diff --git a/tests/test_sniffer_p0f.py b/tests/test_sniffer_p0f.py
new file mode 100644
index 0000000..a8f5715
--- /dev/null
+++ b/tests/test_sniffer_p0f.py
@@ -0,0 +1,117 @@
+"""
+Unit tests for the passive p0f-lite OS fingerprint lookup.
+
+Covers:
+    - initial_ttl() TTL → bucket rounding
+    - hop_distance() upper-bound clamping
+    - guess_os() signature matching for Linux, Windows, macOS, nmap,
+      embedded, and the unknown fallback
+"""
+
+from __future__ import annotations
+
+from decnet.sniffer.p0f import guess_os, hop_distance, initial_ttl
+
+
+# ─── initial_ttl ────────────────────────────────────────────────────────────
+
+class TestInitialTtl:
+    def test_linux_bsd(self):
+        assert initial_ttl(64) == 64
+        assert initial_ttl(59) == 64
+        assert initial_ttl(33) == 64
+
+    def test_windows(self):
+        assert initial_ttl(128) == 128
+        assert initial_ttl(120) == 128
+        assert initial_ttl(65) == 128
+
+    def test_embedded(self):
+        assert initial_ttl(255) == 255
+        assert initial_ttl(254) == 255
+        assert initial_ttl(200) == 255
+
+    def test_very_short(self):
+        # anything <= 32 rounds to 32
+        assert initial_ttl(32) == 32
+        assert initial_ttl(1) == 32
+
+    def test_out_of_range(self):
+        # Packets with TTL > 255 (should never happen) still bucket.
+        assert initial_ttl(300) == 255
+
+
+# ─── hop_distance ───────────────────────────────────────────────────────────
+
+class TestHopDistance:
+    def test_zero_when_local(self):
+        assert hop_distance(64) == 0
+        assert hop_distance(128) == 0
+        assert hop_distance(255) == 0
+
+    def test_typical(self):
+        assert hop_distance(60) == 4  # 4 hops from Linux
+        assert hop_distance(120) == 8  # 8 hops from Windows
+
+    def test_negative_or_weird_still_bucketed(self):
+        # TTL=0 is anomalous but we still return a non-negative distance.
+        # TTL 0 bucket is 32 → distance = 32 - 0 = 32.
+        assert hop_distance(0) == 32
+
+
+# ─── guess_os ───────────────────────────────────────────────────────────────
+
+class TestGuessOs:
+    def test_linux_default(self):
+        # Modern Linux: TTL 64, window 29200+, WScale 7, full options
+        result = guess_os(
+            ttl=64, window=29200, mss=1460, wscale=7,
+            options_sig="M,S,T,N,W",
+        )
+        assert result == "linux"
+
+    def test_windows_default(self):
+        # Windows 10: TTL 128, window 64240, WScale 8, MSS 1460
+        result = guess_os(
+            ttl=128, window=64240, mss=1460, wscale=8,
+            options_sig="M,N,W,N,N,T,S",
+        )
+        assert result == "windows"
+
+    def test_macos_ios(self):
+        # macOS default: TTL 64, window 65535, WScale 6, ends with EOL
+        result = guess_os(
+            ttl=64, window=65535, mss=1460, wscale=6,
+            options_sig="M,N,W,N,N,T,S,E",
+        )
+        assert result == "macos_ios"
+
+    def test_nmap_sYn(self):
+        # nmap -sS uses tiny/distinctive windows like 1024 or 4096
+        result = guess_os(
+            ttl=64, window=1024, mss=1460, wscale=10,
+            options_sig="M,W,T,S,S",
+        )
+        assert result == "nmap"
+
+    def test_nmap_alt_window(self):
+        result = guess_os(
+            ttl=64, window=31337, mss=1460, wscale=10,
+            options_sig="M,W,T,S,S",
+        )
+        assert result == "nmap"
+
+    def test_embedded_ttl255(self):
+        # Any TTL bucket 255 → embedded
+        result = guess_os(
+            ttl=250, window=4128, mss=536, wscale=None,
+            options_sig="M",
+        )
+        assert result == "embedded"
+
+    def test_unknown(self):
+        # Bizarre combo nothing matches
+        result = guess_os(
+            ttl=50, window=100, mss=0, wscale=None, options_sig="",
+        )
+        assert result == "unknown"
diff --git a/tests/test_sniffer_retransmit.py b/tests/test_sniffer_retransmit.py
new file mode 100644
index 0000000..7572886
--- /dev/null
+++ b/tests/test_sniffer_retransmit.py
@@ -0,0 +1,108 @@
+"""
+Unit tests for TCP retransmit detection in the SnifferEngine flow aggregator.
+
+A retransmit is defined as a *forward-direction* (attacker → decky) TCP
+segment carrying payload whose sequence number has already been seen on
+this flow. Empty SYN/ACKs that share seq legitimately are excluded.
+"""
+
+from __future__ import annotations
+
+from scapy.layers.inet import IP, TCP
+
+from decnet.sniffer.fingerprint import SnifferEngine
+
+
+_DECKY_IP = "192.168.1.10"
+_DECKY = "decky-01"
+_ATTACKER_IP = "10.0.0.7"
+
+
+def _mk_engine() -> tuple[SnifferEngine, list[str]]:
+    captured: list[str] = []
+    engine = SnifferEngine(
+        ip_to_decky={_DECKY_IP: _DECKY},
+        write_fn=captured.append,
+        dedup_ttl=0,  # disable dedup for easier assertion
+    )
+    return engine, captured
+
+
+def _data_pkt(seq: int, payload: bytes = b"data", sport: int = 55555):
+    return IP(src=_ATTACKER_IP, dst=_DECKY_IP, ttl=64) / TCP(
+        sport=sport, dport=22, flags="A", seq=seq, window=29200,
+    ) / payload
+
+
+def _rst(sport: int = 55555):
+    return IP(src=_DECKY_IP, dst=_ATTACKER_IP, ttl=64) / TCP(
+        sport=22, dport=sport, flags="R",
+    )
+
+
+def _extract_retransmits(lines: list[str]) -> int:
+    """Pull `retransmits=` from the last tcp_flow_timing line."""
+    import re
+    for line in reversed(lines):
+        if "tcp_flow_timing" not in line:
+            continue
+        m = re.search(r'retransmits="(\d+)"', line)
+        if m:
+            return int(m.group(1))
+    return -1
+
+
+class TestRetransmitDetection:
+    def test_no_retransmits_when_seqs_unique(self):
+        engine, captured = _mk_engine()
+        engine.on_packet(_data_pkt(seq=1000))
+        engine.on_packet(_data_pkt(seq=1004))
+        engine.on_packet(_data_pkt(seq=1008))
+        engine.on_packet(_rst())
+        assert _extract_retransmits(captured) == 0
+
+    def test_single_retransmit(self):
+        engine, captured = _mk_engine()
+        engine.on_packet(_data_pkt(seq=2000))
+        engine.on_packet(_data_pkt(seq=2004))
+        engine.on_packet(_data_pkt(seq=2000))  # retransmitted
+        engine.on_packet(_rst())
+        assert _extract_retransmits(captured) == 1
+
+    def test_multiple_retransmits(self):
+        engine, captured = _mk_engine()
+        engine.on_packet(_data_pkt(seq=3000))
+        engine.on_packet(_data_pkt(seq=3000))
+        engine.on_packet(_data_pkt(seq=3000))
+        engine.on_packet(_data_pkt(seq=3004))
+        engine.on_packet(_rst())
+        # Two retransmits (original + 2 dupes of seq=3000)
+        assert _extract_retransmits(captured) == 2
+
+    def test_reverse_direction_not_counted(self):
+        """Packets from decky → attacker sharing seq should NOT count."""
+        engine, captured = _mk_engine()
+        # Forward data
+        engine.on_packet(_data_pkt(seq=4000))
+        engine.on_packet(_data_pkt(seq=4004))
+        engine.on_packet(_data_pkt(seq=4008))
+        # Reverse response (decky → attacker) with same seq as a forward
+        # packet — different flow direction, must not count as retransmit.
+        reverse = IP(src=_DECKY_IP, dst=_ATTACKER_IP, ttl=64) / TCP(
+            sport=22, dport=55555, flags="A", seq=4000, window=29200,
+        ) / b"resp"
+        engine.on_packet(reverse)
+        engine.on_packet(_rst())
+        assert _extract_retransmits(captured) == 0
+
+    def test_empty_segments_not_counted(self):
+        """Pure ACKs (no payload) are not retransmits even if seqs repeat."""
+        engine, captured = _mk_engine()
+        # Three pure-ACKs with identical seq
+        for _ in range(3):
+            pkt = IP(src=_ATTACKER_IP, dst=_DECKY_IP, ttl=64) / TCP(
+                sport=55555, dport=22, flags="A", seq=5000, window=29200,
+            )
+            engine.on_packet(pkt)
+        engine.on_packet(_rst())
+        assert _extract_retransmits(captured) == 0
diff --git a/tests/test_sniffer_tcp_fingerprint.py b/tests/test_sniffer_tcp_fingerprint.py
new file mode 100644
index 0000000..fc04714
--- /dev/null
+++ b/tests/test_sniffer_tcp_fingerprint.py
@@ -0,0 +1,232 @@
+"""
+Integration tests for TCP-level passive fingerprinting in the SnifferEngine.
+
+Covers end-to-end flow from a scapy packet through `on_packet()` to:
+    - tcp_syn_fingerprint event emission (OS guess, options, hop distance)
+    - tcp_flow_timing event emission (packet count, duration, retransmits)
+    - dedup behavior (one event per unique fingerprint per window)
+    - flow flush on FIN/RST
+"""
+
+from __future__ import annotations
+
+from scapy.layers.inet import IP, TCP
+
+from decnet.sniffer.fingerprint import SnifferEngine
+
+
+# ─── Helpers ────────────────────────────────────────────────────────────────
+
+_DECKY_IP = "192.168.1.10"
+_DECKY = "decky-01"
+_ATTACKER_IP = "10.0.0.7"
+
+
+def _make_engine() -> tuple[SnifferEngine, list[str]]:
+    """Return (engine, captured_syslog_lines)."""
+    captured: list[str] = []
+    engine = SnifferEngine(
+        ip_to_decky={_DECKY_IP: _DECKY},
+        write_fn=captured.append,
+        dedup_ttl=300.0,
+    )
+    return engine, captured
+
+
+def _linux_syn(src_port: int = 45000, dst_port: int = 22, seq: int = 1000):
+    """Build a synthetic SYN that should fingerprint as Linux."""
+    return IP(src=_ATTACKER_IP, dst=_DECKY_IP, ttl=64) / TCP(
+        sport=src_port,
+        dport=dst_port,
+        flags="S",
+        seq=seq,
+        window=29200,
+        options=[
+            ("MSS", 1460),
+            ("SAckOK", b""),
+            ("Timestamp", (123, 0)),
+            ("NOP", None),
+            ("WScale", 7),
+        ],
+    )
+
+
+def _windows_syn(src_port: int = 45001):
+    """Build a synthetic SYN that should fingerprint as Windows."""
+    return IP(src=_ATTACKER_IP, dst=_DECKY_IP, ttl=128) / TCP(
+        sport=src_port,
+        dport=3389,
+        flags="S",
+        window=64240,
+        options=[
+            ("MSS", 1460),
+            ("NOP", None),
+            ("WScale", 8),
+            ("NOP", None),
+            ("NOP", None),
+            ("SAckOK", b""),
+        ],
+    )
+
+
+def _fields_from_line(line: str) -> dict[str, str]:
+    """Parse the SD-params section of an RFC 5424 syslog line into a dict."""
+    import re
+    m = re.search(r"\[decnet@55555 (.*?)\]", line)
+    if not m:
+        return {}
+    body = m.group(1)
+    out: dict[str, str] = {}
+    for k, v in re.findall(r'(\w+)="((?:[^"\\]|\\.)*)"', body):
+        out[k] = v
+    return out
+
+
+def _msgid(line: str) -> str:
+    """Extract MSGID from RFC 5424 line."""
+    parts = line.split(" ", 6)
+    return parts[5] if len(parts) > 5 else ""
+
+
+# ─── tcp_syn_fingerprint emission ──────────────────────────────────────────
+
+class TestSynFingerprintEmission:
+    def test_linux_syn_emits_fingerprint(self):
+        engine, captured = _make_engine()
+        engine.on_packet(_linux_syn())
+        fp_lines = [ln for ln in captured if _msgid(ln) == "tcp_syn_fingerprint"]
+        assert len(fp_lines) == 1
+        f = _fields_from_line(fp_lines[0])
+        assert f["src_ip"] == _ATTACKER_IP
+        assert f["dst_ip"] == _DECKY_IP
+        assert f["os_guess"] == "linux"
+        assert f["ttl"] == "64"
+        assert f["initial_ttl"] == "64"
+        assert f["hop_distance"] == "0"
+        assert f["window"] == "29200"
+        assert f["wscale"] == "7"
+        assert f["mss"] == "1460"
+        assert f["has_sack"] == "true"
+        assert f["has_timestamps"] == "true"
+
+    def test_windows_syn_emits_windows_guess(self):
+        engine, captured = _make_engine()
+        engine.on_packet(_windows_syn())
+        fp_lines = [ln for ln in captured if _msgid(ln) == "tcp_syn_fingerprint"]
+        assert len(fp_lines) == 1
+        f = _fields_from_line(fp_lines[0])
+        assert f["os_guess"] == "windows"
+        assert f["ttl"] == "128"
+        assert f["initial_ttl"] == "128"
+
+    def test_hop_distance_inferred_from_ttl(self):
+        engine, captured = _make_engine()
+        pkt = IP(src=_ATTACKER_IP, dst=_DECKY_IP, ttl=58) / TCP(
+            sport=40000, dport=22, flags="S", window=29200,
+            options=[("MSS", 1460), ("SAckOK", b""), ("Timestamp", (0, 0)),
+                     ("NOP", None), ("WScale", 7)],
+        )
+        engine.on_packet(pkt)
+        fp_lines = [ln for ln in captured if _msgid(ln) == "tcp_syn_fingerprint"]
+        f = _fields_from_line(fp_lines[0])
+        assert f["initial_ttl"] == "64"
+        assert f["hop_distance"] == "6"
+
+    def test_dedup_suppresses_repeated_fingerprints(self):
+        engine, captured = _make_engine()
+        engine.on_packet(_linux_syn(src_port=40001))
+        engine.on_packet(_linux_syn(src_port=40002))
+        engine.on_packet(_linux_syn(src_port=40003))
+        fp_lines = [ln for ln in captured if _msgid(ln) == "tcp_syn_fingerprint"]
+        assert len(fp_lines) == 1  # same OS + options_sig deduped
+
+    def test_different_os_not_deduped(self):
+        engine, captured = _make_engine()
+        engine.on_packet(_linux_syn(src_port=40001))
+        engine.on_packet(_windows_syn(src_port=40002))
+        fp_lines = [ln for ln in captured if _msgid(ln) == "tcp_syn_fingerprint"]
+        assert len(fp_lines) == 2
+
+    def test_decky_source_does_not_emit(self):
+        """Packets originating from a decky (outbound reply) should NOT
+        be classified as an attacker fingerprint."""
+        engine, captured = _make_engine()
+        pkt = IP(src=_DECKY_IP, dst=_ATTACKER_IP, ttl=64) / TCP(
+            sport=22, dport=40000, flags="S", window=29200,
+            options=[("MSS", 1460)],
+        )
+        engine.on_packet(pkt)
+        fp_lines = [ln for ln in captured if _msgid(ln) == "tcp_syn_fingerprint"]
+        assert fp_lines == []
+
+
+# ─── tcp_flow_timing emission ───────────────────────────────────────────────
+
+class TestFlowTiming:
+    def test_flow_flushed_on_fin_if_non_trivial(self):
+        """A session with ≥4 packets triggers a tcp_flow_timing event on FIN."""
+        engine, captured = _make_engine()
+        # SYN + 3 data ACKs + FIN = 5 packets → passes the trivial-flow filter
+        pkts = [_linux_syn(src_port=50000, seq=100)]
+        for i, seq in enumerate((101, 200, 300)):
+            pkts.append(
+                IP(src=_ATTACKER_IP, dst=_DECKY_IP, ttl=64) / TCP(
+                    sport=50000, dport=22, flags="A", seq=seq, window=29200,
+                ) / b"hello-data-here"
+            )
+        pkts.append(
+            IP(src=_ATTACKER_IP, dst=_DECKY_IP, ttl=64) / TCP(
+                sport=50000, dport=22, flags="FA", seq=400, window=29200,
+            )
+        )
+        for p in pkts:
+            engine.on_packet(p)
+
+        flow_lines = [ln for ln in captured if _msgid(ln) == "tcp_flow_timing"]
+        assert len(flow_lines) == 1
+        f = _fields_from_line(flow_lines[0])
+        assert f["src_ip"] == _ATTACKER_IP
+        assert f["dst_ip"] == _DECKY_IP
+        assert int(f["packets"]) == 5
+        assert int(f["retransmits"]) == 0
+
+    def test_trivial_flow_dropped(self):
+        """A 2-packet scan probe (SYN + RST) must NOT emit a timing event."""
+        engine, captured = _make_engine()
+        engine.on_packet(_linux_syn(src_port=50001, seq=200))
+        engine.on_packet(
+            IP(src=_DECKY_IP, dst=_ATTACKER_IP, ttl=64) / TCP(
+                sport=22, dport=50001, flags="R", window=0,
+            )
+        )
+        flow_lines = [ln for ln in captured if _msgid(ln) == "tcp_flow_timing"]
+        assert flow_lines == []  # trivial: packets<4, no retransmits, dur<1s
+
+    def test_retransmit_forces_emission_on_short_flow(self):
+        """Even a 3-packet flow must emit if it contains a retransmit."""
+        engine, captured = _make_engine()
+        engine.on_packet(_linux_syn(src_port=50002, seq=300))
+        # Repeat a forward data seq → retransmit
+        for _ in range(2):
+            engine.on_packet(
+                IP(src=_ATTACKER_IP, dst=_DECKY_IP, ttl=64) / TCP(
+                    sport=50002, dport=22, flags="A", seq=301, window=29200,
+                ) / b"payload"
+            )
+        engine.on_packet(
+            IP(src=_DECKY_IP, dst=_ATTACKER_IP, ttl=64) / TCP(
+                sport=22, dport=50002, flags="R", window=0,
+            )
+        )
+        flow_lines = [ln for ln in captured if _msgid(ln) == "tcp_flow_timing"]
+        assert len(flow_lines) == 1
+        f = _fields_from_line(flow_lines[0])
+        assert int(f["retransmits"]) == 1
+
+    def test_flush_all_flows_helper_drops_trivial(self):
+        """flush_all_flows still filters trivial flows."""
+        engine, captured = _make_engine()
+        engine.on_packet(_linux_syn(src_port=50003, seq=400))
+        engine.flush_all_flows()
+        flow_lines = [ln for ln in captured if _msgid(ln) == "tcp_flow_timing"]
+        assert flow_lines == []  # single packet = trivial