appc/conn25.go

@@ -80,7 +80,7 @@ func AppDNSRoutes(hasCap func(c tailcfg.NodeCapability) bool, self tailcfg.NodeV
 	}
 	m := make(map[string][]*dnstype.Resolver, len(appNamesByDomain))
 	for domain, appName := range appNamesByDomain {
-		m[domain] = []*dnstype.Resolver{{Addr: fmt.Sprintf("%s:%s", DNSAddrScheme, appName)}}
+		m[domain] = []*dnstype.Resolver{{Addr: fmt.Sprintf("%s:%s", DNSAddrScheme, appName), UseWithExitNode: true}}
 	}
 	return m
 }

appc/conn25_test.go

@@ -37,7 +37,7 @@ func TestAppDNSRoutes(t *testing.T) {
 	appSixBytes := getBytesForAttr("app6", []string{"*.Example.com", "EXAMPLE.com", "EXAMPLE.COM"}, []string{"tag:one"})
 
 	resolver := func(appName string) []*dnstype.Resolver {
-		return []*dnstype.Resolver{{Addr: fmt.Sprintf("%s:%s", DNSAddrScheme, appName)}}
+		return []*dnstype.Resolver{{Addr: fmt.Sprintf("%s:%s", DNSAddrScheme, appName), UseWithExitNode: true}}
 	}
 
 	for _, tt := range []struct {

ipn/ipnlocal/dnsconfig_test.go

@@ -429,12 +429,62 @@ func TestDNSConfigForNetmap(t *testing.T) {
 				Hosts:     map[dnsname.FQDN][]netip.Addr{},
 				Routes: map[dnsname.FQDN][]*dnstype.Resolver{
 					dnsname.FQDN("example.com."): {
-						{Addr: "tailscale-app:app1"},
+						{Addr: "tailscale-app:app1", UseWithExitNode: true},
 					},
 				},
 				MagicDNSHostsUnrouted: true,
 			},
 		},
+		{
+			name: "conn25-split-dns-with-exit-node",
+			nm: &netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Name:      "a",
+					Addresses: ipps("100.101.101.101"),
+					CapMap: tailcfg.NodeCapMap{
+						tailcfg.NodeCapability(appc.AppConnectorsExperimentalAttrName): []tailcfg.RawMessage{
+							tailcfg.RawMessage(`{"name":"app1","connectors":["tag:woo"],"domains":["example.com"]}`),
+						},
+					},
+				}).View(),
+				AllCaps: set.Of(tailcfg.NodeCapability(appc.AppConnectorsExperimentalAttrName)),
+			},
+			peers: nodeViews([]*tailcfg.Node{
+				{
+					ID:        1,
+					StableID:  "exit",
+					Name:      "p1",
+					Cap:       26, // can proxy DNS over DoH
+					Addresses: ipps("100.102.0.1"),
+					Tags:      []string{"tag:woo"},
+					Hostinfo: (&tailcfg.Hostinfo{
+						Services: []tailcfg.Service{
+							{
+								Proto: tailcfg.PeerAPI4,
+								Port:  1234,
+							},
+						},
+					}).View(),
+				},
+			}),
+			prefs: &ipn.Prefs{
+				CorpDNS:    true,
+				ExitNodeID: "exit",
+			},
+			want: &dns.Config{
+				AcceptDNS: true,
+				Hosts:     map[dnsname.FQDN][]netip.Addr{},
+				Routes: map[dnsname.FQDN][]*dnstype.Resolver{
+					dnsname.FQDN("example.com."): {
+						{Addr: "tailscale-app:app1", UseWithExitNode: true},
+					},
+				},
+				DefaultResolvers: []*dnstype.Resolver{
+					{Addr: "http://100.102.0.1:1234/dns-query"},
+				},
+				MagicDNSHostsUnrouted: true,
+			},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

ipn/ipnlocal/node_backend.go

@@ -1536,6 +1536,15 @@ func dnsConfigForNetmap(nm *netmap.NetworkMap, peers map[tailcfg.NodeID]tailcfg.
 		}
 	}
 
+	// conn25 split DNS routes are calculated from the domains in the SelfNode.CapMap
+	// section of the netmap, so need to be assembled separately.
+	// TODO(tailscale/corp#37125): make this a hook the extension can add
+	// to reduce dependency from ipnlocal to appc.
+	var conn25AppRoutes map[string][]*dnstype.Resolver
+	if (envknob.UseWIPCode() || testenv.InTest()) && buildfeatures.HasConn25 && !prefs.AppConnector().Advertise {
+		conn25AppRoutes = appc.AppDNSRoutes(nm.HasCap, nm.SelfNode)
+	}
+
 	// If we're using an exit node and that exit node is new enough (1.19.x+)
 	// to run a DoH DNS proxy, then send all our DNS traffic through it,
 	// unless we find resolvers with UseWithExitNode set, in which case we use that.
@@ -1551,6 +1560,7 @@ func dnsConfigForNetmap(nm *netmap.NetworkMap, peers map[tailcfg.NodeID]tailcfg.
 			}
 
 			addSplitDNSRoutes(useWithExitNodeRoutes(nm.DNS.Routes))
+			addSplitDNSRoutes(useWithExitNodeRoutes(conn25AppRoutes))
 			return dcfg
 		}
 	}
@@ -1568,13 +1578,7 @@ func dnsConfigForNetmap(nm *netmap.NetworkMap, peers map[tailcfg.NodeID]tailcfg.
 
 	// Add split DNS routes, with no regard to exit node configuration.
 	addSplitDNSRoutes(nm.DNS.Routes)
-
-	if (envknob.UseWIPCode() || testenv.InTest()) && buildfeatures.HasConn25 && !prefs.AppConnector().Advertise {
-		// Add split DNS routes for conn25
-		if appRoutes := appc.AppDNSRoutes(nm.HasCap, nm.SelfNode); appRoutes != nil {
-			addSplitDNSRoutes(appRoutes)
-		}
-	}
+	addSplitDNSRoutes(conn25AppRoutes)
 
 	// Set FallbackResolvers as the default resolvers in the
 	// scenarios that can't handle a purely split-DNS config. See