net/tstun: finish wiring IPv6 NAT support

Updates https://github.com/tailscale/corp/issues/11202
Updates ENG-991
Signed-off-by: Tom DNetto <tom@tailscale.com>

net/tstun/wrap.go

@@ -98,8 +98,8 @@ type Wrapper struct {
 	// timeNow, if non-nil, will be used to obtain the current time.
 	timeNow func() time.Time
 
-	// natV4Config stores the current IPv4 NAT configuration.
+	// natConfig stores the current NAT configuration.
-	natV4Config atomic.Pointer[natV4Config]
+	natConfig atomic.Pointer[natConfig]
 
 	// vectorBuffer stores the oldest unconsumed packet vector from tdev. It is
 	// allocated in wrap() and the underlying arrays should never grow.
@@ -481,14 +481,9 @@ func (t *Wrapper) sendVectorOutbound(r tunVectorReadResult) {
 	t.vectorOutbound <- r
 }
 
-// snatV4 does SNAT on p if it's an IPv4 packet and the destination
+// snat does SNAT on p if the destination address requires a different source address.
-// address requires a different source address.
+func (t *Wrapper) snat(p *packet.Parsed) {
-func (t *Wrapper) snatV4(p *packet.Parsed) {
+	nc := t.natConfig.Load()
-	if p.IPVersion != 4 {
-		return
-	}
-
-	nc := t.natV4Config.Load()
 	oldSrc := p.Src.Addr()
 	newSrc := nc.selectSrcIP(oldSrc, p.Dst.Addr())
 	if oldSrc != newSrc {
@@ -496,13 +491,9 @@ func (t *Wrapper) snatV4(p *packet.Parsed) {
 	}
 }
 
-// dnatV4 does destination NAT on p if it's an IPv4 packet.
+// dnat does destination NAT on p.
-func (t *Wrapper) dnatV4(p *packet.Parsed) {
+func (t *Wrapper) dnat(p *packet.Parsed) {
-	if p.IPVersion != 4 {
+	nc := t.natConfig.Load()
-		return
-	}
-
-	nc := t.natV4Config.Load()
 	oldDst := p.Dst.Addr()
 	newDst := nc.mapDstIP(oldDst)
 	if newDst != oldDst {
@@ -521,15 +512,79 @@ func findV4(addrs []netip.Prefix) netip.Addr {
 	return netip.Addr{}
 }
 
-// natV4Config is the configuration for IPv4 NAT.
+// findV6 returns the first Tailscale IPv6 address in addrs.
+func findV6(addrs []netip.Prefix) netip.Addr {
+	for _, ap := range addrs {
+		a := ap.Addr()
+		if a.Is6() && tsaddr.IsTailscaleIP(a) {
+			return a
+		}
+	}
+	return netip.Addr{}
+}
+
+// natConfig is the configuration for NAT.
+// It should be treated as immutable.
+//
+// The nil value is a valid configuration.
+type natConfig struct {
+	v4, v6 *natFamilyConfig
+}
+
+func (c *natConfig) String() string {
+	if c == nil {
+		return "<nil>"
+	}
+
+	var b strings.Builder
+	b.WriteString("natConfig{")
+	fmt.Fprintf(&b, "v4: %v, ", c.v4)
+	fmt.Fprintf(&b, "v6: %v", c.v6)
+	b.WriteString("}")
+	return b.String()
+}
+
+// mapDstIP returns the destination IP to use for a packet to dst.
+// If dst is not one of the listen addresses, it is returned as-is,
+// otherwise the native address is returned.
+func (c *natConfig) mapDstIP(oldDst netip.Addr) netip.Addr {
+	if c == nil {
+		return oldDst
+	}
+	if oldDst.Is4() {
+		return c.v4.mapDstIP(oldDst)
+	}
+	if oldDst.Is6() {
+		return c.v6.mapDstIP(oldDst)
+	}
+	return oldDst
+}
+
+// selectSrcIP returns the source IP to use for a packet to dst.
+// If the packet is not from the native address, it is returned as-is.
+func (c *natConfig) selectSrcIP(oldSrc, dst netip.Addr) netip.Addr {
+	if c == nil {
+		return oldSrc
+	}
+	if oldSrc.Is4() {
+		return c.v4.selectSrcIP(oldSrc, dst)
+	}
+	if oldSrc.Is6() {
+		return c.v6.selectSrcIP(oldSrc, dst)
+	}
+	return oldSrc
+}
+
+// natFamilyConfig is the NAT configuration for a particular
+// address family.
 // It should be treated as immutable.
 //
 // The nil value is a valid configuration.
-type natV4Config struct {
+type natFamilyConfig struct {
-	// nativeAddr is the IPv4 Tailscale Address of the current node.
+	// nativeAddr is the Tailscale Address of the current node.
 	nativeAddr netip.Addr
 
-	// listenAddrs is the set of IPv4 addresses that should be
+	// listenAddrs is the set of addresses that should be
 	// mapped to the native address. These are the addresses that
 	// peers will use to connect to this node.
 	listenAddrs views.Map[netip.Addr, struct{}] // masqAddr -> struct{}
@@ -545,12 +600,12 @@ type natV4Config struct {
 	dstAddrToPeerKeyMapper *table.RoutingTable
 }
 
-func (c *natV4Config) String() string {
+func (c *natFamilyConfig) String() string {
 	if c == nil {
-		return "<nil>"
+		return "natFamilyConfig(nil)"
 	}
 	var b strings.Builder
-	b.WriteString("natV4Config{")
+	b.WriteString("natFamilyConfig{")
 	fmt.Fprintf(&b, "nativeAddr: %v, ", c.nativeAddr)
 	fmt.Fprint(&b, "listenAddrs: [")
 
@@ -586,7 +641,7 @@ func (c *natV4Config) String() string {
 // mapDstIP returns the destination IP to use for a packet to dst.
 // If dst is not one of the listen addresses, it is returned as-is,
 // otherwise the native address is returned.
-func (c *natV4Config) mapDstIP(oldDst netip.Addr) netip.Addr {
+func (c *natFamilyConfig) mapDstIP(oldDst netip.Addr) netip.Addr {
 	if c == nil {
 		return oldDst
 	}
@@ -598,7 +653,7 @@ func (c *natV4Config) mapDstIP(oldDst netip.Addr) netip.Addr {
 
 // selectSrcIP returns the source IP to use for a packet to dst.
 // If the packet is not from the native address, it is returned as-is.
-func (c *natV4Config) selectSrcIP(oldSrc, dst netip.Addr) netip.Addr {
+func (c *natFamilyConfig) selectSrcIP(oldSrc, dst netip.Addr) netip.Addr {
 	if c == nil {
 		return oldSrc
 	}
@@ -615,16 +670,25 @@ func (c *natV4Config) selectSrcIP(oldSrc, dst netip.Addr) netip.Addr {
 	return oldSrc
 }
 
-// natV4ConfigFromWGConfig generates a natV4Config from nm.
+// natConfigFromWGConfig generates a natFamilyConfig from nm,
-// If v4 NAT is not required, it returns nil.
+// for the indicated address family.
-func natV4ConfigFromWGConfig(wcfg *wgcfg.Config) *natV4Config {
+// If NAT is not required for that address family, it returns nil.
+func natConfigFromWGConfig(wcfg *wgcfg.Config, addrFam ipproto.IPProtoVersion) *natFamilyConfig {
 	if wcfg == nil {
 		return nil
 	}
-	nativeAddr := findV4(wcfg.Addresses)
+
+	var nativeAddr netip.Addr
+	switch addrFam {
+	case ipproto.IPProtoVersion4:
+		nativeAddr = findV4(wcfg.Addresses)
+	case ipproto.IPProtoVersion6:
+		nativeAddr = findV6(wcfg.Addresses)
+	}
 	if !nativeAddr.IsValid() {
 		return nil
 	}
+
 	var (
 		rt           table.RoutingTableBuilder
 		dstMasqAddrs map[key.NodePublic]netip.Addr
@@ -637,17 +701,25 @@ func natV4ConfigFromWGConfig(wcfg *wgcfg.Config) *natV4Config {
 	exitNodeRequiresMasq := false // true if using an exit node and it requires masquerading
 	for _, p := range wcfg.Peers {
 		isExitNode := slices.Contains(p.AllowedIPs, tsaddr.AllIPv4()) || slices.Contains(p.AllowedIPs, tsaddr.AllIPv6())
-		if isExitNode && p.V4MasqAddr != nil && p.V4MasqAddr.IsValid() {
+		if isExitNode {
+			hasMasqAddrsForFamily := false ||
+				(addrFam == ipproto.IPProtoVersion4 && p.V4MasqAddr != nil && p.V4MasqAddr.IsValid()) ||
+				(addrFam == ipproto.IPProtoVersion6 && p.V6MasqAddr != nil && p.V6MasqAddr.IsValid())
+			if hasMasqAddrsForFamily {
 				exitNodeRequiresMasq = true
+			}
 			break
 		}
 	}
 	for i := range wcfg.Peers {
 		p := &wcfg.Peers[i]
 		var addrToUse netip.Addr
-		if p.V4MasqAddr != nil && p.V4MasqAddr.IsValid() {
+		if addrFam == ipproto.IPProtoVersion4 && p.V4MasqAddr != nil && p.V4MasqAddr.IsValid() {
 			addrToUse = *p.V4MasqAddr
 			mak.Set(&listenAddrs, addrToUse, struct{}{})
+		} else if addrFam == ipproto.IPProtoVersion6 && p.V6MasqAddr != nil && p.V6MasqAddr.IsValid() {
+			addrToUse = *p.V6MasqAddr
+			mak.Set(&listenAddrs, addrToUse, struct{}{})
 		} else if exitNodeRequiresMasq {
 			addrToUse = nativeAddr
 		} else {
@@ -659,7 +731,7 @@ func natV4ConfigFromWGConfig(wcfg *wgcfg.Config) *natV4Config {
 	if len(listenAddrs) == 0 && len(dstMasqAddrs) == 0 {
 		return nil
 	}
-	return &natV4Config{
+	return &natFamilyConfig{
 		nativeAddr:             nativeAddr,
 		listenAddrs:            views.MapOf(listenAddrs),
 		dstMasqAddrs:           views.MapOf(dstMasqAddrs),
@@ -668,10 +740,14 @@ func natV4ConfigFromWGConfig(wcfg *wgcfg.Config) *natV4Config {
 }
 
 // SetNetMap is called when a new NetworkMap is received.
-// It currently (2023-03-01) only updates the IPv4 NAT configuration.
 func (t *Wrapper) SetWGConfig(wcfg *wgcfg.Config) {
-	cfg := natV4ConfigFromWGConfig(wcfg)
+	v4, v6 := natConfigFromWGConfig(wcfg, ipproto.IPProtoVersion4), natConfigFromWGConfig(wcfg, ipproto.IPProtoVersion6)
-	old := t.natV4Config.Swap(cfg)
+	var cfg *natConfig
+	if v4 != nil || v6 != nil {
+		cfg = &natConfig{v4: v4, v6: v6}
+	}
+
+	old := t.natConfig.Swap(cfg)
 	if !reflect.DeepEqual(old, cfg) {
 		t.logf("nat config: %v", cfg)
 	}
@@ -786,7 +862,7 @@ func (t *Wrapper) Read(buffs [][]byte, sizes []int, offset int) (int, error) {
 	for _, data := range res.data {
 		p.Decode(data[res.dataOffset:])
 
-		t.snatV4(p)
+		t.snat(p)
 		if m := t.destIPActivity.Load(); m != nil {
 			if fn := m[p.Dst.Addr()]; fn != nil {
 				fn()
@@ -843,7 +919,7 @@ func (t *Wrapper) injectedRead(res tunInjectedRead, buf []byte, offset int) (int
 	p := parsedPacketPool.Get().(*packet.Parsed)
 	defer parsedPacketPool.Put(p)
 	p.Decode(buf[offset : offset+n])
-	t.snatV4(p)
+	t.snat(p)
 
 	if m := t.destIPActivity.Load(); m != nil {
 		if fn := m[p.Dst.Addr()]; fn != nil {
@@ -965,7 +1041,7 @@ func (t *Wrapper) Write(buffs [][]byte, offset int) (int, error) {
 	captHook := t.captureHook.Load()
 	for _, buff := range buffs {
 		p.Decode(buff[offset:])
-		t.dnatV4(p)
+		t.dnat(p)
 		if !t.disableFilter {
 			if t.filterPacketInboundFromWireGuard(p, captHook) != filter.Accept {
 				metricPacketInDrop.Add(1)
@@ -1030,7 +1106,7 @@ func (t *Wrapper) InjectInboundPacketBuffer(pkt stack.PacketBufferPtr) error {
 	if captHook != nil {
 		captHook(capture.SynthesizedToLocal, t.now(), p.Buffer(), p.CaptureMeta)
 	}
-	t.dnatV4(p)
+	t.dnat(p)
 
 	return t.InjectInboundDirect(buf, PacketStartOffset)
 }

net/tstun/wrap_test.go

@@ -608,11 +608,16 @@ func TestNATCfg(t *testing.T) {
 			AllowedIPs: []netip.Prefix{
 				netip.PrefixFrom(ip, ip.BitLen()),
 			},
-			V4MasqAddr: ptr.To(masqIP),
+		}
+		if masqIP.Is4() {
+			p.V4MasqAddr = ptr.To(masqIP)
+		} else {
+			p.V6MasqAddr = ptr.To(masqIP)
 		}
 		p.AllowedIPs = append(p.AllowedIPs, otherAllowedIPs...)
 		return p
 	}
+	test := func(addrFam ipproto.IPProtoVersion) {
 		var (
 			noIP netip.Addr
 
@@ -630,6 +635,21 @@ func TestNATCfg(t *testing.T) {
 			exitRoute = netip.MustParsePrefix("0.0.0.0/0")
 			publicIP  = netip.MustParseAddr("8.8.8.8")
 		)
+		if addrFam == ipproto.IPProtoVersion6 {
+			selfNativeIP = netip.MustParseAddr("fd7a:115c:a1e0::a")
+			selfEIP1 = netip.MustParseAddr("fd7a:115c:a1e0::1a")
+			selfEIP2 = netip.MustParseAddr("fd7a:115c:a1e0::1b")
+			selfAddrs = []netip.Prefix{netip.PrefixFrom(selfNativeIP, selfNativeIP.BitLen())}
+
+			peer1IP = netip.MustParseAddr("fd7a:115c:a1e0::b")
+			peer2IP = netip.MustParseAddr("fd7a:115c:a1e0::c")
+
+			subnet = netip.MustParsePrefix("2001:db8::/32")
+			subnetIP = netip.MustParseAddr("2001:db8::FFFF")
+
+			exitRoute = netip.MustParsePrefix("::/0")
+			publicIP = netip.MustParseAddr("2001:4860:4860::8888")
+		}
 
 		tests := []struct {
 			name    string
@@ -779,8 +799,8 @@ func TestNATCfg(t *testing.T) {
 		}
 
 		for _, tc := range tests {
-		t.Run(tc.name, func(t *testing.T) {
+			t.Run(fmt.Sprintf("%v/%v", addrFam, tc.name), func(t *testing.T) {
-			ncfg := natV4ConfigFromWGConfig(tc.wcfg)
+				ncfg := natConfigFromWGConfig(tc.wcfg, addrFam)
 				for peer, want := range tc.snatMap {
 					if got := ncfg.selectSrcIP(selfNativeIP, peer); got != want {
 						t.Errorf("selectSrcIP[%v]: got %v; want %v", peer, got, want)
@@ -791,9 +811,15 @@ func TestNATCfg(t *testing.T) {
 						t.Errorf("mapDstIP[%v]: got %v; want %v", dstIP, got, want)
 					}
 				}
+				if t.Failed() {
+					t.Logf("%v", ncfg)
+				}
 			})
 		}
 	}
+	test(ipproto.IPProtoVersion4)
+	test(ipproto.IPProtoVersion6)
+}
 
 // TestCaptureHook verifies that the Wrapper.captureHook callback is called
 // with the correct parameters when various packet operations are performed.

tailcfg/tailcfg.go

@@ -117,7 +117,8 @@ type CapabilityVersion int
 //   - 74: 2023-09-18: Client understands NodeCapMap
 //   - 75: 2023-09-12: Client understands NodeAttrDNSForwarderDisableTCPRetries
 //   - 76: 2023-09-20: Client understands ExitNodeDNSResolvers for IsWireGuardOnly nodes
-const CurrentCapabilityVersion CapabilityVersion = 76
+//   - 77: 2023-10-03: Client understands Peers[].SelfNodeV6MasqAddrForThisPeer
+const CurrentCapabilityVersion CapabilityVersion = 77
 
 type StableID string
 

tstest/integration/integration_test.go

@@ -76,7 +76,7 @@ func TestOneNodeUpNoAuth(t *testing.T) {
 	n1.AwaitResponding()
 	n1.MustUp()
 
-	t.Logf("Got IP: %v", n1.AwaitIP())
+	t.Logf("Got IP: %v", n1.AwaitIP4())
 	n1.AwaitRunning()
 
 	d1.MustCleanShutdown(t)
@@ -130,7 +130,7 @@ func TestControlKnobs(t *testing.T) {
 	n1.AwaitResponding()
 	n1.MustUp()
 
-	t.Logf("Got IP: %v", n1.AwaitIP())
+	t.Logf("Got IP: %v", n1.AwaitIP4())
 	n1.AwaitRunning()
 
 	cmd := n1.Tailscale("debug", "control-knobs")
@@ -212,7 +212,7 @@ func TestStateSavedOnStart(t *testing.T) {
 	n1.AwaitResponding()
 	n1.MustUp()
 
-	t.Logf("Got IP: %v", n1.AwaitIP())
+	t.Logf("Got IP: %v", n1.AwaitIP4())
 	n1.AwaitRunning()
 
 	p1 := n1.diskPrefs()
@@ -271,7 +271,7 @@ func TestOneNodeUpAuth(t *testing.T) {
 	if err := cmd.Run(); err != nil {
 		t.Fatalf("up: %v", err)
 	}
-	t.Logf("Got IP: %v", n1.AwaitIP())
+	t.Logf("Got IP: %v", n1.AwaitIP4())
 
 	n1.AwaitRunning()
 
@@ -574,7 +574,7 @@ func TestNoControlConnWhenDown(t *testing.T) {
 
 	// Come up the first time.
 	n1.MustUp()
-	ip1 := n1.AwaitIP()
+	ip1 := n1.AwaitIP4()
 	n1.AwaitRunning()
 
 	// Then bring it down and stop the daemon.
@@ -590,7 +590,7 @@ func TestNoControlConnWhenDown(t *testing.T) {
 		t.Fatalf("after restart, state = %q; want %q", got, want)
 	}
 
-	ip2 := n1.AwaitIP()
+	ip2 := n1.AwaitIP4()
 	if ip1 != ip2 {
 		t.Errorf("IPs different: %q vs %q", ip1, ip2)
 	}
@@ -615,7 +615,7 @@ func TestOneNodeUpWindowsStyle(t *testing.T) {
 	n1.AwaitResponding()
 	n1.MustUp("--unattended")
 
-	t.Logf("Got IP: %v", n1.AwaitIP())
+	t.Logf("Got IP: %v", n1.AwaitIP4())
 	n1.AwaitRunning()
 
 	d1.MustCleanShutdown(t)
@@ -625,6 +625,7 @@ func TestOneNodeUpWindowsStyle(t *testing.T) {
 // tries to do bi-directional pings between them.
 func TestNATPing(t *testing.T) {
 	t.Parallel()
+	for _, v6 := range []bool{false, true} {
 		env := newTestEnv(t)
 		registerNode := func() (*testNode, key.NodePublic) {
 			n := newTestNode(t, env)
@@ -638,11 +639,21 @@ func TestNATPing(t *testing.T) {
 		n1, k1 := registerNode()
 		n2, k2 := registerNode()
 
-	n1IP := n1.AwaitIP()
+		var n1IP, n2IP netip.Addr
-	n2IP := n2.AwaitIP()
+		if v6 {
+			n1IP = n1.AwaitIP6()
+			n2IP = n2.AwaitIP6()
+		} else {
+			n1IP = n1.AwaitIP4()
+			n2IP = n2.AwaitIP4()
+		}
 
 		n1ExternalIP := netip.MustParseAddr("100.64.1.1")
 		n2ExternalIP := netip.MustParseAddr("100.64.2.1")
+		if v6 {
+			n1ExternalIP = netip.MustParseAddr("fd7a:115c:a1e0::1a")
+			n2ExternalIP = netip.MustParseAddr("fd7a:115c:a1e0::1b")
+		}
 
 		tests := []struct {
 			name       string
@@ -699,18 +710,23 @@ func TestNATPing(t *testing.T) {
 		}
 
 		for _, tc := range tests {
-		t.Run(tc.name, func(t *testing.T) {
+			t.Run(fmt.Sprintf("v6=%t/%v", v6, tc.name), func(t *testing.T) {
 				env.Control.SetMasqueradeAddresses(tc.pairs)
 
+				ipIdx := 0
+				if v6 {
+					ipIdx = 1
+				}
+
 				s1 := n1.MustStatus()
 				n2AsN1Peer := s1.Peer[k2]
-			if got := n2AsN1Peer.TailscaleIPs[0]; got != tc.n1SeesN2IP {
+				if got := n2AsN1Peer.TailscaleIPs[ipIdx]; got != tc.n1SeesN2IP {
 					t.Fatalf("n1 sees n2 as %v; want %v", got, tc.n1SeesN2IP)
 				}
 
 				s2 := n2.MustStatus()
 				n1AsN2Peer := s2.Peer[k1]
-			if got := n1AsN2Peer.TailscaleIPs[0]; got != tc.n2SeesN1IP {
+				if got := n1AsN2Peer.TailscaleIPs[ipIdx]; got != tc.n2SeesN1IP {
 					t.Fatalf("n2 sees n1 as %v; want %v", got, tc.n2SeesN1IP)
 				}
 
@@ -732,6 +748,7 @@ func TestNATPing(t *testing.T) {
 			})
 		}
 	}
+}
 
 func TestLogoutRemovesAllPeers(t *testing.T) {
 	t.Parallel()
@@ -743,7 +760,7 @@ func TestLogoutRemovesAllPeers(t *testing.T) {
 		nodes[i].StartDaemon()
 		nodes[i].AwaitResponding()
 		nodes[i].MustUp()
-		nodes[i].AwaitIP()
+		nodes[i].AwaitIP4()
 		nodes[i].AwaitRunning()
 	}
 	expectedPeers := len(nodes) - 1
@@ -758,7 +775,7 @@ func TestLogoutRemovesAllPeers(t *testing.T) {
 			if err := tstest.WaitFor(20*time.Second, func() error {
 				return nodes[i].Ping(nodes[j])
 			}); err != nil {
-				t.Fatalf("ping %v -> %v: %v", nodes[i].AwaitIP(), nodes[j].AwaitIP(), err)
+				t.Fatalf("ping %v -> %v: %v", nodes[i].AwaitIP4(), nodes[j].AwaitIP4(), err)
 			}
 		}
 	}
@@ -783,7 +800,7 @@ func TestLogoutRemovesAllPeers(t *testing.T) {
 	nodes[0].MustUp() // This will create a new node
 	expectedPeers++
 
-	nodes[0].AwaitIP()
+	nodes[0].AwaitIP4()
 	wantNode0PeerCount(expectedPeers) // all existing peers and the new node
 }
 
@@ -1107,8 +1124,8 @@ func (n *testNode) MustLogOut() {
 
 func (n *testNode) Ping(otherNode *testNode) error {
 	t := n.env.t
-	ip := otherNode.AwaitIP().String()
+	ip := otherNode.AwaitIP4().String()
-	t.Logf("Running ping %v (from %v)...", ip, n.AwaitIP())
+	t.Logf("Running ping %v (from %v)...", ip, n.AwaitIP4())
 	return n.Tailscale("ping", ip).Run()
 }
 
@@ -1162,14 +1179,22 @@ func (n *testNode) AwaitIPs() []netip.Addr {
 	return addrs
 }
 
-// AwaitIP returns the IP address of n.
+// AwaitIP4 returns the IPv4 address of n.
-func (n *testNode) AwaitIP() netip.Addr {
+func (n *testNode) AwaitIP4() netip.Addr {
 	t := n.env.t
 	t.Helper()
 	ips := n.AwaitIPs()
 	return ips[0]
 }
 
+// AwaitIP6 returns the IPv6 address of n.
+func (n *testNode) AwaitIP6() netip.Addr {
+	t := n.env.t
+	t.Helper()
+	ips := n.AwaitIPs()
+	return ips[1]
+}
+
 // AwaitRunning waits for n to reach the IPN state "Running".
 func (n *testNode) AwaitRunning() {
 	t := n.env.t

tstest/integration/testcontrol/testcontrol.go

@@ -900,9 +900,14 @@ func (s *Server) MapResponse(req *tailcfg.MapRequest) (res *tailcfg.MapResponse,
 		peerAddress := s.masquerades[p.Key][node.Key]
 		s.mu.Unlock()
 		if peerAddress.IsValid() {
+			if peerAddress.Is6() {
+				p.Addresses[1] = netip.PrefixFrom(peerAddress, peerAddress.BitLen())
+				p.AllowedIPs[1] = netip.PrefixFrom(peerAddress, peerAddress.BitLen())
+			} else {
 				p.Addresses[0] = netip.PrefixFrom(peerAddress, peerAddress.BitLen())
 				p.AllowedIPs[0] = netip.PrefixFrom(peerAddress, peerAddress.BitLen())
 			}
+		}
 		res.Peers = append(res.Peers, p)
 	}
 

types/ipproto/ipproto.go

@@ -6,6 +6,26 @@ package ipproto
 
 import "fmt"
 
+// IPProtoVersion describes the IP address version.
+type IPProtoVersion uint8
+
+// Valid IPProtoVersion values.
+const (
+	IPProtoVersion4 = 4
+	IPProtoVersion6 = 6
+)
+
+func (p IPProtoVersion) String() string {
+	switch p {
+	case IPProtoVersion4:
+		return "IPv4"
+	case IPProtoVersion6:
+		return "IPv6"
+	default:
+		return fmt.Sprintf("IPProtoVersion-%d", int(p))
+	}
+}
+
 // Proto is an IP subprotocol as defined by the IANA protocol
 // numbers list
 // (https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml),