appc,cmd/sniproxy,ipn/ipnlocal: split sniproxy configuration code out of appc

The design changed during integration and testing, resulting in the
earlier implementation growing in the appc package to be intended now
only for the sniproxy implementation. That code is moved to it's final
location, and the current App Connector code is now renamed.

Updates tailscale/corp#15437

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

appc/appconnector_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 := NewAppConnector(t.Logf, nil)
+	a.UpdateDomains([]string{"example.com"})
+	if got, want := a.Domains().AsSlice(), []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 := NewAppConnector(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
+}