Extract Bonjour hostname from mDNS and use it in arrival/departure alerts

_extract_mdns_hostname scans A records in mDNS payload before on_arrival fires,
so the ARRIVED alert shows the device's actual name instead of the IP.
_parse_mdns_for_names also now updates known_devices hostname on subsequent frames,
so departure alerts get the real name even if the first frame had no A record.

net_alerter/net_alerter.py

@@ -1456,9 +1456,46 @@ def _update_last_seen(mac: str) -> None:
     with last_seen_lock:
         last_seen[mac] = time.time()
 
+def _extract_mdns_hostname(payload: bytes) -> str:
+    """Scan mDNS DNS payload for an A record with a .local name. Returns display name (sans .local) or ''."""
+    try:
+        if len(payload) < 12:
+            return ""
+        qdcount = struct.unpack('!H', payload[4:6])[0]
+        ancount = struct.unpack('!H', payload[6:8])[0]
+        nscount = struct.unpack('!H', payload[8:10])[0]
+        arcount = struct.unpack('!H', payload[10:12])[0]
+        offset = 12
+        for _ in range(min(qdcount, 50)):
+            offset = _skip_dns_name(payload, offset)
+            if offset is None or offset + 4 > len(payload):
+                return ""
+            offset += 4
+        for _ in range(min(ancount + nscount + arcount, 200)):
+            if offset is None or offset + 10 > len(payload):
+                break
+            name_offset = offset
+            offset = _skip_dns_name(payload, offset)
+            if offset is None or offset + 10 > len(payload):
+                break
+            record_type = struct.unpack('!H', payload[offset:offset+2])[0]
+            rdlen = struct.unpack('!H', payload[offset+8:offset+10])[0]
+            offset += 10
+            if record_type == 1:  # A record — name is the device hostname
+                name = _extract_dns_name(payload, name_offset)
+                if name and name.endswith('.local'):
+                    return name[:-6]
+            if offset + rdlen > len(payload):
+                break
+            offset += rdlen
+    except Exception:
+        pass
+    return ""
+
+
 def _parse_mdns_for_names(payload: bytes, mac: str) -> None:
     """Parse mDNS DNS message to extract PTR/SRV/A record names and check against personal_names.
-    
+
     If a record name matches a name in personal_names, add the source MAC to personal_devices.
     """
     try:
@@ -1501,18 +1538,37 @@ def _parse_mdns_for_names(payload: bytes, mac: str) -> None:
             
             # Extract the name string from the record
             name = _extract_dns_name(payload, name_offset)
-            if name and personal_names:
-                with personal_lock:
-                    # Check if the record name matches any personal device names
-                    for pname in personal_names:
-                        if pname.lower() in name.lower():
-                            # Found a match - add this MAC to personal_devices
-                            mac_normalized = _normalize_mac(mac)
-                            if mac_normalized:
-                                personal_devices.add(mac_normalized)
-                                logging.debug(f"mDNS auto-discovery: added {mac_normalized} for name match '{pname}' in '{name}'")
-                            break
-            
+            if name:
+                # A record (type 1): record name IS the Bonjour hostname (e.g. Johns-iPhone.local)
+                # Update known_devices hostname if currently unset or just an IP address
+                if record_type == 1 and name.endswith('.local'):
+                    display_name = name[:-6]  # strip .local
+                    mac_normalized = _normalize_mac(mac)
+                    if mac_normalized and display_name:
+                        with known_lock:
+                            dev = known_devices.get(mac_normalized)
+                            if dev:
+                                current = dev.get('hostname', '')
+                                # Only overwrite if hostname is an IP or unset
+                                try:
+                                    socket.inet_aton(current)
+                                    is_ip = True
+                                except Exception:
+                                    is_ip = not current or current == mac_normalized
+                                if is_ip or not current:
+                                    dev['hostname'] = display_name
+                                    logging.debug(f"mDNS hostname update: {mac_normalized} → {display_name}")
+
+                if personal_names:
+                    with personal_lock:
+                        for pname in personal_names:
+                            if pname.lower() in name.lower():
+                                mac_normalized = _normalize_mac(mac)
+                                if mac_normalized:
+                                    personal_devices.add(mac_normalized)
+                                    logging.debug(f"mDNS auto-discovery: added {mac_normalized} for name match '{pname}' in '{name}'")
+                                break
+
             # Skip RDATA
             if offset + rdlen > len(payload):
                 break
@@ -1637,10 +1693,12 @@ def parse_frame(data: bytes) -> None:
                 with known_lock:
                     already_known = src_mac in known_devices
                 if not already_known:
-                    on_arrival(src_mac, src_ip)
+                    # Try to extract Bonjour hostname from this frame before alerting
+                    mdns_hostname = _extract_mdns_hostname(data[42:]) if len(data) >= 42 else ""
+                    on_arrival(src_mac, src_ip, mdns_hostname)
             # Ingest mDNS signal for multi-source device identity
             _ingest_signal(src_mac, 'mdns')
-            # Parse mDNS DNS message for device names
+            # Parse mDNS DNS message for device names (updates hostname on subsequent frames)
             if len(data) >= 42:
                 _parse_mdns_for_names(data[42:], src_mac)
             return