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/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
+}