Improve tcpdump counter error handling and debugging

- Add detailed error messages when tcpdump fails or produces no output
- Show tcpdump stderr output for debugging filter issues
- Return 0 instead of None when tcpdump finds 0 packets (so it's displayed)
- Handle SIGTERM return code properly (expected when terminating)
- Display tcpdump count even when 0 to help diagnose issues
- Add note when tcpdump counter is unavailable

Co-authored-by: Cursor <cursoragent@cursor.com>

wifi_monitor.py

@@ -427,14 +427,18 @@ class CaptureAnalyzer:
         total_count = len(packets)
         print(f"Total packets captured (scapy): {total_count}")
 
+        scapy_data_count = sum(1 for pkt in packets if pkt.haslayer(Dot11) and pkt[Dot11].type == 2)
+        
         if tcpdump_data_count is not None:
+            print(f"Data frames captured (scapy): {scapy_data_count}")
             print(f"Data frames captured (tcpdump): {tcpdump_data_count}")
-            if total_count > 0:
-                scapy_data_count = sum(1 for pkt in packets if pkt.haslayer(Dot11) and pkt[Dot11].type == 2)
-                print(f"Data frames captured (scapy): {scapy_data_count}")
-                if tcpdump_data_count > 0:
-                    ratio = scapy_data_count / tcpdump_data_count
-                    print(f"Scapy capture ratio: {ratio:.1%} ({scapy_data_count}/{tcpdump_data_count})")
+            if tcpdump_data_count > 0:
+                ratio = scapy_data_count / tcpdump_data_count
+                print(f"Scapy capture ratio: {ratio:.1%} ({scapy_data_count}/{tcpdump_data_count})")
+            elif scapy_data_count > 0:
+                print(f"Note: tcpdump found 0 data frames while scapy found {scapy_data_count}")
+        else:
+            print("Note: tcpdump counter unavailable (tcpdump not found or failed)")
 
         if total_count == 0:
             self._print_no_packets_message()
@@ -786,11 +790,14 @@ class PacketCapture:
             )
             await check_proc.wait()
             if check_proc.returncode != 0:
+                print("Warning: tcpdump not found, skipping concurrent count")
                 return None
             
             # Use tcpdump with BPF filter for data frames
-            # wlan[0] & 0x0C extracts the type field (bits 2-3)
-            # Type 2 (data) = 0x08, so we check wlan[0] & 0x0C == 0x08
+            # For 802.11 frames: wlan[0] contains Frame Control field
+            # Bits 2-3 are the type field: 00=Management, 01=Control, 10=Data
+            # Type 2 (Data) = 0x08 when masked with 0x0C
+            # BPF syntax: use == for equality comparison
             proc = await asyncio.create_subprocess_exec(
                 "tcpdump",
                 "-i", interface,
@@ -817,6 +824,18 @@ class PacketCapture:
             _, stderr = await proc.communicate()
             stderr_text = stderr.decode() if stderr else ""
             
+            # Check return code
+            if proc.returncode != 0 and proc.returncode != -15:  # -15 is SIGTERM, which is expected
+                print(f"Warning: tcpdump exited with code {proc.returncode}")
+                if stderr_text:
+                    print(f"tcpdump stderr: {stderr_text[:300]}")
+                return None
+            
+            # Debug: print stderr if no count found
+            if not stderr_text.strip():
+                print("Warning: tcpdump produced no output")
+                return None
+            
             # Parse tcpdump output for packet count
             # Format: "X packets captured" or "X packets received by filter"
             data_frame_count = 0
@@ -834,11 +853,19 @@ class PacketCapture:
                     if potential_count > data_frame_count:
                         data_frame_count = potential_count
             
-            return data_frame_count if data_frame_count > 0 else None
+            if data_frame_count == 0:
+                # Debug output - but still return 0 so we can show it
+                print(f"Warning: tcpdump found 0 data frames. tcpdump stderr: {stderr_text[:200]}")
+            
+            # Return the count even if 0, so we can display it
+            return data_frame_count
         except FileNotFoundError:
+            print("Warning: tcpdump not found, skipping concurrent count")
             return None
         except Exception as e:
             print(f"Warning: tcpdump counter failed: {e}")
+            import traceback
+            traceback.print_exc()
             return None
 
     async def _main_capture_async(self, interface, duration):