net/stun: unmap IPv4 addresses in 16 byte STUN replies

Updates #5602

Change-Id: I2276ad2bfb415b9ff52f37444f2a1d74b38543b1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/stun/stun.go

@@ -208,7 +208,7 @@ func ParseResponse(b []byte) (tID TxID, addr netip.AddrPort, err error) {
 		b = b[:attrsLen] // trim trailing packet bytes
 	}
 
-	var addr6, fallbackAddr, fallbackAddr6 netip.AddrPort
+	var fallbackAddr netip.AddrPort
 
 	// Read through the attributes.
 	// The the addr+port reported by XOR-MAPPED-ADDRESS
@@ -218,24 +218,20 @@ func ParseResponse(b []byte) (tID TxID, addr netip.AddrPort, err error) {
 	if err := foreachAttr(b, func(attrType uint16, attr []byte) error {
 		switch attrType {
 		case attrXorMappedAddress, attrXorMappedAddressAlt:
-			a, p, err := xorMappedAddress(tID, attr)
+			ipSlice, port, err := xorMappedAddress(tID, attr)
 			if err != nil {
 				return err
 			}
-			if len(a) == 16 {
-				addr6 = netip.AddrPortFrom(netip.AddrFrom16(*(*[16]byte)([]byte(a))), p)
-			} else {
-				addr = netip.AddrPortFrom(netip.AddrFrom4(*(*[4]byte)([]byte(a))), p)
+			if ip, ok := netip.AddrFromSlice(ipSlice); ok {
+				addr = netip.AddrPortFrom(ip.Unmap(), port)
 			}
 		case attrMappedAddress:
-			a, p, err := mappedAddress(attr)
+			ipSlice, port, err := mappedAddress(attr)
 			if err != nil {
 				return ErrMalformedAttrs
 			}
-			if len(a) == 16 {
-				fallbackAddr6 = netip.AddrPortFrom(netip.AddrFrom16(*(*[16]byte)([]byte(a))), p)
-			} else {
-				fallbackAddr = netip.AddrPortFrom(netip.AddrFrom4(*(*[4]byte)([]byte(a))), p)
+			if ip, ok := netip.AddrFromSlice(ipSlice); ok {
+				fallbackAddr = netip.AddrPortFrom(ip.Unmap(), port)
 			}
 		}
 		return nil
@@ -250,12 +246,6 @@ func ParseResponse(b []byte) (tID TxID, addr netip.AddrPort, err error) {
 	if fallbackAddr.IsValid() {
 		return tID, fallbackAddr, nil
 	}
-	if addr6.IsValid() {
-		return tID, addr6, nil
-	}
-	if fallbackAddr6.IsValid() {
-		return tID, fallbackAddr6, nil
-	}
 	return tID, netip.AddrPort{}, ErrMalformedAttrs
 }
 

net/stun/stun_test.go

@@ -6,11 +6,13 @@ package stun_test
 
 import (
 	"bytes"
+	"encoding/hex"
 	"fmt"
 	"net/netip"
 	"testing"
 
 	"tailscale.com/net/stun"
+	"tailscale.com/util/must"
 )
 
 // TODO(bradfitz): fuzz this.
@@ -175,6 +177,13 @@ var responseTests = []struct {
 		wantAddr: netip.AddrFrom4([4]byte{127, 0, 0, 1}),
 		wantPort: 61300,
 	},
+	{
+		name:     "no-4in6",
+		data:     must.Get(hex.DecodeString("010100182112a4424fd5d202dcb37d31fc773306002000140002cd3d2112a4424fd5d202dcb382ce2dc3fcc7")),
+		wantTID:  []byte{79, 213, 210, 2, 220, 179, 125, 49, 252, 119, 51, 6},
+		wantAddr: netip.AddrFrom4([4]byte{209, 180, 207, 193}),
+		wantPort: 60463,
+	},
 }
 
 func TestParseResponse(t *testing.T) {