net/netns, net/dns/resolver, etc: make netmon required in most places

The goal is to move more network state accessors to netmon.Monitor
where they can be cheaper/cached. But first (this change and others)
we need to make sure the one netmon.Monitor is plumbed everywhere.

Some notable bits:

* tsdial.NewDialer is added, taking a now-required netmon

* because a tsdial.Dialer always has a netmon, anything taking both
  a Dialer and a NetMon is now redundant; take only the Dialer and
  get the NetMon from that if/when needed.

* netmon.NewStatic is added, primarily for tests

Updates tailscale/corp#10910
Updates tailscale/corp#18960
Updates #7967
Updates #3299

Change-Id: I877f9cb87618c4eb037cee098241d18da9c01691
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/derper/mesh.go

@@ -15,6 +15,7 @@ import (
 
 	"tailscale.com/derp"
 	"tailscale.com/derp/derphttp"
+	"tailscale.com/net/netmon"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 )
@@ -36,7 +37,8 @@ func startMesh(s *derp.Server) error {
 
 func startMeshWithHost(s *derp.Server, host string) error {
 	logf := logger.WithPrefix(log.Printf, fmt.Sprintf("mesh(%q): ", host))
-	c, err := derphttp.NewClient(s.PrivateKey(), "https://"+host+"/derp", logf)
+	netMon := netmon.NewStatic() // good enough for cmd/derper; no need for netns fanciness
+	c, err := derphttp.NewClient(s.PrivateKey(), "https://"+host+"/derp", logf, netMon)
 	if err != nil {
 		return err
 	}

cmd/tailscaled/tailscaled.go

@@ -393,8 +393,8 @@ func run() (err error) {
 	// Always clean up, even if we're going to run the server. This covers cases
 	// such as when a system was rebooted without shutting down, or tailscaled
 	// crashed, and would for example restore system DNS configuration.
-	dns.CleanUp(logf, args.tunname)
+	dns.CleanUp(logf, netMon, args.tunname)
-	router.CleanUp(logf, args.tunname)
+	router.CleanUp(logf, netMon, args.tunname)
 	// If the cleanUp flag was passed, then exit.
 	if args.cleanUp {
 		return nil

control/controlclient/direct.go

@@ -56,6 +56,7 @@ import (
 	"tailscale.com/util/singleflight"
 	"tailscale.com/util/syspolicy"
 	"tailscale.com/util/systemd"
+	"tailscale.com/util/testenv"
 	"tailscale.com/util/zstdframe"
 )
 
@@ -68,7 +69,7 @@ type Direct struct {
 	serverURL                  string              // URL of the tailcontrol server
 	clock                      tstime.Clock
 	logf                       logger.Logf
-	netMon                     *netmon.Monitor // or nil
+	netMon                     *netmon.Monitor // non-nil
 	health                     *health.Tracker
 	discoPubKey                key.DiscoPublic
 	getMachinePrivKey          func() (key.MachinePrivate, error)
@@ -120,10 +121,9 @@ type Options struct {
 	Hostinfo                   *tailcfg.Hostinfo // non-nil passes ownership, nil means to use default using os.Hostname, etc
 	DiscoPublicKey             key.DiscoPublic
 	Logf                       logger.Logf
-	HTTPTestClient             *http.Client    // optional HTTP client to use (for tests only)
+	HTTPTestClient             *http.Client // optional HTTP client to use (for tests only)
-	NoiseTestClient            *http.Client    // optional HTTP client to use for noise RPCs (tests only)
+	NoiseTestClient            *http.Client // optional HTTP client to use for noise RPCs (tests only)
-	DebugFlags                 []string        // debug settings to send to control
+	DebugFlags                 []string     // debug settings to send to control
-	NetMon                     *netmon.Monitor // optional network monitor
 	HealthTracker              *health.Tracker
 	PopBrowserURL              func(url string)             // optional func to open browser
 	OnClientVersion            func(*tailcfg.ClientVersion) // optional func to inform GUI of client version status
@@ -213,6 +213,19 @@ func NewDirect(opts Options) (*Direct, error) {
 	if opts.GetMachinePrivateKey == nil {
 		return nil, errors.New("controlclient.New: no GetMachinePrivateKey specified")
 	}
+	if opts.Dialer == nil {
+		if testenv.InTest() {
+			panic("no Dialer")
+		}
+		return nil, errors.New("controlclient.New: no Dialer specified")
+	}
+	netMon := opts.Dialer.NetMon()
+	if netMon == nil {
+		if testenv.InTest() {
+			panic("no NetMon in Dialer")
+		}
+		return nil, errors.New("controlclient.New: Dialer has nil NetMon")
+	}
 	if opts.ControlKnobs == nil {
 		opts.ControlKnobs = &controlknobs.Knobs{}
 	}
@@ -233,9 +246,8 @@ func NewDirect(opts Options) (*Direct, error) {
 	dnsCache := &dnscache.Resolver{
 		Forward:          dnscache.Get().Forward, // use default cache's forwarder
 		UseLastGood:      true,
-		LookupIPFallback: dnsfallback.MakeLookupFunc(opts.Logf, opts.NetMon),
+		LookupIPFallback: dnsfallback.MakeLookupFunc(opts.Logf, netMon),
 		Logf:             opts.Logf,
-		NetMon:           opts.NetMon,
 	}
 
 	httpc := opts.HTTPTestClient
@@ -272,7 +284,7 @@ func NewDirect(opts Options) (*Direct, error) {
 		authKey:                    opts.AuthKey,
 		discoPubKey:                opts.DiscoPublicKey,
 		debugFlags:                 opts.DebugFlags,
-		netMon:                     opts.NetMon,
+		netMon:                     netMon,
 		health:                     opts.HealthTracker,
 		skipIPForwardingCheck:      opts.SkipIPForwardingCheck,
 		pinger:                     opts.Pinger,

control/controlclient/direct_test.go

@@ -14,6 +14,7 @@ import (
 
 	"tailscale.com/hostinfo"
 	"tailscale.com/ipn/ipnstate"
+	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
@@ -31,7 +32,7 @@ func TestNewDirect(t *testing.T) {
 		GetMachinePrivateKey: func() (key.MachinePrivate, error) {
 			return k, nil
 		},
-		Dialer: new(tsdial.Dialer),
+		Dialer: tsdial.NewDialer(netmon.NewStatic()),
 	}
 	c, err := NewDirect(opts)
 	if err != nil {
@@ -107,7 +108,7 @@ func TestTsmpPing(t *testing.T) {
 		GetMachinePrivateKey: func() (key.MachinePrivate, error) {
 			return k, nil
 		},
-		Dialer: new(tsdial.Dialer),
+		Dialer: tsdial.NewDialer(netmon.NewStatic()),
 	}
 
 	c, err := NewDirect(opts)

control/controlclient/noise_test.go

@@ -16,6 +16,7 @@ import (
 
 	"golang.org/x/net/http2"
 	"tailscale.com/control/controlhttp"
+	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
@@ -73,7 +74,7 @@ func (tt noiseClientTest) run(t *testing.T) {
 	})
 	defer hs.Close()
 
-	dialer := new(tsdial.Dialer)
+	dialer := tsdial.NewDialer(netmon.NewStatic())
 	nc, err := NewNoiseClient(NoiseOpts{
 		PrivKey:      clientPrivate,
 		ServerPubKey: serverPrivate.Public(),

control/controlhttp/client.go

@@ -393,7 +393,6 @@ func (a *Dialer) resolver() *dnscache.Resolver {
 		LookupIPFallback: dnsfallback.MakeLookupFunc(a.logf, a.NetMon),
 		UseLastGood:      true,
 		Logf:             a.Logf, // not a.logf method; we want to propagate nil-ness
-		NetMon:           a.NetMon,
 	}
 }
 
@@ -412,7 +411,6 @@ func (a *Dialer) tryURLUpgrade(ctx context.Context, u *url.URL, addr netip.Addr,
 			SingleHostStaticResult: []netip.Addr{addr},
 			SingleHost:             u.Hostname(),
 			Logf:                   a.Logf, // not a.logf method; we want to propagate nil-ness
-			NetMon:                 a.NetMon,
 		}
 	} else {
 		dns = a.resolver()

control/controlhttp/http_test.go

@@ -22,6 +22,7 @@ import (
 
 	"tailscale.com/control/controlbase"
 	"tailscale.com/net/dnscache"
+	"tailscale.com/net/netmon"
 	"tailscale.com/net/socks5"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/tailcfg"
@@ -199,14 +200,17 @@ func testControlHTTP(t *testing.T, param httpTestParam) {
 		defer cancel()
 	}
 
+	netMon := netmon.NewStatic()
+	dialer := tsdial.NewDialer(netMon)
 	a := &Dialer{
 		Hostname:             "localhost",
 		HTTPPort:             strconv.Itoa(httpLn.Addr().(*net.TCPAddr).Port),
 		HTTPSPort:            strconv.Itoa(httpsLn.Addr().(*net.TCPAddr).Port),
 		MachineKey:           client,
 		ControlKey:           server.Public(),
+		NetMon:               netMon,
 		ProtocolVersion:      testProtocolVersion,
-		Dialer:               new(tsdial.Dialer).SystemDial,
+		Dialer:               dialer.SystemDial,
 		Logf:                 t.Logf,
 		omitCertErrorLogging: true,
 		testFallbackDelay:    fallbackDelay,
@@ -643,7 +647,7 @@ func TestDialPlan(t *testing.T) {
 
 			dialer := closeTrackDialer{
 				t:     t,
-				inner: new(tsdial.Dialer).SystemDial,
+				inner: tsdial.NewDialer(netmon.NewStatic()).SystemDial,
 				conns: make(map[*closeTrackConn]bool),
 			}
 			defer dialer.Done()

derp/derphttp/derphttp_client.go

@@ -71,7 +71,7 @@ type Client struct {
 
 	privateKey key.NodePrivate
 	logf       logger.Logf
-	netMon     *netmon.Monitor // optional; nil means interfaces will be looked up on-demand
+	netMon     *netmon.Monitor // always non-nil
 	dialer     func(ctx context.Context, network, addr string) (net.Conn, error)
 
 	// Either url or getRegion is non-nil:
@@ -116,9 +116,11 @@ func (c *Client) String() string {
 
 // NewRegionClient returns a new DERP-over-HTTP client. It connects lazily.
 // To trigger a connection, use Connect.
-// The netMon parameter is optional; if non-nil it's used to do faster interface lookups.
 // The healthTracker parameter is also optional.
 func NewRegionClient(privateKey key.NodePrivate, logf logger.Logf, netMon *netmon.Monitor, getRegion func() *tailcfg.DERPRegion) *Client {
+	if netMon == nil {
+		panic("nil netMon")
+	}
 	ctx, cancel := context.WithCancel(context.Background())
 	c := &Client{
 		privateKey: privateKey,
@@ -140,7 +142,10 @@ func NewNetcheckClient(logf logger.Logf) *Client {
 
 // NewClient returns a new DERP-over-HTTP client. It connects lazily.
 // To trigger a connection, use Connect.
-func NewClient(privateKey key.NodePrivate, serverURL string, logf logger.Logf) (*Client, error) {
+func NewClient(privateKey key.NodePrivate, serverURL string, logf logger.Logf, netMon *netmon.Monitor) (*Client, error) {
+	if netMon == nil {
+		panic("nil netMon")
+	}
 	u, err := url.Parse(serverURL)
 	if err != nil {
 		return nil, fmt.Errorf("derphttp.NewClient: %v", err)
@@ -157,6 +162,7 @@ func NewClient(privateKey key.NodePrivate, serverURL string, logf logger.Logf) (
 		ctx:        ctx,
 		cancelCtx:  cancel,
 		clock:      tstime.StdClock{},
+		netMon:     netMon,
 	}
 	return c, nil
 }

derp/derphttp/derphttp_test.go

@@ -16,12 +16,15 @@ import (
 	"time"
 
 	"tailscale.com/derp"
+	"tailscale.com/net/netmon"
 	"tailscale.com/types/key"
 )
 
 func TestSendRecv(t *testing.T) {
 	serverPrivateKey := key.NewNode()
 
+	netMon := netmon.NewStatic()
+
 	const numClients = 3
 	var clientPrivateKeys []key.NodePrivate
 	var clientKeys []key.NodePublic
@@ -68,7 +71,7 @@ func TestSendRecv(t *testing.T) {
 	}()
 	for i := range numClients {
 		key := clientPrivateKeys[i]
-		c, err := NewClient(key, serverURL, t.Logf)
+		c, err := NewClient(key, serverURL, t.Logf, netMon)
 		if err != nil {
 			t.Fatalf("client %d: %v", i, err)
 		}
@@ -183,7 +186,7 @@ func TestPing(t *testing.T) {
 		}
 	}()
 
-	c, err := NewClient(key.NewNode(), serverURL, t.Logf)
+	c, err := NewClient(key.NewNode(), serverURL, t.Logf, netmon.NewStatic())
 	if err != nil {
 		t.Fatalf("NewClient: %v", err)
 	}
@@ -236,7 +239,7 @@ func newTestServer(t *testing.T, k key.NodePrivate) (serverURL string, s *derp.S
 }
 
 func newWatcherClient(t *testing.T, watcherPrivateKey key.NodePrivate, serverToWatchURL string) (c *Client) {
-	c, err := NewClient(watcherPrivateKey, serverToWatchURL, t.Logf)
+	c, err := NewClient(watcherPrivateKey, serverToWatchURL, t.Logf, netmon.NewStatic())
 	if err != nil {
 		t.Fatal(err)
 	}

ipn/ipnlocal/local.go

@@ -353,6 +353,12 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 	e := sys.Engine.Get()
 	store := sys.StateStore.Get()
 	dialer := sys.Dialer.Get()
+	if dialer == nil {
+		return nil, errors.New("dialer to NewLocalBackend must be set")
+	}
+	if dialer.NetMon() == nil {
+		return nil, errors.New("dialer to NewLocalBackend must have a NetMon")
+	}
 	_ = sys.MagicSock.Get() // or panic
 
 	goos := envknob.GOOS()
@@ -1762,7 +1768,6 @@ func (b *LocalBackend) Start(opts ipn.Options) error {
 		HTTPTestClient:             httpTestClient,
 		DiscoPublicKey:             discoPublic,
 		DebugFlags:                 debugFlags,
-		NetMon:                     b.sys.NetMon.Get(),
 		HealthTracker:              b.health,
 		Pinger:                     b,
 		PopBrowserURL:              b.tellClientToBrowseToURL,

ipn/ipnlocal/local_test.go

@@ -2273,7 +2273,6 @@ func TestOnTailnetDefaultAutoUpdate(t *testing.T) {
 		t.Skip("test broken on macOS; see https://github.com/tailscale/tailscale/issues/11894")
 	}
 	tests := []struct {
-		desc           string
 		before, after  opt.Bool
 		tailnetDefault bool
 	}{
@@ -2309,7 +2308,7 @@ func TestOnTailnetDefaultAutoUpdate(t *testing.T) {
 		},
 	}
 	for _, tt := range tests {
-		t.Run(fmt.Sprintf("before=%s after=%s", tt.before, tt.after), func(t *testing.T) {
+		t.Run(fmt.Sprintf("before=%s,after=%s", tt.before, tt.after), func(t *testing.T) {
 			b := newTestBackend(t)
 			p := ipn.NewPrefs()
 			p.AutoUpdate.Apply = tt.before

ipn/ipnlocal/network-lock_test.go

@@ -20,6 +20,8 @@ import (
 	"tailscale.com/hostinfo"
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/store/mem"
+	"tailscale.com/net/netmon"
+	"tailscale.com/net/tsdial"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tka"
 	"tailscale.com/types/key"
@@ -50,6 +52,7 @@ func fakeControlClient(t *testing.T, c *http.Client) *controlclient.Auto {
 		HTTPTestClient:  c,
 		NoiseTestClient: c,
 		Observer:        observerFunc(func(controlclient.Status) {}),
+		Dialer:          tsdial.NewDialer(netmon.NewStatic()),
 	}
 
 	cc, err := controlclient.NewNoStart(opts)

ipn/ipnlocal/serve_test.go

@@ -28,6 +28,7 @@ import (
 	"tailscale.com/ipn/store/mem"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tsd"
+	"tailscale.com/types/logger"
 	"tailscale.com/types/logid"
 	"tailscale.com/types/netmap"
 	"tailscale.com/util/mak"
@@ -663,14 +664,21 @@ func mustCreateURL(t *testing.T, u string) url.URL {
 }
 
 func newTestBackend(t *testing.T) *LocalBackend {
+	var logf logger.Logf = logger.Discard
+	const debug = true
+	if debug {
+		logf = logger.WithPrefix(t.Logf, "... ")
+	}
+
 	sys := &tsd.System{}
-	e, err := wgengine.NewUserspaceEngine(t.Logf, wgengine.Config{SetSubsystem: sys.Set})
+	e, err := wgengine.NewUserspaceEngine(logf, wgengine.Config{SetSubsystem: sys.Set})
 	if err != nil {
 		t.Fatal(err)
 	}
 	sys.Set(e)
 	sys.Set(new(mem.Store))
-	b, err := NewLocalBackend(t.Logf, logid.PublicID{}, sys, 0)
+
+	b, err := NewLocalBackend(logf, logid.PublicID{}, sys, 0)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -678,7 +686,7 @@ func newTestBackend(t *testing.T) *LocalBackend {
 	dir := t.TempDir()
 	b.SetVarRoot(dir)
 
-	pm := must.Get(newProfileManager(new(mem.Store), t.Logf))
+	pm := must.Get(newProfileManager(new(mem.Store), logf))
 	pm.currentProfile = &ipn.LoginProfile{ID: "id0"}
 	b.pm = pm
 

log/sockstatlog/logger.go

@@ -93,11 +93,16 @@ func SockstatLogID(logID logid.PublicID) logid.PrivateID {
 // On platforms that do not support sockstat logging, a nil Logger will be returned.
 // The returned Logger is not yet enabled, and must be shut down with Shutdown when it is no longer needed.
 // Logs will be uploaded to the log server using a new log ID derived from the provided backend logID.
-// The netMon parameter is optional; if non-nil it's used to do faster interface lookups.
+//
+// The netMon parameter is optional. It should be specified in environments where
+// Tailscaled is manipulating the routing table.
 func NewLogger(logdir string, logf logger.Logf, logID logid.PublicID, netMon *netmon.Monitor, health *health.Tracker) (*Logger, error) {
 	if !sockstats.IsAvailable {
 		return nil, nil
 	}
+	if netMon == nil {
+		netMon = netmon.NewStatic()
+	}
 
 	if err := os.MkdirAll(logdir, 0755); err != nil && !os.IsExist(err) {
 		return nil, err

logpolicy/logpolicy.go

@@ -447,8 +447,8 @@ func tryFixLogStateLocation(dir, cmdname string, logf logger.Logf) {
 // New returns a new log policy (a logger and its instance ID) for a given
 // collection name.
 //
-// The netMon parameter is optional; if non-nil it's used to do faster
+// The netMon parameter is optional. It should be specified in environments where
-// interface lookups.
+// Tailscaled is manipulating the routing table.
 //
 // The logf parameter is optional; if non-nil, information logs (e.g. when
 // migrating state) are sent to that logger, and global changes to the log
@@ -459,6 +459,9 @@ func New(collection string, netMon *netmon.Monitor, health *health.Tracker, logf
 
 // NewWithConfigPath is identical to New, but uses the specified directory and
 // command name. If either is empty, it derives them automatically.
+//
+// The netMon parameter is optional. It should be specified in environments where
+// Tailscaled is manipulating the routing table.
 func NewWithConfigPath(collection, dir, cmdName string, netMon *netmon.Monitor, health *health.Tracker, logf logger.Logf) *Policy {
 	var lflags int
 	if term.IsTerminal(2) || runtime.GOOS == "windows" {
@@ -681,8 +684,12 @@ func (p *Policy) Shutdown(ctx context.Context) error {
 //   - If TLS connection fails, try again using LetsEncrypt's built-in root certificate,
 //     for the benefit of older OS platforms which might not include it.
 //
-// The netMon parameter is optional; if non-nil it's used to do faster interface lookups.
+// The netMon parameter is optional. It should be specified in environments where
+// Tailscaled is manipulating the routing table.
 func MakeDialFunc(netMon *netmon.Monitor, logf logger.Logf) func(ctx context.Context, netw, addr string) (net.Conn, error) {
+	if netMon == nil {
+		netMon = netmon.NewStatic()
+	}
 	return func(ctx context.Context, netw, addr string) (net.Conn, error) {
 		return dialContext(ctx, netw, addr, netMon, logf)
 	}
@@ -725,7 +732,6 @@ func dialContext(ctx context.Context, netw, addr string, netMon *netmon.Monitor,
 		Forward:          dnscache.Get().Forward, // use default cache's forwarder
 		UseLastGood:      true,
 		LookupIPFallback: dnsfallback.MakeLookupFunc(logf, netMon),
-		NetMon:           netMon,
 	}
 	dialer := dnscache.Dialer(nd.DialContext, dnsCache)
 	c, err = dialer(ctx, netw, addr)
@@ -738,7 +744,8 @@ func dialContext(ctx context.Context, netw, addr string, netMon *netmon.Monitor,
 // NewLogtailTransport returns an HTTP Transport particularly suited to uploading
 // logs to the given host name. See DialContext for details on how it works.
 //
-// The netMon parameter is optional; if non-nil it's used to do faster interface lookups.
+// The netMon parameter is optional. It should be specified in environments where
+// Tailscaled is manipulating the routing table.
 //
 // The logf parameter is optional; if non-nil, logs are printed using the
 // provided function; if nil, log.Printf will be used instead.
@@ -746,6 +753,9 @@ func NewLogtailTransport(host string, netMon *netmon.Monitor, health *health.Tra
 	if testenv.InTest() {
 		return noopPretendSuccessTransport{}
 	}
+	if netMon == nil {
+		netMon = netmon.NewStatic()
+	}
 	// Start with a copy of http.DefaultTransport and tweak it a bit.
 	tr := http.DefaultTransport.(*http.Transport).Clone()
 

logtail/logtail.go

@@ -232,7 +232,7 @@ func (l *Logger) SetVerbosityLevel(level int) {
 	atomic.StoreInt64(&l.stderrLevel, int64(level))
 }
 
-// SetNetMon sets the optional the network monitor.
+// SetNetMon sets the network monitor.
 //
 // It should not be changed concurrently with log writes and should
 // only be set once.

net/dns/manager.go

@@ -55,15 +55,17 @@ type Manager struct {
 }
 
 // NewManagers created a new manager from the given config.
-// The netMon parameter is optional; if non-nil it's used to do faster interface lookups.
+func NewManager(logf logger.Logf, oscfg OSConfigurator, health *health.Tracker, dialer *tsdial.Dialer, linkSel resolver.ForwardLinkSelector, knobs *controlknobs.Knobs) *Manager {
-func NewManager(logf logger.Logf, oscfg OSConfigurator, netMon *netmon.Monitor, health *health.Tracker, dialer *tsdial.Dialer, linkSel resolver.ForwardLinkSelector, knobs *controlknobs.Knobs) *Manager {
 	if dialer == nil {
 		panic("nil Dialer")
 	}
+	if dialer.NetMon() == nil {
+		panic("Dialer has nil NetMon")
+	}
 	logf = logger.WithPrefix(logf, "dns: ")
 	m := &Manager{
 		logf:     logf,
-		resolver: resolver.New(logf, netMon, linkSel, dialer, knobs),
+		resolver: resolver.New(logf, linkSel, dialer, knobs),
 		os:       oscfg,
 		health:   health,
 	}
@@ -454,13 +456,15 @@ func (m *Manager) FlushCaches() error {
 // CleanUp restores the system DNS configuration to its original state
 // 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, interfaceName string) {
+func CleanUp(logf logger.Logf, netMon *netmon.Monitor, interfaceName string) {
 	oscfg, err := NewOSConfigurator(logf, nil, interfaceName)
 	if err != nil {
 		logf("creating dns cleanup: %v", err)
 		return
 	}
-	dns := NewManager(logf, oscfg, nil, nil, &tsdial.Dialer{Logf: logf}, nil, nil)
+	d := &tsdial.Dialer{Logf: logf}
+	d.SetNetMon(netMon)
+	dns := NewManager(logf, oscfg, nil, d, nil, nil)
 	if err := dns.Down(); err != nil {
 		logf("dns down: %v", err)
 	}

net/dns/manager_tcp_test.go

@@ -15,6 +15,7 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	dns "golang.org/x/net/dns/dnsmessage"
+	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/tstest"
 	"tailscale.com/util/dnsname"
@@ -87,7 +88,7 @@ func TestDNSOverTCP(t *testing.T) {
 			SearchDomains: fqdns("coffee.shop"),
 		},
 	}
-	m := NewManager(t.Logf, &f, nil, nil, new(tsdial.Dialer), nil, nil)
+	m := NewManager(t.Logf, &f, nil, tsdial.NewDialer(netmon.NewStatic()), nil, nil)
 	m.resolver.TestOnlySetHook(f.SetResolver)
 	m.Set(Config{
 		Hosts: hosts(
@@ -172,7 +173,7 @@ func TestDNSOverTCP_TooLarge(t *testing.T) {
 			SearchDomains: fqdns("coffee.shop"),
 		},
 	}
-	m := NewManager(log, &f, nil, nil, new(tsdial.Dialer), nil, nil)
+	m := NewManager(log, &f, nil, tsdial.NewDialer(netmon.NewStatic()), nil, nil)
 	m.resolver.TestOnlySetHook(f.SetResolver)
 	m.Set(Config{
 		Hosts:         hosts("andrew.ts.com.", "1.2.3.4"),

net/dns/manager_test.go

@@ -12,6 +12,7 @@ import (
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
 	"tailscale.com/net/dns/resolver"
+	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/types/dnstype"
 	"tailscale.com/util/dnsname"
@@ -613,7 +614,7 @@ func TestManager(t *testing.T) {
 				SplitDNS:   test.split,
 				BaseConfig: test.bs,
 			}
-			m := NewManager(t.Logf, &f, nil, nil, new(tsdial.Dialer), nil, nil)
+			m := NewManager(t.Logf, &f, nil, tsdial.NewDialer(netmon.NewStatic()), nil, nil)
 			m.resolver.TestOnlySetHook(f.SetResolver)
 
 			if err := m.Set(test.in); err != nil {

net/dns/resolver/forwarder.go

@@ -10,7 +10,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"math/rand"
 	"net"
 	"net/http"
 	"net/netip"
@@ -186,7 +185,7 @@ type resolverAndDelay struct {
 // forwarder forwards DNS packets to a number of upstream nameservers.
 type forwarder struct {
 	logf    logger.Logf
-	netMon  *netmon.Monitor
+	netMon  *netmon.Monitor     // always non-nil
 	linkSel ForwardLinkSelector // TODO(bradfitz): remove this when tsdial.Dialer absorbs it
 	dialer  *tsdial.Dialer
 
@@ -214,11 +213,10 @@ type forwarder struct {
 	cloudHostFallback []resolverAndDelay
 }
 
-func init() {
-	rand.Seed(time.Now().UnixNano())
-}
-
 func newForwarder(logf logger.Logf, netMon *netmon.Monitor, linkSel ForwardLinkSelector, dialer *tsdial.Dialer, knobs *controlknobs.Knobs) *forwarder {
+	if netMon == nil {
+		panic("nil netMon")
+	}
 	f := &forwarder{
 		logf:         logger.WithPrefix(logf, "forward: "),
 		netMon:       netMon,
@@ -410,7 +408,6 @@ func (f *forwarder) getKnownDoHClientForProvider(urlBase string) (c *http.Client
 		SingleHost:             dohURL.Hostname(),
 		SingleHostStaticResult: allIPs,
 		Logf:                   f.logf,
-		NetMon:                 f.netMon,
 	})
 	c = &http.Client{
 		Transport: &http.Transport{
@@ -584,7 +581,7 @@ func (f *forwarder) send(ctx context.Context, fq *forwardQuery, rr resolverAndDe
 	}
 
 	// Kick off the race between the UDP and TCP queries.
-	rh := race.New[[]byte](timeout, firstUDP, thenTCP)
+	rh := race.New(timeout, firstUDP, thenTCP)
 	resp, err := rh.Start(ctx)
 	if err == nil {
 		return resp, nil

net/dns/resolver/tsdns.go

@@ -181,7 +181,7 @@ func WriteRoutes(w *bufio.Writer, routes map[dnsname.FQDN][]*dnstype.Resolver) {
 // it delegates to upstream nameservers if any are set.
 type Resolver struct {
 	logf               logger.Logf
-	netMon             *netmon.Monitor  // or nil
+	netMon             *netmon.Monitor  // non-nil
 	dialer             *tsdial.Dialer   // non-nil
 	saveConfigForTests func(cfg Config) // used in tests to capture resolver config
 	// forwarder forwards requests to upstream nameservers.
@@ -205,11 +205,14 @@ type ForwardLinkSelector interface {
 }
 
 // New returns a new resolver.
-// netMon optionally specifies a network monitor to use for socket rebinding.
+func New(logf logger.Logf, linkSel ForwardLinkSelector, dialer *tsdial.Dialer, knobs *controlknobs.Knobs) *Resolver {
-func New(logf logger.Logf, netMon *netmon.Monitor, linkSel ForwardLinkSelector, dialer *tsdial.Dialer, knobs *controlknobs.Knobs) *Resolver {
 	if dialer == nil {
 		panic("nil Dialer")
 	}
+	netMon := dialer.NetMon()
+	if netMon == nil {
+		logf("nil netMon")
+	}
 	r := &Resolver{
 		logf:     logger.WithPrefix(logf, "resolver: "),
 		netMon:   netMon,

net/dns/resolver/tsdns_test.go

@@ -28,6 +28,7 @@ import (
 	"tailscale.com/net/tsdial"
 	"tailscale.com/tstest"
 	"tailscale.com/types/dnstype"
+	"tailscale.com/types/logger"
 	"tailscale.com/util/dnsname"
 )
 
@@ -313,7 +314,11 @@ func TestRDNSNameToIPv6(t *testing.T) {
 }
 
 func newResolver(t testing.TB) *Resolver {
-	return New(t.Logf, nil /* no network monitor */, nil /* no link selector */, new(tsdial.Dialer), nil /* no control knobs */)
+	return New(t.Logf,
+		nil, // no link selector
+		tsdial.NewDialer(netmon.NewStatic()),
+		nil, // no control knobs
+	)
 }
 
 func TestResolveLocal(t *testing.T) {
@@ -1009,7 +1014,13 @@ func TestForwardLinkSelection(t *testing.T) {
 	// routes differently.
 	specialIP := netaddr.IPv4(1, 2, 3, 4)
 
-	fwd := newForwarder(t.Logf, nil, linkSelFunc(func(ip netip.Addr) string {
+	netMon, err := netmon.New(logger.WithPrefix(t.Logf, ".... netmon: "))
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { netMon.Close() })
+
+	fwd := newForwarder(t.Logf, netMon, linkSelFunc(func(ip netip.Addr) string {
 		if ip == netaddr.IPv4(1, 2, 3, 4) {
 			return "special"
 		}

net/dnscache/dnscache.go

@@ -19,7 +19,6 @@ import (
 	"time"
 
 	"tailscale.com/envknob"
-	"tailscale.com/net/netmon"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/cloudenv"
 	"tailscale.com/util/singleflight"
@@ -90,11 +89,6 @@ type Resolver struct {
 	// be added to all log messages printed with this logger.
 	Logf logger.Logf
 
-	// NetMon optionally provides a netmon.Monitor to use to get the current
-	// (cached) network interface.
-	// If nil, the interface will be looked up dynamically.
-	NetMon *netmon.Monitor
-
 	sf singleflight.Group[string, ipRes]
 
 	mu      sync.Mutex

net/netcheck/netcheck.go

@@ -1657,7 +1657,6 @@ func (c *Client) nodeAddr(ctx context.Context, n *tailcfg.DERPNode, proto probeP
 				Forward:     net.DefaultResolver,
 				UseLastGood: true,
 				Logf:        c.logf,
-				NetMon:      c.NetMon,
 			}
 		}
 		resolver := c.resolver

net/netcheck/netcheck_test.go

@@ -24,7 +24,6 @@ import (
 	"tailscale.com/net/stun/stuntest"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tstest"
-	"tailscale.com/types/logger"
 )
 
 func TestHairpinSTUN(t *testing.T) {
@@ -157,14 +156,8 @@ func TestHairpinWait(t *testing.T) {
 }
 
 func newTestClient(t testing.TB) *Client {
-	netMon, err := netmon.New(logger.WithPrefix(t.Logf, "... netmon: "))
-	if err != nil {
-		t.Fatalf("netmon.New: %v", err)
-	}
-	t.Cleanup(func() { netMon.Close() })
-
 	c := &Client{
-		NetMon: netMon,
+		NetMon: netmon.NewStatic(),
 		Logf:   t.Logf,
 	}
 	return c

net/netcheck/standalone.go

@@ -24,12 +24,15 @@ import (
 // to bind, errors will be returned, if one or both protocols can bind no error
 // is returned.
 func (c *Client) Standalone(ctx context.Context, bindAddr string) error {
+	if c.NetMon == nil {
+		panic("netcheck.Client.NetMon must be set")
+	}
 	if bindAddr == "" {
 		bindAddr = ":0"
 	}
 	var errs []error
 
-	u4, err := nettype.MakePacketListenerWithNetIP(netns.Listener(c.logf, nil)).ListenPacket(ctx, "udp4", bindAddr)
+	u4, err := nettype.MakePacketListenerWithNetIP(netns.Listener(c.logf, c.NetMon)).ListenPacket(ctx, "udp4", bindAddr)
 	if err != nil {
 		c.logf("udp4: %v", err)
 		errs = append(errs, err)
@@ -37,7 +40,7 @@ func (c *Client) Standalone(ctx context.Context, bindAddr string) error {
 		go readPackets(ctx, c.logf, u4, c.ReceiveSTUNPacket)
 	}
 
-	u6, err := nettype.MakePacketListenerWithNetIP(netns.Listener(c.logf, nil)).ListenPacket(ctx, "udp6", bindAddr)
+	u6, err := nettype.MakePacketListenerWithNetIP(netns.Listener(c.logf, c.NetMon)).ListenPacket(ctx, "udp6", bindAddr)
 	if err != nil {
 		c.logf("udp6: %v", err)
 		errs = append(errs, err)

net/netmon/netmon.go

@@ -55,6 +55,7 @@ type Monitor struct {
 	om     osMon         // nil means not supported on this platform
 	change chan bool     // send false to wake poller, true to also force ChangeDeltas be sent
 	stop   chan struct{} // closed on Stop
+	static bool          // static Monitor that doesn't actually monitor
 
 	// Things that must be set early, before use,
 	// and not change at runtime.
@@ -139,6 +140,17 @@ func New(logf logger.Logf) (*Monitor, error) {
 	return m, nil
 }
 
+// NewStatic returns a Monitor that's a one-time snapshot of the network state
+// but doesn't actually monitor for changes. It should only be used in tests
+// and situations like cleanups or short-lived CLI programs.
+func NewStatic() *Monitor {
+	m := &Monitor{static: true}
+	if st, err := m.interfaceStateUncached(); err == nil {
+		m.ifState = st
+	}
+	return m
+}
+
 // InterfaceState returns the latest snapshot of the machine's network
 // interfaces.
 //
@@ -168,6 +180,10 @@ func (m *Monitor) SetTailscaleInterfaceName(ifName string) {
 // It's the same as interfaces.LikelyHomeRouterIP, but it caches the
 // result until the monitor detects a network change.
 func (m *Monitor) GatewayAndSelfIP() (gw, myIP netip.Addr, ok bool) {
+	if m.static {
+		return
+	}
+
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	if m.gwValid {
@@ -190,6 +206,9 @@ func (m *Monitor) GatewayAndSelfIP() (gw, myIP netip.Addr, ok bool) {
 // notified (in their own goroutine) when the network state changes.
 // To remove this callback, call unregister (or close the monitor).
 func (m *Monitor) RegisterChangeCallback(callback ChangeFunc) (unregister func()) {
+	if m.static {
+		return func() {}
+	}
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	handle := m.cbs.Add(callback)
@@ -210,6 +229,9 @@ type RuleDeleteCallback func(table uint8, priority uint32)
 // notified (in their own goroutine) when a Linux ip rule is deleted.
 // To remove this callback, call unregister (or close the monitor).
 func (m *Monitor) RegisterRuleDeleteCallback(callback RuleDeleteCallback) (unregister func()) {
+	if m.static {
+		return func() {}
+	}
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	handle := m.ruleDelCB.Add(callback)
@@ -223,6 +245,9 @@ func (m *Monitor) RegisterRuleDeleteCallback(callback RuleDeleteCallback) (unreg
 // Start starts the monitor.
 // A monitor can only be started & closed once.
 func (m *Monitor) Start() {
+	if m.static {
+		return
+	}
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	if m.started || m.closed {
@@ -244,6 +269,9 @@ func (m *Monitor) Start() {
 
 // Close closes the monitor.
 func (m *Monitor) Close() error {
+	if m.static {
+		return nil
+	}
 	m.mu.Lock()
 	if m.closed {
 		m.mu.Unlock()
@@ -275,6 +303,9 @@ func (m *Monitor) Close() error {
 // ChangeFunc callbacks will be called within the event coalescing
 // period (under a fraction of a second).
 func (m *Monitor) InjectEvent() {
+	if m.static {
+		return
+	}
 	select {
 	case m.change <- true:
 	default:
@@ -290,6 +321,9 @@ func (m *Monitor) InjectEvent() {
 // This is like InjectEvent but only fires ChangeFunc callbacks
 // if the network state differed at all.
 func (m *Monitor) Poll() {
+	if m.static {
+		return
+	}
 	select {
 	case m.change <- false:
 	default:

net/netns/netns.go

@@ -56,8 +56,10 @@ func SetDisableBindConnToInterface(v bool) {
 // Listener returns a new net.Listener with its Control hook func
 // initialized as necessary to run in logical network namespace that
 // doesn't route back into Tailscale.
-// The netMon parameter is optional; if non-nil it's used to do faster interface lookups.
 func Listener(logf logger.Logf, netMon *netmon.Monitor) *net.ListenConfig {
+	if netMon == nil {
+		panic("netns.Listener called with nil netMon")
+	}
 	if disabled.Load() {
 		return new(net.ListenConfig)
 	}
@@ -68,8 +70,10 @@ func Listener(logf logger.Logf, netMon *netmon.Monitor) *net.ListenConfig {
 // hook func initialized as necessary to run in a logical network
 // namespace that doesn't route back into Tailscale. It also handles
 // using a SOCKS if configured in the environment with ALL_PROXY.
-// The netMon parameter is optional; if non-nil it's used to do faster interface lookups.
 func NewDialer(logf logger.Logf, netMon *netmon.Monitor) Dialer {
+	if netMon == nil {
+		panic("netns.NewDialer called with nil netMon")
+	}
 	return FromDialer(logf, netMon, &net.Dialer{
 		KeepAlive: netknob.PlatformTCPKeepAlive(),
 	})
@@ -79,8 +83,10 @@ func NewDialer(logf logger.Logf, netMon *netmon.Monitor) Dialer {
 // network namespace that doesn't route back into Tailscale. It also
 // handles using a SOCKS if configured in the environment with
 // ALL_PROXY.
-// The netMon parameter is optional; if non-nil it's used to do faster interface lookups.
 func FromDialer(logf logger.Logf, netMon *netmon.Monitor, d *net.Dialer) Dialer {
+	if netMon == nil {
+		panic("netns.FromDialer called with nil netMon")
+	}
 	if disabled.Load() {
 		return d
 	}

net/portmapper/igd_test.go

@@ -16,6 +16,7 @@ import (
 
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/net/netaddr"
+	"tailscale.com/net/netmon"
 	"tailscale.com/syncs"
 	"tailscale.com/types/logger"
 )
@@ -259,12 +260,13 @@ func (d *TestIGD) handlePCPQuery(pkt []byte, src netip.AddrPort) {
 
 func newTestClient(t *testing.T, igd *TestIGD) *Client {
 	var c *Client
-	c = NewClient(t.Logf, nil, nil, new(controlknobs.Knobs), func() {
+	c = NewClient(t.Logf, netmon.NewStatic(), nil, new(controlknobs.Knobs), func() {
 		t.Logf("port map changed")
 		t.Logf("have mapping: %v", c.HaveMapping())
 	})
 	c.testPxPPort = igd.TestPxPPort()
 	c.testUPnPPort = igd.TestUPnPPort()
+	c.netMon = netmon.NewStatic()
 	c.SetGatewayLookupFunc(testIPAndGateway)
 	return c
 }

net/portmapper/portmapper.go

@@ -198,8 +198,7 @@ func (m *pmpMapping) Release(ctx context.Context) {
 
 // NewClient returns a new portmapping client.
 //
-// The netMon parameter is optional; if non-nil it's used to do faster interface
+// The netMon parameter is required.
-// lookups.
 //
 // The debug argument allows configuring the behaviour of the portmapper for
 // debugging; if nil, a sensible set of defaults will be used.
@@ -211,10 +210,13 @@ func (m *pmpMapping) Release(ctx context.Context) {
 // whenever the port mapping status has changed. If nil, it doesn't make a
 // callback.
 func NewClient(logf logger.Logf, netMon *netmon.Monitor, debug *DebugKnobs, controlKnobs *controlknobs.Knobs, onChange func()) *Client {
+	if netMon == nil {
+		panic("nil netMon")
+	}
 	ret := &Client{
 		logf:         logf,
 		netMon:       netMon,
-		ipAndGateway: interfaces.LikelyHomeRouterIP,
+		ipAndGateway: interfaces.LikelyHomeRouterIP, // TODO(bradfitz): move this to netMon
 		onChange:     onChange,
 		controlKnobs: controlKnobs,
 	}

net/tsdial/tsdial.go

@@ -26,13 +26,27 @@ import (
 	"tailscale.com/types/netmap"
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/mak"
+	"tailscale.com/util/testenv"
 	"tailscale.com/version"
 )
 
+// NewDialer returns a new Dialer that can dial out of tailscaled.
+// Its exported fields should be set before use, if any.
+func NewDialer(netMon *netmon.Monitor) *Dialer {
+	if netMon == nil {
+		panic("NewDialer: netMon is nil")
+	}
+	d := &Dialer{}
+	d.SetNetMon(netMon)
+	return d
+}
+
 // Dialer dials out of tailscaled, while taking care of details while
 // handling the dozens of edge cases depending on the server mode
 // (TUN, netstack), the OS network sandboxing style (macOS/iOS
 // Extension, none), user-selected route acceptance prefs, etc.
+//
+// Before use, SetNetMon should be called with a netmon.Monitor.
 type Dialer struct {
 	Logf logger.Logf
 	// UseNetstackForIP if non-nil is whether NetstackDialTCP (if
@@ -130,9 +144,14 @@ func (d *Dialer) Close() error {
 	return nil
 }
 
+// SetNetMon sets d's network monitor to netMon.
+// It is a no-op to call SetNetMon with the same netMon as the current one.
 func (d *Dialer) SetNetMon(netMon *netmon.Monitor) {
 	d.mu.Lock()
 	defer d.mu.Unlock()
+	if d.netMon == netMon {
+		return
+	}
 	if d.netMonUnregister != nil {
 		go d.netMonUnregister()
 		d.netMonUnregister = nil
@@ -141,6 +160,14 @@ func (d *Dialer) SetNetMon(netMon *netmon.Monitor) {
 	d.netMonUnregister = d.netMon.RegisterChangeCallback(d.linkChanged)
 }
 
+// NetMon returns the Dialer's network monitor.
+// It returns nil if SetNetMon has not been called.
+func (d *Dialer) NetMon() *netmon.Monitor {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	return d.netMon
+}
+
 var (
 	metricLinkChangeConnClosed      = clientmetric.NewCounter("tsdial_linkchange_closes")
 	metricChangeDeltaNoDefaultRoute = clientmetric.NewCounter("tsdial_changedelta_no_default_route")
@@ -314,6 +341,13 @@ func (d *Dialer) logf(format string, args ...any) {
 // Control and (in the future, as of 2022-04-27) DERPs..
 func (d *Dialer) SystemDial(ctx context.Context, network, addr string) (net.Conn, error) {
 	d.mu.Lock()
+	if d.netMon == nil {
+		d.mu.Unlock()
+		if testenv.InTest() {
+			panic("SystemDial requires a netmon.Monitor; call SetNetMon first")
+		}
+		return nil, errors.New("SystemDial requires a netmon.Monitor; call SetNetMon first")
+	}
 	closed := d.closed
 	d.mu.Unlock()
 	if closed {

prober/derp_test.go

@@ -16,6 +16,7 @@ import (
 
 	"tailscale.com/derp"
 	"tailscale.com/derp/derphttp"
+	"tailscale.com/net/netmon"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
 )
@@ -140,7 +141,7 @@ func TestRunDerpProbeNodePair(t *testing.T) {
 		}
 	}()
 	newClient := func() *derphttp.Client {
-		c, err := derphttp.NewClient(key.NewNode(), serverURL, t.Logf)
+		c, err := derphttp.NewClient(key.NewNode(), serverURL, t.Logf, netmon.NewStatic())
 		if err != nil {
 			t.Fatalf("NewClient: %v", err)
 		}

wgengine/router/router.go

@@ -53,7 +53,7 @@ func New(logf logger.Logf, tundev tun.Device, netMon *netmon.Monitor, health *he
 // CleanUp restores the system network configuration to its original state
 // 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, interfaceName string) {
+func CleanUp(logf logger.Logf, netMon *netmon.Monitor, interfaceName string) {
 	cleanUp(logf, interfaceName)
 }
 

wgengine/userspace.go

@@ -193,7 +193,7 @@ type Config struct {
 	HealthTracker *health.Tracker
 
 	// Dialer is the dialer to use for outbound connections.
-	// If nil, a new Dialer is created
+	// If nil, a new Dialer is created.
 	Dialer *tsdial.Dialer
 
 	// ControlKnobs is the set of control plane-provied knobs
@@ -341,7 +341,7 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 	tunName, _ := conf.Tun.Name()
 	conf.Dialer.SetTUNName(tunName)
 	conf.Dialer.SetNetMon(e.netMon)
-	e.dns = dns.NewManager(logf, conf.DNS, e.netMon, e.health, conf.Dialer, fwdDNSLinkSelector{e, tunName}, conf.ControlKnobs)
+	e.dns = dns.NewManager(logf, conf.DNS, e.health, conf.Dialer, fwdDNSLinkSelector{e, tunName}, conf.ControlKnobs)
 
 	// TODO: there's probably a better place for this
 	sockstats.SetNetMon(e.netMon)