vnet: fix port mapping (w/ maisem + andrew)

Co-authored-by: Maisem Ali <maisem@tailscale.com>
Co-authored-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I703b39f05af2e3e1a979be8e77091586cb9ec3eb
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/integration/nat/nat_test.go

@@ -94,6 +94,13 @@ func easy(c *vnet.Config) *vnet.Node {
 		fmt.Sprintf("192.168.%d.1/24", n), vnet.EasyNAT))
 }
 
+func easyPMP(c *vnet.Config) *vnet.Node {
+	n := c.NumNodes() + 1
+	return c.AddNode(c.AddNetwork(
+		fmt.Sprintf("2.%d.%d.%d", n, n, n), // public IP
+		fmt.Sprintf("192.168.%d.1/24", n), vnet.EasyNAT, vnet.NATPMP))
+}
+
 func hard(c *vnet.Config) *vnet.Node {
 	n := c.NumNodes() + 1
 	return c.AddNode(c.AddNetwork(
@@ -161,7 +168,7 @@ func (nt *natTest) runTest(node1, node2 addNodeFunc) {
 
 		cmd := exec.Command("qemu-system-x86_64",
 			"-M", "microvm,isa-serial=off",
-			"-m", "1G",
+			"-m", "384M",
 			"-nodefaults", "-no-user-config", "-nographic",
 			"-kernel", nt.kernel,
 			"-append", "console=hvc0 root=PARTUUID=60c24cc1-f3f9-427a-8199-dd02023b0001/PARTNROFF=1 ro init=/gokrazy/init panic=10 oops=panic pci=off nousb tsc=unstable clocksource=hpet tailscale-tta=1",
@@ -252,7 +259,7 @@ func streamDaemonLogs(ctx context.Context, t testing.TB, c *vnet.NodeAgentClient
 			Text string `json:"text"`
 		}
 		if err := dec.Decode(&logEntry); err != nil {
-			if err == io.EOF {
+			if err == io.EOF || errors.Is(err, context.Canceled) {
 				return
 			}
 			t.Errorf("log entry: %v", err)
@@ -324,3 +331,8 @@ func TestEasyHardPMP(t *testing.T) {
 	nt := newNatTest(t)
 	nt.runTest(easy, hardPMP)
 }
+
+func TestEasyPMPHard(t *testing.T) {
+	nt := newNatTest(t)
+	nt.runTest(easyPMP, hard)
+}

tstest/natlab/vnet/vnet.go

@@ -412,10 +412,11 @@ type network struct {
 	ns     *stack.Stack
 	linkEP *channel.Endpoint
 
-	natStyle syncs.AtomicValue[NAT]
-	natMu    sync.Mutex // held while using + changing natTable
-	natTable NATTable
-	portMap  map[netip.AddrPort]portMapping // WAN ip:port -> LAN ip:port
+	natStyle    syncs.AtomicValue[NAT]
+	natMu       sync.Mutex // held while using + changing natTable
+	natTable    NATTable
+	portMap     map[netip.AddrPort]portMapping    // WAN ip:port -> LAN ip:port
+	portMapFlow map[portmapFlowKey]netip.AddrPort // (lanAP, peerWANAP) -> portmapped wanAP
 
 	// writeFunc is a map of MAC -> func to write to that MAC.
 	// It contains entries for connected nodes only.
@@ -1197,13 +1198,27 @@ func (s *Server) createDNSResponse(pkt gopacket.Packet) ([]byte, error) {
 // doNATOut performs NAT on an outgoing packet from src to dst, where
 // src is a LAN IP and dst is a WAN IP.
 //
-// It returns the souce WAN ip:port to use.
+// It returns the source WAN ip:port to use.
 func (n *network) doNATOut(src, dst netip.AddrPort) (newSrc netip.AddrPort) {
 	n.natMu.Lock()
 	defer n.natMu.Unlock()
+
+	// First see if there's a port mapping, before doing NAT.
+	if wanAP, ok := n.portMapFlow[portmapFlowKey{
+		peerWAN: dst,
+		lanAP:   src,
+	}]; ok {
+		return wanAP
+	}
+
 	return n.natTable.PickOutgoingSrc(src, dst, time.Now())
 }
 
+type portmapFlowKey struct {
+	peerWAN netip.AddrPort // the peer's WAN ip:port
+	lanAP   netip.AddrPort
+}
+
 // doNATIn performs NAT on an incoming packet from WAN src to WAN dst, returning
 // a new destination LAN ip:port to use.
 func (n *network) doNATIn(src, dst netip.AddrPort) (newDst netip.AddrPort) {
@@ -1215,6 +1230,10 @@ func (n *network) doNATIn(src, dst netip.AddrPort) (newDst netip.AddrPort) {
 	// First see if there's a port mapping, before doing NAT.
 	if lanAP, ok := n.portMap[dst]; ok {
 		if now.Before(lanAP.expiry) {
+			mak.Set(&n.portMapFlow, portmapFlowKey{
+				peerWAN: src,
+				lanAP:   lanAP.dst,
+			}, dst)
 			n.logf("XXX NAT: doNatIn: port mapping %v=>%v", dst, lanAP.dst)
 			return lanAP.dst
 		}