feat(dns): global upstream forward rate limit with sinkhole fallback

Adds DNS_FORWARD_BUDGET (default 50) and DNS_FORWARD_WINDOW (default 1.0s)
env vars. _can_forward() maintains a rolling deque of upstream call
timestamps; queries that exceed the budget within the window are answered
with the sinkhole (127.x) instead of being forwarded, making the honeypot
ineligible as a sustained amp vector even when real_recursive is enabled.
Rate limit is global (not per-source) so IP-spoofed amplification floods
hit the ceiling regardless of how many source addresses are rotated.

decnet/services/dns.py

@@ -65,6 +65,20 @@ class DNSService(BaseService):
             placeholder="8.8.8.8:53",
             help="Upstream DNS resolver used when real_recursive is enabled (host:port).",
         ),
+        ServiceConfigField(
+            key="forward_budget",
+            label="Forward budget (queries/window)",
+            type="string",
+            default="50",
+            help="Maximum upstream forwarding calls allowed within the rate window. Excess queries fall back to sinkhole.",
+        ),
+        ServiceConfigField(
+            key="forward_window",
+            label="Forward budget window (seconds)",
+            type="string",
+            default="1.0",
+            help="Rolling window in seconds for the forward budget counter.",
+        ),
     ]
 
     def compose_fragment(
@@ -83,6 +97,8 @@ class DNSService(BaseService):
             "DNS_EXTRA_RECORDS":    str(cfg.get("extra_records",    "")),
             "DNS_REAL_RECURSIVE":   "true" if cfg.get("real_recursive") else "false",
             "DNS_UPSTREAM":         str(cfg.get("upstream",        "8.8.8.8:53")),
+            "DNS_FORWARD_BUDGET":   str(cfg.get("forward_budget",  "50")),
+            "DNS_FORWARD_WINDOW":   str(cfg.get("forward_window",  "1.0")),
         }
         if log_target:
             env["LOG_TARGET"] = log_target

decnet/templates/dns/server.py

@@ -313,6 +313,10 @@ _RECON_SIGNAL_TYPES       = frozenset({"fingerprint_probe", "zone_transfer", "am
 # Eviction telemetry
 _EVICT_EVENT_EVERY = 100
 
+# Global upstream forwarding budget
+_FORWARD_BUDGET_MAX = int(os.environ.get("DNS_FORWARD_BUDGET", "50"))
+_FORWARD_BUDGET_WIN = float(os.environ.get("DNS_FORWARD_WINDOW", "1.0"))
+
 # ── Per-src state ─────────────────────────────────────────────────────────────
 
 # Tunneling: src_ip -> deque of recent TXT timestamps
@@ -332,6 +336,18 @@ _recon_cooldown: dict[str, float] = {}
 
 _evictions_total = 0
 
+# Global forward budget: timestamps of recent upstream calls
+_forward_timestamps: collections.deque[float] = collections.deque()
+
+
+def _can_forward() -> bool:
+    """Return True and consume one budget slot if under the global forward limit."""
+    now = time.monotonic()
+    _forward_timestamps.append(now)
+    while _forward_timestamps[0] < now - _FORWARD_BUDGET_WIN:
+        _forward_timestamps.popleft()
+    return len(_forward_timestamps) <= _FORWARD_BUDGET_MAX
+
 
 def _note_eviction(tracker_name: str) -> None:
     global _evictions_total
@@ -619,7 +635,7 @@ async def _dispatch(data: bytes, src_ip: str, src_port: int, transport: str) ->
     """Async dispatcher: runs sync _handle (logging + detection), then overlays
     upstream forwarding for real-recursive out-of-zone queries."""
     sinkhole = _handle(data, src_ip, src_port, transport)
-    if _is_upstream_candidate(data):
+    if _is_upstream_candidate(data) and _can_forward():
         upstream = await _forward_upstream(data)
         if upstream is not None:
             return upstream

tests/service_testing/test_dns.py

@@ -576,6 +576,66 @@ class TestRealRecursive:
         assert frag["environment"]["DNS_REAL_RECURSIVE"] == "false"
 
 
+class TestForwardBudget:
+    def _load_with_budget(self, budget: int = 3):
+        mod, events = _load_dns({
+            "DNS_ZONE_MODE": "recursive",
+            "DNS_REAL_RECURSIVE": "true",
+            "DNS_FORWARD_BUDGET": str(budget),
+            "DNS_FORWARD_WINDOW": "60",  # wide window so nothing expires mid-test
+        })
+        mod._forward_timestamps.clear()
+        return mod, events
+
+    def test_within_budget_forwards(self):
+        mod, _ = self._load_with_budget(budget=5)
+        import asyncio
+        from unittest.mock import AsyncMock, patch
+        fake_resp = b"\x12\x34" + b"\x81\x80" + b"\x00" * 8
+        mock_fwd = AsyncMock(return_value=fake_resp)
+        with patch.object(mod, "_forward_upstream", mock_fwd):
+            query = _build_query("evil.example.com", mod.TYPE_A)
+            for _ in range(5):
+                asyncio.get_event_loop().run_until_complete(
+                    mod._dispatch(query, "1.1.1.1", 1234, "udp")
+                )
+        assert mock_fwd.await_count == 5
+
+    def test_over_budget_falls_back_to_sinkhole(self):
+        mod, _ = self._load_with_budget(budget=2)
+        import asyncio
+        from unittest.mock import AsyncMock, patch
+        fake_resp = b"\x12\x34" + b"\x81\x80" + b"\x00" * 8
+        mock_fwd = AsyncMock(return_value=fake_resp)
+        with patch.object(mod, "_forward_upstream", mock_fwd):
+            query = _build_query("evil.example.com", mod.TYPE_A)
+            responses = []
+            for _ in range(5):
+                resp = asyncio.get_event_loop().run_until_complete(
+                    mod._dispatch(query, "1.1.1.1", 1234, "udp")
+                )
+                responses.append(resp)
+        # Upstream called at most budget+1 times (budget check appends before pruning)
+        assert mock_fwd.await_count <= 3
+        # All responses are non-None (sinkhole for over-budget ones)
+        assert all(r is not None for r in responses)
+
+    def test_budget_is_global_not_per_src(self):
+        """Budget counts all upstream calls regardless of source IP."""
+        mod, _ = self._load_with_budget(budget=2)
+        import asyncio
+        from unittest.mock import AsyncMock, patch
+        fake_resp = b"\x12\x34" + b"\x81\x80" + b"\x00" * 8
+        mock_fwd = AsyncMock(return_value=fake_resp)
+        with patch.object(mod, "_forward_upstream", mock_fwd):
+            query = _build_query("evil.example.com", mod.TYPE_A)
+            for i in range(5):
+                asyncio.get_event_loop().run_until_complete(
+                    mod._dispatch(query, f"10.0.0.{i+1}", 1234, "udp")
+                )
+        assert mock_fwd.await_count <= 3
+
+
 class TestZoneModeRecursive:
     def test_recursive_mode_sets_ra_flag(self):
         mod, _ = _load_dns({"DNS_ZONE_MODE": "recursive"})