mapper: send a disable-ipv4 holder its peers over IPv6 only

As SaaS. Peers' 100.x addresses routed into tailscale0 and hid a local
host with the same address, which is what the attr is meant to avoid.

hscontrol/mapper/builder.go

@@ -281,6 +281,8 @@ func (b *MapResponseBuilder) buildTailPeers(peers views.Slice[types.NodeView]) (
 	// Snapshot these peers' policy caps under one policy-manager lock.
 	peerCapMaps := b.mapper.state.NodeCapMapsForPeers(peers)
 
+	_, ipv4Disabled := b.mapper.state.NodeCapMap(node.ID())[types.NodeAttrDisableIPv4]
+
 	// Build tail nodes with per-peer via-aware route function.
 	tailPeers := make([]*tailcfg.Node, 0, peers.Len())
 
@@ -327,6 +329,16 @@ func (b *MapResponseBuilder) buildTailPeers(peers views.Slice[types.NodeView]) (
 		tn.CapMap = policyv2.PeerCapMap(peer, peerCapMaps[peer.ID()])
 		tn.Hostinfo = types.PeerHostinfo(tn.Hostinfo)
 
+		// A disable-ipv4 holder sees its peers over IPv6 only, as on SaaS, so
+		// their 100.x addresses stay off its interface. Routes stay.
+		if ipv4Disabled {
+			ownIPv4 := func(p netip.Prefix) bool {
+				return p.Addr().Is4() && slices.Contains(tn.Addresses, p)
+			}
+			tn.AllowedIPs = slices.DeleteFunc(tn.AllowedIPs, ownIPv4)
+			tn.Addresses = slices.DeleteFunc(tn.Addresses, func(p netip.Prefix) bool { return p.Addr().Is4() })
+		}
+
 		// As on Tailscale SaaS, a connected expired peer shows online, but
 		// without the endpoints and home DERP region that would reach it.
 		if peer.IsExpired() {

hscontrol/mapper/mapper_test.go

@@ -1812,6 +1812,14 @@ func (b *MapResponseBuilder) buildTailPeersCapMapsReference(peers views.Slice[ty
 		tn.CapMap = policyv2.PeerCapMap(peer, allCapMaps[peer.ID()])
 		tn.Hostinfo = types.PeerHostinfo(tn.Hostinfo)
 
+		if _, ipv4Disabled := allCapMaps[node.ID()][types.NodeAttrDisableIPv4]; ipv4Disabled {
+			ownIPv4 := func(p netip.Prefix) bool {
+				return p.Addr().Is4() && slices.Contains(tn.Addresses, p)
+			}
+			tn.AllowedIPs = slices.DeleteFunc(tn.AllowedIPs, ownIPv4)
+			tn.Addresses = slices.DeleteFunc(tn.Addresses, func(p netip.Prefix) bool { return p.Addr().Is4() })
+		}
+
 		if peer.IsExpired() {
 			if peer.ActiveSessions() > 0 {
 				tn.Online, tn.LastSeen = new(true), nil

hscontrol/servertest/via_compat_test.go

@@ -353,6 +353,29 @@ func runViaMapCompat(t *testing.T, c *testcapture.Capture) {
 	compareViaMapCompat(t, c, clients)
 }
 
+// TestDisableIPv4PeersCompat: a disable-ipv4 holder sees its peers over IPv6
+// only, in both recordings of such a viewer.
+func TestDisableIPv4PeersCompat(t *testing.T) {
+	t.Parallel()
+
+	for _, id := range []string{"nodeattrs-attr-c15-disable-ipv4", "nodeattrs-attr-c16-disable-ipv4-router"} {
+		t.Run(id, func(t *testing.T) {
+			t.Parallel()
+
+			c, err := testcapture.Read(filepath.Join("..", "policy", "v2", "testdata", "nodeattrs_results", id+".hujson"))
+			require.NoError(t, err)
+
+			_, clients := startViaMapCompat(t, c, true)
+
+			for viewer, want := range c.Captures {
+				assert.EventuallyWithT(t, func(collect *assert.CollectT) {
+					comparePeers(collect, clients[viewer].Netmap(), want, clients, id+"/"+viewer, nil)
+				}, 20*time.Second, 200*time.Millisecond, "viewer %s", viewer)
+			}
+		})
+	}
+}
+
 // startViaMapCompat starts a server holding the capture's policy and a
 // client per captured node, approving routes as [startCaptureClients] does,
 // and waits for every client's first netmap and the peers SaaS gave it.

hscontrol/types/node.go

@@ -44,11 +44,11 @@ var (
 // [netip.Prefix] values representing the routes for that node.
 type RouteFunc func(id NodeID) []netip.Prefix
 
-// nodeAttrDisableIPv4 is the policy nodeAttr key that suppresses the
+// NodeAttrDisableIPv4 is the policy nodeAttr key that suppresses the
 // node's own IPv4 CGNAT prefix in [tailcfg.Node.Addresses] and
 // [tailcfg.Node.AllowedIPs]. Subnet routes the node advertises remain.
 // See https://tailscale.com/docs/reference/troubleshooting/network-configuration/cgnat-conflicts.
-const nodeAttrDisableIPv4 nodecap.Cap = "disable-ipv4"
+const NodeAttrDisableIPv4 nodecap.Cap = "disable-ipv4"
 
 // filterIPv4 returns ps with every IPv4 prefix dropped. Used by
 // [NodeView.TailNode] when the node carries the disable-ipv4 nodeAttr.
@@ -1402,7 +1402,7 @@ func (nv NodeView) TailNode(
 	// subnet routes -- even IPv4 ones -- survive: routes belong to
 	// the routing layer, not the node's identity. Mirrors the SaaS
 	// captures in testdata/nodeattrs_results/nodeattrs-attr-c1{5,6}-disable-ipv4*.
-	_, ipv4Disabled := selfPolicyCaps[nodeAttrDisableIPv4]
+	_, ipv4Disabled := selfPolicyCaps[NodeAttrDisableIPv4]
 
 	addresses := nv.Prefixes()
 	if ipv4Disabled {