appc,ipn/ipnlocal,net/dns/resolver: add App Connector wiring when enabled in prefs

An EmbeddedAppConnector is added that when configured observes DNS
responses from the PeerAPI. If a response is found matching a configured
domain, routes are advertised when necessary.

The wiring from a configuration in the netmap capmap is not yet done, so
while the connector can be enabled, no domains can yet be added.

Updates tailscale/corp#15437

Signed-off-by: James Tucker <james@tailscale.com>

appc/appc.go

@@ -31,7 +31,7 @@ type target struct {
 	Matching tailcfg.ProtoPortRange
 }
 
-// Server implements an App Connector.
+// Server implements an App Connector as expressed in sniproxy.
 type Server struct {
 	mu         sync.RWMutex // mu guards following fields
 	connectors map[appctype.ConfigID]connector

appc/embedded.go

@@ -0,0 +1,165 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package appc implements App Connectors. An AppConnector provides domain
+// oriented routing of traffic.
+package appc
+
+import (
+	"net/netip"
+	"slices"
+	"strings"
+	"sync"
+
+	"golang.org/x/net/dns/dnsmessage"
+	"tailscale.com/types/logger"
+)
+
+/*
+ * TODO(raggi): the sniproxy servicing portions of this package will be moved
+ * into the sniproxy or deprecated at some point, when doing so is not
+ * disruptive. At that time EmbeddedAppConnector can be renamed to AppConnector.
+ */
+
+// RouteAdvertiser is an interface that allows the AppConnector to advertise
+// newly discovered routes that need to be served through the AppConnector.
+type RouteAdvertiser interface {
+	// AdvertiseRoute adds a new route advertisement if the route is not already
+	// being advertised.
+	AdvertiseRoute(netip.Prefix) error
+}
+
+// EmbeddedAppConnector is an implementation of an AppConnector that performs
+// its function as a subsystem inside of a tailscale node. At the control plane
+// side App Connector routing is configured in terms of domains rather than IP
+// addresses.
+// The AppConnectors responsibility inside tailscaled is to apply the routing
+// and domain configuration as supplied in the map response.
+// DNS requests for configured domains are observed. If the domains resolve to
+// routes not yet served by the AppConnector the local node configuration is
+// updated to advertise the new route.
+type EmbeddedAppConnector struct {
+	logf            logger.Logf
+	routeAdvertiser RouteAdvertiser
+
+	// mu guards the fields that follow
+	mu sync.Mutex
+	// domains is a map of lower case domain names with no trailing dot, to a
+	// list of resolved IP addresses.
+	domains map[string][]netip.Addr
+}
+
+// NewEmbeddedAppConnector creates a new EmbeddedAppConnector.
+func NewEmbeddedAppConnector(logf logger.Logf, routeAdvertiser RouteAdvertiser) *EmbeddedAppConnector {
+	return &EmbeddedAppConnector{
+		logf:            logger.WithPrefix(logf, "appc: "),
+		routeAdvertiser: routeAdvertiser,
+	}
+}
+
+// UpdateDomains replaces the current set of configured domains with the
+// supplied set of domains. Domains must not contain a trailing dot, and should
+// be lower case.
+func (e *EmbeddedAppConnector) UpdateDomains(domains []string) {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+
+	var old map[string][]netip.Addr
+	old, e.domains = e.domains, make(map[string][]netip.Addr, len(domains))
+	for _, d := range domains {
+		d = strings.ToLower(d)
+		e.domains[d] = old[d]
+	}
+}
+
+// ObserveDNSResponse is a callback invoked by the DNS resolver when a DNS
+// response is being returned over the PeerAPI. The response is parsed and
+// matched against the configured domains, if matched the routeAdvertiser is
+// advised to advertise the discovered route.
+func (e *EmbeddedAppConnector) ObserveDNSResponse(res []byte) {
+	var p dnsmessage.Parser
+	if _, err := p.Start(res); err != nil {
+		return
+	}
+	if err := p.SkipAllQuestions(); err != nil {
+		return
+	}
+
+	for {
+		h, err := p.AnswerHeader()
+		if err == dnsmessage.ErrSectionDone {
+			break
+		}
+		if err != nil {
+			return
+		}
+
+		if h.Class != dnsmessage.ClassINET {
+			if err := p.SkipAnswer(); err != nil {
+				return
+			}
+			continue
+		}
+		if h.Type != dnsmessage.TypeA && h.Type != dnsmessage.TypeAAAA {
+			if err := p.SkipAnswer(); err != nil {
+				return
+			}
+			continue
+		}
+
+		domain := h.Name.String()
+		if len(domain) == 0 {
+			return
+		}
+		if domain[len(domain)-1] == '.' {
+			domain = domain[:len(domain)-1]
+		}
+		domain = strings.ToLower(domain)
+		e.logf("[v2] observed DNS response for %s", domain)
+
+		e.mu.Lock()
+		addrs, ok := e.domains[domain]
+		e.mu.Unlock()
+		if !ok {
+			if err := p.SkipAnswer(); err != nil {
+				return
+			}
+			continue
+		}
+
+		var addr netip.Addr
+		switch h.Type {
+		case dnsmessage.TypeA:
+			r, err := p.AResource()
+			if err != nil {
+				return
+			}
+			addr = netip.AddrFrom4(r.A)
+		case dnsmessage.TypeAAAA:
+			r, err := p.AAAAResource()
+			if err != nil {
+				return
+			}
+			addr = netip.AddrFrom16(r.AAAA)
+		default:
+			if err := p.SkipAnswer(); err != nil {
+				return
+			}
+			continue
+		}
+		if slices.Contains(addrs, addr) {
+			continue
+		}
+		// TODO(raggi): check for existing prefixes
+		if err := e.routeAdvertiser.AdvertiseRoute(netip.PrefixFrom(addr, addr.BitLen())); err != nil {
+			e.logf("failed to advertise route for %v: %v", addr, err)
+			continue
+		}
+		e.logf("[v2] advertised route for %v: %v", domain, addr)
+
+		e.mu.Lock()
+		e.domains[domain] = append(addrs, addr)
+		e.mu.Unlock()
+	}
+
+}

appc/embedded_test.go

@@ -0,0 +1,118 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package appc
+
+import (
+	"net/netip"
+	"slices"
+	"testing"
+
+	xmaps "golang.org/x/exp/maps"
+	"golang.org/x/net/dns/dnsmessage"
+	"tailscale.com/util/must"
+)
+
+func TestUpdateDomains(t *testing.T) {
+	a := NewEmbeddedAppConnector(t.Logf, nil)
+	a.UpdateDomains([]string{"example.com"})
+	if got, want := xmaps.Keys(a.domains), []string{"example.com"}; !slices.Equal(got, want) {
+		t.Errorf("got %v; want %v", got, want)
+	}
+
+	addr := netip.MustParseAddr("192.0.0.8")
+	a.domains["example.com"] = append(a.domains["example.com"], addr)
+	a.UpdateDomains([]string{"example.com"})
+
+	if got, want := a.domains["example.com"], []netip.Addr{addr}; !slices.Equal(got, want) {
+		t.Errorf("got %v; want %v", got, want)
+	}
+
+	// domains are explicitly downcased on set.
+	a.UpdateDomains([]string{"UP.EXAMPLE.COM"})
+	if got, want := xmaps.Keys(a.domains), []string{"up.example.com"}; !slices.Equal(got, want) {
+		t.Errorf("got %v; want %v", got, want)
+	}
+}
+
+func TestObserveDNSResponse(t *testing.T) {
+	rc := &routeCollector{}
+	a := NewEmbeddedAppConnector(t.Logf, rc)
+
+	// a has no domains configured, so it should not advertise any routes
+	a.ObserveDNSResponse(dnsResponse("example.com.", "192.0.0.8"))
+	if got, want := rc.routes, ([]netip.Prefix)(nil); !slices.Equal(got, want) {
+		t.Errorf("got %v; want %v", got, want)
+	}
+
+	wantRoutes := []netip.Prefix{netip.MustParsePrefix("192.0.0.8/32")}
+
+	a.UpdateDomains([]string{"example.com"})
+	a.ObserveDNSResponse(dnsResponse("example.com.", "192.0.0.8"))
+	if got, want := rc.routes, wantRoutes; !slices.Equal(got, want) {
+		t.Errorf("got %v; want %v", got, want)
+	}
+
+	wantRoutes = append(wantRoutes, netip.MustParsePrefix("2001:db8::1/128"))
+
+	a.ObserveDNSResponse(dnsResponse("example.com.", "2001:db8::1"))
+	if got, want := rc.routes, wantRoutes; !slices.Equal(got, want) {
+		t.Errorf("got %v; want %v", got, want)
+	}
+
+	// don't re-advertise routes that have already been advertised
+	a.ObserveDNSResponse(dnsResponse("example.com.", "2001:db8::1"))
+	if !slices.Equal(rc.routes, wantRoutes) {
+		t.Errorf("got %v; want %v", rc.routes, wantRoutes)
+	}
+}
+
+// dnsResponse is a test helper that creates a DNS response buffer for the given domain and address
+func dnsResponse(domain, address string) []byte {
+	addr := netip.MustParseAddr(address)
+	b := dnsmessage.NewBuilder(nil, dnsmessage.Header{})
+	b.EnableCompression()
+	b.StartAnswers()
+	switch addr.BitLen() {
+	case 32:
+		b.AResource(
+			dnsmessage.ResourceHeader{
+				Name:  dnsmessage.MustNewName(domain),
+				Type:  dnsmessage.TypeA,
+				Class: dnsmessage.ClassINET,
+				TTL:   0,
+			},
+			dnsmessage.AResource{
+				A: addr.As4(),
+			},
+		)
+	case 128:
+		b.AAAAResource(
+			dnsmessage.ResourceHeader{
+				Name:  dnsmessage.MustNewName(domain),
+				Type:  dnsmessage.TypeAAAA,
+				Class: dnsmessage.ClassINET,
+				TTL:   0,
+			},
+			dnsmessage.AAAAResource{
+				AAAA: addr.As16(),
+			},
+		)
+	default:
+		panic("invalid address length")
+	}
+	return must.Get(b.Finish())
+}
+
+// routeCollector is a test helper that collects the list of routes advertised
+type routeCollector struct {
+	routes []netip.Prefix
+}
+
+// routeCollector implements RouteAdvertiser
+var _ RouteAdvertiser = (*routeCollector)(nil)
+
+func (rc *routeCollector) AdvertiseRoute(pfx netip.Prefix) error {
+	rc.routes = append(rc.routes, pfx)
+	return nil
+}

cmd/tailscaled/depaware.txt

@@ -216,11 +216,13 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         gvisor.dev/gvisor/pkg/tcpip/transport/udp                    from tailscale.com/net/tstun+
         gvisor.dev/gvisor/pkg/waiter                                 from gvisor.dev/gvisor/pkg/context+
         inet.af/peercred                                             from tailscale.com/ipn/ipnauth
+        inet.af/tcpproxy                                             from tailscale.com/appc
    W 💣 inet.af/wf                                                   from tailscale.com/wf
         nhooyr.io/websocket                                          from tailscale.com/derp/derphttp+
         nhooyr.io/websocket/internal/errd                            from nhooyr.io/websocket
         nhooyr.io/websocket/internal/xsync                           from nhooyr.io/websocket
         tailscale.com                                                from tailscale.com/version
+        tailscale.com/appc                                           from tailscale.com/ipn/ipnlocal
         tailscale.com/atomicfile                                     from tailscale.com/ipn+
   LD    tailscale.com/chirp                                          from tailscale.com/cmd/tailscaled
         tailscale.com/client/tailscale                               from tailscale.com/derp+
@@ -269,7 +271,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/net/dns/publicdns                              from tailscale.com/net/dns/resolver+
         tailscale.com/net/dns/recursive                              from tailscale.com/net/dnsfallback
         tailscale.com/net/dns/resolvconffile                         from tailscale.com/net/dns+
-        tailscale.com/net/dns/resolver                               from tailscale.com/ipn/ipnlocal+
+        tailscale.com/net/dns/resolver                               from tailscale.com/net/dns
         tailscale.com/net/dnscache                                   from tailscale.com/control/controlclient+
         tailscale.com/net/dnsfallback                                from tailscale.com/control/controlclient+
         tailscale.com/net/flowtrack                                  from tailscale.com/net/packet+
@@ -319,6 +321,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/tstime/mono                                    from tailscale.com/net/tstun+
         tailscale.com/tstime/rate                                    from tailscale.com/wgengine/filter+
         tailscale.com/tsweb/varz                                     from tailscale.com/cmd/tailscaled
+        tailscale.com/types/appctype                                 from tailscale.com/appc
         tailscale.com/types/dnstype                                  from tailscale.com/ipn/ipnlocal+
         tailscale.com/types/empty                                    from tailscale.com/ipn+
         tailscale.com/types/flagtype                                 from tailscale.com/cmd/tailscaled

ipn/ipnlocal/local.go

@@ -32,6 +32,7 @@
 	"go4.org/netipx"
 	xmaps "golang.org/x/exp/maps"
 	"gvisor.dev/gvisor/pkg/tcpip"
+	"tailscale.com/appc"
 	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/control/controlknobs"
@@ -203,9 +204,10 @@ type LocalBackend struct {
 	conf           *conffile.Config // latest parsed config, or nil if not in declarative mode
 	pm             *profileManager  // mu guards access
 	filterHash     deephash.Sum
-	httpTestClient *http.Client // for controlclient. nil by default, used by tests.
+	httpTestClient *http.Client               // for controlclient. nil by default, used by tests.
-	ccGen          clientGen    // function for producing controlclient; lazily populated
+	ccGen          clientGen                  // function for producing controlclient; lazily populated
-	sshServer      SSHServer    // or nil, initialized lazily.
+	sshServer      SSHServer                  // or nil, initialized lazily.
+	appConnector   *appc.EmbeddedAppConnector // or nil, initialized when configured.
 	webClient      webClient
 	notify         func(ipn.Notify)
 	cc             controlclient.Client
@@ -2995,6 +2997,9 @@ func (b *LocalBackend) setPrefsLockedOnEntry(caller string, newp *ipn.Prefs) ipn
 
 	oldHi := b.hostinfo
 	newHi := oldHi.Clone()
+	if newHi == nil {
+		newHi = new(tailcfg.Hostinfo)
+	}
 	b.applyPrefsToHostinfoLocked(newHi, newp.View())
 	b.hostinfo = newHi
 	hostInfoChanged := !oldHi.Equal(newHi)
@@ -3240,6 +3245,10 @@ func (b *LocalBackend) authReconfig() {
 	disableSubnetsIfPAC := hasCapability(nm, tailcfg.NodeAttrDisableSubnetsIfPAC)
 	dohURL, dohURLOK := exitNodeCanProxyDNS(nm, b.peers, prefs.ExitNodeID())
 	dcfg := dnsConfigForNetmap(nm, b.peers, prefs, b.logf, version.OS())
+	// If the current node is an app connector, ensure the app connector machine is started
+	if prefs.AppConnector().Advertise && b.appConnector == nil {
+		b.appConnector = appc.NewEmbeddedAppConnector(b.logf, b)
+	}
 	b.mu.Unlock()
 
 	if blocked {
@@ -4812,6 +4821,14 @@ func (b *LocalBackend) OfferingExitNode() bool {
 	return def4 && def6
 }
 
+// OfferingAppConnector reports whether b is currently offering app
+// connector services.
+func (b *LocalBackend) OfferingAppConnector() bool {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.appConnector != nil
+}
+
 // allowExitNodeDNSProxyToServeName reports whether the Exit Node DNS
 // proxy is allowed to serve responses for the provided DNS name.
 func (b *LocalBackend) allowExitNodeDNSProxyToServeName(name string) bool {
@@ -5398,6 +5415,39 @@ func (b *LocalBackend) DebugBreakDERPConns() error {
 	return b.magicConn().DebugBreakDERPConns()
 }
 
+// ObserveDNSResponse passes a DNS response from the PeerAPI DNS server to the
+// App Connector to enable route discovery.
+func (b *LocalBackend) ObserveDNSResponse(res []byte) {
+	var appConnector *appc.EmbeddedAppConnector
+	b.mu.Lock()
+	if b.appConnector == nil {
+		b.mu.Unlock()
+		return
+	}
+	appConnector = b.appConnector
+	b.mu.Unlock()
+
+	appConnector.ObserveDNSResponse(res)
+}
+
+// AdvertiseRoute implements the appc.RouteAdvertiser interface. It sets a new
+// route advertisement if one is not already present in the existing routes.
+func (b *LocalBackend) AdvertiseRoute(ipp netip.Prefix) error {
+	currentRoutes := b.Prefs().AdvertiseRoutes()
+	// TODO(raggi): check if the new route is a subset of an existing route.
+	if currentRoutes.ContainsFunc(func(r netip.Prefix) bool { return r == ipp }) {
+		return nil
+	}
+	routes := append(currentRoutes.AsSlice(), ipp)
+	_, err := b.EditPrefs(&ipn.MaskedPrefs{
+		Prefs: ipn.Prefs{
+			AdvertiseRoutes: routes,
+		},
+		AdvertiseRoutesSet: true,
+	})
+	return err
+}
+
 // mayDeref dereferences p if non-nil, otherwise it returns the zero value.
 func mayDeref[T any](p *T) (v T) {
 	if p == nil {

ipn/ipnlocal/local_test.go

@@ -10,10 +10,13 @@
 	"net/http"
 	"net/netip"
 	"reflect"
+	"slices"
 	"testing"
 	"time"
 
 	"go4.org/netipx"
+	"golang.org/x/net/dns/dnsmessage"
+	"tailscale.com/appc"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/store/mem"
@@ -30,6 +33,7 @@
 	"tailscale.com/types/ptr"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/util/mak"
+	"tailscale.com/util/must"
 	"tailscale.com/util/set"
 	"tailscale.com/wgengine"
 	"tailscale.com/wgengine/filter"
@@ -1142,6 +1146,47 @@ type tc struct {
 	}
 }
 
+func TestOfferingAppConnector(t *testing.T) {
+	b := newTestBackend(t)
+	if b.OfferingAppConnector() {
+		t.Fatal("unexpected offering app connector")
+	}
+	b.appConnector = appc.NewEmbeddedAppConnector(t.Logf, nil)
+	if !b.OfferingAppConnector() {
+		t.Fatal("unexpected not offering app connector")
+	}
+}
+
+func TestRouteAdvertiser(t *testing.T) {
+	b := newTestBackend(t)
+	testPrefix := netip.MustParsePrefix("192.0.0.8/32")
+
+	ra := appc.RouteAdvertiser(b)
+	must.Do(ra.AdvertiseRoute(testPrefix))
+
+	routes := b.Prefs().AdvertiseRoutes()
+	if routes.Len() != 1 || routes.At(0) != testPrefix {
+		t.Fatalf("got routes %v, want %v", routes, []netip.Prefix{testPrefix})
+	}
+}
+
+func TestObserveDNSResponse(t *testing.T) {
+	b := newTestBackend(t)
+
+	// ensure no error when no app connector is configured
+	b.ObserveDNSResponse(dnsResponse("example.com.", "192.0.0.8"))
+
+	rc := &routeCollector{}
+	b.appConnector = appc.NewEmbeddedAppConnector(t.Logf, rc)
+	b.appConnector.UpdateDomains([]string{"example.com"})
+
+	b.ObserveDNSResponse(dnsResponse("example.com.", "192.0.0.8"))
+	wantRoutes := []netip.Prefix{netip.MustParsePrefix("192.0.0.8/32")}
+	if !slices.Equal(rc.routes, wantRoutes) {
+		t.Fatalf("got routes %v, want %v", rc.routes, wantRoutes)
+	}
+}
+
 func resolversEqual(t *testing.T, a, b []*dnstype.Resolver) bool {
 	if a == nil && b == nil {
 		return true
@@ -1176,3 +1221,50 @@ func routesEqual(t *testing.T, a, b map[dnsname.FQDN][]*dnstype.Resolver) bool {
 	}
 	return true
 }
+
+// dnsResponse is a test helper that creates a DNS response buffer for the given domain and address
+func dnsResponse(domain, address string) []byte {
+	addr := netip.MustParseAddr(address)
+	b := dnsmessage.NewBuilder(nil, dnsmessage.Header{})
+	b.EnableCompression()
+	b.StartAnswers()
+	switch addr.BitLen() {
+	case 32:
+		b.AResource(
+			dnsmessage.ResourceHeader{
+				Name:  dnsmessage.MustNewName(domain),
+				Type:  dnsmessage.TypeA,
+				Class: dnsmessage.ClassINET,
+				TTL:   0,
+			},
+			dnsmessage.AResource{
+				A: addr.As4(),
+			},
+		)
+	case 128:
+		b.AAAAResource(
+			dnsmessage.ResourceHeader{
+				Name:  dnsmessage.MustNewName(domain),
+				Type:  dnsmessage.TypeAAAA,
+				Class: dnsmessage.ClassINET,
+				TTL:   0,
+			},
+			dnsmessage.AAAAResource{
+				AAAA: addr.As16(),
+			},
+		)
+	default:
+		panic("invalid address length")
+	}
+	return must.Get(b.Finish())
+}
+
+// routeCollector is a test helper that collects the list of routes advertised
+type routeCollector struct {
+	routes []netip.Prefix
+}
+
+func (rc *routeCollector) AdvertiseRoute(pfx netip.Prefix) error {
+	rc.routes = append(rc.routes, pfx)
+	return nil
+}

ipn/ipnlocal/peerapi.go

@@ -32,7 +32,6 @@
 	"tailscale.com/health"
 	"tailscale.com/hostinfo"
 	"tailscale.com/ipn"
-	"tailscale.com/net/dns/resolver"
 	"tailscale.com/net/interfaces"
 	"tailscale.com/net/netaddr"
 	"tailscale.com/net/netutil"
@@ -51,9 +50,14 @@
 // ("cleartext" HTTP/2) support to the peerAPI.
 var addH2C func(*http.Server)
 
+// peerDNSQueryHandler is implemented by tsdns.Resolver.
+type peerDNSQueryHandler interface {
+	HandlePeerDNSQuery(context.Context, []byte, netip.AddrPort, func(name string) bool) (res []byte, err error)
+}
+
 type peerAPIServer struct {
 	b        *LocalBackend
-	resolver *resolver.Resolver
+	resolver peerDNSQueryHandler
 
 	taildrop *taildrop.Manager
 }
@@ -861,9 +865,9 @@ func (h *peerAPIHandler) replyToDNSQueries() bool {
 		return true
 	}
 	b := h.ps.b
-	if !b.OfferingExitNode() {
+	if !b.OfferingExitNode() && !b.OfferingAppConnector() {
-		// If we're not an exit node, there's no point to
+		// If we're not an exit node or app connector, there's
-		// being a DNS server for somebody.
+		// no point to being a DNS server for somebody.
 		return false
 	}
 	if !h.remoteAddr.IsValid() {
@@ -927,7 +931,7 @@ func (h *peerAPIHandler) handleDNSQuery(w http.ResponseWriter, r *http.Request)
 
 	ctx, cancel := context.WithTimeout(r.Context(), arbitraryTimeout)
 	defer cancel()
-	res, err := h.ps.resolver.HandleExitNodeDNSQuery(ctx, q, h.remoteAddr, h.ps.b.allowExitNodeDNSProxyToServeName)
+	res, err := h.ps.resolver.HandlePeerDNSQuery(ctx, q, h.remoteAddr, h.ps.b.allowExitNodeDNSProxyToServeName)
 	if err != nil {
 		h.logf("handleDNS fwd error: %v", err)
 		if err := ctx.Err(); err != nil {
@@ -937,6 +941,13 @@ func (h *peerAPIHandler) handleDNSQuery(w http.ResponseWriter, r *http.Request)
 		}
 		return
 	}
+	// TODO(raggi): consider pushing the integration down into the resolver
+	// instead to avoid re-parsing the DNS response for improved performance in
+	// the future.
+	if h.ps.b.OfferingAppConnector() {
+		h.ps.b.ObserveDNSResponse(res)
+	}
+
 	if pretty {
 		// Non-standard response for interactive debugging.
 		w.Header().Set("Content-Type", "application/json")

ipn/ipnlocal/peerapi_test.go

@@ -5,6 +5,7 @@
 
 import (
 	"bytes"
+	"context"
 	"fmt"
 	"io"
 	"io/fs"
@@ -14,11 +15,14 @@
 	"net/netip"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
 	"go4.org/netipx"
+	"golang.org/x/net/dns/dnsmessage"
+	"tailscale.com/appc"
 	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/store/mem"
@@ -680,3 +684,63 @@ func TestPeerAPIReplyToDNSQueries(t *testing.T) {
 		t.Errorf("unexpectedly IPv6 deny; wanted to be a DNS server")
 	}
 }
+
+func TestPeerAPIReplyToDNSQueriesAreObserved(t *testing.T) {
+	var h peerAPIHandler
+	h.remoteAddr = netip.MustParseAddrPort("100.150.151.152:12345")
+
+	rc := &routeCollector{}
+	eng, _ := wgengine.NewFakeUserspaceEngine(logger.Discard, 0)
+	pm := must.Get(newProfileManager(new(mem.Store), t.Logf))
+	h.ps = &peerAPIServer{
+		b: &LocalBackend{
+			e:            eng,
+			pm:           pm,
+			store:        pm.Store(),
+			appConnector: appc.NewEmbeddedAppConnector(t.Logf, rc),
+		},
+	}
+	h.ps.b.appConnector.UpdateDomains([]string{"example.com"})
+
+	h.ps.resolver = &fakeResolver{}
+	f := filter.NewAllowAllForTest(logger.Discard)
+	h.ps.b.setFilter(f)
+
+	if !h.ps.b.OfferingAppConnector() {
+		t.Fatal("expecting to be offering app connector")
+	}
+	if !h.replyToDNSQueries() {
+		t.Errorf("unexpectedly deny; wanted to be a DNS server")
+	}
+
+	w := httptest.NewRecorder()
+	h.handleDNSQuery(w, httptest.NewRequest("GET", "/dns-query?q=true&t=example.com.", nil))
+	if w.Code != http.StatusOK {
+		t.Errorf("unexpected status code: %v", w.Code)
+	}
+
+	wantRoutes := []netip.Prefix{netip.MustParsePrefix("192.0.0.8/32")}
+	if !slices.Equal(rc.routes, wantRoutes) {
+		t.Errorf("got %v; want %v", rc.routes, wantRoutes)
+	}
+}
+
+type fakeResolver struct{}
+
+func (f *fakeResolver) HandlePeerDNSQuery(ctx context.Context, q []byte, from netip.AddrPort, allowName func(name string) bool) (res []byte, err error) {
+	b := dnsmessage.NewBuilder(nil, dnsmessage.Header{})
+	b.EnableCompression()
+	b.StartAnswers()
+	b.AResource(
+		dnsmessage.ResourceHeader{
+			Name:  dnsmessage.MustNewName("example.com."),
+			Type:  dnsmessage.TypeA,
+			Class: dnsmessage.ClassINET,
+			TTL:   0,
+		},
+		dnsmessage.AResource{
+			A: [4]byte{192, 0, 0, 8},
+		},
+	)
+	return b.Finish()
+}

net/dns/resolver/tsdns.go

@@ -314,9 +314,9 @@ func parseExitNodeQuery(q []byte) *response {
 	return p.response()
 }
 
-// HandleExitNodeDNSQuery handles a DNS query that arrived from a peer
+// HandlePeerDNSQuery handles a DNS query that arrived from a peer
-// via the peerapi's DoH server. This is only used when the local
+// via the peerapi's DoH server. This is used when the local
-// node is being an exit node.
+// node is being an exit node or an app connector.
 //
 // The provided allowName callback is whether a DNS query for a name
 // (as found by parsing q) is allowed.
@@ -325,7 +325,7 @@ func parseExitNodeQuery(q []byte) *response {
 // still result in a response DNS packet (saying there's a failure)
 // and a nil error.
 // TODO: figure out if we even need an error result.
-func (r *Resolver) HandleExitNodeDNSQuery(ctx context.Context, q []byte, from netip.AddrPort, allowName func(name string) bool) (res []byte, err error) {
+func (r *Resolver) HandlePeerDNSQuery(ctx context.Context, q []byte, from netip.AddrPort, allowName func(name string) bool) (res []byte, err error) {
 	metricDNSExitProxyQuery.Add(1)
 	ch := make(chan packet, 1)