fix: skip NSEC records in ServiceBrowser to suppress spurious updates (#1762)

bluetoothbot · web-flow · commit 137a5d6c2938 · 2026-05-20T17:58:17.000-05:00
diff --git a/src/zeroconf/_services/browser.pxd b/src/zeroconf/_services/browser.pxd
@@ -14,6 +14,7 @@ cdef bint TYPE_CHECKING
 cdef object cached_possible_types
 cdef cython.uint _EXPIRE_REFRESH_TIME_PERCENT, _MAX_MSG_TYPICAL, _DNS_PACKET_HEADER_LEN
 cdef cython.uint _TYPE_PTR
+cdef cython.uint _TYPE_NSEC
 cdef object _CLASS_IN
 cdef object SERVICE_STATE_CHANGE_ADDED, SERVICE_STATE_CHANGE_REMOVED, SERVICE_STATE_CHANGE_UPDATED
 cdef cython.set _ADDRESS_RECORD_TYPES
diff --git a/src/zeroconf/_services/browser.py b/src/zeroconf/_services/browser.py
@@ -63,6 +63,7 @@
     _MDNS_ADDR,
     _MDNS_ADDR6,
     _MDNS_PORT,
+    _TYPE_NSEC,
     _TYPE_PTR,
 )
 
@@ -678,7 +679,12 @@ def async_update_records(self, zc: Zeroconf, now: float_, records: list[RecordUp
             old_record = record_update.old
             record_type = record.type
 
-            if record_type is _TYPE_PTR:
+            # NSEC records assert non-existence of a record type
+            # (RFC 6762 §6.1); skip so we do not fire spurious updates.
+            if record_type == _TYPE_NSEC:
+                continue
+
+            if record_type == _TYPE_PTR:
                 if TYPE_CHECKING:
                     record = cast(DNSPointer, record)
                 pointer = record
diff --git a/tests/services/test_browser.py b/tests/services/test_browser.py
@@ -1085,6 +1085,73 @@ def update_service(self, zc, type_, name) -> None:  # type: ignore[no-untyped-de
     zc.close()
 
 
+def test_service_browser_nsec_record_does_not_trigger_update():
+    """NSEC records assert non-existence and must not fire ServiceStateChange.Updated."""
+    zc = Zeroconf(interfaces=["127.0.0.1"])
+    type_ = "_hap._tcp.local."
+    registration_name = f"xxxyyy.{type_}"
+    callbacks: list[tuple[str, str, str]] = []
+    service_added = Event()
+
+    class MyServiceListener(r.ServiceListener):
+        def add_service(self, zc, type_, name) -> None:  # type: ignore[no-untyped-def]
+            if name == registration_name:
+                callbacks.append(("add", type_, name))
+                service_added.set()
+
+        def remove_service(self, zc, type_, name) -> None:  # type: ignore[no-untyped-def]
+            if name == registration_name:
+                callbacks.append(("remove", type_, name))
+
+        def update_service(self, zc, type_, name) -> None:  # type: ignore[no-untyped-def]
+            if name == registration_name:
+                callbacks.append(("update", type_, name))
+
+    listener = MyServiceListener()
+    browser = r.ServiceBrowser(zc, type_, None, listener)
+    try:
+        desc = {"path": "/~paulsm/"}
+        address = socket.inet_aton("10.0.1.2")
+        info = ServiceInfo(type_, registration_name, 80, 0, 0, desc, "ash-2.local.", addresses=[address])
+
+        _inject_response(
+            zc,
+            mock_incoming_msg(
+                [
+                    info.dns_pointer(),
+                    info.dns_service(),
+                    info.dns_text(),
+                    *info.dns_addresses(),
+                ]
+            ),
+        )
+        assert service_added.wait(timeout=5), "add_service callback never fired"
+
+        # NSEC inject runs synchronously through the event loop; once
+        # _inject_response returns, async_update_records has already
+        # decided not to enqueue a callback for the NSEC record.
+        _inject_response(
+            zc,
+            mock_incoming_msg(
+                [
+                    r.DNSNsec(
+                        registration_name,
+                        const._TYPE_NSEC,
+                        const._CLASS_IN | const._CLASS_UNIQUE,
+                        const._DNS_OTHER_TTL,
+                        registration_name,
+                        [const._TYPE_AAAA],
+                    ),
+                ]
+            ),
+        )
+
+        assert callbacks == [("add", type_, registration_name)]
+    finally:
+        browser.cancel()
+        zc.close()
+
+
 def test_service_browser_uses_non_strict_names():
     """Verify we can look for technically invalid names as we cannot change what others do."""
 
