various: create a catch-all NRPT rule when "Override local DNS" is enabled on Windows

Without this rule, Windows 8.1 and newer devices issue parallel DNS requests to DNS servers
associated with all network adapters, even when "Override local DNS" is enabled and/or
a Mullvad exit node is being used, resulting in DNS leaks.

This also adds "disable-local-dns-override-via-nrpt" nodeAttr that can be used to disable
the new behavior if needed.

Fixes tailscale/corp#20718

Signed-off-by: Nick Khyl <nickk@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -698,7 +698,7 @@ func tryEngine(logf logger.Logf, sys *tsd.System, name string) (onlyNetstack boo
 			// configuration being unavailable (from the noop
 			// manager). More in Issue 4017.
 			// TODO(bradfitz): add a Synology-specific DNS manager.
-			conf.DNS, err = dns.NewOSConfigurator(logf, sys.HealthTracker(), "") // empty interface name
+			conf.DNS, err = dns.NewOSConfigurator(logf, sys.HealthTracker(), sys.ControlKnobs(), "") // empty interface name
 			if err != nil {
 				return false, fmt.Errorf("dns.NewOSConfigurator: %w", err)
 			}
@@ -726,7 +726,7 @@ func tryEngine(logf logger.Logf, sys *tsd.System, name string) (onlyNetstack boo
 			return false, fmt.Errorf("creating router: %w", err)
 		}
 
-		d, err := dns.NewOSConfigurator(logf, sys.HealthTracker(), devName)
+		d, err := dns.NewOSConfigurator(logf, sys.HealthTracker(), sys.ControlKnobs(), devName)
 		if err != nil {
 			dev.Close()
 			r.Close()

control/controlknobs/controlknobs.go

@@ -90,6 +90,15 @@ type Knobs struct {
 	// This is for now (2024-06-06) an iOS-specific battery life optimization,
 	// and this knob allows us to disable the optimization remotely if needed.
 	DisableSplitDNSWhenNoCustomResolvers atomic.Bool
+
+	// DisableLocalDNSOverrideViaNRPT indicates that the node's DNS manager should not
+	// create a default (catch-all) Windows NRPT rule when "Override local DNS" is enabled.
+	// Without this rule, Windows 8.1 and newer devices issue parallel DNS requests to DNS servers
+	// associated with all network adapters, even when "Override local DNS" is enabled and/or
+	// a Mullvad exit node is being used, resulting in DNS leaks.
+	// We began creating this rule on 2024-06-14, and this knob
+	// allows us to disable the new behavior remotely if needed.
+	DisableLocalDNSOverrideViaNRPT atomic.Bool
 }
 
 // UpdateFromNodeAttributes updates k (if non-nil) based on the provided self
@@ -117,6 +126,7 @@ func (k *Knobs) UpdateFromNodeAttributes(capMap tailcfg.NodeCapMap) {
 		appCStoreRoutes                      = has(tailcfg.NodeAttrStoreAppCRoutes)
 		userDialUseRoutes                    = has(tailcfg.NodeAttrUserDialUseRoutes)
 		disableSplitDNSWhenNoCustomResolvers = has(tailcfg.NodeAttrDisableSplitDNSWhenNoCustomResolvers)
+		disableLocalDNSOverrideViaNRPT       = has(tailcfg.NodeAttrDisableLocalDNSOverrideViaNRPT)
 	)
 
 	if has(tailcfg.NodeAttrOneCGNATEnable) {
@@ -142,6 +152,7 @@ func (k *Knobs) UpdateFromNodeAttributes(capMap tailcfg.NodeCapMap) {
 	k.AppCStoreRoutes.Store(appCStoreRoutes)
 	k.UserDialUseRoutes.Store(userDialUseRoutes)
 	k.DisableSplitDNSWhenNoCustomResolvers.Store(disableSplitDNSWhenNoCustomResolvers)
+	k.DisableLocalDNSOverrideViaNRPT.Store(disableLocalDNSOverrideViaNRPT)
 }
 
 // AsDebugJSON returns k as something that can be marshalled with json.Marshal
@@ -168,5 +179,6 @@ func (k *Knobs) AsDebugJSON() map[string]any {
 		"AppCStoreRoutes":                      k.AppCStoreRoutes.Load(),
 		"UserDialUseRoutes":                    k.UserDialUseRoutes.Load(),
 		"DisableSplitDNSWhenNoCustomResolvers": k.DisableSplitDNSWhenNoCustomResolvers.Load(),
+		"DisableLocalDNSOverrideViaNRPT":       k.DisableLocalDNSOverrideViaNRPT.Load(),
 	}
 }

net/dns/manager.go

@@ -485,7 +485,7 @@ func (m *Manager) FlushCaches() error {
 // in case the Tailscale daemon terminated without closing the router.
 // No other state needs to be instantiated before this runs.
 func CleanUp(logf logger.Logf, netMon *netmon.Monitor, interfaceName string) {
-	oscfg, err := NewOSConfigurator(logf, nil, interfaceName)
+	oscfg, err := NewOSConfigurator(logf, nil, nil, interfaceName)
 	if err != nil {
 		logf("creating dns cleanup: %v", err)
 		return

net/dns/manager_darwin.go

@@ -8,12 +8,16 @@ import (
 	"os"
 
 	"go4.org/mem"
+	"tailscale.com/control/controlknobs"
 	"tailscale.com/health"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/mak"
 )
 
-func NewOSConfigurator(logf logger.Logf, health *health.Tracker, ifName string) (OSConfigurator, error) {
+// NewOSConfigurator creates a new OS configurator.
+//
+// The health tracker and the knobs may be nil and are ignored on this platform.
+func NewOSConfigurator(logf logger.Logf, _ *health.Tracker, _ *controlknobs.Knobs, ifName string) (OSConfigurator, error) {
 	return &darwinConfigurator{logf: logf, ifName: ifName}, nil
 }
 

net/dns/manager_default.go

@@ -6,10 +6,14 @@
 package dns
 
 import (
+	"tailscale.com/control/controlknobs"
 	"tailscale.com/health"
 	"tailscale.com/types/logger"
 )
 
-func NewOSConfigurator(logger.Logf, *health.Tracker, string) (OSConfigurator, error) {
+// NewOSConfigurator creates a new OS configurator.
+//
+// The health tracker and the knobs may be nil and are ignored on this platform.
+func NewOSConfigurator(logger.Logf, *health.Tracker, *controlknobs.Knobs, string) (OSConfigurator, error) {
 	return NewNoopManager()
 }

net/dns/manager_freebsd.go

@@ -7,11 +7,15 @@ import (
 	"fmt"
 	"os"
 
+	"tailscale.com/control/controlknobs"
 	"tailscale.com/health"
 	"tailscale.com/types/logger"
 )
 
-func NewOSConfigurator(logf logger.Logf, health *health.Tracker, _ string) (OSConfigurator, error) {
+// NewOSConfigurator creates a new OS configurator.
+//
+// The health tracker may be nil; the knobs may be nil and are ignored on this platform.
+func NewOSConfigurator(logf logger.Logf, health *health.Tracker, _ *controlknobs.Knobs, _ string) (OSConfigurator, error) {
 	bs, err := os.ReadFile("/etc/resolv.conf")
 	if os.IsNotExist(err) {
 		return newDirectManager(logf, health), nil

net/dns/manager_linux.go

@@ -14,6 +14,7 @@ import (
 	"time"
 
 	"github.com/godbus/dbus/v5"
+	"tailscale.com/control/controlknobs"
 	"tailscale.com/health"
 	"tailscale.com/net/netaddr"
 	"tailscale.com/types/logger"
@@ -31,7 +32,10 @@ func (kv kv) String() string {
 
 var publishOnce sync.Once
 
-func NewOSConfigurator(logf logger.Logf, health *health.Tracker, interfaceName string) (ret OSConfigurator, err error) {
+// NewOSConfigurator created a new OS configurator.
+//
+// The health tracker may be nil; the knobs may be nil and are ignored on this platform.
+func NewOSConfigurator(logf logger.Logf, health *health.Tracker, _ *controlknobs.Knobs, interfaceName string) (ret OSConfigurator, err error) {
 	env := newOSConfigEnv{
 		fs:                directFS{},
 		dbusPing:          dbusPing,

net/dns/manager_openbsd.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"os"
 
+	"tailscale.com/control/controlknobs"
 	"tailscale.com/health"
 	"tailscale.com/types/logger"
 )
@@ -20,7 +21,10 @@ func (kv kv) String() string {
 	return fmt.Sprintf("%s=%s", kv.k, kv.v)
 }
 
-func NewOSConfigurator(logf logger.Logf, health *health.Tracker, interfaceName string) (OSConfigurator, error) {
+// NewOSConfigurator created a new OS configurator.
+//
+// The health tracker may be nil; the knobs may be nil and are ignored on this platform.
+func NewOSConfigurator(logf logger.Logf, health *health.Tracker, _ *controlknobs.Knobs, interfaceName string) (OSConfigurator, error) {
 	return newOSConfigurator(logf, health, interfaceName,
 		newOSConfigEnv{
 			rcIsResolvd: rcIsResolvd,

net/dns/manager_windows.go

@@ -22,6 +22,7 @@ import (
 	"golang.org/x/sys/windows/registry"
 	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
 	"tailscale.com/atomicfile"
+	"tailscale.com/control/controlknobs"
 	"tailscale.com/envknob"
 	"tailscale.com/health"
 	"tailscale.com/types/logger"
@@ -38,6 +39,7 @@ var configureWSL = envknob.RegisterBool("TS_DEBUG_CONFIGURE_WSL")
 type windowsManager struct {
 	logf       logger.Logf
 	guid       string
+	knobs      *controlknobs.Knobs // or nil
 	nrptDB     *nrptRuleDatabase
 	wslManager *wslManager
 
@@ -45,10 +47,14 @@ type windowsManager struct {
 	closing bool
 }
 
-func NewOSConfigurator(logf logger.Logf, health *health.Tracker, interfaceName string) (OSConfigurator, error) {
+// NewOSConfigurator created a new OS configurator.
+//
+// The health tracker and the knobs may be nil.
+func NewOSConfigurator(logf logger.Logf, health *health.Tracker, knobs *controlknobs.Knobs, interfaceName string) (OSConfigurator, error) {
 	ret := &windowsManager{
 		logf:       logf,
 		guid:       interfaceName,
+		knobs:      knobs,
 		wslManager: newWSLManager(logf, health),
 	}
 
@@ -288,6 +294,10 @@ func (m *windowsManager) setPrimaryDNS(resolvers []netip.Addr, domains []dnsname
 	return nil
 }
 
+func (m *windowsManager) disableLocalDNSOverrideViaNRPT() bool {
+	return m.knobs != nil && m.knobs.DisableLocalDNSOverrideViaNRPT.Load()
+}
+
 func (m *windowsManager) SetDNS(cfg OSConfig) error {
 	// We can configure Windows DNS in one of two ways:
 	//
@@ -322,7 +332,17 @@ func (m *windowsManager) SetDNS(cfg OSConfig) error {
 	}
 
 	if len(cfg.MatchDomains) == 0 {
-		if err := m.setSplitDNS(nil, nil); err != nil {
+		var resolvers []netip.Addr
+		var domains []dnsname.FQDN
+		if !m.disableLocalDNSOverrideViaNRPT() {
+			// Create a default catch-all rule to make ourselves the actual primary resolver.
+			// Without this rule, Windows 8.1 and newer devices issue parallel DNS requests to DNS servers
+			// associated with all network adapters, even when "Override local DNS" is enabled and/or
+			// a Mullvad exit node is being used, resulting in DNS leaks.
+			resolvers = cfg.Nameservers
+			domains = []dnsname.FQDN{"."}
+		}
+		if err := m.setSplitDNS(resolvers, domains); err != nil {
 			return err
 		}
 		if err := m.setHosts(nil); err != nil {
@@ -331,8 +351,6 @@ func (m *windowsManager) SetDNS(cfg OSConfig) error {
 		if err := m.setPrimaryDNS(cfg.Nameservers, cfg.SearchDomains); err != nil {
 			return err
 		}
-	} else if m.nrptDB == nil {
-		return errors.New("cannot set per-domain resolvers on Windows 7")
 	} else {
 		if err := m.setSplitDNS(cfg.Nameservers, cfg.MatchDomains); err != nil {
 			return err

net/dns/manager_windows_test.go

@@ -84,7 +84,7 @@ func TestManagerWindowsGPCopy(t *testing.T) {
 	}
 	defer delIfKey()
 
-	cfg, err := NewOSConfigurator(logf, nil, fakeInterface.String())
+	cfg, err := NewOSConfigurator(logf, nil, nil, fakeInterface.String())
 	if err != nil {
 		t.Fatalf("NewOSConfigurator: %v\n", err)
 	}
@@ -213,7 +213,7 @@ func runTest(t *testing.T, isLocal bool) {
 	}
 	defer delIfKey()
 
-	cfg, err := NewOSConfigurator(logf, nil, fakeInterface.String())
+	cfg, err := NewOSConfigurator(logf, nil, nil, fakeInterface.String())
 	if err != nil {
 		t.Fatalf("NewOSConfigurator: %v\n", err)
 	}

tailcfg/tailcfg.go

@@ -139,7 +139,8 @@ type CapabilityVersion int
 //   - 96: 2024-05-29: Client understands NodeAttrSSHBehaviorV1
 //   - 97: 2024-06-06: Client understands NodeAttrDisableSplitDNSWhenNoCustomResolvers
 //   - 98: 2024-06-13: iOS/tvOS clients may provide serial number as part of posture information
-const CurrentCapabilityVersion CapabilityVersion = 98
+//   - 99: 2024-06-14: Client understands NodeAttrDisableLocalDNSOverrideViaNRPT
+const CurrentCapabilityVersion CapabilityVersion = 99
 
 type StableID string
 
@@ -2306,6 +2307,15 @@ const (
 	// and this node attribute allows us to disable the optimization remotely
 	// if needed.
 	NodeAttrDisableSplitDNSWhenNoCustomResolvers NodeCapability = "disable-split-dns-when-no-custom-resolvers"
+
+	// NodeAttrDisableLocalDNSOverrideViaNRPT indicates that the node's DNS manager should not
+	// create a default (catch-all) Windows NRPT rule when "Override local DNS" is enabled.
+	// Without this rule, Windows 8.1 and newer devices issue parallel DNS requests to DNS servers
+	// associated with all network adapters, even when "Override local DNS" is enabled and/or
+	// a Mullvad exit node is being used, resulting in DNS leaks.
+	// We began creating this rule on 2024-06-14, and this node attribute
+	// allows us to disable the new behavior remotely if needed.
+	NodeAttrDisableLocalDNSOverrideViaNRPT NodeCapability = "disable-local-dns-override-via-nrpt"
 )
 
 // SetDNSRequest is a request to add a DNS record.