diff --git a/net/portmapper/igd_test.go b/net/portmapper/igd_test.go
new file mode 100644
index 000000000..2defb21f6
--- /dev/null
+++ b/net/portmapper/igd_test.go
@@ -0,0 +1,155 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package portmapper
+
+import (
+	"bytes"
+	"fmt"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"sync"
+
+	"inet.af/netaddr"
+)
+
+// TestIGD is an IGD (Intenet Gateway Device) for testing. It supports fake
+// implementations of NAT-PMP, PCP, and/or UPnP to test clients against.
+type TestIGD struct {
+	upnpConn net.PacketConn // for UPnP discovery
+	pxpConn  net.PacketConn // for NAT-PMP and/or PCP
+	ts       *httptest.Server
+
+	doPMP  bool
+	doPCP  bool
+	doUPnP bool // TODO: more options for 3 flavors of UPnP services
+
+	mu       sync.Mutex // guards below
+	counters igdCounters
+}
+
+type igdCounters struct {
+	numUPnPDiscoRecv     int32
+	numUPnPOtherUDPRecv  int32
+	numUPnPHTTPRecv      int32
+	numPMPRecv           int32
+	numPMPDiscoRecv      int32
+	numPCPRecv           int32
+	numPCPDiscoRecv      int32
+	numPMPPublicAddrRecv int32
+	numPMPBogusRecv      int32
+}
+
+func NewTestIGD() (*TestIGD, error) {
+	d := &TestIGD{
+		doPMP:  true,
+		doPCP:  true,
+		doUPnP: true,
+	}
+	var err error
+	if d.upnpConn, err = net.ListenPacket("udp", "127.0.0.1:1900"); err != nil {
+		return nil, err
+	}
+	if d.pxpConn, err = net.ListenPacket("udp", "127.0.0.1:5351"); err != nil {
+		return nil, err
+	}
+	d.ts = httptest.NewServer(http.HandlerFunc(d.serveUPnPHTTP))
+	go d.serveUPnPDiscovery()
+	go d.servePxP()
+	return d, nil
+}
+
+func (d *TestIGD) Close() error {
+	d.ts.Close()
+	d.upnpConn.Close()
+	d.pxpConn.Close()
+	return nil
+}
+
+func (d *TestIGD) inc(p *int32) {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	(*p)++
+}
+
+func (d *TestIGD) stats() igdCounters {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	return d.counters
+}
+
+func (d *TestIGD) serveUPnPHTTP(w http.ResponseWriter, r *http.Request) {
+	http.NotFound(w, r) // TODO
+}
+
+func (d *TestIGD) serveUPnPDiscovery() {
+	buf := make([]byte, 1500)
+	for {
+		n, src, err := d.upnpConn.ReadFrom(buf)
+		if err != nil {
+			return
+		}
+		pkt := buf[:n]
+		if bytes.Equal(pkt, uPnPPacket) { // a super lazy "parse"
+			d.inc(&d.counters.numUPnPDiscoRecv)
+			resPkt := []byte(fmt.Sprintf("HTTP/1.1 200 OK\r\nCACHE-CONTROL: max-age=120\r\nST: urn:schemas-upnp-org:device:InternetGatewayDevice:1\r\nUSN: uuid:bee7052b-49e8-3597-b545-55a1e38ac11::urn:schemas-upnp-org:device:InternetGatewayDevice:1\r\nEXT:\r\nSERVER: Tailscale-Test/1.0 UPnP/1.1 MiniUPnPd/2.2.1\r\nLOCATION: %s\r\nOPT: \"http://schemas.upnp.org/upnp/1/0/\"; ns=01\r\n01-NLS: 1627958564\r\nBOOTID.UPNP.ORG: 1627958564\r\nCONFIGID.UPNP.ORG: 1337\r\n\r\n", d.ts.URL+"/rootDesc.xml"))
+			d.upnpConn.WriteTo(resPkt, src)
+		} else {
+			d.inc(&d.counters.numUPnPOtherUDPRecv)
+		}
+	}
+}
+
+// servePxP serves NAT-PMP and PCP, which share a port number.
+func (d *TestIGD) servePxP() {
+	buf := make([]byte, 1500)
+	for {
+		n, a, err := d.pxpConn.ReadFrom(buf)
+		if err != nil {
+			return
+		}
+		ua := a.(*net.UDPAddr)
+		src, ok := netaddr.FromStdAddr(ua.IP, ua.Port, ua.Zone)
+		if !ok {
+			panic("bogus addr")
+		}
+		pkt := buf[:n]
+		if len(pkt) < 2 {
+			continue
+		}
+		ver := pkt[0]
+		switch ver {
+		default:
+			continue
+		case pmpVersion:
+			d.handlePMPQuery(pkt, src)
+		case pcpVersion:
+			d.handlePCPQuery(pkt, src)
+		}
+	}
+}
+
+func (d *TestIGD) handlePMPQuery(pkt []byte, src netaddr.IPPort) {
+	d.inc(&d.counters.numPMPRecv)
+	if len(pkt) < 2 {
+		return
+	}
+	op := pkt[1]
+	switch op {
+	case pmpOpMapPublicAddr:
+		if len(pkt) != 2 {
+			d.inc(&d.counters.numPMPBogusRecv)
+			return
+		}
+		d.inc(&d.counters.numPMPPublicAddrRecv)
+
+	}
+	// TODO
+}
+
+func (d *TestIGD) handlePCPQuery(pkt []byte, src netaddr.IPPort) {
+	d.inc(&d.counters.numPCPRecv)
+	// TODO
+}
diff --git a/net/portmapper/portmapper.go b/net/portmapper/portmapper.go
index ba180510c..33fb805ac 100644
--- a/net/portmapper/portmapper.go
+++ b/net/portmapper/portmapper.go
@@ -726,7 +726,7 @@ func (c *Client) Probe(ctx context.Context) (res ProbeResult, err error) {
 	}
 }
 
-var pmpReqExternalAddrPacket = []byte{0, 0} // version 0, opcode 0 = "Public address request"
+var pmpReqExternalAddrPacket = []byte{pmpVersion, pmpOpMapPublicAddr} // 0, 0
 
 const (
 	upnpPort = 1900 // for UDP discovery only; TCP port discovered later
diff --git a/net/portmapper/portmapper_test.go b/net/portmapper/portmapper_test.go
index e25c751d8..7d5d77f1f 100644
--- a/net/portmapper/portmapper_test.go
+++ b/net/portmapper/portmapper_test.go
@@ -10,6 +10,9 @@ import (
 	"strconv"
 	"testing"
 	"time"
+
+	"inet.af/netaddr"
+	"tailscale.com/types/logger"
 )
 
 func TestCreateOrGetMapping(t *testing.T) {
@@ -55,3 +58,30 @@ func TestClientProbeThenMap(t *testing.T) {
 	ext, err := c.createOrGetMapping(context.Background())
 	t.Logf("createOrGetMapping: %v, %v", ext, err)
 }
+
+func TestProbeIntegration(t *testing.T) {
+	igd, err := NewTestIGD()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer igd.Close()
+
+	logf := t.Logf
+	var c *Client
+	c = NewClient(logger.WithPrefix(logf, "portmapper: "), func() {
+		logf("portmapping changed.")
+		logf("have mapping: %v", c.HaveMapping())
+	})
+
+	c.SetGatewayLookupFunc(func() (gw, self netaddr.IP, ok bool) {
+		return netaddr.IPv4(127, 0, 0, 1), netaddr.IPv4(1, 2, 3, 4), true
+	})
+
+	res, err := c.Probe(context.Background())
+	if err != nil {
+		t.Fatalf("Probe: %v", err)
+	}
+	t.Logf("Probe: %+v", res)
+	t.Logf("IGD stats: %+v", igd.stats())
+	// TODO(bradfitz): finish
+}