net/tsdial: add VIP service names to dnsmap with correct address family selection

Adds VIP service name resolution to the MagicDNS map so that service
names like "mydb" and "mydb.<tailnet>" resolve to the service VIP address.

Uses the same address family iteration as the peer loop to avoid inserting
unreachable IPv4 addresses on IPv6-only nodes.

Fixes #19097

Signed-off-by: Raj Singh <raj@tailscale.com>

net/tsdial/dnsmap.go

@@ -74,6 +74,32 @@ func dnsMapFromNetworkMap(nm *netmap.NetworkMap) dnsMap {
 		}
 		ret[canonMapKey(rec.Name)] = ip
 	}
+	// Map VIP service names into the DNS map so that MagicDNS can
+	// resolve e.g. "mydb" or "mydb.<tailnet>" to the service VIP.
+	// suffix is derived from the self node name; it's only empty when
+	// the self node is invalid, in which case there's nothing to do.
+	if suffix != "" {
+		for svcName, svcAddrs := range nm.GetVIPServiceIPMap() {
+			bare := svcName.WithoutPrefix()
+			if bare == "" {
+				continue
+			}
+			var ip netip.Addr
+			for _, a := range svcAddrs {
+				if a.Is4() && !have4 {
+					continue
+				}
+				ip = a
+				break
+			}
+			if !ip.IsValid() {
+				continue
+			}
+			fqdn := bare + "." + suffix
+			ret[canonMapKey(fqdn)] = ip
+			ret[canonMapKey(bare)] = ip
+		}
+	}
 	return ret
 }
 

net/tsdial/dnsmap_test.go

@@ -4,6 +4,7 @@
 package tsdial
 
 import (
+	"encoding/json"
 	"net/netip"
 	"reflect"
 	"testing"
@@ -12,6 +13,15 @@
 	"tailscale.com/types/netmap"
 )
 
+func mustMarshal(t *testing.T, v any) []byte {
+	t.Helper()
+	b, err := json.Marshal(v)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return b
+}
+
 func nodeViews(v []*tailcfg.Node) []tailcfg.NodeView {
 	nv := make([]tailcfg.NodeView, len(v))
 	for i, n := range v {
@@ -113,6 +123,127 @@ func TestDNSMapFromNetworkMap(t *testing.T) {
 				"b.tailnet":   ip("100::202"),
 			},
 		},
+		{
+			name: "vip_services",
+			nm: &netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Name: "foo.tailnet.",
+					Addresses: []netip.Prefix{
+						pfx("100.102.103.104/32"),
+						pfx("100::123/128"),
+					},
+					CapMap: tailcfg.NodeCapMap{
+						tailcfg.NodeAttrServiceHost: []tailcfg.RawMessage{
+							tailcfg.RawMessage(mustMarshal(t, tailcfg.ServiceIPMappings{
+								"svc:mydb": {
+									netip.MustParseAddr("100.65.32.1"),
+									netip.MustParseAddr("fd7a:115c:a1e0::1234"),
+								},
+							})),
+						},
+					},
+				}).View(),
+			},
+			want: dnsMap{
+				"foo":          ip("100.102.103.104"),
+				"foo.tailnet":  ip("100.102.103.104"),
+				"mydb.tailnet": ip("100.65.32.1"),
+				"mydb":         ip("100.65.32.1"),
+			},
+		},
+		{
+			name: "vip_services_v6_only_self",
+			nm: &netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Name: "foo.tailnet.",
+					Addresses: []netip.Prefix{
+						pfx("100::123/128"),
+					},
+					CapMap: tailcfg.NodeCapMap{
+						tailcfg.NodeAttrServiceHost: []tailcfg.RawMessage{
+							tailcfg.RawMessage(mustMarshal(t, tailcfg.ServiceIPMappings{
+								"svc:mydb": {
+									netip.MustParseAddr("100.65.32.1"),
+									netip.MustParseAddr("fd7a:115c:a1e0::1234"),
+								},
+							})),
+						},
+					},
+				}).View(),
+			},
+			want: dnsMap{
+				"foo":          ip("100::123"),
+				"foo.tailnet":  ip("100::123"),
+				"mydb.tailnet": ip("fd7a:115c:a1e0::1234"),
+				"mydb":         ip("fd7a:115c:a1e0::1234"),
+			},
+		},
+		{
+			// VIP service has only IPv4 addrs but self is IPv6-only.
+			// Should be excluded entirely since no reachable address exists.
+			name: "vip_services_v4_only_addrs_v6_only_self",
+			nm: &netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Name: "foo.tailnet.",
+					Addresses: []netip.Prefix{
+						pfx("100::123/128"),
+					},
+					CapMap: tailcfg.NodeCapMap{
+						tailcfg.NodeAttrServiceHost: []tailcfg.RawMessage{
+							tailcfg.RawMessage(mustMarshal(t, tailcfg.ServiceIPMappings{
+								"svc:mydb": {
+									netip.MustParseAddr("100.65.32.1"),
+									netip.MustParseAddr("100.65.32.2"),
+								},
+							})),
+						},
+					},
+				}).View(),
+			},
+			want: dnsMap{
+				"foo":         ip("100::123"),
+				"foo.tailnet": ip("100::123"),
+				// mydb should NOT appear — both addrs are IPv4 and self is v6-only
+			},
+		},
+		{
+			// VIP service name collides with a peer name.
+			// VIP runs after peers so it overwrites.
+			name: "vip_service_overwrites_peer",
+			nm: &netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Name: "foo.tailnet.",
+					Addresses: []netip.Prefix{
+						pfx("100.102.103.104/32"),
+						pfx("100::123/128"),
+					},
+					CapMap: tailcfg.NodeCapMap{
+						tailcfg.NodeAttrServiceHost: []tailcfg.RawMessage{
+							tailcfg.RawMessage(mustMarshal(t, tailcfg.ServiceIPMappings{
+								"svc:a": {
+									netip.MustParseAddr("100.65.32.1"),
+								},
+							})),
+						},
+					},
+				}).View(),
+				Peers: nodeViews([]*tailcfg.Node{
+					{
+						Name: "a.tailnet",
+						Addresses: []netip.Prefix{
+							pfx("100.0.0.201/32"),
+							pfx("100::201/128"),
+						},
+					},
+				}),
+			},
+			want: dnsMap{
+				"foo":         ip("100.102.103.104"),
+				"foo.tailnet": ip("100.102.103.104"),
+				"a":           ip("100.65.32.1"), // VIP overwrites peer
+				"a.tailnet":   ip("100.65.32.1"), // VIP overwrites peer
+			},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {