python-zeroconf · bdraco · May 20, 2026 · May 20, 2026
diff --git a/src/zeroconf/_listener.pxd b/src/zeroconf/_listener.pxd
@@ -15,6 +15,8 @@ cdef bint TYPE_CHECKING
 cdef cython.uint _MAX_MSG_ABSOLUTE
 cdef cython.uint _DUPLICATE_PACKET_SUPPRESSION_INTERVAL
 cdef cython.uint _RECENT_PACKETS_MAX
+cdef cython.uint _MAX_DEFERRED_ADDRS
+cdef cython.uint _MAX_DEFERRED_PER_ADDR
 
 
 cdef class AsyncListener:
@@ -41,6 +43,8 @@ cdef class AsyncListener:
 
     cdef _cancel_any_timers_for_addr(self, object addr)
 
+    cdef _evict_oldest_deferred(self)
+
     @cython.locals(deadline=object, fire_at=double)
     cdef double _compute_deferred_fire_at(self, object addr, double now, double delay)
 

diff --git a/src/zeroconf/_listener.py b/src/zeroconf/_listener.py
@@ -32,7 +32,13 @@
 from ._protocol.incoming import DNSIncoming
 from ._transport import _WrappedTransport, make_wrapped_transport
 from ._utils.time import current_time_millis, millis_to_seconds
-from .const import _DUPLICATE_PACKET_SUPPRESSION_INTERVAL, _MAX_MSG_ABSOLUTE, _RECENT_PACKETS_MAX
+from .const import (
+    _DUPLICATE_PACKET_SUPPRESSION_INTERVAL,
+    _MAX_DEFERRED_ADDRS,
+    _MAX_DEFERRED_PER_ADDR,
+    _MAX_MSG_ABSOLUTE,
+    _RECENT_PACKETS_MAX,
+)
 
 if TYPE_CHECKING:
     from ._core import Zeroconf
@@ -240,7 +246,17 @@ def handle_query_or_defer(
             self._respond_query(msg, addr, port, transport, v6_flow_scope)
             return
 
+        if addr not in self._deferred and len(self._deferred) >= _MAX_DEFERRED_ADDRS:
+            # Bound total deferred addrs so a spoofed-source flood
+            # cannot keep adding distinct entries; evict the oldest
+            # (insertion-order) entry and discard its in-flight queue.
+            self._evict_oldest_deferred()
+
         deferred = self._deferred.setdefault(addr, [])
+        if len(deferred) >= _MAX_DEFERRED_PER_ADDR:
+            # Bound per-addr queue length; further fragments from the
+            # same source are dropped until the timer flushes.
+            return
         # If we get the same packet we ignore it
         for incoming in reversed(deferred):
             if incoming.data == msg.data:
@@ -293,6 +309,21 @@ def _cancel_any_timers_for_addr(self, addr: _str) -> None:
         if addr in self._timers:
             self._timers.pop(addr).cancel()
 
+    def _evict_oldest_deferred(self) -> None:
+        """Discard the oldest deferred addr's reassembly state.
+
+        Used when ``_MAX_DEFERRED_ADDRS`` would be exceeded; the
+        evicted addr's queue and timer are dropped without firing, so
+        the bound holds even when an attacker rotates source IPs.
+        Eviction is FIFO (oldest by first-seen, via dict insertion
+        order) rather than LRU so an active flooder cannot pin its
+        slots by re-sending into the same addr.
+        """
+        oldest_addr = next(iter(self._deferred))
+        self._cancel_any_timers_for_addr(oldest_addr)
+        self._deferred_deadlines.pop(oldest_addr, None)
+        del self._deferred[oldest_addr]
+
     def _respond_query(
         self,
         msg: DNSIncoming | None,

diff --git a/src/zeroconf/const.py b/src/zeroconf/const.py
@@ -77,6 +77,20 @@
 # flooding distinct questions (RFC 6762 §7.3, defense-in-depth).
 _MAX_QUESTION_HISTORY_ENTRIES = 10000
 
+# Per-addr cap on the number of truncated (TC-bit) packets retained for
+# RFC 6762 §18.5 reassembly. The spec anticipates only a handful of
+# segments per truncated query; 16 is well above legitimate need and
+# keeps the per-arrival dedup scan a constant-time cost under a flood.
+_MAX_DEFERRED_PER_ADDR = 16
+
+# Per-listener cap on the number of distinct addrs with in-flight
+# TC-deferral state. Each entry can hold up to _MAX_DEFERRED_PER_ADDR
+# packets of up to _MAX_MSG_ABSOLUTE bytes; 512 leaves headroom for a
+# legitimate burst (LAN-wide power-resume / boot storm where many
+# devices announce at once) while bounding worst-case memory at
+# ~72 MB even when a peer floods with spoofed source IPs.
+_MAX_DEFERRED_ADDRS = 512
+
 _DNS_PACKET_HEADER_LEN = 12
 
 _MAX_MSG_TYPICAL = 1460  # unused

diff --git a/tests/test_core.py b/tests/test_core.py
@@ -19,7 +19,7 @@
 import pytest
 
 import zeroconf as r
-from zeroconf import NotRunningException, Zeroconf, const, current_time_millis
+from zeroconf import NotRunningException, Zeroconf, _listener, const, current_time_millis
 from zeroconf._listener import _TC_DELAY_RANDOM_INTERVAL, AsyncListener, _WrappedTransport
 from zeroconf._protocol.incoming import DNSIncoming
 from zeroconf.asyncio import AsyncZeroconf
@@ -794,6 +794,101 @@ def test_tc_bit_defer_window_is_bounded():
     zc.close()
 
 
+def _make_distinct_tc_packets(count: int, name_prefix: str = "q") -> list[bytes]:
+    """Generate ``count`` byte-distinct TC-flagged query packets for flood inputs."""
+    packets = []
+    for i in range(count):
+        out = r.DNSOutgoing(const._FLAGS_QR_QUERY | const._FLAGS_TC)
+        out.add_question(r.DNSQuestion(f"{name_prefix}{i}._tcp.local.", const._TYPE_PTR, const._CLASS_IN))
+        packets.append(out.packets()[0])
+    return packets
+
+
+def _synthetic_source_ip(i: int) -> str:
+    """Distinct synthetic source IPs from the documentation ranges."""
+    if i < 256:
+        return f"203.0.113.{i}"
+    if i < 512:
+        return f"198.51.100.{i - 256}"
+    return f"192.0.2.{i - 512}"
+
+
+def test_tc_bit_per_addr_queue_is_bounded(quick_timing: None) -> None:
+    """Per-addr deferred queue must not grow past ``_MAX_DEFERRED_PER_ADDR``."""
+    zc = Zeroconf(interfaces=["127.0.0.1"])
+    _wait_for_start(zc)
+    protocol = zc.engine.protocols[0]
+    source_ip = "203.0.113.21"
+
+    extra = 4
+    packets = _make_distinct_tc_packets(const._MAX_DEFERRED_PER_ADDR + extra)
+
+    # Push the reassembly timer well past any possible test runtime
+    # so the bound under test is the only thing that can drop entries.
+    with patch.object(_listener, "_TC_DELAY_RANDOM_INTERVAL", (60_000, 60_001)):
+        for raw in packets:
+            threadsafe_query(zc, protocol, r.DNSIncoming(raw), source_ip, const._MDNS_PORT, Mock(), ())
+
+        assert len(protocol._deferred[source_ip]) == const._MAX_DEFERRED_PER_ADDR
+        # Last ``extra`` packets must have been dropped, not displaced; the
+        # earlier ``_MAX_DEFERRED_PER_ADDR`` entries are the ones retained.
+        retained = [incoming.data for incoming in protocol._deferred[source_ip]]
+        assert retained == packets[: const._MAX_DEFERRED_PER_ADDR]
+
+    zc.close()
+
+
+def test_tc_bit_total_addrs_is_bounded(quick_timing: None) -> None:
+    """Distinct addrs with deferred state must not exceed ``_MAX_DEFERRED_ADDRS``."""
+    zc = Zeroconf(interfaces=["127.0.0.1"])
+    _wait_for_start(zc)
+    protocol = zc.engine.protocols[0]
+
+    raw = _make_distinct_tc_packets(1)[0]
+    extra = 4
+    addrs = [_synthetic_source_ip(i) for i in range(const._MAX_DEFERRED_ADDRS + extra)]
+
+    # Push the reassembly timer well past any possible test runtime
+    # so the bound under test is the only thing that can drop entries;
+    # without this, PyPy / slow runners can fire timers between the
+    # last enqueue and the assertion.
+    with patch.object(_listener, "_TC_DELAY_RANDOM_INTERVAL", (60_000, 60_001)):
+        for source_ip in addrs:
+            threadsafe_query(zc, protocol, r.DNSIncoming(raw), source_ip, const._MDNS_PORT, Mock(), ())
+
+        assert len(protocol._deferred) == const._MAX_DEFERRED_ADDRS
+        assert len(protocol._timers) == const._MAX_DEFERRED_ADDRS
+
+    zc.close()
+
+
+def test_tc_bit_eviction_drops_oldest_addr(quick_timing: None) -> None:
+    """Adding a new addr at capacity drops the oldest insertion (FIFO)."""
+    zc = Zeroconf(interfaces=["127.0.0.1"])
+    _wait_for_start(zc)
+    protocol = zc.engine.protocols[0]
+
+    raw = _make_distinct_tc_packets(1)[0]
+    fillers = [_synthetic_source_ip(i) for i in range(const._MAX_DEFERRED_ADDRS)]
+    new_addr = _synthetic_source_ip(const._MAX_DEFERRED_ADDRS)
+    oldest = fillers[0]
+
+    with patch.object(_listener, "_TC_DELAY_RANDOM_INTERVAL", (60_000, 60_001)):
+        for source_ip in fillers:
+            threadsafe_query(zc, protocol, r.DNSIncoming(raw), source_ip, const._MDNS_PORT, Mock(), ())
+        assert len(protocol._deferred) == const._MAX_DEFERRED_ADDRS
+        assert oldest in protocol._deferred
+
+        # One more distinct addr must evict the oldest insertion-order entry.
+        threadsafe_query(zc, protocol, r.DNSIncoming(raw), new_addr, const._MDNS_PORT, Mock(), ())
+        assert oldest not in protocol._deferred
+        assert oldest not in protocol._timers
+        assert new_addr in protocol._deferred
+        assert len(protocol._deferred) == const._MAX_DEFERRED_ADDRS
+
+    zc.close()
+
+
 @pytest.mark.asyncio
 async def test_open_close_twice_from_async() -> None:
     """Test we can close twice from a coroutine when using Zeroconf.