net/{interfaces,netmon}: remove "interesting", EqualFiltered API

This removes a lot of API from net/interfaces (including all the
filter types, EqualFiltered, active Tailscale interface func, etc) and
moves the "major" change detection to net/netmon which knows more
about the world and the previous/new states.

Updates #9040

Change-Id: I7fe66a23039c6347ae5458745b709e7ebdcce245
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/nm.go

@@ -8,13 +8,14 @@
 import (
 	"context"
 	"fmt"
+	"net"
 	"net/netip"
 	"sort"
 	"time"
 
 	"github.com/godbus/dbus/v5"
 	"github.com/josharian/native"
-	"tailscale.com/net/interfaces"
+	"tailscale.com/net/tsaddr"
 	"tailscale.com/util/dnsname"
 )
 
@@ -139,14 +140,18 @@ func (m *nmManager) trySet(ctx context.Context, config OSConfig) error {
 	// tell it explicitly to keep it. Read out the current interface
 	// settings and mirror them out to NetworkManager.
 	var addrs6 []map[string]any
-	addrs, _, err := interfaces.Tailscale()
+	if tsIf, err := net.InterfaceByName(m.interfaceName); err == nil {
-	if err == nil {
+		addrs, _ := tsIf.Addrs()
 		for _, a := range addrs {
-			if a.Is6() {
+			if ipnet, ok := a.(*net.IPNet); ok {
-				addrs6 = append(addrs6, map[string]any{
+				nip, ok := netip.AddrFromSlice(ipnet.IP)
-					"address": a.String(),
+				nip = nip.Unmap()
-					"prefix":  uint32(128),
+				if ok && tsaddr.IsTailscaleIP(nip) && nip.Is6() {
-				})
+					addrs6 = append(addrs6, map[string]any{
+						"address": nip.String(),
+						"prefix":  uint32(128),
+					})
+				}
 			}
 		}
 	}

net/interfaces/interfaces.go

@@ -11,6 +11,7 @@
 	"net/http"
 	"net/netip"
 	"runtime"
+	"slices"
 	"sort"
 	"strings"
 
@@ -24,49 +25,6 @@
 // which HTTP proxy the system should use.
 var LoginEndpointForProxyDetermination = "https://controlplane.tailscale.com/"
 
-// Tailscale returns the current machine's Tailscale interface, if any.
-// If none is found, all zero values are returned.
-// A non-nil error is only returned on a problem listing the system interfaces.
-func Tailscale() ([]netip.Addr, *net.Interface, error) {
-	ifs, err := netInterfaces()
-	if err != nil {
-		return nil, nil, err
-	}
-	for _, iface := range ifs {
-		if !maybeTailscaleInterfaceName(iface.Name) {
-			continue
-		}
-		addrs, err := iface.Addrs()
-		if err != nil {
-			continue
-		}
-		var tsIPs []netip.Addr
-		for _, a := range addrs {
-			if ipnet, ok := a.(*net.IPNet); ok {
-				nip, ok := netip.AddrFromSlice(ipnet.IP)
-				nip = nip.Unmap()
-				if ok && tsaddr.IsTailscaleIP(nip) {
-					tsIPs = append(tsIPs, nip)
-				}
-			}
-		}
-		if len(tsIPs) > 0 {
-			return tsIPs, iface.Interface, nil
-		}
-	}
-	return nil, nil, nil
-}
-
-// maybeTailscaleInterfaceName reports whether s is an interface
-// name that might be used by Tailscale.
-func maybeTailscaleInterfaceName(s string) bool {
-	return s == "Tailscale" ||
-		strings.HasPrefix(s, "wg") ||
-		strings.HasPrefix(s, "ts") ||
-		strings.HasPrefix(s, "tailscale") ||
-		strings.HasPrefix(s, "utun")
-}
-
 func isUp(nif *net.Interface) bool       { return nif.Flags&net.FlagUp != 0 }
 func isLoopback(nif *net.Interface) bool { return nif.Flags&net.FlagLoopback != 0 }
 
@@ -300,9 +258,9 @@ func (s *State) String() string {
 		}
 	}
 	sb.WriteString("ifs={")
-	ifs := make([]string, 0, len(s.Interface))
+	var ifs []string
 	for k := range s.Interface {
-		if anyInterestingIP(s.InterfaceIPs[k]) {
+		if s.keepInterfaceInStringSummary(k) {
 			ifs = append(ifs, k)
 		}
 	}
@@ -318,23 +276,40 @@ func (s *State) String() string {
 		if i > 0 {
 			sb.WriteString(" ")
 		}
-		if s.Interface[ifName].IsUp() {
+		iface := s.Interface[ifName]
-			fmt.Fprintf(&sb, "%s:[", ifName)
+		if iface.Interface == nil {
-			needSpace := false
+			fmt.Fprintf(&sb, "%s:nil", ifName)
-			for _, pfx := range s.InterfaceIPs[ifName] {
+			continue
-				if !isInterestingIP(pfx.Addr()) {
-					continue
-				}
-				if needSpace {
-					sb.WriteString(" ")
-				}
-				fmt.Fprintf(&sb, "%s", pfx)
-				needSpace = true
-			}
-			sb.WriteString("]")
-		} else {
-			fmt.Fprintf(&sb, "%s:down", ifName)
 		}
+		if !iface.IsUp() {
+			fmt.Fprintf(&sb, "%s:down", ifName)
+			continue
+		}
+		fmt.Fprintf(&sb, "%s:[", ifName)
+		needSpace := false
+		for _, pfx := range s.InterfaceIPs[ifName] {
+			a := pfx.Addr()
+			if a.IsMulticast() {
+				continue
+			}
+			fam := "4"
+			if a.Is6() {
+				fam = "6"
+			}
+			if needSpace {
+				sb.WriteString(" ")
+			}
+			needSpace = true
+			switch {
+			case a.IsLoopback():
+				fmt.Fprintf(&sb, "lo%s", fam)
+			case a.IsLinkLocalUnicast():
+				fmt.Fprintf(&sb, "llu%s", fam)
+			default:
+				fmt.Fprintf(&sb, "%s", pfx)
+			}
+		}
+		sb.WriteString("]")
 	}
 	sb.WriteString("}")
 
@@ -351,18 +326,8 @@ func (s *State) String() string {
 	return sb.String()
 }
 
-// An InterfaceFilter indicates whether EqualFiltered should use i when deciding whether two States are equal.
+// Equal reports whether s and s2 are exactly equal.
-// ips are all the IPPrefixes associated with i.
+func (s *State) Equal(s2 *State) bool {
-type InterfaceFilter func(i Interface, ips []netip.Prefix) bool
-
-// An IPFilter indicates whether EqualFiltered should use ip when deciding whether two States are equal.
-// ip is an ip address associated with some interface under consideration.
-type IPFilter func(ip netip.Addr) bool
-
-// EqualFiltered reports whether s and s2 are equal,
-// considering only interfaces in s for which filter returns true,
-// and considering only IPs for those interfaces for which filterIP returns true.
-func (s *State) EqualFiltered(s2 *State, useInterface InterfaceFilter, useIP IPFilter) bool {
 	if s == nil && s2 == nil {
 		return true
 	}
@@ -378,19 +343,16 @@ func (s *State) EqualFiltered(s2 *State, useInterface InterfaceFilter, useIP IPF
 		return false
 	}
 	for iname, i := range s.Interface {
-		ips := s.InterfaceIPs[iname]
-		if !useInterface(i, ips) {
-			continue
-		}
 		i2, ok := s2.Interface[iname]
 		if !ok {
 			return false
 		}
-		ips2, ok := s2.InterfaceIPs[iname]
+		if !i.Equal(i2) {
-		if !ok {
 			return false
 		}
-		if !interfacesEqual(i, i2) || !prefixesEqualFiltered(ips, ips2, useIP) {
+	}
+	for iname, vv := range s.InterfaceIPs {
+		if !slices.Equal(vv, s2.InterfaceIPs[iname]) {
 			return false
 		}
 	}
@@ -413,60 +375,23 @@ func (s *State) HasIP(ip netip.Addr) bool {
 	return false
 }
 
-func interfacesEqual(a, b Interface) bool {
+func (a Interface) Equal(b Interface) bool {
-	return a.Index == b.Index &&
+	if (a.Interface == nil) != (b.Interface == nil) {
+		return false
+	}
+	if !(a.Desc == b.Desc && netAddrsEqual(a.AltAddrs, b.AltAddrs)) {
+		return false
+	}
+	if a.Interface != nil && !(a.Index == b.Index &&
 		a.MTU == b.MTU &&
 		a.Name == b.Name &&
 		a.Flags == b.Flags &&
-		bytes.Equal([]byte(a.HardwareAddr), []byte(b.HardwareAddr))
+		bytes.Equal([]byte(a.HardwareAddr), []byte(b.HardwareAddr))) {
-}
-
-func filteredIPPs(ipps []netip.Prefix, useIP IPFilter) []netip.Prefix {
-	// TODO: rewrite prefixesEqualFiltered to avoid making copies
-	x := make([]netip.Prefix, 0, len(ipps))
-	for _, ipp := range ipps {
-		if useIP(ipp.Addr()) {
-			x = append(x, ipp)
-		}
-	}
-	return x
-}
-
-func prefixesEqualFiltered(a, b []netip.Prefix, useIP IPFilter) bool {
-	return prefixesEqual(filteredIPPs(a, useIP), filteredIPPs(b, useIP))
-}
-
-func prefixesEqual(a, b []netip.Prefix) bool {
-	if len(a) != len(b) {
 		return false
 	}
-	for i, v := range a {
-		if b[i] != v {
-			return false
-		}
-	}
 	return true
 }
 
-// UseInterestingInterfaces is an InterfaceFilter that reports whether i is an interesting interface.
-// An interesting interface if it is (a) not owned by Tailscale and (b) routes interesting IP addresses.
-// See UseInterestingIPs for the definition of an interesting IP address.
-func UseInterestingInterfaces(i Interface, ips []netip.Prefix) bool {
-	return !isTailscaleInterface(i.Name, ips) && anyInterestingIP(ips)
-}
-
-// UseInterestingIPs is an IPFilter that reports whether ip is an interesting IP address.
-// An IP address is interesting if it is neither a loopback nor a link local unicast IP address.
-func UseInterestingIPs(ip netip.Addr) bool {
-	return isInterestingIP(ip)
-}
-
-// UseAllInterfaces is an InterfaceFilter that includes all interfaces.
-func UseAllInterfaces(i Interface, ips []netip.Prefix) bool { return true }
-
-// UseAllIPs is an IPFilter that includes all IPs.
-func UseAllIPs(ips netip.Addr) bool { return true }
-
 func (s *State) HasPAC() bool { return s != nil && s.PAC != "" }
 
 // AnyInterfaceUp reports whether any interface seems like it has Internet access.
@@ -477,6 +402,18 @@ func (s *State) AnyInterfaceUp() bool {
 	return s != nil && (s.HaveV4 || s.HaveV6)
 }
 
+func netAddrsEqual(a, b []net.Addr) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i, av := range a {
+		if av.Network() != b[i].Network() || av.String() != b[i].String() {
+			return false
+		}
+	}
+	return true
+}
+
 func hasTailscaleIP(pfxs []netip.Prefix) bool {
 	for _, pfx := range pfxs {
 		if tsaddr.IsTailscaleIP(pfx.Addr()) {
@@ -664,11 +601,23 @@ func isUsableV6(ip netip.Addr) bool {
 	v6Global1 = netip.MustParsePrefix("2000::/3")
 )
 
-// anyInterestingIP reports whether pfxs contains any IP that matches
+// keepInterfaceInStringSummary reports whether the named interface should be included
-// isInterestingIP.
+// in the String method's summary string.
-func anyInterestingIP(pfxs []netip.Prefix) bool {
+func (s *State) keepInterfaceInStringSummary(ifName string) bool {
-	for _, pfx := range pfxs {
+	iface, ok := s.Interface[ifName]
-		if isInterestingIP(pfx.Addr()) {
+	if !ok || iface.Interface == nil {
+		return false
+	}
+	if ifName == s.DefaultRouteInterface {
+		return true
+	}
+	up := iface.IsUp()
+	for _, p := range s.InterfaceIPs[ifName] {
+		a := p.Addr()
+		if a.IsLinkLocalUnicast() || a.IsLoopback() {
+			continue
+		}
+		if up || a.IsGlobalUnicast() || a.IsPrivate() {
 			return true
 		}
 	}
@@ -677,9 +626,12 @@ func anyInterestingIP(pfxs []netip.Prefix) bool {
 
 // isInterestingIP reports whether ip is an interesting IP that we
 // should log in interfaces.State logging. We don't need to show
-// localhost or link-local addresses.
+// loopback, link-local addresses, or non-Tailscale ULA addresses.
 func isInterestingIP(ip netip.Addr) bool {
-	return !ip.IsLoopback() && !ip.IsLinkLocalUnicast()
+	if ip.IsLoopback() || ip.IsLinkLocalUnicast() {
+		return false
+	}
+	return true
 }
 
 var altNetInterfaces func() ([]Interface, error)

net/interfaces/interfaces_test.go

@@ -23,19 +23,6 @@ func TestGetState(t *testing.T) {
 	}
 	t.Logf("Got: %s", j)
 	t.Logf("As string: %s", st)
-
-	st2, err := GetState()
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	if !st.EqualFiltered(st2, UseAllInterfaces, UseAllIPs) {
-		// let's assume nobody was changing the system network interfaces between
-		// the two GetState calls.
-		t.Fatal("two States back-to-back were not equal")
-	}
-
-	t.Logf("As string:\n\t%s", st)
 }
 
 func TestLikelyHomeRouterIP(t *testing.T) {
@@ -154,7 +141,7 @@ func TestIsUsableV6(t *testing.T) {
 		{"first ULA", "fc00::1", true},
 		{"Tailscale", "fd7a:115c:a1e0::1", false},
 		{"Cloud Run", "fddf:3978:feb1:d745::1", true},
-		{"zeros", "0000:0000:0000:0000:0000:0000:0000:0000", false},
+		{"zeros", "0::0", false},
 		{"Link Local", "fe80::1", false},
 		{"Global", "2602::1", true},
 		{"IPv4 public", "192.0.2.1", false},
@@ -168,41 +155,6 @@ func TestIsUsableV6(t *testing.T) {
 	}
 }
 
-func TestStateEqualFilteredIPFilter(t *testing.T) {
-	// s1 and s2 are identical, except that an "interesting" interface
-	// has gained an "uninteresting" IP address.
-
-	s1 := &State{
-		InterfaceIPs: map[string][]netip.Prefix{"x": {
-			netip.MustParsePrefix("42.0.0.0/8"),
-			netip.MustParsePrefix("169.254.0.0/16"), // link local unicast
-		}},
-		Interface: map[string]Interface{"x": {Interface: &net.Interface{Name: "x"}}},
-	}
-
-	s2 := &State{
-		InterfaceIPs: map[string][]netip.Prefix{"x": {
-			netip.MustParsePrefix("42.0.0.0/8"),
-			netip.MustParsePrefix("169.254.0.0/16"), // link local unicast
-			netip.MustParsePrefix("127.0.0.0/8"),    // loopback (added)
-		}},
-		Interface: map[string]Interface{"x": {Interface: &net.Interface{Name: "x"}}},
-	}
-
-	// s1 and s2 are different...
-	if s1.EqualFiltered(s2, UseAllInterfaces, UseAllIPs) {
-		t.Errorf("%+v != %+v", s1, s2)
-	}
-	// ...and they look different if you only restrict to interesting interfaces...
-	if s1.EqualFiltered(s2, UseInterestingInterfaces, UseAllIPs) {
-		t.Errorf("%+v != %+v when restricting to interesting interfaces _but not_ IPs", s1, s2)
-	}
-	// ...but because the additional IP address is uninteresting, we should treat them as the same.
-	if !s1.EqualFiltered(s2, UseInterestingInterfaces, UseInterestingIPs) {
-		t.Errorf("%+v == %+v when restricting to interesting interfaces and IPs", s1, s2)
-	}
-}
-
 func TestStateString(t *testing.T) {
 	tests := []struct {
 		name string
@@ -222,11 +174,15 @@ func TestStateString(t *testing.T) {
 					"wlan0": {
 						Interface: &net.Interface{},
 					},
+					"lo": {
+						Interface: &net.Interface{},
+					},
 				},
 				InterfaceIPs: map[string][]netip.Prefix{
 					"eth0": {
 						netip.MustParsePrefix("10.0.0.2/8"),
 					},
+					"lo": {},
 				},
 				HaveV4: true,
 			},
@@ -239,10 +195,13 @@ func TestStateString(t *testing.T) {
 				Interface: map[string]Interface{
 					"foo": {
 						Desc: "a foo thing",
+						Interface: &net.Interface{
+							Flags: net.FlagUp,
+						},
 					},
 				},
 			},
-			want: `interfaces.State{defaultRoute=foo (a foo thing) ifs={} v4=false v6=false}`,
+			want: `interfaces.State{defaultRoute=foo (a foo thing) ifs={foo:[]} v4=false v6=false}`,
 		},
 	}
 	for _, tt := range tests {
@@ -254,3 +213,112 @@ func TestStateString(t *testing.T) {
 		})
 	}
 }
+
+func TestIsInterestingIP(t *testing.T) {
+	tests := []struct {
+		ip   string
+		want bool
+	}{
+		{"fd7a:115c:a1e0:ab12:4843:cd96:624a:4603", true}, // Tailscale private ULA
+		{"fd15:bbfa:c583:4fce:f4fb:4ff:fe1a:4148", true},  // Other private ULA
+		{"10.2.3.4", true},
+		{"127.0.0.1", false},
+		{"::1", false},
+		{"2001::2", true},
+		{"169.254.1.2", false},
+		{"fe80::1", false},
+	}
+	for _, tt := range tests {
+		if got := isInterestingIP(netip.MustParseAddr(tt.ip)); got != tt.want {
+			t.Errorf("isInterestingIP(%q) = %v, want %v", tt.ip, got, tt.want)
+		}
+	}
+}
+
+// tests (*State).Equal
+func TestEqual(t *testing.T) {
+	tests := []struct {
+		name   string
+		s1, s2 *State
+		want   bool // implies !wantMajor
+	}{
+		{
+			name: "eq_nil",
+			want: true,
+		},
+		{
+			name: "nil_mix",
+			s2:   new(State),
+			want: false,
+		},
+		{
+			name: "eq",
+			s1: &State{
+				DefaultRouteInterface: "foo",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {netip.MustParsePrefix("10.0.1.2/16")},
+				},
+			},
+			s2: &State{
+				DefaultRouteInterface: "foo",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {netip.MustParsePrefix("10.0.1.2/16")},
+				},
+			},
+			want: true,
+		},
+		{
+			name: "default-route-changed",
+			s1: &State{
+				DefaultRouteInterface: "foo",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {netip.MustParsePrefix("10.0.1.2/16")},
+				},
+			},
+			s2: &State{
+				DefaultRouteInterface: "bar",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {netip.MustParsePrefix("10.0.1.2/16")},
+				},
+			},
+			want: false,
+		},
+		{
+			name: "some-interface-ips-changed",
+			s1: &State{
+				DefaultRouteInterface: "foo",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {netip.MustParsePrefix("10.0.1.2/16")},
+				},
+			},
+			s2: &State{
+				DefaultRouteInterface: "foo",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {netip.MustParsePrefix("10.0.1.3/16")},
+				},
+			},
+			want: false,
+		},
+		{
+			name: "altaddrs-changed",
+			s1: &State{
+				Interface: map[string]Interface{
+					"foo": {AltAddrs: []net.Addr{&net.TCPAddr{IP: net.ParseIP("1.2.3.4")}}},
+				},
+			},
+			s2: &State{
+				Interface: map[string]Interface{
+					"foo": {AltAddrs: []net.Addr{&net.TCPAddr{IP: net.ParseIP("5.6.7.8")}}},
+				},
+			},
+			want: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.s2.Equal(tt.s1); got != tt.want {
+				t.Errorf("Equal = %v; want %v", got, tt.want)
+			}
+		})
+	}
+}

net/netmon/netmon.go

@@ -55,6 +55,10 @@ type Monitor struct {
 	change chan bool     // send false to wake poller, true to also force ChangeDeltas be sent
 	stop   chan struct{} // closed on Stop
 
+	// Things that must be set early, before use,
+	// and not change at runtime.
+	tsIfName string // tailscale interface name, if known/set ("tailscale0", "utun3", ...)
+
 	mu         sync.Mutex // guards all following fields
 	cbs        set.HandleSet[ChangeFunc]
 	ruleDelCB  set.HandleSet[RuleDeleteCallback]
@@ -76,6 +80,9 @@ type Monitor struct {
 
 // ChangeDelta describes the difference between two network states.
 type ChangeDelta struct {
+	// Monitor is the network monitor that sent this delta.
+	Monitor *Monitor
+
 	// Old is the old interface state, if known.
 	// It's nil if the old state is unknown.
 	// Do not mutate it.
@@ -145,6 +152,15 @@ func (m *Monitor) interfaceStateUncached() (*interfaces.State, error) {
 	return interfaces.GetState()
 }
 
+// SetTailscaleInterfaceName sets the name of the Tailscale interface. For
+// example, "tailscale0", "tun0", "utun3", etc.
+//
+// This must be called only early in tailscaled startup before the monitor is
+// used.
+func (m *Monitor) SetTailscaleInterfaceName(ifName string) {
+	m.tsIfName = ifName
+}
+
 // GatewayAndSelfIP returns the current network's default gateway, and
 // the machine's default IP for that gateway.
 //
@@ -320,7 +336,11 @@ func (m *Monitor) notifyRuleDeleted(rdm ipRuleDeletedMessage) {
 // considered when checking for network state changes.
 // The ips parameter should be the IPs of the provided interface.
 func (m *Monitor) isInterestingInterface(i interfaces.Interface, ips []netip.Prefix) bool {
-	return m.om.IsInterestingInterface(i.Name) && interfaces.UseInterestingInterfaces(i, ips)
+	if !m.om.IsInterestingInterface(i.Name) {
+		return false
+	}
+
+	return true
 }
 
 // debounce calls the callback function with a delay between events
@@ -358,18 +378,19 @@ func (m *Monitor) handlePotentialChange(newState *interfaces.State, forceCallbac
 	defer m.mu.Unlock()
 	oldState := m.ifState
 	timeJumped := shouldMonitorTimeJump && m.checkWallTimeAdvanceLocked()
-	if !timeJumped && !forceCallbacks && oldState.EqualFiltered(newState, interfaces.UseAllInterfaces, interfaces.UseAllIPs) {
+	if !timeJumped && !forceCallbacks && oldState.Equal(newState) {
 		// Exactly equal. Nothing to do.
 		return
 	}
 
 	delta := &ChangeDelta{
+		Monitor:    m,
 		Old:        oldState,
 		New:        newState,
 		TimeJumped: timeJumped,
 	}
 
-	delta.Major = !newState.EqualFiltered(oldState, m.isInterestingInterface, interfaces.UseInterestingIPs)
+	delta.Major = m.IsMajorChangeFrom(oldState, newState)
 	if delta.Major {
 		m.gwValid = false
 		m.ifState = newState
@@ -394,6 +415,79 @@ func (m *Monitor) handlePotentialChange(newState *interfaces.State, forceCallbac
 	}
 }
 
+// IsMajorChangeFrom reports whether the transition from s1 to s2 is
+// a "major" change, where major roughly means it's worth tearing down
+// a bunch of connections and rebinding.
+//
+// TODO(bradiftz): tigten this definition.
+func (m *Monitor) IsMajorChangeFrom(s1, s2 *interfaces.State) bool {
+	if s1 == nil && s2 == nil {
+		return false
+	}
+	if s1 == nil || s2 == nil {
+		return true
+	}
+	if s1.HaveV6 != s2.HaveV6 ||
+		s1.HaveV4 != s2.HaveV4 ||
+		s1.IsExpensive != s2.IsExpensive ||
+		s1.DefaultRouteInterface != s2.DefaultRouteInterface ||
+		s1.HTTPProxy != s2.HTTPProxy ||
+		s1.PAC != s2.PAC {
+		return true
+	}
+	for iname, i := range s1.Interface {
+		if iname == m.tsIfName {
+			// Ignore changes in the Tailscale interface itself.
+			continue
+		}
+		ips := s1.InterfaceIPs[iname]
+		if !m.isInterestingInterface(i, ips) {
+			continue
+		}
+		i2, ok := s2.Interface[iname]
+		if !ok {
+			return true
+		}
+		ips2, ok := s2.InterfaceIPs[iname]
+		if !ok {
+			return true
+		}
+		if !i.Equal(i2) || !prefixesMajorEqual(ips, ips2) {
+			return true
+		}
+	}
+	return false
+}
+
+// prefixesMajorEqual reports whether a and b are equal after ignoring
+// boring things like link-local, loopback, and multicast addresses.
+func prefixesMajorEqual(a, b []netip.Prefix) bool {
+	// trim returns a subslice of p with link local unicast,
+	// loopback, and multicast prefixes removed from the front.
+	trim := func(p []netip.Prefix) []netip.Prefix {
+		for len(p) > 0 {
+			a := p[0].Addr()
+			if a.IsLinkLocalUnicast() || a.IsLoopback() || a.IsMulticast() {
+				p = p[1:]
+				continue
+			}
+			break
+		}
+		return p
+	}
+	for {
+		a = trim(a)
+		b = trim(b)
+		if len(a) == 0 || len(b) == 0 {
+			return len(a) == 0 && len(b) == 0
+		}
+		if a[0] != b[0] {
+			return false
+		}
+		a, b = a[1:], b[1:]
+	}
+}
+
 func jsonSummary(x any) any {
 	j, err := json.Marshal(x)
 	if err != nil {

net/netmon/netmon_test.go

@@ -5,9 +5,14 @@
 
 import (
 	"flag"
+	"net"
+	"net/netip"
 	"sync/atomic"
 	"testing"
 	"time"
+
+	"tailscale.com/net/interfaces"
+	"tailscale.com/util/mak"
 )
 
 func TestMonitorStartClose(t *testing.T) {
@@ -108,3 +113,130 @@ func TestMonitorMode(t *testing.T) {
 		t.Logf("%v callbacks", n)
 	}
 }
+
+// tests (*State).IsMajorChangeFrom
+func TestIsMajorChangeFrom(t *testing.T) {
+	type State = interfaces.State
+	type Interface = interfaces.Interface
+	tests := []struct {
+		name   string
+		s1, s2 *State
+		want   bool
+	}{
+		{
+			name: "eq_nil",
+			want: false,
+		},
+		{
+			name: "nil_mix",
+			s2:   new(State),
+			want: true,
+		},
+		{
+			name: "eq",
+			s1: &State{
+				DefaultRouteInterface: "foo",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {netip.MustParsePrefix("10.0.1.2/16")},
+				},
+			},
+			s2: &State{
+				DefaultRouteInterface: "foo",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {netip.MustParsePrefix("10.0.1.2/16")},
+				},
+			},
+			want: false,
+		},
+		{
+			name: "default-route-changed",
+			s1: &State{
+				DefaultRouteInterface: "foo",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {netip.MustParsePrefix("10.0.1.2/16")},
+				},
+			},
+			s2: &State{
+				DefaultRouteInterface: "bar",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {netip.MustParsePrefix("10.0.1.2/16")},
+				},
+			},
+			want: true,
+		},
+		{
+			name: "some-interesting-ip-changed",
+			s1: &State{
+				DefaultRouteInterface: "foo",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {netip.MustParsePrefix("10.0.1.2/16")},
+				},
+			},
+			s2: &State{
+				DefaultRouteInterface: "foo",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {netip.MustParsePrefix("10.0.1.3/16")},
+				},
+			},
+			want: true,
+		},
+		{
+			name: "ipv6-ula-addressed-appeared",
+			s1: &State{
+				DefaultRouteInterface: "foo",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {netip.MustParsePrefix("10.0.1.2/16")},
+				},
+			},
+			s2: &State{
+				DefaultRouteInterface: "foo",
+				InterfaceIPs: map[string][]netip.Prefix{
+					"foo": {
+						netip.MustParsePrefix("10.0.1.2/16"),
+						// Brad saw this address coming & going on his home LAN, possibly
+						// via an Apple TV Thread routing advertisement? (Issue 9040)
+						netip.MustParsePrefix("fd15:bbfa:c583:4fce:f4fb:4ff:fe1a:4148/64"),
+					},
+				},
+			},
+			want: true, // TODO(bradfitz): want false (ignore the IPv6 ULA address on foo)
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Populate dummy interfaces where missing.
+			for _, s := range []*State{tt.s1, tt.s2} {
+				if s == nil {
+					continue
+				}
+				for name := range s.InterfaceIPs {
+					if _, ok := s.Interface[name]; !ok {
+						mak.Set(&s.Interface, name, Interface{Interface: &net.Interface{
+							Name: name,
+						}})
+					}
+				}
+			}
+
+			var m Monitor
+			m.om = &testOSMon{
+				Interesting: func(name string) bool { return true },
+			}
+			if got := m.IsMajorChangeFrom(tt.s1, tt.s2); got != tt.want {
+				t.Errorf("IsMajorChange = %v; want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+type testOSMon struct {
+	osMon
+	Interesting func(name string) bool
+}
+
+func (m *testOSMon) IsInterestingInterface(name string) bool {
+	if m.Interesting == nil {
+		return true
+	}
+	return m.Interesting(name)
+}

net/netns/netns_darwin.go

@@ -20,6 +20,7 @@
 	"tailscale.com/envknob"
 	"tailscale.com/net/interfaces"
 	"tailscale.com/net/netmon"
+	"tailscale.com/net/tsaddr"
 	"tailscale.com/types/logger"
 )
 
@@ -110,7 +111,7 @@ func getInterfaceIndex(logf logger.Logf, netMon *netmon.Monitor, address string)
 
 	// Verify that we didn't just choose the Tailscale interface;
 	// if so, we fall back to binding from the default.
-	_, tsif, err2 := interfaces.Tailscale()
+	tsif, err2 := tailscaleInterface()
 	if err2 == nil && tsif != nil && tsif.Index == idx {
 		logf("[unexpected] netns: interfaceIndexFor returned Tailscale interface")
 		return defaultIdx()
@@ -119,6 +120,34 @@ func getInterfaceIndex(logf logger.Logf, netMon *netmon.Monitor, address string)
 	return idx, err
 }
 
+// tailscaleInterface returns the current machine's Tailscale interface, if any.
+// If none is found, (nil, nil) is returned.
+// A non-nil error is only returned on a problem listing the system interfaces.
+func tailscaleInterface() (*net.Interface, error) {
+	ifs, err := net.Interfaces()
+	if err != nil {
+		return nil, err
+	}
+	for _, iface := range ifs {
+		if !strings.HasPrefix(iface.Name, "utun") {
+			continue
+		}
+		addrs, err := iface.Addrs()
+		if err != nil {
+			continue
+		}
+		for _, a := range addrs {
+			if ipnet, ok := a.(*net.IPNet); ok {
+				nip, ok := netip.AddrFromSlice(ipnet.IP)
+				if ok && tsaddr.IsTailscaleIP(nip.Unmap()) {
+					return &iface, nil
+				}
+			}
+		}
+	}
+	return nil, nil
+}
+
 // interfaceIndexFor returns the interface index that we should bind to in
 // order to send traffic to the provided address.
 func interfaceIndexFor(addr netip.Addr, canRecurse bool) (int, error) {

net/netns/netns_darwin_test.go

@@ -55,7 +55,7 @@ func TestGetInterfaceIndex(t *testing.T) {
 	}
 
 	t.Run("NoTailscale", func(t *testing.T) {
-		_, tsif, err := interfaces.Tailscale()
+		tsif, err := tailscaleInterface()
 		if err != nil {
 			t.Fatal(err)
 		}