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tailscale/net/portmapper/portmapper.go

775 lines
22 KiB
Go

// 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 is a UDP port mapping client. It currently allows for mapping over
// NAT-PMP, UPnP, and PCP.
package portmapper
import (
"context"
"encoding/binary"
"errors"
"fmt"
"io"
"net"
"net/http"
"os"
"sync"
"time"
"go4.org/mem"
"inet.af/netaddr"
"tailscale.com/net/interfaces"
"tailscale.com/net/netns"
"tailscale.com/types/logger"
)
// Debug knobs for "tailscaled debug --portmap".
var (
VerboseLogs bool
// Disable* disables a specific service from mapping.
DisableUPnP bool
DisablePMP bool
DisablePCP bool
)
// References:
//
// NAT-PMP: https://tools.ietf.org/html/rfc6886
// portMapServiceTimeout is the time we wait for port mapping
// services (UPnP, NAT-PMP, PCP) to respond before we give up and
// decide that they're not there. Since these services are on the
// same LAN as this machine and a single L3 hop away, we don't
// give them much time to respond.
const portMapServiceTimeout = 250 * time.Millisecond
// trustServiceStillAvailableDuration is how often we re-verify a port
// mapping service is available.
const trustServiceStillAvailableDuration = 10 * time.Minute
// Client is a port mapping client.
type Client struct {
logf logger.Logf
ipAndGateway func() (gw, ip netaddr.IP, ok bool)
onChange func() // or nil
testPxPPort uint16 // if non-zero, pxpPort to use for tests
testUPnPPort uint16 // if non-zero, uPnPPort to use for tests
mu sync.Mutex // guards following, and all fields thereof
// runningCreate is whether we're currently working on creating
// a port mapping (whether GetCachedMappingOrStartCreatingOne kicked
// off a createMapping goroutine).
runningCreate bool
lastMyIP netaddr.IP
lastGW netaddr.IP
closed bool
lastProbe time.Time
pmpPubIP netaddr.IP // non-zero if known
pmpPubIPTime time.Time // time pmpPubIP last verified
pmpLastEpoch uint32
pcpSawTime time.Time // time we last saw PCP was available
uPnPSawTime time.Time // time we last saw UPnP was available
uPnPMeta uPnPDiscoResponse // Location header from UPnP UDP discovery response
uPnPHTTPClient *http.Client // netns-configured HTTP client for UPnP; nil until needed
localPort uint16
mapping mapping // non-nil if we have a mapping
}
// mapping represents a created port-mapping over some protocol. It specifies a lease duration,
// how to release the mapping, and whether the map is still valid.
//
// After a mapping is created, it should be immutable, and thus reads should be safe across
// concurrent goroutines.
type mapping interface {
// Release will attempt to unmap the established port mapping. It will block until completion,
// but can be called asynchronously. Release should be idempotent, and thus even if called
// multiple times should not cause additional side-effects.
Release(context.Context)
// goodUntil will return the lease time that the mapping is valid for.
GoodUntil() time.Time
// renewAfter returns the earliest time that the mapping should be renewed.
RenewAfter() time.Time
// externalIPPort indicates what port the mapping can be reached from on the outside.
External() netaddr.IPPort
}
// HaveMapping reports whether we have a current valid mapping.
func (c *Client) HaveMapping() bool {
c.mu.Lock()
defer c.mu.Unlock()
return c.mapping != nil && c.mapping.GoodUntil().After(time.Now())
}
// pmpMapping is an already-created PMP mapping.
//
// All fields are immutable once created.
type pmpMapping struct {
c *Client
gw netaddr.IPPort
external netaddr.IPPort
internal netaddr.IPPort
renewAfter time.Time // the time at which we want to renew the mapping
goodUntil time.Time // the mapping's total lifetime
epoch uint32
}
// externalValid reports whether m.external is valid, with both its IP and Port populated.
func (m *pmpMapping) externalValid() bool {
return !m.external.IP().IsZero() && m.external.Port() != 0
}
func (p *pmpMapping) GoodUntil() time.Time { return p.goodUntil }
func (p *pmpMapping) RenewAfter() time.Time { return p.renewAfter }
func (p *pmpMapping) External() netaddr.IPPort { return p.external }
// Release does a best effort fire-and-forget release of the PMP mapping m.
func (m *pmpMapping) Release(ctx context.Context) {
uc, err := m.c.listenPacket(ctx, "udp4", ":0")
if err != nil {
return
}
defer uc.Close()
pkt := buildPMPRequestMappingPacket(m.internal.Port(), m.external.Port(), pmpMapLifetimeDelete)
uc.WriteTo(pkt, m.gw.UDPAddr())
}
// NewClient returns a new portmapping client.
//
// The optional onChange argument specifies a func to run in a new
// goroutine whenever the port mapping status has changed. If nil,
// it doesn't make a callback.
func NewClient(logf logger.Logf, onChange func()) *Client {
return &Client{
logf: logf,
ipAndGateway: interfaces.LikelyHomeRouterIP,
onChange: onChange,
}
}
// SetGatewayLookupFunc set the func that returns the machine's default gateway IP, and
// the primary IP address for that gateway. It must be called before the client is used.
// If not called, interfaces.LikelyHomeRouterIP is used.
func (c *Client) SetGatewayLookupFunc(f func() (gw, myIP netaddr.IP, ok bool)) {
c.ipAndGateway = f
}
// NoteNetworkDown should be called when the network has transitioned to a down state.
// It's too late to release port mappings at this point (the user might've just turned off
// their wifi), but we can make sure we invalidate mappings for later when the network
// comes back.
func (c *Client) NoteNetworkDown() {
c.mu.Lock()
defer c.mu.Unlock()
c.invalidateMappingsLocked(false)
}
func (c *Client) Close() error {
c.mu.Lock()
defer c.mu.Unlock()
if c.closed {
return nil
}
c.closed = true
c.invalidateMappingsLocked(true)
// TODO: close some future ever-listening UDP socket(s),
// waiting for multicast announcements from router.
return nil
}
// SetLocalPort updates the local port number to which we want to port
// map UDP traffic.
func (c *Client) SetLocalPort(localPort uint16) {
c.mu.Lock()
defer c.mu.Unlock()
if c.localPort == localPort {
return
}
c.localPort = localPort
c.invalidateMappingsLocked(true)
}
func (c *Client) gatewayAndSelfIP() (gw, myIP netaddr.IP, ok bool) {
gw, myIP, ok = c.ipAndGateway()
if !ok {
gw = netaddr.IP{}
myIP = netaddr.IP{}
}
c.mu.Lock()
defer c.mu.Unlock()
if gw != c.lastGW || myIP != c.lastMyIP || !ok {
c.lastMyIP = myIP
c.lastGW = gw
c.invalidateMappingsLocked(true)
}
return
}
// pxpPort returns the NAT-PMP and PCP port number.
// It returns 5351, except for in tests where it varies by run.
func (c *Client) pxpPort() uint16 {
if c.testPxPPort != 0 {
return c.testPxPPort
}
return pmpDefaultPort
}
// upnpPort returns the UPnP discovery port number.
// It returns 1900, except for in tests where it varies by run.
func (c *Client) upnpPort() uint16 {
if c.testUPnPPort != 0 {
return c.testUPnPPort
}
return upnpDefaultPort
}
func (c *Client) listenPacket(ctx context.Context, network, addr string) (net.PacketConn, error) {
if (c.testPxPPort != 0 || c.testUPnPPort != 0) && os.Getenv("GITHUB_ACTIONS") == "true" {
var lc net.ListenConfig
return lc.ListenPacket(ctx, network, addr)
}
return netns.Listener().ListenPacket(ctx, network, addr)
}
func (c *Client) invalidateMappingsLocked(releaseOld bool) {
if c.mapping != nil {
if releaseOld {
c.mapping.Release(context.Background())
}
c.mapping = nil
}
c.pmpPubIP = netaddr.IP{}
c.pmpPubIPTime = time.Time{}
c.pcpSawTime = time.Time{}
c.uPnPSawTime = time.Time{}
c.uPnPMeta = uPnPDiscoResponse{}
}
func (c *Client) sawPMPRecently() bool {
c.mu.Lock()
defer c.mu.Unlock()
return c.sawPMPRecentlyLocked()
}
func (c *Client) sawPMPRecentlyLocked() bool {
return !c.pmpPubIP.IsZero() && c.pmpPubIPTime.After(time.Now().Add(-trustServiceStillAvailableDuration))
}
func (c *Client) sawPCPRecently() bool {
c.mu.Lock()
defer c.mu.Unlock()
return c.sawPCPRecentlyLocked()
}
func (c *Client) sawPCPRecentlyLocked() bool {
return c.pcpSawTime.After(time.Now().Add(-trustServiceStillAvailableDuration))
}
func (c *Client) sawUPnPRecently() bool {
c.mu.Lock()
defer c.mu.Unlock()
return c.uPnPSawTime.After(time.Now().Add(-trustServiceStillAvailableDuration))
}
// closeCloserOnContextDone starts a new goroutine to call c.Close
// if/when ctx becomes done.
// To stop the goroutine, call the returned stop func.
func closeCloserOnContextDone(ctx context.Context, c io.Closer) (stop func()) {
// Close uc on ctx being done.
ctxDone := ctx.Done()
if ctxDone == nil {
return func() {}
}
stopWaitDone := make(chan struct{})
go func() {
select {
case <-stopWaitDone:
case <-ctxDone:
c.Close()
}
}()
return func() { close(stopWaitDone) }
}
// NoMappingError is returned when no NAT mapping could be done.
type NoMappingError struct {
err error
}
func (nme NoMappingError) Unwrap() error { return nme.err }
func (nme NoMappingError) Error() string { return fmt.Sprintf("no NAT mapping available: %v", nme.err) }
// IsNoMappingError reports whether err is of type NoMappingError.
func IsNoMappingError(err error) bool {
_, ok := err.(NoMappingError)
return ok
}
var (
ErrNoPortMappingServices = errors.New("no port mapping services were found")
ErrGatewayRange = errors.New("skipping portmap; gateway range likely lacks support")
)
// GetCachedMappingOrStartCreatingOne quickly returns with our current cached portmapping, if any.
// If there's not one, it starts up a background goroutine to create one.
// If the background goroutine ends up creating one, the onChange hook registered with the
// NewClient constructor (if any) will fire.
func (c *Client) GetCachedMappingOrStartCreatingOne() (external netaddr.IPPort, ok bool) {
c.mu.Lock()
defer c.mu.Unlock()
// Do we have an existing mapping that's valid?
now := time.Now()
if m := c.mapping; m != nil {
if now.Before(m.GoodUntil()) {
if now.After(m.RenewAfter()) {
c.maybeStartMappingLocked()
}
return m.External(), true
}
}
c.maybeStartMappingLocked()
return netaddr.IPPort{}, false
}
// maybeStartMappingLocked starts a createMapping goroutine up, if one isn't already running.
//
// c.mu must be held.
func (c *Client) maybeStartMappingLocked() {
if !c.runningCreate {
c.runningCreate = true
go c.createMapping()
}
}
func (c *Client) createMapping() {
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel()
defer func() {
c.mu.Lock()
defer c.mu.Unlock()
c.runningCreate = false
}()
if _, err := c.createOrGetMapping(ctx); err == nil && c.onChange != nil {
go c.onChange()
} else if err != nil && !IsNoMappingError(err) {
c.logf("createOrGetMapping: %v", err)
}
}
// wildcardIP is used when the previous external IP is not known for PCP port mapping.
var wildcardIP = netaddr.MustParseIP("0.0.0.0")
// createOrGetMapping either creates a new mapping or returns a cached
// valid one.
//
// If no mapping is available, the error will be of type
// NoMappingError; see IsNoMappingError.
func (c *Client) createOrGetMapping(ctx context.Context) (external netaddr.IPPort, err error) {
if DisableUPnP && DisablePCP && DisablePMP {
return netaddr.IPPort{}, NoMappingError{ErrNoPortMappingServices}
}
gw, myIP, ok := c.gatewayAndSelfIP()
if !ok {
return netaddr.IPPort{}, NoMappingError{ErrGatewayRange}
}
c.mu.Lock()
localPort := c.localPort
internalAddr := netaddr.IPPortFrom(myIP, localPort)
// prevPort is the port we had most previously, if any. We try
// to ask for the same port. 0 means to give us any port.
var prevPort uint16
// Do we have an existing mapping that's valid?
now := time.Now()
if m := c.mapping; m != nil {
if now.Before(m.RenewAfter()) {
defer c.mu.Unlock()
return m.External(), nil
}
// The mapping might still be valid, so just try to renew it.
prevPort = m.External().Port()
}
if DisablePCP && DisablePMP {
c.mu.Unlock()
if external, ok := c.getUPnPPortMapping(ctx, gw, internalAddr, prevPort); ok {
return external, nil
}
return netaddr.IPPort{}, NoMappingError{ErrNoPortMappingServices}
}
// If we just did a Probe (e.g. via netchecker) but didn't
// find a PMP service, bail out early rather than probing
// again. Cuts down latency for most clients.
haveRecentPMP := c.sawPMPRecentlyLocked()
haveRecentPCP := c.sawPCPRecentlyLocked()
// Since PMP mapping may require multiple calls, and it's not clear from the outset
// whether we're doing a PCP or PMP call, initialize the PMP mapping here,
// and only return it once completed.
//
// PCP returns all the information necessary for a mapping in a single packet, so we can
// construct it upon receiving that packet.
m := &pmpMapping{
c: c,
gw: netaddr.IPPortFrom(gw, c.pxpPort()),
internal: internalAddr,
}
if haveRecentPMP {
m.external = m.external.WithIP(c.pmpPubIP)
}
if c.lastProbe.After(now.Add(-5*time.Second)) && !haveRecentPMP && !haveRecentPCP {
c.mu.Unlock()
// fallback to UPnP portmapping
if external, ok := c.getUPnPPortMapping(ctx, gw, internalAddr, prevPort); ok {
return external, nil
}
return netaddr.IPPort{}, NoMappingError{ErrNoPortMappingServices}
}
c.mu.Unlock()
uc, err := c.listenPacket(ctx, "udp4", ":0")
if err != nil {
return netaddr.IPPort{}, err
}
defer uc.Close()
uc.SetReadDeadline(time.Now().Add(portMapServiceTimeout))
defer closeCloserOnContextDone(ctx, uc)()
pxpAddr := netaddr.IPPortFrom(gw, c.pxpPort())
pxpAddru := pxpAddr.UDPAddr()
preferPCP := !DisablePCP && (DisablePMP || (!haveRecentPMP && haveRecentPCP))
// Create a mapping, defaulting to PMP unless only PCP was seen recently.
if preferPCP {
// TODO replace wildcardIP here with previous external if known.
// Only do PCP mapping in the case when PMP did not appear to be available recently.
pkt := buildPCPRequestMappingPacket(myIP, localPort, prevPort, pcpMapLifetimeSec, wildcardIP)
if _, err := uc.WriteTo(pkt, pxpAddru); err != nil {
return netaddr.IPPort{}, err
}
} else {
// Ask for our external address if needed.
if m.external.IP().IsZero() {
if _, err := uc.WriteTo(pmpReqExternalAddrPacket, pxpAddru); err != nil {
return netaddr.IPPort{}, err
}
}
pkt := buildPMPRequestMappingPacket(localPort, prevPort, pmpMapLifetimeSec)
if _, err := uc.WriteTo(pkt, pxpAddru); err != nil {
return netaddr.IPPort{}, err
}
}
res := make([]byte, 1500)
for {
n, srci, err := uc.ReadFrom(res)
if err != nil {
if ctx.Err() == context.Canceled {
return netaddr.IPPort{}, err
}
// fallback to UPnP portmapping
if mapping, ok := c.getUPnPPortMapping(ctx, gw, internalAddr, prevPort); ok {
return mapping, nil
}
return netaddr.IPPort{}, NoMappingError{ErrNoPortMappingServices}
}
srcu := srci.(*net.UDPAddr)
src, ok := netaddr.FromStdAddr(srcu.IP, srcu.Port, srcu.Zone)
if !ok {
continue
}
if src == pxpAddr {
version := res[0]
switch version {
case pmpVersion:
pres, ok := parsePMPResponse(res[:n])
if !ok {
c.logf("unexpected PMP response: % 02x", res[:n])
continue
}
if pres.ResultCode != 0 {
return netaddr.IPPort{}, NoMappingError{fmt.Errorf("PMP response Op=0x%x,Res=0x%x", pres.OpCode, pres.ResultCode)}
}
if pres.OpCode == pmpOpReply|pmpOpMapPublicAddr {
m.external = m.external.WithIP(pres.PublicAddr)
}
if pres.OpCode == pmpOpReply|pmpOpMapUDP {
m.external = m.external.WithPort(pres.ExternalPort)
d := time.Duration(pres.MappingValidSeconds) * time.Second
now := time.Now()
m.goodUntil = now.Add(d)
m.renewAfter = now.Add(d / 2) // renew in half the time
m.epoch = pres.SecondsSinceEpoch
}
case pcpVersion:
pcpMapping, err := parsePCPMapResponse(res[:n])
if err != nil {
c.logf("failed to get PCP mapping: %v", err)
// PCP should only have a single packet response
return netaddr.IPPort{}, NoMappingError{ErrNoPortMappingServices}
}
pcpMapping.c = c
pcpMapping.internal = m.internal
pcpMapping.gw = netaddr.IPPortFrom(gw, c.pxpPort())
c.mu.Lock()
defer c.mu.Unlock()
c.mapping = pcpMapping
return pcpMapping.external, nil
default:
c.logf("unknown PMP/PCP version number: %d %v", version, res[:n])
return netaddr.IPPort{}, NoMappingError{ErrNoPortMappingServices}
}
}
if m.externalValid() {
c.mu.Lock()
defer c.mu.Unlock()
c.mapping = m
return m.external, nil
}
}
}
type pmpResultCode uint16
// NAT-PMP constants.
const (
pmpDefaultPort = 5351
pmpMapLifetimeSec = 7200 // RFC recommended 2 hour map duration
pmpMapLifetimeDelete = 0 // 0 second lifetime deletes
pmpVersion = 0
pmpOpMapPublicAddr = 0
pmpOpMapUDP = 1
pmpOpReply = 0x80 // OR'd into request's op code on response
pmpCodeOK pmpResultCode = 0
pmpCodeUnsupportedVersion pmpResultCode = 1
pmpCodeNotAuthorized pmpResultCode = 2 // "e.g., box supports mapping, but user has turned feature off"
pmpCodeNetworkFailure pmpResultCode = 3 // "e.g., NAT box itself has not obtained a DHCP lease"
pmpCodeOutOfResources pmpResultCode = 4
pmpCodeUnsupportedOpcode pmpResultCode = 5
)
func buildPMPRequestMappingPacket(localPort, prevPort uint16, lifetimeSec uint32) (pkt []byte) {
pkt = make([]byte, 12)
pkt[1] = pmpOpMapUDP
binary.BigEndian.PutUint16(pkt[4:], localPort)
binary.BigEndian.PutUint16(pkt[6:], prevPort)
binary.BigEndian.PutUint32(pkt[8:], lifetimeSec)
return pkt
}
type pmpResponse struct {
OpCode uint8
ResultCode pmpResultCode
SecondsSinceEpoch uint32
// For Map ops:
MappingValidSeconds uint32
InternalPort uint16
ExternalPort uint16
// For public addr ops:
PublicAddr netaddr.IP
}
func parsePMPResponse(pkt []byte) (res pmpResponse, ok bool) {
if len(pkt) < 12 {
return
}
ver := pkt[0]
if ver != 0 {
return
}
res.OpCode = pkt[1]
res.ResultCode = pmpResultCode(binary.BigEndian.Uint16(pkt[2:]))
res.SecondsSinceEpoch = binary.BigEndian.Uint32(pkt[4:])
if res.OpCode == pmpOpReply|pmpOpMapUDP {
if len(pkt) != 16 {
return res, false
}
res.InternalPort = binary.BigEndian.Uint16(pkt[8:])
res.ExternalPort = binary.BigEndian.Uint16(pkt[10:])
res.MappingValidSeconds = binary.BigEndian.Uint32(pkt[12:])
}
if res.OpCode == pmpOpReply|pmpOpMapPublicAddr {
if len(pkt) != 12 {
return res, false
}
res.PublicAddr = netaddr.IPv4(pkt[8], pkt[9], pkt[10], pkt[11])
}
return res, true
}
type ProbeResult struct {
PCP bool
PMP bool
UPnP bool
}
// Probe returns a summary of which port mapping services are
// available on the network.
//
// If a probe has run recently and there haven't been any network changes since,
// the returned result might be server from the Client's cache, without
// sending any network traffic.
func (c *Client) Probe(ctx context.Context) (res ProbeResult, err error) {
gw, myIP, ok := c.gatewayAndSelfIP()
if !ok {
return res, ErrGatewayRange
}
defer func() {
if err == nil {
c.mu.Lock()
defer c.mu.Unlock()
c.lastProbe = time.Now()
}
}()
uc, err := c.listenPacket(context.Background(), "udp4", ":0")
if err != nil {
c.logf("ProbePCP: %v", err)
return res, err
}
defer uc.Close()
ctx, cancel := context.WithTimeout(ctx, 250*time.Millisecond)
defer cancel()
defer closeCloserOnContextDone(ctx, uc)()
pxpAddr := netaddr.IPPortFrom(gw, c.pxpPort()).UDPAddr()
upnpAddr := netaddr.IPPortFrom(gw, c.upnpPort()).UDPAddr()
// Don't send probes to services that we recently learned (for
// the same gw/myIP) are available. See
// https://github.com/tailscale/tailscale/issues/1001
if c.sawPMPRecently() {
res.PMP = true
} else if !DisablePMP {
uc.WriteTo(pmpReqExternalAddrPacket, pxpAddr)
}
if c.sawPCPRecently() {
res.PCP = true
} else if !DisablePCP {
uc.WriteTo(pcpAnnounceRequest(myIP), pxpAddr)
}
if c.sawUPnPRecently() {
res.UPnP = true
} else if !DisableUPnP {
uc.WriteTo(uPnPPacket, upnpAddr)
}
buf := make([]byte, 1500)
pcpHeard := false // true when we get any PCP response
for {
if pcpHeard && res.PMP && res.UPnP {
// Nothing more to discover.
return res, nil
}
n, addr, err := uc.ReadFrom(buf)
if err != nil {
if ctx.Err() == context.DeadlineExceeded {
err = nil
}
return res, err
}
port := uint16(addr.(*net.UDPAddr).Port)
switch port {
case c.upnpPort():
if mem.Contains(mem.B(buf[:n]), mem.S(":InternetGatewayDevice:")) {
meta, err := parseUPnPDiscoResponse(buf[:n])
if err != nil {
c.logf("unrecognized UPnP discovery response; ignoring")
}
if VerboseLogs {
c.logf("UPnP reply %+v, %q", meta, buf[:n])
}
res.UPnP = true
c.mu.Lock()
c.uPnPSawTime = time.Now()
if c.uPnPMeta != meta {
c.logf("UPnP meta changed: %+v", meta)
c.uPnPMeta = meta
}
c.mu.Unlock()
}
case c.pxpPort(): // same value for PMP and PCP
if pres, ok := parsePCPResponse(buf[:n]); ok {
if pres.OpCode == pcpOpReply|pcpOpAnnounce {
pcpHeard = true
c.mu.Lock()
c.pcpSawTime = time.Now()
c.mu.Unlock()
switch pres.ResultCode {
case pcpCodeOK:
c.logf("Got PCP response: epoch: %v", pres.Epoch)
res.PCP = true
continue
case pcpCodeNotAuthorized:
// A PCP service is running, but refuses to
// provide port mapping services.
res.PCP = false
continue
default:
// Fall through to unexpected log line.
}
}
c.logf("unexpected PCP probe response: %+v", pres)
}
if pres, ok := parsePMPResponse(buf[:n]); ok {
if pres.OpCode == pmpOpReply|pmpOpMapPublicAddr && pres.ResultCode == pmpCodeOK {
c.logf("Got PMP response; IP: %v, epoch: %v", pres.PublicAddr, pres.SecondsSinceEpoch)
res.PMP = true
c.mu.Lock()
c.pmpPubIP = pres.PublicAddr
c.pmpPubIPTime = time.Now()
c.pmpLastEpoch = pres.SecondsSinceEpoch
c.mu.Unlock()
continue
}
c.logf("unexpected PMP probe response: %+v", pres)
}
}
}
}
var pmpReqExternalAddrPacket = []byte{pmpVersion, pmpOpMapPublicAddr} // 0, 0
const (
upnpDefaultPort = 1900 // for UDP discovery only; TCP port discovered later
)
// uPnPPacket is the UPnP UDP discovery packet's request body.
var uPnPPacket = []byte("M-SEARCH * HTTP/1.1\r\n" +
"HOST: 239.255.255.250:1900\r\n" +
"ST: ssdp:all\r\n" +
"MAN: \"ssdp:discover\"\r\n" +
"MX: 2\r\n\r\n")