V3.3

src/main.go

@@ -444,7 +444,8 @@ func (t *Tracer) sendProbeUDP(ttl int) {
 	t.hops[ttl-1].recordSent()
 
 	t.sendMu.Lock()
-	_ = t.udpPC.SetTTL(ttl)
+	_ = setUDPSocketTTL(t.udpPC, t.udpSend, ttl)
+
 	_, err := t.udpSend.WriteTo(payload, &net.UDPAddr{IP: t.targetIP, Port: dstPort})
 	t.sendMu.Unlock()
 
@@ -523,6 +524,8 @@ func (t *Tracer) receiveLoop() {
 			}
 		}
 
+		fmt.Fprintf(os.Stderr, "[debug] icmp recv %d bytes from %s\n", n, peer)
+
 		from := peerString(peer)
 		now := time.Now()
 
@@ -624,19 +627,22 @@ func extractOriginalICMP(data []byte) (seq, id int, ok bool) {
 // extractOriginalUDPPort reads the original UDP destination port from the
 // embedded original IP+UDP header in a Time Exceeded or Dest Unreachable reply.
 func extractOriginalUDPPort(data []byte) (dstPort int, ok bool) {
-	if len(data) < 28 {
-		return 0, false
-	}
-	ihl := int(data[0]&0x0f) * 4
-	if ihl < 20 || len(data) < ihl+8 {
-		return 0, false
-	}
-	if data[9] != 17 { // protocol must be UDP (17)
-		return 0, false
-	}
-	// UDP: srcPort(2) dstPort(2) length(2) checksum(2)
-	dstPort = int(binary.BigEndian.Uint16(data[ihl+2 : ihl+4]))
-	return dstPort, true
+    if len(data) < 28 {
+        fmt.Fprintf(os.Stderr, "[debug] extractUDP: too short (%d bytes)\n", len(data))
+        return 0, false
+    }
+    ihl := int(data[0]&0x0f) * 4
+    if ihl < 20 || len(data) < ihl+8 {
+        fmt.Fprintf(os.Stderr, "[debug] extractUDP: bad IHL (%d)\n", ihl)
+        return 0, false
+    }
+    if data[9] != 17 {
+        fmt.Fprintf(os.Stderr, "[debug] extractUDP: proto=%d (not UDP)\n", data[9])
+        return 0, false
+    }
+    dstPort = int(binary.BigEndian.Uint16(data[ihl+2 : ihl+4]))
+    fmt.Fprintf(os.Stderr, "[debug] extractUDP: dstPort=%d\n", dstPort)
+    return dstPort, true
 }
 
 // ─── Reply handler ────────────────────────────────────────────────────────────

src/winapi_other.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"net"
 	"time"
+        "golang.org/x/net/ipv4"
 )
 
 func winAPIProbe(_ net.IP, _ int, _ time.Duration, _ []byte) (float64, string, uint32, error) {
@@ -15,3 +16,7 @@ func winAPIProbe(_ net.IP, _ int, _ time.Duration, _ []byte) (float64, string, u
 func (t *Tracer) sendProbeWinAPI(_ int) {}
 
 func udpModeWarning() {}
+
+func setUDPSocketTTL(pc *ipv4.PacketConn, conn net.PacketConn, ttl int) error {
+    return pc.SetTTL(ttl)
+}

src/winapi_windows.go

@@ -9,6 +9,7 @@ import (
 	"time"
 	"unsafe"
 	"os"
+	"golang.org/x/net/ipv4"
 )
 
 var (
@@ -142,3 +143,20 @@ func (t *Tracer) sendProbeWinAPI(ttl int) {
 		}
 	}()
 }
+
+func setUDPSocketTTL(pc *ipv4.PacketConn, conn net.PacketConn, ttl int) error {
+    udpConn, ok := conn.(*net.UDPConn)
+    if !ok {
+        return pc.SetTTL(ttl)
+    }
+    raw, err := udpConn.SyscallConn()
+    if err != nil {
+        return pc.SetTTL(ttl)
+    }
+    var sockErr error
+    raw.Control(func(fd uintptr) {
+        sockErr = syscall.SetsockoptInt(syscall.Handle(fd),
+            syscall.IPPROTO_IP, syscall.IP_TTL, ttl)
+    })
+    return sockErr
+}