From 1aeeeb7e45e346cf8942a0364b85c1c7194f09e9 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Tue, 23 Feb 2021 14:46:17 -0800
Subject: [PATCH] net/netcheck: do PCP discovery without side effects

Manually cherry-picked subset of c64bd587aee9709bc6a6203de58361e2bafc2dbf
back into the 1.4 branch.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 net/netcheck/netcheck.go | 50 ++++++++++------------------------------
 1 file changed, 12 insertions(+), 38 deletions(-)

diff --git a/net/netcheck/netcheck.go b/net/netcheck/netcheck.go
index 1b0194830..04fda240d 100644
--- a/net/netcheck/netcheck.go
+++ b/net/netcheck/netcheck.go
@@ -8,9 +8,7 @@ package netcheck
 import (
 	"bufio"
 	"context"
-	"crypto/rand"
 	"crypto/tls"
-	"encoding/binary"
 	"errors"
 	"fmt"
 	"io"
@@ -701,16 +699,14 @@ func (rs *reportState) probePortMapServices() {
 		return
 	}
 	defer uc.Close()
-	tempPort := uc.LocalAddr().(*net.UDPAddr).Port
 	uc.SetReadDeadline(time.Now().Add(portMapServiceProbeTimeout))
 
 	// Send request packets for all three protocols.
 	uc.WriteTo(uPnPPacket, port1900)
 	uc.WriteTo(pmpPacket, port5351)
-	uc.WriteTo(pcpPacket(myIP, tempPort, false), port5351)
+	uc.WriteTo(pcpAnnounceRequest(myIP), port5351)
 
 	res := make([]byte, 1500)
-	sentPCPDelete := false
 	for {
 		n, addr, err := uc.ReadFrom(res)
 		if err != nil {
@@ -725,17 +721,8 @@ func (rs *reportState) probePortMapServices() {
 			if n == 12 && res[0] == 0x00 { // right length and version 0
 				rs.setOptBool(&rs.report.PMP, true)
 			}
-			if n == 60 && res[0] == 0x02 { // right length and version 2
+			if n == 24 && res[0] == 0x02 { // right length and version 2
 				rs.setOptBool(&rs.report.PCP, true)
-
-				if !sentPCPDelete {
-					sentPCPDelete = true
-					// And now delete the mapping.
-					// (PCP is the only protocol of the three that requires
-					// we cause a side effect to detect whether it's present,
-					// so we need to redo that side effect now.)
-					uc.WriteTo(pcpPacket(myIP, tempPort, true), port5351)
-				}
 			}
 		}
 	}
@@ -751,32 +738,19 @@ var uPnPPacket = []byte("M-SEARCH * HTTP/1.1\r\n" +
 
 var v4unspec, _ = netaddr.ParseIP("0.0.0.0")
 
-// pcpPacket generates a PCP packet with a MAP opcode.
-func pcpPacket(myIP netaddr.IP, mapToLocalPort int, delete bool) []byte {
-	const udpProtoNumber = 17
-	lifetimeSeconds := uint32(1)
-	if delete {
-		lifetimeSeconds = 0
-	}
-	const opMap = 1
-
-	// 24 byte header + 36 byte map opcode
-	pkt := make([]byte, (32+32+128)/8+(96+8+24+16+16+128)/8)
+const (
+	pcpVersion    = 2
+	pcpOpAnnounce = 0
+)
 
-	// The header (https://tools.ietf.org/html/rfc6887#section-7.1)
-	pkt[0] = 2 // version
-	pkt[1] = opMap
-	binary.BigEndian.PutUint32(pkt[4:8], lifetimeSeconds)
+// pcpAnnounceRequest generates a PCP packet with an ANNOUNCE opcode.
+func pcpAnnounceRequest(myIP netaddr.IP) []byte {
+	// See https://tools.ietf.org/html/rfc6887#section-7.1
+	pkt := make([]byte, 24)
+	pkt[0] = pcpVersion // version
+	pkt[1] = pcpOpAnnounce
 	myIP16 := myIP.As16()
 	copy(pkt[8:], myIP16[:])
-
-	// The map opcode body (https://tools.ietf.org/html/rfc6887#section-11.1)
-	mapOp := pkt[24:]
-	rand.Read(mapOp[:12]) // 96 bit mappping nonce
-	mapOp[12] = udpProtoNumber
-	binary.BigEndian.PutUint16(mapOp[16:], uint16(mapToLocalPort))
-	v4unspec16 := v4unspec.As16()
-	copy(mapOp[20:], v4unspec16[:])
 	return pkt
 }