V3.2

src/main.go

@@ -295,14 +295,17 @@ func NewTracer(
 func (t *Tracer) Done() <-chan struct{} { return t.stopCh }
 
 func (t *Tracer) Start() error {
-	if t.proto == "winapi" {
-		// Windows ICMP API manages its own sockets per probe.
-		// No raw socket, no receive loop, no cleanup loop needed.
-		go t.probeLoop()
-		return nil
-	}
+    if t.proto == "winapi" {
+        go t.probeLoop()
+        return nil
+    }
 
-	var err error
+    // ADD THIS:
+    if t.proto == "udp" {
+        udpModeWarning()
+    }
+
+    var err error
 	t.icmpConn, err = icmp.ListenPacket("ip4:icmp", "0.0.0.0")
 	if err != nil {
 		t.icmpConn, err = icmp.ListenPacket("udp4", "0.0.0.0:0")

src/winapi_other.go

@@ -13,3 +13,5 @@ func winAPIProbe(_ net.IP, _ int, _ time.Duration, _ []byte) (float64, string, u
 }
 
 func (t *Tracer) sendProbeWinAPI(_ int) {}
+
+func udpModeWarning() {}

src/winapi_windows.go

@@ -8,6 +8,7 @@ import (
 	"syscall"
 	"time"
 	"unsafe"
+	"os"
 )
 
 var (
@@ -23,6 +24,12 @@ const (
 	ipReqTimedOut       uint32 = 11010
 )
 
+func udpModeWarning() {
+    fmt.Fprintln(os.Stderr,
+        "Warning: UDP mode on Windows requires an inbound Windows Firewall rule "+
+            "to allow ICMP Time Exceeded (Type 11). Consider using -proto winapi instead.")
+}
+
 // mirrors IP_OPTION_INFORMATION from <ipexport.h>
 type ipOptInfo struct {
 	Ttl         uint8