, fmt.Errorf("get default nexthop for %s: %w", unspec, err)
+	}
+	if nexthop.Intf == nil || !nexthop.IP.IsValid() {
+		return false, fmt.Errorf("unusable default nexthop for %s (iface=%v gw=%v)",
+			unspec, nexthop.Intf, nexthop.IP)
+	}
+
+	if err := r.addScopedDefault(unspec, nexthop); err != nil {
+		return false, fmt.Errorf("add scoped default via %s on %s: %w",
+			nexthop.IP, nexthop.Intf.Name, err)
+	}
+
+	af := unix.AF_INET
+	if unspec.Is6() {
+		af = unix.AF_INET6
+	}
+	nbnet.SetBoundInterface(af, nexthop.Intf)
+	log.Infof("installed scoped default route via %s on %s for %s",
+		nexthop.IP, nexthop.Intf.Name, afOf(unspec))
+	return true, nil
+}
+
+func (r *SysOps) cleanupAdvancedRouting() error {
+	nbnet.ClearBoundInterfaces()
+	return r.flushScopedDefaults()
+}
+
+// flushPlatformExtras runs darwin-specific residual cleanup hooked into the
+// generic FlushMarkedRoutes path, so a crashed daemon's scoped defaults get
+// removed on the next boot regardless of whether a profile is brought up.
+func (r *SysOps) flushPlatformExtras() error {
+	return r.flushScopedDefaults()
+}
+
+// flushScopedDefaults removes any scoped default routes tagged with routeProtoFlag.
+// Safe to call at startup to clear residual entries from a prior session.
+func (r *SysOps) flushScopedDefaults() error {
+	rib, err := retryFetchRIB()
+	if err != nil {
+		return fmt.Errorf("fetch routing table: %w", err)
+	}
+
+	msgs, err := route.ParseRIB(route.RIBTypeRoute, rib)
+	if err != nil {
+		return fmt.Errorf("parse routing table: %w", err)
+	}
+
+	var merr *multierror.Error
+	removed := 0
+
+	for _, msg := range msgs {
+		rtMsg, ok := msg.(*route.RouteMessage)
+		if !ok {
+			continue
+		}
+		if rtMsg.Flags&routeProtoFlag == 0 {
+			continue
+		}
+		if rtMsg.Flags&unix.RTF_IFSCOPE == 0 {
+			continue
+		}
+
+		info, err := MsgToRoute(rtMsg)
+		if err != nil {
+			log.Debugf("skip scoped flush: %v", err)
+			continue
+		}
+		if !info.Dst.IsValid() || info.Dst.Bits() != 0 {
+			continue
+		}
+
+		if err := r.deleteScopedRoute(rtMsg); err != nil {
+			merr = multierror.Append(merr, fmt.Errorf("delete scoped default %s on index %d: %w",
+				info.Dst, rtMsg.Index, err))
+			continue
+		}
+		removed++
+		log.Debugf("flushed residual scoped default %s on index %d", info.Dst, rtMsg.Index)
+	}
+
+	if removed > 0 {
+		log.Infof("flushed %d residual scoped default route(s)", removed)
+	}
+	return nberrors.FormatErrorOrNil(merr)
+}
+
+func (r *SysOps) addScopedDefault(unspec netip.Addr, nexthop Nexthop) error {
+	return r.scopedRouteSocket(unix.RTM_ADD, unspec, nexthop)
+}
+
+func (r *SysOps) deleteScopedRoute(rtMsg *route.RouteMessage) error {
+	// Preserve identifying flags from the stored route (including RTF_GATEWAY
+	// only if present); kernel-set bits like RTF_DONE don't belong on RTM_DELETE.
+	keep := unix.RTF_UP | unix.RTF_STATIC | unix.RTF_GATEWAY | unix.RTF_IFSCOPE | routeProtoFlag
+	del := &route.RouteMessage{
+		Type:    unix.RTM_DELETE,
+		Flags:   rtMsg.Flags & keep,
+		Version: unix.RTM_VERSION,
+		Seq:     r.getSeq(),
+		Index:   rtMsg.Index,
+		Addrs:   rtMsg.Addrs,
+	}
+	return r.writeRouteMessage(del)
+}
+
+func (r *SysOps) scopedRouteSocket(action int, unspec netip.Addr, nexthop Nexthop) error {
+	flags := unix.RTF_UP | unix.RTF_STATIC | unix.RTF_GATEWAY | unix.RTF_IFSCOPE | routeProtoFlag
+
+	msg := &route.RouteMessage{
+		Type:    action,
+		Flags:   flags,
+		Version: unix.RTM_VERSION,
+		Seq:     r.getSeq(),
+		Index:   nexthop.Intf.Index,
+	}
+
+	const numAddrs = unix.RTAX_NETMASK + 1
+	addrs := make([]route.Addr, numAddrs)
+
+	dst, err := addrToRouteAddr(unspec)
+	if err != nil {
+		return fmt.Errorf("build destination: %w", err)
+	}
+	mask, err := prefixToRouteNetmask(netip.PrefixFrom(unspec, 0))
+	if err != nil {
+		return fmt.Errorf("build netmask: %w", err)
+	}
+	gw, err := addrToRouteAddr(nexthop.IP.Unmap())
+	if err != nil {
+		return fmt.Errorf("build gateway: %w", err)
+	}
+	addrs[unix.RTAX_DST] = dst
+	addrs[unix.RTAX_NETMASK] = mask
+	addrs[unix.RTAX_GATEWAY] = gw
+	msg.Addrs = addrs
+
+	return r.writeRouteMessage(msg)
+}
+
+func afOf(a netip.Addr) string {
+	if a.Is4() {
+		return "IPv4"
+	}
+	return "IPv6"
+}

client/internal/routemanager/systemops/systemops_unix.go

@@ -41,26 +41,42 @@ func init() {
 }
 
 func (r *SysOps) SetupRouting(initAddresses []net.IP, stateManager *statemanager.Manager, advancedRouting bool) error {
+	if advancedRouting {
+		return r.setupAdvancedRouting()
+	}
+
+	log.Infof("Using legacy routing setup with ref counters")
 	return r.setupRefCounter(initAddresses, stateManager)
 }
 
 func (r *SysOps) CleanupRouting(stateManager *statemanager.Manager, advancedRouting bool) error {
+	if advancedRouting {
+		return r.cleanupAdvancedRouting()
+	}
+
 	return r.cleanupRefCounter(stateManager)
 }
 
 // FlushMarkedRoutes removes single IP exclusion routes marked with the configured RTF_PROTO flag.
+// On darwin it also flushes residual RTF_IFSCOPE scoped default routes so a
+// crashed prior session can't leave crud in the table.
 func (r *SysOps) FlushMarkedRoutes() error {
+	var merr *multierror.Error
+
+	if err := r.flushPlatformExtras(); err != nil {
+		merr = multierror.Append(merr, fmt.Errorf("flush platform extras: %w", err))
+	}
+
 	rib, err := retryFetchRIB()
 	if err != nil {
-		return fmt.Errorf("fetch routing table: %w", err)
+		return nberrors.FormatErrorOrNil(multierror.Append(merr, fmt.Errorf("fetch routing table: %w", err)))
 	}
 
 	msgs, err := route.ParseRIB(route.RIBTypeRoute, rib)
 	if err != nil {
-		return fmt.Errorf("parse routing table: %w", err)
+		return nberrors.FormatErrorOrNil(multierror.Append(merr, fmt.Errorf("parse routing table: %w", err)))
 	}
 
-	var merr *multierror.Error
 	flushedCount := 0
 
 	for _, msg := range msgs {
@@ -117,12 +133,12 @@ func (r *SysOps) routeSocket(action int, prefix netip.Prefix, nexthop Nexthop) e
 		return fmt.Errorf("invalid prefix: %s", prefix)
 	}
 
-	expBackOff := backoff.NewExponentialBackOff()
-	expBackOff.InitialInterval = 50 * time.Millisecond
-	expBackOff.MaxInterval = 500 * time.Millisecond
-	expBackOff.MaxElapsedTime = 1 * time.Second
+	msg, err := r.buildRouteMessage(action, prefix, nexthop)
+	if err != nil {
+		return fmt.Errorf("build route message: %w", err)
+	}
 
-	if err := backoff.Retry(r.routeOp(action, prefix, nexthop), expBackOff); err != nil {
+	if err := r.writeRouteMessage(msg); err != nil {
 		a := "add"
 		if action == unix.RTM_DELETE {
 			a = "remove"
@@ -132,50 +148,79 @@ func (r *SysOps) routeSocket(action int, prefix netip.Prefix, nexthop Nexthop) e
 	return nil
 }
 
-func (r *SysOps) routeOp(action int, prefix netip.Prefix, nexthop Nexthop) func() error {
-	operation := func() error {
-		fd, err := unix.Socket(syscall.AF_ROUTE, syscall.SOCK_RAW, syscall.AF_UNSPEC)
-		if err != nil {
-			return fmt.Errorf("open routing socket: %w", err)
+// writeRouteMessage sends a route message over AF_ROUTE and waits for the
+// kernel's matching reply, retrying transient failures. Callers do not need to
+// manage sockets or seq numbers themselves.
+func (r *SysOps) writeRouteMessage(msg *route.RouteMessage) error {
+	expBackOff := backoff.NewExponentialBackOff()
+	expBackOff.InitialInterval = 50 * time.Millisecond
+	expBackOff.MaxInterval = 500 * time.Millisecond
+	expBackOff.MaxElapsedTime = 1 * time.Second
+
+	return backoff.Retry(func() error { return routeMessageRoundtrip(msg) }, expBackOff)
+}
+
+func routeMessageRoundtrip(msg *route.RouteMessage) error {
+	fd, err := unix.Socket(syscall.AF_ROUTE, syscall.SOCK_RAW, syscall.AF_UNSPEC)
+	if err != nil {
+		return fmt.Errorf("open routing socket: %w", err)
+	}
+	defer func() {
+		if err := unix.Close(fd); err != nil && !errors.Is(err, unix.EBADF) {
+			log.Warnf("close routing socket: %v", err)
 		}
-		defer func() {
-			if err := unix.Close(fd); err != nil && !errors.Is(err, unix.EBADF) {
-				log.Warnf("failed to close routing socket: %v", err)
+	}()
+
+	tv := unix.Timeval{Sec: 1}
+	if err := unix.SetsockoptTimeval(fd, unix.SOL_SOCKET, unix.SO_RCVTIMEO, &tv); err != nil {
+		return backoff.Permanent(fmt.Errorf("set recv timeout: %w", err))
+	}
+
+	bytes, err := msg.Marshal()
+	if err != nil {
+		return backoff.Permanent(fmt.Errorf("marshal: %w", err))
+	}
+
+	if _, err = unix.Write(fd, bytes); err != nil {
+		if errors.Is(err, unix.ENOBUFS) || errors.Is(err, unix.EAGAIN) {
+			return fmt.Errorf("write: %w", err)
+		}
+		return backoff.Permanent(fmt.Errorf("write: %w", err))
+	}
+	return readRouteResponse(fd, msg.Type, msg.Seq)
+}
+
+// readRouteResponse reads from the AF_ROUTE socket until it sees a reply
+// matching our write (same type, seq, and pid). AF_ROUTE SOCK_RAW is a
+// broadcast channel: interface up/down, third-party route changes and neighbor
+// discovery events can all land between our write and read, so we must filter.
+func readRouteResponse(fd, wantType, wantSeq int) error {
+	pid := int32(os.Getpid())
+	resp := make([]byte, 2048)
+	deadline := time.Now().Add(time.Second)
+	for {
+		if time.Now().After(deadline) {
+			return backoff.Permanent(fmt.Errorf("read: timeout waiting for route reply type=%d seq=%d", wantType, wantSeq))
+		}
+		n, err := unix.Read(fd, resp)
+		if err != nil {
+			if errors.Is(err, unix.EAGAIN) || errors.Is(err, unix.EWOULDBLOCK) {
+				return fmt.Errorf("read: %w", err)
 			}
-		}()
-
-		msg, err := r.buildRouteMessage(action, prefix, nexthop)
-		if err != nil {
-			return backoff.Permanent(fmt.Errorf("build route message: %w", err))
+			return backoff.Permanent(fmt.Errorf("read: %w", err))
 		}
-
-		msgBytes, err := msg.Marshal()
-		if err != nil {
-			return backoff.Permanent(fmt.Errorf("marshal route message: %w", err))
+		if n < int(unsafe.Sizeof(unix.RtMsghdr{})) {
+			continue
 		}
-
-		if _, err = unix.Write(fd, msgBytes); err != nil {
-			if errors.Is(err, unix.ENOBUFS) || errors.Is(err, unix.EAGAIN) {
-				return fmt.Errorf("write: %w", err)
-			}
-			return backoff.Permanent(fmt.Errorf("write: %w", err))
+		hdr := (*unix.RtMsghdr)(unsafe.Pointer(&resp[0]))
+		if int(hdr.Type) != wantType || int(hdr.Seq) != wantSeq || hdr.Pid != pid {
+			continue
 		}
-
-		respBuf := make([]byte, 2048)
-		n, err := unix.Read(fd, respBuf)
-		if err != nil {
-			return backoff.Permanent(fmt.Errorf("read route response: %w", err))
+		if hdr.Errno != 0 {
+			return backoff.Permanent(fmt.Errorf("kernel errno %d", hdr.Errno))
 		}
-
-		if n > 0 {
-			if err := r.parseRouteResponse(respBuf[:n]); err != nil {
-				return backoff.Permanent(err)
-			}
-		}
-
 		return nil
 	}
-	return operation
 }
 
 func (r *SysOps) buildRouteMessage(action int, prefix netip.Prefix, nexthop Nexthop) (msg *route.RouteMessage, err error) {
@@ -221,19 +266,6 @@ func (r *SysOps) buildRouteMessage(action int, prefix netip.Prefix, nexthop Next
 	return msg, nil
 }
 
-func (r *SysOps) parseRouteResponse(buf []byte) error {
-	if len(buf) < int(unsafe.Sizeof(unix.RtMsghdr{})) {
-		return nil
-	}
-
-	rtMsg := (*unix.RtMsghdr)(unsafe.Pointer(&buf[0]))
-	if rtMsg.Errno != 0 {
-		return fmt.Errorf("parse: %d", rtMsg.Errno)
-	}
-
-	return nil
-}
-
 // addrToRouteAddr converts a netip.Addr to the appropriate route.Addr (*route.Inet4Addr or *route.Inet6Addr).
 func addrToRouteAddr(addr netip.Addr) (route.Addr, error) {
 	if addr.Is4() {

client/net/dialer_init_darwin.go

@@ -0,0 +1,5 @@
+package net
+
+func (d *Dialer) init() {
+	d.Dialer.Control = applyBoundIfToSocket
+}

client/net/dialer_init_generic.go

@@ -1,4 +1,4 @@
-//go:build !linux && !windows
+//go:build !linux && !windows && !darwin
 
 package net
 

client/net/env_darwin.go

@@ -0,0 +1,67 @@
+//go:build darwin && !ios
+
+package net
+
+import (
+	"os"
+	"strconv"
+	"sync"
+
+	log "github.com/sirupsen/logrus"
+
+	"github.com/netbirdio/netbird/client/iface/netstack"
+)
+
+var (
+	vpnInterfaceName string
+	vpnInitMutex     sync.RWMutex
+
+	advancedRoutingSupported bool
+)
+
+func Init() {
+	advancedRoutingSupported = checkAdvancedRoutingSupport()
+}
+
+func checkAdvancedRoutingSupport() bool {
+	var err error
+	var legacyRouting bool
+	if val := os.Getenv(envUseLegacyRouting); val != "" {
+		legacyRouting, err = strconv.ParseBool(val)
+		if err != nil {
+			log.Warnf("failed to parse %s: %v", envUseLegacyRouting, err)
+		}
+	}
+
+	if legacyRouting || netstack.IsEnabled() {
+		log.Info("advanced routing has been requested to be disabled")
+		return false
+	}
+
+	log.Info("system supports advanced routing")
+
+	return true
+}
+
+// AdvancedRouting reports whether routing loops can be avoided without using exclusion routes
+func AdvancedRouting() bool {
+	return advancedRoutingSupported
+}
+
+// GetVPNInterfaceName returns the stored VPN interface name
+func GetVPNInterfaceName() string {
+	vpnInitMutex.RLock()
+	defer vpnInitMutex.RUnlock()
+	return vpnInterfaceName
+}
+
+// SetVPNInterfaceName sets the VPN interface name for lazy initialization
+func SetVPNInterfaceName(name string) {
+	vpnInitMutex.Lock()
+	defer vpnInitMutex.Unlock()
+	vpnInterfaceName = name
+
+	if name != "" {
+		log.Infof("VPN interface name set to %s for route exclusion", name)
+	}
+}

client/net/env_generic.go

@@ -1,4 +1,4 @@
-//go:build !linux && !windows && !android
+//go:build !linux && !windows && !android && (ios || !darwin)
 
 package net
 

client/net/listener_init_darwin.go

@@ -0,0 +1,5 @@
+package net
+
+func (l *ListenerConfig) init() {
+	l.ListenConfig.Control = applyBoundIfToSocket
+}

client/net/listener_init_generic.go

@@ -1,4 +1,4 @@
-//go:build !linux && !windows
+//go:build !linux && !windows && !darwin
 
 package net
 

client/net/net_darwin.go

@@ -0,0 +1,150 @@
+package net
+
+import (
+	"fmt"
+	"net"
+	"net/netip"
+	"strconv"
+	"strings"
+	"sync"
+	"syscall"
+
+	log "github.com/sirupsen/logrus"
+	"golang.org/x/sys/unix"
+)
+
+// On darwin IPV6_BOUND_IF also scopes v4-mapped egress from dual-stack
+// (IPV6_V6ONLY=0) AF_INET6 sockets, so a single setsockopt on "udp6"/"tcp6"
+// covers both families. Setting IP_BOUND_IF on an AF_INET6 socket returns
+// EINVAL regardless of V6ONLY because the IPPROTO_IP ctloutput path is
+// dispatched by socket domain (AF_INET only) not by inp_vflag.
+
+// boundIface holds the physical interface chosen at routing setup time. Sockets
+// created via nbnet.NewDialer / nbnet.NewListener bind to it via IP_BOUND_IF
+// (IPv4) or IPV6_BOUND_IF (IPv6 / dual-stack) so their scoped route lookup
+// hits the RTF_IFSCOPE default installed by the routemanager, rather than
+// following the VPN's split default.
+var (
+	boundIfaceMu sync.RWMutex
+	boundIface4  *net.Interface
+	boundIface6  *net.Interface
+)
+
+// SetBoundInterface records the egress interface for an address family. Called
+// by the routemanager after a scoped default route has been installed.
+// af must be unix.AF_INET or unix.AF_INET6; other values are ignored.
+// nil iface is rejected — use ClearBoundInterfaces to clear all slots.
+func SetBoundInterface(af int, iface *net.Interface) {
+	if iface == nil {
+		log.Warnf("SetBoundInterface: nil iface for AF %d, ignored", af)
+		return
+	}
+	boundIfaceMu.Lock()
+	defer boundIfaceMu.Unlock()
+	switch af {
+	case unix.AF_INET:
+		boundIface4 = iface
+	case unix.AF_INET6:
+		boundIface6 = iface
+	default:
+		log.Warnf("SetBoundInterface: unsupported address family %d", af)
+	}
+}
+
+// ClearBoundInterfaces resets the cached egress interfaces. Called by the
+// routemanager during cleanup.
+func ClearBoundInterfaces() {
+	boundIfaceMu.Lock()
+	defer boundIfaceMu.Unlock()
+	boundIface4 = nil
+	boundIface6 = nil
+}
+
+func boundInterfaceFor(network, address string) *net.Interface {
+	if iface := zoneInterface(address); iface != nil {
+		return iface
+	}
+
+	boundIfaceMu.RLock()
+	defer boundIfaceMu.RUnlock()
+
+	if isV6Network(network) {
+		return boundIface6
+	}
+	return boundIface4
+}
+
+func isV6Network(network string) bool {
+	return strings.HasSuffix(network, "6")
+}
+
+// zoneInterface extracts an explicit interface from an IPv6 link-local zone (e.g. fe80::1%en0).
+func zoneInterface(address string) *net.Interface {
+	if address == "" {
+		return nil
+	}
+	addr, err := netip.ParseAddrPort(address)
+	if err != nil {
+		a, err := netip.ParseAddr(address)
+		if err != nil {
+			return nil
+		}
+		addr = netip.AddrPortFrom(a, 0)
+	}
+	zone := addr.Addr().Zone()
+	if zone == "" {
+		return nil
+	}
+	if iface, err := net.InterfaceByName(zone); err == nil {
+		return iface
+	}
+	if idx, err := strconv.Atoi(zone); err == nil {
+		if iface, err := net.InterfaceByIndex(idx); err == nil {
+			return iface
+		}
+	}
+	return nil
+}
+
+func setIPv4BoundIf(fd uintptr, iface *net.Interface) error {
+	if err := unix.SetsockoptInt(int(fd), unix.IPPROTO_IP, unix.IP_BOUND_IF, iface.Index); err != nil {
+		return fmt.Errorf("set IP_BOUND_IF: %w (interface: %s, index: %d)", err, iface.Name, iface.Index)
+	}
+	return nil
+}
+
+func setIPv6BoundIf(fd uintptr, iface *net.Interface) error {
+	if err := unix.SetsockoptInt(int(fd), unix.IPPROTO_IPV6, unix.IPV6_BOUND_IF, iface.Index); err != nil {
+		return fmt.Errorf("set IPV6_BOUND_IF: %w (interface: %s, index: %d)", err, iface.Name, iface.Index)
+	}
+	return nil
+}
+
+// applyBoundIfToSocket binds the socket to the cached physical egress interface
+// so scoped route lookup avoids the VPN utun and egresses the underlay directly.
+func applyBoundIfToSocket(network, address string, c syscall.RawConn) error {
+	if !AdvancedRouting() {
+		return nil
+	}
+
+	iface := boundInterfaceFor(network, address)
+	if iface == nil {
+		return nil
+	}
+
+	isV6 := isV6Network(network)
+	var controlErr error
+	if err := c.Control(func(fd uintptr) {
+		if isV6 {
+			controlErr = setIPv6BoundIf(fd, iface)
+		} else {
+			controlErr = setIPv4BoundIf(fd, iface)
+		}
+		if controlErr == nil {
+			log.Debugf("set BOUND_IF=%d on %s for %s to %s", iface.Index, iface.Name, network, address)
+		}
+	}); err != nil {
+		return fmt.Errorf("control: %w", err)
+	}
+	return controlErr
+}