diff --git a/hbd/server/udp.py b/hbd/server/udp.py
index e6034a4..50a7e29 100644
--- a/hbd/server/udp.py
+++ b/hbd/server/udp.py
@@ -422,6 +422,12 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
         except Exception as e:
             logger.debug(f"Failed to log message to journal: {e}")
     
+    # Only named HTB/PLG datagrams come from clients; anything else (e.g. an internet scanner's
+    # SIP probe) is journaled above but must not sign on a host.
+    if msg.get("ID") not in ("HTB", "PLG") or not msg.get("name"):
+        logger.debug("Ignoring non-client datagram from %s", addr)
+        return
+
     cfg = ctx.get("config", {})
     hbdcls = ctx.get("hbdclass")
     msg_to_websockets = ctx.get("msg_to_websockets")
@@ -430,7 +436,7 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
 
     # normalize addr (ip, port)
     ip = addr[0] if isinstance(addr, (list, tuple)) else addr
-    name = msg.get("name", "unknown")
+    name = msg["name"]
     from ..common.utils import shortname
     from . import config as config_mod
 
diff --git a/tests/test_udp_stray_datagrams.py b/tests/test_udp_stray_datagrams.py
new file mode 100644
index 0000000..2e1cc74
--- /dev/null
+++ b/tests/test_udp_stray_datagrams.py
@@ -0,0 +1,40 @@
+"""Datagrams that are not heartbeat traffic (internet scanners) must not create hosts."""
+from hbd.common.proto import dicttos
+from hbd.server import hbdclass
+from hbd.server.udp import handle_datagram, parse_message
+
+
+class _FakeTransport:
+    sent = 0
+
+    def sendto(self, data, addr):
+        _FakeTransport.sent += 1
+
+
+def _ctx():
+    return {"config": {}, "hbdclass": hbdclass, "msg_to_websockets": None,
+            "DEBUG": 0, "verbose": False}
+
+
+def _handle(raw):
+    handle_datagram(parse_message(raw), ("192.0.2.9", 50000), _FakeTransport(), _ctx())
+
+
+def test_sip_invite_does_not_create_a_host():
+    hbdclass.Host.hosts.pop("unknown", None)
+    _handle(b"INVITE sip:10000@192.0.2.1:50003 SIP/2.0\r\nTo: <sip:10000@192.0.2.1>\r\n\r\n")
+    assert "unknown" not in hbdclass.Host.hosts
+
+
+def test_nameless_heartbeat_does_not_create_a_host_or_get_acked():
+    hbdclass.Host.hosts.pop("unknown", None)
+    _FakeTransport.sent = 0
+    _handle(dicttos("HTB", {"id": 0}))
+    assert "unknown" not in hbdclass.Host.hosts
+    assert _FakeTransport.sent == 0
+
+
+def test_named_heartbeat_still_creates_its_host():
+    hbdclass.Host.hosts.pop("stray-ok", None)
+    _handle(dicttos("HTB", {"name": "stray-ok", "id": 0}))
+    assert "stray-ok" in hbdclass.Host.hosts