tailscale/tstest/natlab/vnet/vnet.go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause

// Package vnet simulates a virtual Internet containing a set of networks with various
// NAT behaviors. You can then plug VMs into the virtual internet at different points
// to test Tailscale working end-to-end in various network conditions.
//
// See https://github.com/tailscale/tailscale/issues/13038
package vnet

// TODO:
// - [ ] port mapping actually working
// - [ ] conf to let you firewall things
// - [ ] tests for NAT tables

import (
	"bufio"
	"bytes"
	"context"
	"crypto/tls"
	"encoding/binary"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log"
	"math/rand/v2"
	"net"
	"net/http"
	"net/http/httptest"
	"net/netip"
	"os/exec"
	"strconv"
	"sync"
	"sync/atomic"
	"time"

	"github.com/google/gopacket"
	"github.com/google/gopacket/layers"
	"go4.org/mem"
	"gvisor.dev/gvisor/pkg/buffer"
	"gvisor.dev/gvisor/pkg/tcpip"
	"gvisor.dev/gvisor/pkg/tcpip/adapters/gonet"
	"gvisor.dev/gvisor/pkg/tcpip/header"
	"gvisor.dev/gvisor/pkg/tcpip/link/channel"
	"gvisor.dev/gvisor/pkg/tcpip/network/arp"
	"gvisor.dev/gvisor/pkg/tcpip/network/ipv4"
	"gvisor.dev/gvisor/pkg/tcpip/stack"
	"gvisor.dev/gvisor/pkg/tcpip/transport/icmp"
	"gvisor.dev/gvisor/pkg/tcpip/transport/tcp"
	"gvisor.dev/gvisor/pkg/waiter"
	"tailscale.com/client/tailscale"
	"tailscale.com/derp"
	"tailscale.com/derp/derphttp"
	"tailscale.com/net/netutil"
	"tailscale.com/net/stun"
	"tailscale.com/syncs"
	"tailscale.com/tailcfg"
	"tailscale.com/tstest/integration/testcontrol"
	"tailscale.com/types/key"
	"tailscale.com/types/logger"
	"tailscale.com/util/mak"
	"tailscale.com/util/must"
	"tailscale.com/util/set"
	"tailscale.com/util/zstdframe"
)

const nicID = 1
const stunPort = 3478

func (s *Server) PopulateDERPMapIPs() error {
	out, err := exec.Command("tailscale", "debug", "derp-map").Output()
	if err != nil {
		return fmt.Errorf("tailscale debug derp-map: %v", err)
	}
	var dm tailcfg.DERPMap
	if err := json.Unmarshal(out, &dm); err != nil {
		return fmt.Errorf("unmarshal DERPMap: %v", err)
	}
	for _, r := range dm.Regions {
		for _, n := range r.Nodes {
			if n.IPv4 != "" {
				s.derpIPs.Add(netip.MustParseAddr(n.IPv4))
			}
		}
	}
	return nil
}

func (n *network) InitNAT(natType NAT) error {
	ctor, ok := natTypes[natType]
	if !ok {
		return fmt.Errorf("unknown NAT type %q", natType)
	}
	t, err := ctor(n)
	if err != nil {
		return fmt.Errorf("error creating NAT type %q for network %v: %w", natType, n.wanIP, err)
	}
	n.setNATTable(t)
	n.natStyle.Store(natType)
	return nil
}

func (n *network) setNATTable(nt NATTable) {
	n.natMu.Lock()
	defer n.natMu.Unlock()
	n.natTable = nt
}

// SoleLANIP implements [IPPool].
func (n *network) SoleLANIP() (netip.Addr, bool) {
	if len(n.nodesByIP) != 1 {
		return netip.Addr{}, false
	}
	for ip := range n.nodesByIP {
		return ip, true
	}
	return netip.Addr{}, false
}

// WANIP implements [IPPool].
func (n *network) WANIP() netip.Addr { return n.wanIP }

func (n *network) initStack() error {
	n.ns = stack.New(stack.Options{
		NetworkProtocols: []stack.NetworkProtocolFactory{
			ipv4.NewProtocol,
			arp.NewProtocol,
		},
		TransportProtocols: []stack.TransportProtocolFactory{
			tcp.NewProtocol,
			icmp.NewProtocol4,
		},
	})
	sackEnabledOpt := tcpip.TCPSACKEnabled(true) // TCP SACK is disabled by default
	tcpipErr := n.ns.SetTransportProtocolOption(tcp.ProtocolNumber, &sackEnabledOpt)
	if tcpipErr != nil {
		return fmt.Errorf("SetTransportProtocolOption SACK: %v", tcpipErr)
	}
	n.linkEP = channel.New(512, 1500, tcpip.LinkAddress(n.mac.HWAddr()))
	if tcpipProblem := n.ns.CreateNIC(nicID, n.linkEP); tcpipProblem != nil {
		return fmt.Errorf("CreateNIC: %v", tcpipProblem)
	}
	n.ns.SetPromiscuousMode(nicID, true)
	n.ns.SetSpoofing(nicID, true)

	prefix := tcpip.AddrFrom4Slice(n.lanIP.Addr().AsSlice()).WithPrefix()
	prefix.PrefixLen = n.lanIP.Bits()
	if tcpProb := n.ns.AddProtocolAddress(nicID, tcpip.ProtocolAddress{
		Protocol:          ipv4.ProtocolNumber,
		AddressWithPrefix: prefix,
	}, stack.AddressProperties{}); tcpProb != nil {
		return errors.New(tcpProb.String())
	}

	ipv4Subnet, err := tcpip.NewSubnet(tcpip.AddrFromSlice(make([]byte, 4)), tcpip.MaskFromBytes(make([]byte, 4)))
	if err != nil {
		return fmt.Errorf("could not create IPv4 subnet: %v", err)
	}
	n.ns.SetRouteTable([]tcpip.Route{
		{
			Destination: ipv4Subnet,
			NIC:         nicID,
		},
	})

	const tcpReceiveBufferSize = 0 // default
	const maxInFlightConnectionAttempts = 8192
	tcpFwd := tcp.NewForwarder(n.ns, tcpReceiveBufferSize, maxInFlightConnectionAttempts, n.acceptTCP)
	n.ns.SetTransportProtocolHandler(tcp.ProtocolNumber, func(tei stack.TransportEndpointID, pb *stack.PacketBuffer) (handled bool) {
		return tcpFwd.HandlePacket(tei, pb)
	})

	go func() {
		for {
			pkt := n.linkEP.ReadContext(n.s.shutdownCtx)
			if pkt == nil {
				if n.s.shutdownCtx.Err() != nil {
					// Return without logging.
					return
				}
				continue
			}

			ipRaw := pkt.ToView().AsSlice()
			goPkt := gopacket.NewPacket(
				ipRaw,
				layers.LayerTypeIPv4, gopacket.Lazy)
			layerV4 := goPkt.Layer(layers.LayerTypeIPv4).(*layers.IPv4)

			dstIP, _ := netip.AddrFromSlice(layerV4.DstIP)
			node, ok := n.nodesByIP[dstIP]
			if !ok {
				log.Printf("no MAC for dest IP %v", dstIP)
				continue
			}
			eth := &layers.Ethernet{
				SrcMAC:       n.mac.HWAddr(),
				DstMAC:       node.mac.HWAddr(),
				EthernetType: layers.EthernetTypeIPv4,
			}
			buffer := gopacket.NewSerializeBuffer()
			options := gopacket.SerializeOptions{FixLengths: true, ComputeChecksums: true}
			sls := []gopacket.SerializableLayer{
				eth,
			}
			for _, layer := range goPkt.Layers() {
				sl, ok := layer.(gopacket.SerializableLayer)
				if !ok {
					log.Fatalf("layer %s is not serializable", layer.LayerType().String())
				}
				switch gl := layer.(type) {
				case *layers.TCP:
					gl.SetNetworkLayerForChecksum(layerV4)
				case *layers.UDP:
					gl.SetNetworkLayerForChecksum(layerV4)
				}
				sls = append(sls, sl)
			}

			if err := gopacket.SerializeLayers(buffer, options, sls...); err != nil {
				log.Printf("Serialize error: %v", err)
				continue
			}
			if writeFunc, ok := n.writeFunc.Load(node.mac); ok {
				writeFunc(buffer.Bytes())
			} else {
				log.Printf("No writeFunc for %v", node.mac)
			}
		}
	}()
	return nil
}

func netaddrIPFromNetstackIP(s tcpip.Address) netip.Addr {
	switch s.Len() {
	case 4:
		return netip.AddrFrom4(s.As4())
	case 16:
		return netip.AddrFrom16(s.As16()).Unmap()
	}
	return netip.Addr{}
}

func stringifyTEI(tei stack.TransportEndpointID) string {
	localHostPort := net.JoinHostPort(tei.LocalAddress.String(), strconv.Itoa(int(tei.LocalPort)))
	remoteHostPort := net.JoinHostPort(tei.RemoteAddress.String(), strconv.Itoa(int(tei.RemotePort)))
	return fmt.Sprintf("%s -> %s", remoteHostPort, localHostPort)
}

func (n *network) acceptTCP(r *tcp.ForwarderRequest) {
	reqDetails := r.ID()

	clientRemoteIP := netaddrIPFromNetstackIP(reqDetails.RemoteAddress)
	destIP := netaddrIPFromNetstackIP(reqDetails.LocalAddress)
	destPort := reqDetails.LocalPort
	if !clientRemoteIP.IsValid() {
		r.Complete(true) // sends a RST
		return
	}

	var wq waiter.Queue
	ep, err := r.CreateEndpoint(&wq)
	if err != nil {
		log.Printf("CreateEndpoint error for %s: %v", stringifyTEI(reqDetails), err)
		r.Complete(true) // sends a RST
		return
	}
	ep.SocketOptions().SetKeepAlive(true)

	if destPort == 123 {
		r.Complete(false)
		tc := gonet.NewTCPConn(&wq, ep)
		io.WriteString(tc, "Hello from Go\nGoodbye.\n")
		tc.Close()
		return
	}

	if destPort == 124 {
		node, ok := n.nodesByIP[clientRemoteIP]
		if !ok {
			log.Printf("no node for TCP 124 connection from %v", clientRemoteIP)
			r.Complete(true)
			return
		}
		r.Complete(false)
		tc := gonet.NewTCPConn(&wq, ep)

		go func() {
			defer tc.Close()
			bs := bufio.NewScanner(tc)
			for bs.Scan() {
				line := bs.Text()
				log.Printf("LOG from %v: %s", node, line)
			}
		}()
		return
	}

	if destPort == 8008 && destIP == fakeTestAgentIP {
		r.Complete(false)
		tc := gonet.NewTCPConn(&wq, ep)
		node := n.nodesByIP[clientRemoteIP]
		ac := &agentConn{node, tc}
		n.s.addIdleAgentConn(ac)
		return
	}

	if destPort == 80 && destIP == fakeControlIP {
		r.Complete(false)
		tc := gonet.NewTCPConn(&wq, ep)
		hs := &http.Server{Handler: n.s.control}
		go hs.Serve(netutil.NewOneConnListener(tc, nil))
		return
	}

	if destPort == 443 && (destIP == fakeDERP1IP || destIP == fakeDERP2IP) {
		ds := n.s.derps[0]
		if destIP == fakeDERP2IP {
			ds = n.s.derps[1]
		}

		r.Complete(false)
		tc := gonet.NewTCPConn(&wq, ep)
		tlsConn := tls.Server(tc, ds.tlsConfig)
		hs := &http.Server{Handler: ds.handler}
		go hs.Serve(netutil.NewOneConnListener(tlsConn, nil))
		return
	}
	if destPort == 80 && (destIP == fakeDERP1IP || destIP == fakeDERP2IP) {
		r.Complete(false)
		tc := gonet.NewTCPConn(&wq, ep)
		hs := &http.Server{Handler: n.s.derps[0].handler}
		go hs.Serve(netutil.NewOneConnListener(tc, nil))
		return
	}
	if destPort == 443 && destIP == fakeLogCatcherIP {

		r.Complete(false)
		tc := gonet.NewTCPConn(&wq, ep)
		go n.serveLogCatcherConn(clientRemoteIP, tc)
		return
	}

	log.Printf("vnet-AcceptTCP: %v", stringifyTEI(reqDetails))

	var targetDial string
	if n.s.derpIPs.Contains(destIP) {
		targetDial = destIP.String() + ":" + strconv.Itoa(int(destPort))
	} else if destIP == fakeProxyControlplaneIP {
		targetDial = "controlplane.tailscale.com:" + strconv.Itoa(int(destPort))
	}
	if targetDial != "" {
		c, err := net.Dial("tcp", targetDial)
		if err != nil {
			r.Complete(true)
			log.Printf("Dial controlplane: %v", err)
			return
		}
		defer c.Close()
		tc := gonet.NewTCPConn(&wq, ep)
		defer tc.Close()
		r.Complete(false)
		errc := make(chan error, 2)
		go func() { _, err := io.Copy(tc, c); errc <- err }()
		go func() { _, err := io.Copy(c, tc); errc <- err }()
		<-errc
	} else {
		r.Complete(true) // sends a RST
	}
}

// serveLogCatchConn serves a TCP connection to "log.tailscale.io", speaking the
// logtail/logcatcher protocol.
//
// We terminate TLS with an arbitrary cert; the client is configured to not
// validate TLS certs for this hostname when running under these integration
// tests.
func (n *network) serveLogCatcherConn(clientRemoteIP netip.Addr, c net.Conn) {
	tlsConfig := n.s.derps[0].tlsConfig // self-signed (stealing DERP's); test client configure to not check
	tlsConn := tls.Server(c, tlsConfig)
	var handler http.Handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		all, _ := io.ReadAll(r.Body)
		if r.Header.Get("Content-Encoding") == "zstd" {
			var err error
			all, err = zstdframe.AppendDecode(nil, all)
			if err != nil {
				log.Printf("LOGS DECODE ERROR zstd decode: %v", err)
				http.Error(w, "zstd decode error", http.StatusBadRequest)
				return
			}
		}
		var logs []struct {
			Logtail struct {
				Client_Time time.Time
			}
			Text string
		}
		if err := json.Unmarshal(all, &logs); err != nil {
			log.Printf("Logs decode error: %v", err)
			return
		}
		node := n.nodesByIP[clientRemoteIP]
		if node != nil {
			node.logMu.Lock()
			defer node.logMu.Unlock()
			node.logCatcherWrites++
			for _, lg := range logs {
				tStr := lg.Logtail.Client_Time.Round(time.Millisecond).Format(time.RFC3339Nano)
				fmt.Fprintf(&node.logBuf, "[%v] %s\n", tStr, lg.Text)
			}
		}
	})
	hs := &http.Server{Handler: handler}
	hs.Serve(netutil.NewOneConnListener(tlsConn, nil))
}

var (
	fakeDNSIP               = netip.AddrFrom4([4]byte{4, 11, 4, 11})
	fakeProxyControlplaneIP = netip.AddrFrom4([4]byte{52, 52, 0, 1}) // real controlplane.tailscale.com proxy
	fakeTestAgentIP         = netip.AddrFrom4([4]byte{52, 52, 0, 2})
	fakeControlIP           = netip.AddrFrom4([4]byte{52, 52, 0, 3}) // 3=C for "Control"
	fakeDERP1IP             = netip.AddrFrom4([4]byte{33, 4, 0, 1})  // 3340=DERP; 1=derp 1
	fakeDERP2IP             = netip.AddrFrom4([4]byte{33, 4, 0, 2})  // 3340=DERP; 1=derp 1
	fakeLogCatcherIP        = netip.AddrFrom4([4]byte{52, 52, 0, 4})
	fakeSyslogIP            = netip.AddrFrom4([4]byte{52, 52, 0, 9})
)

type EthernetPacket struct {
	le *layers.Ethernet
	gp gopacket.Packet
}

func (ep EthernetPacket) SrcMAC() MAC {
	return MAC(ep.le.SrcMAC)
}

func (ep EthernetPacket) DstMAC() MAC {
	return MAC(ep.le.DstMAC)
}

type MAC [6]byte

func (m MAC) IsBroadcast() bool {
	return m == MAC{0xff, 0xff, 0xff, 0xff, 0xff, 0xff}
}

func macOf(hwa net.HardwareAddr) (_ MAC, ok bool) {
	if len(hwa) != 6 {
		return MAC{}, false
	}
	return MAC(hwa), true
}

func (m MAC) HWAddr() net.HardwareAddr {
	return net.HardwareAddr(m[:])
}

func (m MAC) String() string {
	return fmt.Sprintf("%02x:%02x:%02x:%02x:%02x:%02x", m[0], m[1], m[2], m[3], m[4], m[5])
}

type portMapping struct {
	dst    netip.AddrPort // LAN IP:port
	expiry time.Time
}

type network struct {
	s              *Server
	mac            MAC
	portmap        bool
	lanInterfaceID int
	wanInterfaceID int
	wanIP          netip.Addr
	lanIP          netip.Prefix // with host bits set (e.g. 192.168.2.1/24)
	nodesByIP      map[netip.Addr]*node
	logf           func(format string, args ...any)

	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
	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.
	writeFunc syncs.Map[MAC, func([]byte)] // MAC -> func to write to that MAC
}

func (n *network) registerWriter(mac MAC, f func([]byte)) {
	if f != nil {
		n.writeFunc.Store(mac, f)
	} else {
		n.writeFunc.Delete(mac)
	}
}

func (n *network) MACOfIP(ip netip.Addr) (_ MAC, ok bool) {
	if n.lanIP.Addr() == ip {
		return n.mac, true
	}
	if n, ok := n.nodesByIP[ip]; ok {
		return n.mac, true
	}
	return MAC{}, false
}

type node struct {
	mac           MAC
	num           int // 1-based node number
	interfaceID   int
	net           *network
	lanIP         netip.Addr // must be in net.lanIP prefix + unique in net
	verboseSyslog bool

	// logMu guards logBuf.
	// TODO(bradfitz): conditionally write these out to separate files at the end?
	// Currently they only hold logcatcher logs.
	logMu            sync.Mutex
	logBuf           bytes.Buffer
	logCatcherWrites int
}

// String returns the string "nodeN" where N is the 1-based node number.
func (n *node) String() string {
	return fmt.Sprintf("node%d", n.num)
}

type derpServer struct {
	srv       *derp.Server
	handler   http.Handler
	tlsConfig *tls.Config
}

func newDERPServer() *derpServer {
	// Just to get a self-signed TLS cert:
	ts := httptest.NewTLSServer(nil)
	ts.Close()

	ds := &derpServer{
		srv:       derp.NewServer(key.NewNode(), logger.Discard),
		tlsConfig: ts.TLS, // self-signed; test client configure to not check
	}
	var mux http.ServeMux
	mux.Handle("/derp", derphttp.Handler(ds.srv))
	mux.HandleFunc("/generate_204", derphttp.ServeNoContent)

	ds.handler = &mux
	return ds
}

type Server struct {
	shutdownCtx    context.Context
	shutdownCancel context.CancelFunc
	shuttingDown   atomic.Bool
	wg             sync.WaitGroup
	blendReality   bool

	derpIPs set.Set[netip.Addr]

	nodes        []*node
	nodeByMAC    map[MAC]*node
	networks     set.Set[*network]
	networkByWAN map[netip.Addr]*network

	control    *testcontrol.Server
	derps      []*derpServer
	pcapWriter *pcapWriter

	mu              sync.Mutex
	agentConnWaiter map[*node]chan<- struct{} // signaled after added to set
	agentConns      set.Set[*agentConn]       //  not keyed by node; should be small/cheap enough to scan all
	agentDialer     map[*node]DialFunc
}

type DialFunc func(ctx context.Context, network, address string) (net.Conn, error)

var derpMap = &tailcfg.DERPMap{
	Regions: map[int]*tailcfg.DERPRegion{
		1: {
			RegionID:   1,
			RegionCode: "atlantis",
			RegionName: "Atlantis",
			Nodes: []*tailcfg.DERPNode{
				{
					Name:             "1a",
					RegionID:         1,
					HostName:         "derp1.tailscale",
					IPv4:             fakeDERP1IP.String(),
					InsecureForTests: true,
					CanPort80:        true,
				},
			},
		},
		2: {
			RegionID:   2,
			RegionCode: "northpole",
			RegionName: "North Pole",
			Nodes: []*tailcfg.DERPNode{
				{
					Name:             "2a",
					RegionID:         2,
					HostName:         "derp2.tailscale",
					IPv4:             fakeDERP2IP.String(),
					InsecureForTests: true,
					CanPort80:        true,
				},
			},
		},
	},
}

func New(c *Config) (*Server, error) {
	ctx, cancel := context.WithCancel(context.Background())
	s := &Server{
		shutdownCtx:    ctx,
		shutdownCancel: cancel,

		control: &testcontrol.Server{
			DERPMap:         derpMap,
			ExplicitBaseURL: "http://control.tailscale",
		},

		derpIPs: set.Of[netip.Addr](),

		nodeByMAC:    map[MAC]*node{},
		networkByWAN: map[netip.Addr]*network{},
		networks:     set.Of[*network](),
	}
	for range 2 {
		s.derps = append(s.derps, newDERPServer())
	}
	if err := s.initFromConfig(c); err != nil {
		return nil, err
	}
	for n := range s.networks {
		if err := n.initStack(); err != nil {
			return nil, fmt.Errorf("newServer: initStack: %v", err)
		}
	}

	return s, nil
}

func (s *Server) Close() {
	if shutdown := s.shuttingDown.Swap(true); !shutdown {
		s.shutdownCancel()
		s.pcapWriter.Close()
	}
	s.wg.Wait()
}

func (s *Server) HWAddr(mac MAC) net.HardwareAddr {
	// TODO: cache
	return net.HardwareAddr(mac[:])
}

// IPv4ForDNS returns the IP address for the given DNS query name (for IPv4 A
// queries only).
func (s *Server) IPv4ForDNS(qname string) (netip.Addr, bool) {
	switch qname {
	case "dns":
		return fakeDNSIP, true
	case "log.tailscale.io":
		return fakeLogCatcherIP, true
	case "test-driver.tailscale":
		return fakeTestAgentIP, true
	case "controlplane.tailscale.com":
		return fakeProxyControlplaneIP, true
	case "control.tailscale":
		return fakeControlIP, true
	case "derp1.tailscale":
		return fakeDERP1IP, true
	case "derp2.tailscale":
		return fakeDERP2IP, true
	}
	return netip.Addr{}, false
}

type Protocol int

const (
	ProtocolQEMU      = Protocol(iota + 1)
	ProtocolUnixDGRAM // for macOS Hypervisor.Framework and VZFileHandleNetworkDeviceAttachment
)

// serveConn serves a single connection from a client.
func (s *Server) ServeUnixConn(uc *net.UnixConn, proto Protocol) {
	if s.shuttingDown.Load() {
		return
	}
	s.wg.Add(1)
	defer s.wg.Done()
	context.AfterFunc(s.shutdownCtx, func() {
		uc.SetDeadline(time.Now())
	})
	log.Printf("Got conn %T %p", uc, uc)
	defer uc.Close()

	bw := bufio.NewWriterSize(uc, 2<<10)
	var writeMu sync.Mutex
	var srcNode *node
	writePkt := func(pkt []byte) {
		if pkt == nil {
			return
		}
		writeMu.Lock()
		defer writeMu.Unlock()
		if proto == ProtocolQEMU {
			hdr := binary.BigEndian.AppendUint32(bw.AvailableBuffer()[:0], uint32(len(pkt)))
			if _, err := bw.Write(hdr); err != nil {
				log.Printf("Write hdr: %v", err)
				return
			}
		}
		if _, err := bw.Write(pkt); err != nil {
			log.Printf("Write pkt: %v", err)
			return
		}
		if err := bw.Flush(); err != nil {
			log.Printf("Flush: %v", err)
		}
		must.Do(s.pcapWriter.WritePacket(gopacket.CaptureInfo{
			Timestamp:      time.Now(),
			CaptureLength:  len(pkt),
			Length:         len(pkt),
			InterfaceIndex: srcNode.interfaceID,
		}, pkt))
	}

	buf := make([]byte, 16<<10)
	var netw *network // non-nil after first packet
	for {
		var packetRaw []byte
		if proto == ProtocolUnixDGRAM {
			n, _, err := uc.ReadFromUnix(buf)
			if err != nil {
				log.Printf("ReadFromUnix: %v", err)
				continue
			}
			packetRaw = buf[:n]
		} else if proto == ProtocolQEMU {
			if _, err := io.ReadFull(uc, buf[:4]); err != nil {
				if s.shutdownCtx.Err() != nil {
					// Return without logging.
					return
				}
				log.Printf("ReadFull header: %v", err)
				return
			}
			n := binary.BigEndian.Uint32(buf[:4])

			if _, err := io.ReadFull(uc, buf[4:4+n]); err != nil {
				if s.shutdownCtx.Err() != nil {
					// Return without logging.
					return
				}
				log.Printf("ReadFull pkt: %v", err)
				return
			}
			packetRaw = buf[4 : 4+n] // raw ethernet frame
		}

		packet := gopacket.NewPacket(packetRaw, layers.LayerTypeEthernet, gopacket.Lazy)
		le, ok := packet.LinkLayer().(*layers.Ethernet)
		if !ok || len(le.SrcMAC) != 6 || len(le.DstMAC) != 6 {
			continue
		}
		ep := EthernetPacket{le, packet}

		srcMAC := ep.SrcMAC()
		if srcNode == nil {
			srcNode, ok = s.nodeByMAC[srcMAC]
			if !ok {
				log.Printf("[conn %p] ignoring frame from unknown MAC %v", uc, srcMAC)
				continue
			}
			log.Printf("[conn %p] MAC %v is node %v", uc, srcMAC, srcNode.lanIP)
			netw = srcNode.net
			netw.registerWriter(srcMAC, writePkt)
			defer netw.registerWriter(srcMAC, nil)
		} else {
			if srcMAC != srcNode.mac {
				log.Printf("[conn %p] ignoring frame from MAC %v, expected %v", uc, srcMAC, srcNode.mac)
				continue
			}
		}
		must.Do(s.pcapWriter.WritePacket(gopacket.CaptureInfo{
			Timestamp:      time.Now(),
			CaptureLength:  len(packetRaw),
			Length:         len(packetRaw),
			InterfaceIndex: srcNode.interfaceID,
		}, packetRaw))
		netw.HandleEthernetPacket(ep)
	}
}

func (s *Server) routeUDPPacket(up UDPPacket) {
	// Find which network owns this based on the destination IP
	// and all the known networks' wan IPs.

	// But certain things (like STUN) we do in-process.
	if up.Dst.Port() == stunPort {
		// TODO(bradfitz): fake latency; time.AfterFunc the response
		if res, ok := makeSTUNReply(up); ok {
			//log.Printf("STUN reply: %+v", res)
			s.routeUDPPacket(res)
		} else {
			log.Printf("weird: STUN packet not handled")
		}
		return
	}

	dstIP := up.Dst.Addr()
	netw, ok := s.networkByWAN[dstIP]
	if !ok {
		if dstIP.IsPrivate() {
			// Not worth spamming logs. RFC 1918 space doesn't route.
			return
		}
		log.Printf("no network to route UDP packet for %v", up.Dst)
		return
	}
	netw.HandleUDPPacket(up)
}

// writeEth writes a raw Ethernet frame to all (0, 1, or multiple) connected
// clients on the network.
//
// This only delivers to client devices and not the virtual router/gateway
// device.
func (n *network) writeEth(res []byte) {
	if len(res) < 12 {
		return
	}
	dstMAC := MAC(res[0:6])
	srcMAC := MAC(res[6:12])
	if dstMAC.IsBroadcast() {
		n.writeFunc.Range(func(mac MAC, writeFunc func([]byte)) bool {
			writeFunc(res)
			return true
		})
		return
	}
	if srcMAC == dstMAC {
		n.logf("dropping write of packet from %v to itself", srcMAC)
		return
	}
	if writeFunc, ok := n.writeFunc.Load(dstMAC); ok {
		writeFunc(res)
		return
	}
}

func (n *network) HandleEthernetPacket(ep EthernetPacket) {
	packet := ep.gp
	dstMAC := ep.DstMAC()
	isBroadcast := dstMAC.IsBroadcast()
	forRouter := dstMAC == n.mac || isBroadcast

	switch ep.le.EthernetType {
	default:
		n.logf("Dropping non-IP packet: %v", ep.le.EthernetType)
		return
	case layers.EthernetTypeARP:
		res, err := n.createARPResponse(packet)
		if err != nil {
			n.logf("createARPResponse: %v", err)
		} else {
			n.writeEth(res)
		}
		return
	case layers.EthernetTypeIPv6:
		// One day. Low value for now. IPv4 NAT modes is the main thing
		// this project wants to test.
		return
	case layers.EthernetTypeIPv4:
		// Below
	}

	// Send ethernet broadcasts and unicast ethernet frames to peers
	// on the same network. This is all LAN traffic that isn't meant
	// for the router/gw itself:
	n.writeEth(ep.gp.Data())

	if forRouter {
		n.HandleEthernetIPv4PacketForRouter(ep)
	}
}

// HandleUDPPacket handles a UDP packet arriving from the internet,
// addressed to the router's WAN IP. It is then NATed back to a
// LAN IP here and wrapped in an ethernet layer and delivered
// to the network.
func (n *network) HandleUDPPacket(p UDPPacket) {
	buf, err := n.serializedUDPPacket(p.Src, p.Dst, p.Payload, nil)
	if err != nil {
		n.logf("serializing UDP packet: %v", err)
		return
	}
	n.s.pcapWriter.WritePacket(gopacket.CaptureInfo{
		Timestamp:      time.Now(),
		CaptureLength:  len(buf),
		Length:         len(buf),
		InterfaceIndex: n.wanInterfaceID,
	}, buf)
	dst := n.doNATIn(p.Src, p.Dst)
	if !dst.IsValid() {
		n.logf("Warning: NAT dropped packet; no mapping for %v=>%v", p.Src, p.Dst)
		return
	}
	p.Dst = dst
	buf, err = n.serializedUDPPacket(p.Src, p.Dst, p.Payload, nil)
	if err != nil {
		n.logf("serializing UDP packet: %v", err)
		return
	}
	n.s.pcapWriter.WritePacket(gopacket.CaptureInfo{
		Timestamp:      time.Now(),
		CaptureLength:  len(buf),
		Length:         len(buf),
		InterfaceIndex: n.lanInterfaceID,
	}, buf)
	n.WriteUDPPacketNoNAT(p)
}

// WriteUDPPacketNoNAT writes a UDP packet to the network, without
// doing any NAT translation.
//
// The packet will always have the ethernet src MAC of the router
// so this should not be used for packets between clients on the
// same ethernet segment.
func (n *network) WriteUDPPacketNoNAT(p UDPPacket) {
	src, dst := p.Src, p.Dst
	node, ok := n.nodesByIP[dst.Addr()]
	if !ok {
		n.logf("no node for dest IP %v in UDP packet %v=>%v", dst.Addr(), p.Src, p.Dst)
		return
	}

	eth := &layers.Ethernet{
		SrcMAC:       n.mac.HWAddr(), // of gateway
		DstMAC:       node.mac.HWAddr(),
		EthernetType: layers.EthernetTypeIPv4,
	}
	ethRaw, err := n.serializedUDPPacket(src, dst, p.Payload, eth)
	if err != nil {
		n.logf("serializing UDP packet: %v", err)
		return
	}
	n.writeEth(ethRaw)
}

// serializedUDPPacket serializes a UDP packet with the given source and
// destination IP:port pairs, and payload.
//
// If eth is non-nil, it will be used as the Ethernet layer, otherwise the
// Ethernet layer will be omitted from the serialization.
func (n *network) serializedUDPPacket(src, dst netip.AddrPort, payload []byte, eth *layers.Ethernet) ([]byte, error) {
	ip := &layers.IPv4{
		Version:  4,
		TTL:      64,
		Protocol: layers.IPProtocolUDP,
		SrcIP:    src.Addr().AsSlice(),
		DstIP:    dst.Addr().AsSlice(),
	}
	udp := &layers.UDP{
		SrcPort: layers.UDPPort(src.Port()),
		DstPort: layers.UDPPort(dst.Port()),
	}
	udp.SetNetworkLayerForChecksum(ip)

	buffer := gopacket.NewSerializeBuffer()
	options := gopacket.SerializeOptions{FixLengths: true, ComputeChecksums: true}
	layers := []gopacket.SerializableLayer{eth, ip, udp, gopacket.Payload(payload)}
	if eth == nil {
		layers = layers[1:]
	}
	if err := gopacket.SerializeLayers(buffer, options, layers...); err != nil {
		return nil, fmt.Errorf("serializing UDP: %v", err)
	}
	return buffer.Bytes(), nil
}

// HandleEthernetIPv4PacketForRouter handles an IPv4 packet that is
// directed to the router/gateway itself. The packet may be to the
// broadcast MAC address, or to the router's MAC address. The target
// IP may be the router's IP, or an internet (routed) IP.
func (n *network) HandleEthernetIPv4PacketForRouter(ep EthernetPacket) {
	packet := ep.gp

	v4, ok := packet.Layer(layers.LayerTypeIPv4).(*layers.IPv4)
	if !ok {
		return
	}
	srcIP, _ := netip.AddrFromSlice(v4.SrcIP)
	dstIP, _ := netip.AddrFromSlice(v4.DstIP)
	toForward := dstIP != n.lanIP.Addr() && dstIP != netip.IPv4Unspecified()
	udp, isUDP := packet.Layer(layers.LayerTypeUDP).(*layers.UDP)

	if isDHCPRequest(packet) {
		res, err := n.s.createDHCPResponse(packet)
		if err != nil {
			n.logf("createDHCPResponse: %v", err)
			return
		}
		n.writeEth(res)
		return
	}

	if isMDNSQuery(packet) || isIGMP(packet) {
		// Don't log. Spammy for now.
		return
	}

	if isDNSRequest(packet) {
		// TODO(bradfitz): restrict this to 4.11.4.11? add DNS
		// on gateway instead?
		res, err := n.s.createDNSResponse(packet)
		if err != nil {
			n.logf("createDNSResponse: %v", err)
			return
		}
		n.writeEth(res)
		return
	}

	if isUDP && dstIP == fakeSyslogIP {
		node, ok := n.nodesByIP[srcIP]
		if !ok {
			return
		}
		if node.verboseSyslog {
			// TODO(bradfitz): parse this and capture it, structured, into
			// node's log buffer.
			log.Printf("syslog from %v: %s", node, udp.Payload)
		}
		return
	}

	if !toForward && isNATPMP(packet) {
		n.handleNATPMPRequest(UDPPacket{
			Src:     netip.AddrPortFrom(srcIP, uint16(udp.SrcPort)),
			Dst:     netip.AddrPortFrom(dstIP, uint16(udp.DstPort)),
			Payload: udp.Payload,
		})
		return
	}

	if toForward && isUDP {
		src := netip.AddrPortFrom(srcIP, uint16(udp.SrcPort))
		dst := netip.AddrPortFrom(dstIP, uint16(udp.DstPort))
		buf, err := n.serializedUDPPacket(src, dst, udp.Payload, nil)
		if err != nil {
			n.logf("serializing UDP packet: %v", err)
			return
		}
		n.s.pcapWriter.WritePacket(gopacket.CaptureInfo{
			Timestamp:      time.Now(),
			CaptureLength:  len(buf),
			Length:         len(buf),
			InterfaceIndex: n.lanInterfaceID,
		}, buf)

		src = n.doNATOut(src, dst)
		buf, err = n.serializedUDPPacket(src, dst, udp.Payload, nil)
		if err != nil {
			n.logf("serializing UDP packet: %v", err)
			return
		}
		n.s.pcapWriter.WritePacket(gopacket.CaptureInfo{
			Timestamp:      time.Now(),
			CaptureLength:  len(buf),
			Length:         len(buf),
			InterfaceIndex: n.wanInterfaceID,
		}, buf)

		n.s.routeUDPPacket(UDPPacket{
			Src:     src,
			Dst:     dst,
			Payload: udp.Payload,
		})
		return
	}

	if toForward && n.s.shouldInterceptTCP(packet) {
		ipp := packet.Layer(layers.LayerTypeIPv4).(*layers.IPv4)
		pktCopy := make([]byte, 0, len(ipp.Contents)+len(ipp.Payload))
		pktCopy = append(pktCopy, ipp.Contents...)
		pktCopy = append(pktCopy, ipp.Payload...)
		packetBuf := stack.NewPacketBuffer(stack.PacketBufferOptions{
			Payload: buffer.MakeWithData(pktCopy),
		})
		n.linkEP.InjectInbound(header.IPv4ProtocolNumber, packetBuf)
		packetBuf.DecRef()
		return
	}

	//log.Printf("Got packet: %v", packet)
}

func (s *Server) createDHCPResponse(request gopacket.Packet) ([]byte, error) {
	ethLayer := request.Layer(layers.LayerTypeEthernet).(*layers.Ethernet)
	srcMAC, ok := macOf(ethLayer.SrcMAC)
	if !ok {
		return nil, nil
	}
	node, ok := s.nodeByMAC[srcMAC]
	if !ok {
		log.Printf("DHCP request from unknown node %v; ignoring", srcMAC)
		return nil, nil
	}
	gwIP := node.net.lanIP.Addr()

	ipLayer := request.Layer(layers.LayerTypeIPv4).(*layers.IPv4)
	udpLayer := request.Layer(layers.LayerTypeUDP).(*layers.UDP)
	dhcpLayer := request.Layer(layers.LayerTypeDHCPv4).(*layers.DHCPv4)

	response := &layers.DHCPv4{
		Operation:    layers.DHCPOpReply,
		HardwareType: layers.LinkTypeEthernet,
		HardwareLen:  6,
		Xid:          dhcpLayer.Xid,
		ClientHWAddr: dhcpLayer.ClientHWAddr,
		Flags:        dhcpLayer.Flags,
		YourClientIP: node.lanIP.AsSlice(),
		Options: []layers.DHCPOption{
			{
				Type:   layers.DHCPOptServerID,
				Data:   gwIP.AsSlice(), // DHCP server's IP
				Length: 4,
			},
		},
	}

	var msgType layers.DHCPMsgType
	for _, opt := range dhcpLayer.Options {
		if opt.Type == layers.DHCPOptMessageType && opt.Length > 0 {
			msgType = layers.DHCPMsgType(opt.Data[0])
		}
	}
	switch msgType {
	case layers.DHCPMsgTypeDiscover:
		response.Options = append(response.Options, layers.DHCPOption{
			Type:   layers.DHCPOptMessageType,
			Data:   []byte{byte(layers.DHCPMsgTypeOffer)},
			Length: 1,
		})
	case layers.DHCPMsgTypeRequest:
		response.Options = append(response.Options,
			layers.DHCPOption{
				Type:   layers.DHCPOptMessageType,
				Data:   []byte{byte(layers.DHCPMsgTypeAck)},
				Length: 1,
			},
			layers.DHCPOption{
				Type:   layers.DHCPOptLeaseTime,
				Data:   binary.BigEndian.AppendUint32(nil, 3600), // hour? sure.
				Length: 4,
			},
			layers.DHCPOption{
				Type:   layers.DHCPOptRouter,
				Data:   gwIP.AsSlice(),
				Length: 4,
			},
			layers.DHCPOption{
				Type:   layers.DHCPOptDNS,
				Data:   fakeDNSIP.AsSlice(),
				Length: 4,
			},
			layers.DHCPOption{
				Type:   layers.DHCPOptSubnetMask,
				Data:   net.CIDRMask(node.net.lanIP.Bits(), 32),
				Length: 4,
			},
		)
	}

	eth := &layers.Ethernet{
		SrcMAC:       node.net.mac.HWAddr(),
		DstMAC:       ethLayer.SrcMAC,
		EthernetType: layers.EthernetTypeIPv4,
	}

	ip := &layers.IPv4{
		Version:  4,
		TTL:      64,
		Protocol: layers.IPProtocolUDP,
		SrcIP:    ipLayer.DstIP,
		DstIP:    ipLayer.SrcIP,
	}

	udp := &layers.UDP{
		SrcPort: udpLayer.DstPort,
		DstPort: udpLayer.SrcPort,
	}
	udp.SetNetworkLayerForChecksum(ip)

	buffer := gopacket.NewSerializeBuffer()
	options := gopacket.SerializeOptions{FixLengths: true, ComputeChecksums: true}
	if err := gopacket.SerializeLayers(buffer, options,
		eth,
		ip,
		udp,
		response,
	); err != nil {
		return nil, err
	}

	return buffer.Bytes(), nil
}

func isDHCPRequest(pkt gopacket.Packet) bool {
	v4, ok := pkt.Layer(layers.LayerTypeIPv4).(*layers.IPv4)
	if !ok || v4.Protocol != layers.IPProtocolUDP {
		return false
	}
	udp, ok := pkt.Layer(layers.LayerTypeUDP).(*layers.UDP)
	return ok && udp.DstPort == 67 && udp.SrcPort == 68
}

func isIGMP(pkt gopacket.Packet) bool {
	return pkt.Layer(layers.LayerTypeIGMP) != nil
}

func isMDNSQuery(pkt gopacket.Packet) bool {
	udp, ok := pkt.Layer(layers.LayerTypeUDP).(*layers.UDP)
	// TODO(bradfitz): also check IPv4 DstIP=224.0.0.251 (or whatever)
	return ok && udp.SrcPort == 5353 && udp.DstPort == 5353
}

func (s *Server) shouldInterceptTCP(pkt gopacket.Packet) bool {
	tcp, ok := pkt.Layer(layers.LayerTypeTCP).(*layers.TCP)
	if !ok {
		return false
	}
	ipv4, ok := pkt.Layer(layers.LayerTypeIPv4).(*layers.IPv4)
	if !ok {
		return false
	}
	if tcp.DstPort == 123 || tcp.DstPort == 124 {
		return true
	}
	dstIP, _ := netip.AddrFromSlice(ipv4.DstIP.To4())
	if tcp.DstPort == 80 || tcp.DstPort == 443 {
		switch dstIP {
		case fakeControlIP, fakeDERP1IP, fakeDERP2IP, fakeLogCatcherIP:
			return true
		}
		if dstIP == fakeProxyControlplaneIP {
			return s.blendReality
		}
		if s.derpIPs.Contains(dstIP) {
			return true
		}
	}
	if tcp.DstPort == 8008 && dstIP == fakeTestAgentIP {
		// Connection from cmd/tta.
		return true
	}
	return false
}

// isDNSRequest reports whether pkt is a DNS request to the fake DNS server.
func isDNSRequest(pkt gopacket.Packet) bool {
	udp, ok := pkt.Layer(layers.LayerTypeUDP).(*layers.UDP)
	if !ok || udp.DstPort != 53 {
		return false
	}
	ip, ok := pkt.Layer(layers.LayerTypeIPv4).(*layers.IPv4)
	if !ok {
		return false
	}
	dstIP, ok := netip.AddrFromSlice(ip.DstIP)
	if !ok || dstIP != fakeDNSIP {
		return false
	}
	dns, ok := pkt.Layer(layers.LayerTypeDNS).(*layers.DNS)
	return ok && dns.QR == false && len(dns.Questions) > 0
}

func isNATPMP(pkt gopacket.Packet) bool {
	udp, ok := pkt.Layer(layers.LayerTypeUDP).(*layers.UDP)
	return ok && udp.DstPort == 5351 && len(udp.Payload) > 0 && udp.Payload[0] == 0 // version 0, not 2 for PCP
}

func makeSTUNReply(req UDPPacket) (res UDPPacket, ok bool) {
	txid, err := stun.ParseBindingRequest(req.Payload)
	if err != nil {
		log.Printf("invalid STUN request: %v", err)
		return res, false
	}
	return UDPPacket{
		Src:     req.Dst,
		Dst:     req.Src,
		Payload: stun.Response(txid, req.Src),
	}, true
}

func (s *Server) createDNSResponse(pkt gopacket.Packet) ([]byte, error) {
	ethLayer := pkt.Layer(layers.LayerTypeEthernet).(*layers.Ethernet)
	ipLayer := pkt.Layer(layers.LayerTypeIPv4).(*layers.IPv4)
	udpLayer := pkt.Layer(layers.LayerTypeUDP).(*layers.UDP)
	dnsLayer := pkt.Layer(layers.LayerTypeDNS).(*layers.DNS)

	if dnsLayer.OpCode != layers.DNSOpCodeQuery || dnsLayer.QR || len(dnsLayer.Questions) == 0 {
		return nil, nil
	}

	response := &layers.DNS{
		ID:           dnsLayer.ID,
		QR:           true,
		AA:           true,
		TC:           false,
		RD:           dnsLayer.RD,
		RA:           true,
		OpCode:       layers.DNSOpCodeQuery,
		ResponseCode: layers.DNSResponseCodeNoErr,
	}

	var names []string
	for _, q := range dnsLayer.Questions {
		response.QDCount++
		response.Questions = append(response.Questions, q)

		if mem.HasSuffix(mem.B(q.Name), mem.S(".pool.ntp.org")) {
			// Just drop DNS queries for NTP servers. For Debian/etc guests used
			// during development. Not needed. Assume VM guests get correct time
			// via their hypervisor.
			return nil, nil
		}

		names = append(names, q.Type.String()+"/"+string(q.Name))
		if q.Class != layers.DNSClassIN || q.Type != layers.DNSTypeA {
			continue
		}

		if ip, ok := s.IPv4ForDNS(string(q.Name)); ok {
			response.ANCount++
			response.Answers = append(response.Answers, layers.DNSResourceRecord{
				Name:  q.Name,
				Type:  q.Type,
				Class: q.Class,
				IP:    ip.AsSlice(),
				TTL:   60,
			})
		}
	}

	eth2 := &layers.Ethernet{
		SrcMAC:       ethLayer.DstMAC,
		DstMAC:       ethLayer.SrcMAC,
		EthernetType: layers.EthernetTypeIPv4,
	}
	ip2 := &layers.IPv4{
		Version:  4,
		TTL:      64,
		Protocol: layers.IPProtocolUDP,
		SrcIP:    ipLayer.DstIP,
		DstIP:    ipLayer.SrcIP,
	}
	udp2 := &layers.UDP{
		SrcPort: udpLayer.DstPort,
		DstPort: udpLayer.SrcPort,
	}
	udp2.SetNetworkLayerForChecksum(ip2)

	buffer := gopacket.NewSerializeBuffer()
	options := gopacket.SerializeOptions{FixLengths: true, ComputeChecksums: true}
	if err := gopacket.SerializeLayers(buffer, options, eth2, ip2, udp2, response); err != nil {
		return nil, err
	}

	const debugDNS = false
	if debugDNS {
		if len(response.Answers) > 0 {
			back := gopacket.NewPacket(buffer.Bytes(), layers.LayerTypeEthernet, gopacket.Lazy)
			log.Printf("Generated: %v", back)
		} else {
			log.Printf("made empty response for %q", names)
		}
	}

	return buffer.Bytes(), nil
}

// 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 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) {
	n.natMu.Lock()
	defer n.natMu.Unlock()

	now := time.Now()

	// 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("NAT: doNatIn: port mapping %v=>%v", dst, lanAP.dst)
			return lanAP.dst
		}
		n.logf("NAT: doNatIn: port mapping EXPIRED for %v=>%v", dst, lanAP.dst)
		delete(n.portMap, dst)
		return netip.AddrPort{}
	}

	return n.natTable.PickIncomingDst(src, dst, now)
}

// IsPublicPortUsed reports whether the given public port is currently in use.
//
// n.natMu must be held by the caller. (It's only called by nat implementations
// which are always called with natMu held))
func (n *network) IsPublicPortUsed(ap netip.AddrPort) bool {
	_, ok := n.portMap[ap]
	return ok
}

func (n *network) doPortMap(src netip.Addr, dstLANPort, wantExtPort uint16, sec int) (gotPort uint16, ok bool) {
	n.natMu.Lock()
	defer n.natMu.Unlock()

	if !n.portmap {
		return 0, false
	}

	wanAP := netip.AddrPortFrom(n.wanIP, wantExtPort)
	dst := netip.AddrPortFrom(src, dstLANPort)

	if sec == 0 {
		lanAP, ok := n.portMap[wanAP]
		if ok && lanAP.dst.Addr() == src {
			delete(n.portMap, wanAP)
		}
		return 0, false
	}

	// See if they already have a mapping and extend expiry if so.
	for k, v := range n.portMap {
		if v.dst == dst {
			n.portMap[k] = portMapping{
				dst:    dst,
				expiry: time.Now().Add(time.Duration(sec) * time.Second),
			}
			return k.Port(), true
		}
	}

	for try := 0; try < 20_000; try++ {
		if wanAP.Port() > 0 && !n.natTable.IsPublicPortUsed(wanAP) {
			mak.Set(&n.portMap, wanAP, portMapping{
				dst:    dst,
				expiry: time.Now().Add(time.Duration(sec) * time.Second),
			})
			n.logf("vnet: allocated NAT mapping from %v to %v", wanAP, dst)
			return wanAP.Port(), true
		}
		wantExtPort = rand.N(uint16(32<<10)) + 32<<10
		wanAP = netip.AddrPortFrom(n.wanIP, wantExtPort)
	}
	return 0, false
}

func (n *network) createARPResponse(pkt gopacket.Packet) ([]byte, error) {
	ethLayer, ok := pkt.Layer(layers.LayerTypeEthernet).(*layers.Ethernet)
	if !ok {
		return nil, nil
	}
	arpLayer, ok := pkt.Layer(layers.LayerTypeARP).(*layers.ARP)
	if !ok ||
		arpLayer.Operation != layers.ARPRequest ||
		arpLayer.AddrType != layers.LinkTypeEthernet ||
		arpLayer.Protocol != layers.EthernetTypeIPv4 ||
		arpLayer.HwAddressSize != 6 ||
		arpLayer.ProtAddressSize != 4 ||
		len(arpLayer.DstProtAddress) != 4 {
		return nil, nil
	}

	wantIP := netip.AddrFrom4([4]byte(arpLayer.DstProtAddress))
	foundMAC, ok := n.MACOfIP(wantIP)
	if !ok {
		return nil, nil
	}

	eth := &layers.Ethernet{
		SrcMAC:       foundMAC.HWAddr(),
		DstMAC:       ethLayer.SrcMAC,
		EthernetType: layers.EthernetTypeARP,
	}

	a2 := &layers.ARP{
		AddrType:          layers.LinkTypeEthernet,
		Protocol:          layers.EthernetTypeIPv4,
		HwAddressSize:     6,
		ProtAddressSize:   4,
		Operation:         layers.ARPReply,
		SourceHwAddress:   foundMAC.HWAddr(),
		SourceProtAddress: arpLayer.DstProtAddress,
		DstHwAddress:      ethLayer.SrcMAC,
		DstProtAddress:    arpLayer.SourceProtAddress,
	}

	buffer := gopacket.NewSerializeBuffer()
	options := gopacket.SerializeOptions{FixLengths: true, ComputeChecksums: true}
	if err := gopacket.SerializeLayers(buffer, options, eth, a2); err != nil {
		return nil, err
	}

	return buffer.Bytes(), nil
}

func (n *network) handleNATPMPRequest(req UDPPacket) {
	if !n.portmap {
		return
	}
	if string(req.Payload) == "\x00\x00" {
		// https://www.rfc-editor.org/rfc/rfc6886#section-3.2

		res := make([]byte, 0, 12)
		res = append(res,
			0,    // version 0 (NAT-PMP)
			128,  // response to op 0 (128+0)
			0, 0, // result code success
		)
		res = binary.BigEndian.AppendUint32(res, uint32(time.Now().Unix()))
		wan4 := n.wanIP.As4()
		res = append(res, wan4[:]...)
		n.WriteUDPPacketNoNAT(UDPPacket{
			Src:     req.Dst,
			Dst:     req.Src,
			Payload: res,
		})
		return
	}

	// Map UDP request
	if len(req.Payload) == 12 && req.Payload[0] == 0 && req.Payload[1] == 1 {
		// https://www.rfc-editor.org/rfc/rfc6886#section-3.3
		// "00 01 00 00 ed 40 00 00 00 00 1c 20" =>
		//   00 ver
		//   01 op=map UDP
		//   00 00 reserved  (0 in request; in response, this is the result code)
		//   ed 40 internal port 60736
		//   00 00 suggested external port
		//   00 00 1c 20 suggested lifetime in seconds (7200 sec = 2 hours)
		internalPort := binary.BigEndian.Uint16(req.Payload[4:6])
		wantExtPort := binary.BigEndian.Uint16(req.Payload[6:8])
		lifetimeSec := binary.BigEndian.Uint32(req.Payload[8:12])
		gotPort, ok := n.doPortMap(req.Src.Addr(), internalPort, wantExtPort, int(lifetimeSec))
		if !ok {
			n.logf("NAT-PMP map request for %v:%d failed", req.Src.Addr(), internalPort)
			return
		}
		res := make([]byte, 0, 16)
		res = append(res,
			0,     // version 0 (NAT-PMP)
			1+128, // response to op 1
			0, 0,  // result code success
		)
		res = binary.BigEndian.AppendUint32(res, uint32(time.Now().Unix()))
		res = binary.BigEndian.AppendUint16(res, internalPort)
		res = binary.BigEndian.AppendUint16(res, gotPort)
		res = binary.BigEndian.AppendUint32(res, lifetimeSec)
		n.WriteUDPPacketNoNAT(UDPPacket{
			Src:     req.Dst,
			Dst:     req.Src,
			Payload: res,
		})
		return
	}

	n.logf("TODO: handle NAT-PMP packet % 02x", req.Payload)
}

// UDPPacket is a UDP packet.
//
// For the purposes of this project, a UDP packet
// (not a general IP packet) is the unit to be NAT'ed,
// as that's all that Tailscale uses.
type UDPPacket struct {
	Src     netip.AddrPort
	Dst     netip.AddrPort
	Payload []byte // everything after UDP header
}

func (s *Server) WriteStartingBanner(w io.Writer) {
	fmt.Fprintf(w, "vnet serving clients:\n")

	for _, n := range s.nodes {
		fmt.Fprintf(w, "  %v %15v (%v, %v)\n", n.mac, n.lanIP, n.net.wanIP, n.net.natStyle.Load())
	}
}

type agentConn struct {
	node *node
	tc   *gonet.TCPConn
}

func (s *Server) addIdleAgentConn(ac *agentConn) {
	//log.Printf("got agent conn from %v", ac.node.mac)
	s.mu.Lock()
	defer s.mu.Unlock()

	s.agentConns.Make()
	s.agentConns.Add(ac)

	if waiter, ok := s.agentConnWaiter[ac.node]; ok {
		select {
		case waiter <- struct{}{}:
		default:
		}
	}
}

func (s *Server) takeAgentConn(ctx context.Context, n *node) (_ *agentConn, ok bool) {
	for {
		ac, ok := s.takeAgentConnOne(n)
		if ok {
			//log.Printf("got agent conn for %v", n.mac)
			return ac, true
		}
		s.mu.Lock()
		ready := make(chan struct{})
		mak.Set(&s.agentConnWaiter, n, ready)
		s.mu.Unlock()

		//log.Printf("waiting for agent conn for %v", n.mac)
		select {
		case <-ctx.Done():
			return nil, false
		case <-ready:
		case <-time.After(time.Second):
			// Try again regularly anyway, in case we have multiple clients
			// trying to hit the same node, or if a race means we weren't in the
			// select by the time addIdleAgentConn tried to signal us.
		}
	}
}

func (s *Server) takeAgentConnOne(n *node) (_ *agentConn, ok bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	for ac := range s.agentConns {
		if ac.node == n {
			s.agentConns.Delete(ac)
			return ac, true
		}
	}
	return nil, false
}

type NodeAgentClient struct {
	*tailscale.LocalClient
	HTTPClient *http.Client
}

func (s *Server) NodeAgentDialer(n *Node) DialFunc {
	s.mu.Lock()
	defer s.mu.Unlock()

	if d, ok := s.agentDialer[n.n]; ok {
		return d
	}
	d := func(ctx context.Context, network, addr string) (net.Conn, error) {
		ac, ok := s.takeAgentConn(ctx, n.n)
		if !ok {
			return nil, ctx.Err()
		}
		return ac.tc, nil
	}
	mak.Set(&s.agentDialer, n.n, d)
	return d
}

func (s *Server) NodeAgentClient(n *Node) *NodeAgentClient {
	d := s.NodeAgentDialer(n)
	return &NodeAgentClient{
		LocalClient: &tailscale.LocalClient{
			UseSocketOnly: true,
			OmitAuth:      true,
			Dial:          d,
		},
		HTTPClient: &http.Client{
			Transport: &http.Transport{
				DialContext: d,
			},
		},
	}
}

// EnableHostFirewall enables the host's stateful firewall.
func (c *NodeAgentClient) EnableHostFirewall(ctx context.Context) error {
	req, err := http.NewRequestWithContext(ctx, "GET", "http://unused/fw", nil)
	if err != nil {
		return err
	}
	res, err := c.HTTPClient.Do(req)
	if err != nil {
		return err
	}
	defer res.Body.Close()
	all, _ := io.ReadAll(res.Body)
	if res.StatusCode != 200 {
		return fmt.Errorf("unexpected status code %v: %s", res.Status, all)
	}
	return nil
}