wgengine/netstack: implement netstack loopback (#13301)

When the TS_DEBUG_NETSTACK_LOOPBACK_PORT environment variable is set,
netstack will loop back (dnat to addressFamilyLoopback:loopbackPort)
TCP & UDP flows originally destined to localServicesIP:loopbackPort.
localServicesIP is quad-100 or the IPv6 equivalent.

Updates tailscale/corp#22713

Signed-off-by: Jordan Whited <jordan@tailscale.com>

tstest/integration/integration_test.go

@@ -23,6 +23,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"runtime"
+	"strconv"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -1204,6 +1205,149 @@ func TestDNSOverTCPIntervalResolver(t *testing.T) {
 	d1.MustCleanShutdown(t)
 }
 
+// TestNetstackTCPLoopback tests netstack loopback of a TCP stream, in both
+// directions.
+// TODO(jwhited): do the same for UDP
+func TestNetstackTCPLoopback(t *testing.T) {
+	tstest.Shard(t)
+	if os.Getuid() != 0 {
+		t.Skip("skipping when not root")
+	}
+
+	env := newTestEnv(t)
+	env.tunMode = true
+	loopbackPort := uint16(5201)
+	env.loopbackPort = &loopbackPort
+	loopbackPortStr := strconv.Itoa(int(loopbackPort))
+	n1 := newTestNode(t, env)
+	d1 := n1.StartDaemon()
+
+	n1.AwaitResponding()
+	n1.MustUp()
+
+	n1.AwaitIP4()
+	n1.AwaitRunning()
+
+	cases := []struct {
+		lisAddr  string
+		network  string
+		dialAddr string
+	}{
+		{
+			lisAddr:  net.JoinHostPort("127.0.0.1", loopbackPortStr),
+			network:  "tcp4",
+			dialAddr: net.JoinHostPort(tsaddr.TailscaleServiceIPString, loopbackPortStr),
+		},
+		{
+			lisAddr:  net.JoinHostPort("::1", loopbackPortStr),
+			network:  "tcp6",
+			dialAddr: net.JoinHostPort(tsaddr.TailscaleServiceIPv6String, loopbackPortStr),
+		},
+	}
+
+	writeBufSize := 128 << 10 // 128KiB, exercise GSO if enabled
+	writeBufIterations := 100 // allow TCP send window to open up
+	wantTotal := writeBufSize * writeBufIterations
+
+	for _, c := range cases {
+		lis, err := net.Listen(c.network, c.lisAddr)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer lis.Close()
+
+		writeFn := func(conn net.Conn) error {
+			for i := 0; i < writeBufIterations; i++ {
+				toWrite := make([]byte, writeBufSize)
+				var wrote int
+				for {
+					n, err := conn.Write(toWrite)
+					if err != nil {
+						return err
+					}
+					wrote += n
+					if wrote == len(toWrite) {
+						break
+					}
+				}
+			}
+			return nil
+		}
+
+		readFn := func(conn net.Conn) error {
+			var read int
+			for {
+				b := make([]byte, writeBufSize)
+				n, err := conn.Read(b)
+				if err != nil {
+					return err
+				}
+				read += n
+				if read == wantTotal {
+					return nil
+				}
+			}
+		}
+
+		lisStepCh := make(chan error)
+		go func() {
+			conn, err := lis.Accept()
+			if err != nil {
+				lisStepCh <- err
+				return
+			}
+			lisStepCh <- readFn(conn)
+			lisStepCh <- writeFn(conn)
+		}()
+
+		var conn net.Conn
+		err = tstest.WaitFor(time.Second*5, func() error {
+			conn, err = net.DialTimeout(c.network, c.dialAddr, time.Second*1)
+			if err != nil {
+				return err
+			}
+			return nil
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer conn.Close()
+
+		dialerStepCh := make(chan error)
+		go func() {
+			dialerStepCh <- writeFn(conn)
+			dialerStepCh <- readFn(conn)
+		}()
+
+		var (
+			dialerSteps int
+			lisSteps    int
+		)
+		for {
+			select {
+			case lisErr := <-lisStepCh:
+				if lisErr != nil {
+					t.Fatal(err)
+				}
+				lisSteps++
+				if dialerSteps == 2 && lisSteps == 2 {
+					return
+				}
+			case dialerErr := <-dialerStepCh:
+				if dialerErr != nil {
+					t.Fatal(err)
+				}
+				dialerSteps++
+				if dialerSteps == 2 && lisSteps == 2 {
+					return
+				}
+			}
+		}
+	}
+
+	d1.MustCleanShutdown(t)
+}
+
 // testEnv contains the test environment (set of servers) used by one
 // or more nodes.
 type testEnv struct {
@@ -1211,6 +1355,7 @@ type testEnv struct {
 	tunMode      bool
 	cli          string
 	daemon       string
+	loopbackPort *uint16
 
 	LogCatcher       *LogCatcher
 	LogCatcherServer *httptest.Server
@@ -1511,6 +1656,9 @@ func (n *testNode) StartDaemonAsIPNGOOS(ipnGOOS string) *Daemon {
 		"TS_DISABLE_PORTMAPPER=1", // shouldn't be needed; test is all localhost
 		"TS_DEBUG_LOG_RATE=all",
 	)
+	if n.env.loopbackPort != nil {
+		cmd.Env = append(cmd.Env, "TS_DEBUG_NETSTACK_LOOPBACK_PORT="+strconv.Itoa(int(*n.env.loopbackPort)))
+	}
 	if version.IsRace() {
 		cmd.Env = append(cmd.Env, "GORACE=halt_on_error=1")
 	}

wgengine/netstack/netstack.go

@@ -187,6 +187,11 @@ type Impl struct {
 	dns           *dns.Manager
 	driveForLocal drive.FileSystemForLocal // or nil
 
+	// loopbackPort, if non-nil, will enable Impl to loop back (dnat to
+	// <address-family-loopback>:loopbackPort) TCP & UDP flows originally
+	// destined to serviceIP{v6}:loopbackPort.
+	loopbackPort *int
+
 	peerapiPort4Atomic atomic.Uint32 // uint16 port number for IPv4 peerapi
 	peerapiPort6Atomic atomic.Uint32 // uint16 port number for IPv6 peerapi
 
@@ -378,6 +383,10 @@ func Create(logf logger.Logf, tundev *tstun.Wrapper, e wgengine.Engine, mc *magi
 		dns:                   dns,
 		driveForLocal:         driveForLocal,
 	}
+	loopbackPort, ok := envknob.LookupInt("TS_DEBUG_NETSTACK_LOOPBACK_PORT")
+	if ok && loopbackPort >= 0 && loopbackPort <= math.MaxUint16 {
+		ns.loopbackPort = &loopbackPort
+	}
 	ns.ctx, ns.ctxCancel = context.WithCancel(context.Background())
 	ns.atomicIsLocalIPFunc.Store(ipset.FalseContainsIPFunc())
 	ns.tundev.PostFilterPacketInboundFromWireGuard = ns.injectInbound
@@ -706,6 +715,13 @@ func (ns *Impl) UpdateNetstackIPs(nm *netmap.NetworkMap) {
 	}
 }
 
+func (ns *Impl) isLoopbackPort(port uint16) bool {
+	if ns.loopbackPort != nil && int(port) == *ns.loopbackPort {
+		return true
+	}
+	return false
+}
+
 // handleLocalPackets is hooked into the tun datapath for packets leaving
 // the host and arriving at tailscaled. This method returns filter.DropSilently
 // to intercept a packet for handling, for instance traffic to quad-100.
@@ -724,11 +740,11 @@ func (ns *Impl) handleLocalPackets(p *packet.Parsed, t *tstun.Wrapper) filter.Re
 		// 80, and 8080.
 		switch p.IPProto {
 		case ipproto.TCP:
-			if port := p.Dst.Port(); port != 53 && port != 80 && port != 8080 {
+			if port := p.Dst.Port(); port != 53 && port != 80 && port != 8080 && !ns.isLoopbackPort(port) {
 				return filter.Accept
 			}
 		case ipproto.UDP:
-			if port := p.Dst.Port(); port != 53 {
+			if port := p.Dst.Port(); port != 53 && !ns.isLoopbackPort(port) {
 				return filter.Accept
 			}
 		}
@@ -1169,6 +1185,11 @@ func netaddrIPFromNetstackIP(s tcpip.Address) netip.Addr {
 	return netip.Addr{}
 }
 
+var (
+	ipv4Loopback = netip.MustParseAddr("127.0.0.1")
+	ipv6Loopback = netip.MustParseAddr("::1")
+)
+
 func (ns *Impl) acceptTCP(r *tcp.ForwarderRequest) {
 	reqDetails := r.ID()
 	if debugNetstack() {
@@ -1305,8 +1326,15 @@ func (ns *Impl) acceptTCP(r *tcp.ForwarderRequest) {
 			return
 		}
 	}
-	if isTailscaleIP {
+	switch {
-		dialIP = netaddr.IPv4(127, 0, 0, 1)
+	case hittingServiceIP && ns.isLoopbackPort(reqDetails.LocalPort):
+		if dialIP == serviceIPv6 {
+			dialIP = ipv6Loopback
+		} else {
+			dialIP = ipv4Loopback
+		}
+	case isTailscaleIP:
+		dialIP = ipv4Loopback
 	}
 	dialAddr := netip.AddrPortFrom(dialIP, uint16(reqDetails.LocalPort))
 
@@ -1457,16 +1485,23 @@ func (ns *Impl) acceptUDP(r *udp.ForwarderRequest) {
 		return
 	}
 
-	// Handle magicDNS traffic (via UDP) here.
+	// Handle magicDNS and loopback traffic (via UDP) here.
 	if dst := dstAddr.Addr(); dst == serviceIP || dst == serviceIPv6 {
-		if dstAddr.Port() != 53 {
+		switch {
-			ep.Close()
+		case dstAddr.Port() == 53:
-			return // Only MagicDNS traffic runs on the service IPs for now.
-		}
-
 			c := gonet.NewUDPConn(&wq, ep)
 			go ns.handleMagicDNSUDP(srcAddr, c)
 			return
+		case ns.isLoopbackPort(dstAddr.Port()):
+			if dst == serviceIPv6 {
+				dstAddr = netip.AddrPortFrom(ipv6Loopback, dstAddr.Port())
+			} else {
+				dstAddr = netip.AddrPortFrom(ipv4Loopback, dstAddr.Port())
+			}
+		default:
+			ep.Close()
+			return // Only MagicDNS and loopback traffic runs on the service IPs for now.
+		}
 	}
 
 	if get := ns.GetUDPHandlerForFlow; get != nil {
@@ -1545,9 +1580,17 @@ func (ns *Impl) forwardUDP(client *gonet.UDPConn, clientAddr, dstAddr netip.Addr
 	var backendListenAddr *net.UDPAddr
 	var backendRemoteAddr *net.UDPAddr
 	isLocal := ns.isLocalIP(dstAddr.Addr())
+	isLoopback := dstAddr.Addr() == ipv4Loopback || dstAddr.Addr() == ipv6Loopback
 	if isLocal {
 		backendRemoteAddr = &net.UDPAddr{IP: net.ParseIP("127.0.0.1"), Port: int(port)}
 		backendListenAddr = &net.UDPAddr{IP: net.ParseIP("127.0.0.1"), Port: int(srcPort)}
+	} else if isLoopback {
+		ip := net.IP(ipv4Loopback.AsSlice())
+		if dstAddr.Addr() == ipv6Loopback {
+			ip = ipv6Loopback.AsSlice()
+		}
+		backendRemoteAddr = &net.UDPAddr{IP: ip, Port: int(port)}
+		backendListenAddr = &net.UDPAddr{IP: ip, Port: int(srcPort)}
 	} else {
 		if dstIP := dstAddr.Addr(); viaRange.Contains(dstIP) {
 			dstAddr = netip.AddrPortFrom(tsaddr.UnmapVia(dstIP), dstAddr.Port())