DECNET/decnet/sniffer/fingerprint.py

"""
TLS fingerprinting engine for the fleet-wide MACVLAN sniffer.

Extracted from templates/sniffer/server.py. All pure-Python TLS parsing,
JA3/JA3S/JA4/JA4S/JA4L computation, session tracking, and dedup logic
lives here. The packet callback is parameterized to accept an IP-to-decky
mapping and a write function, so it works for fleet-wide sniffing.
"""

from __future__ import annotations

import hashlib
import struct
import time
from pathlib import Path
from typing import Any, Callable

from decnet.sniffer.syslog import SEVERITY_INFO, SEVERITY_WARNING, syslog_line

# ─── Constants ───────────────────────────────────────────────────────────────

SERVICE_NAME: str = "sniffer"

_SESSION_TTL: float = 60.0
_DEDUP_TTL: float = 300.0

_GREASE: frozenset[int] = frozenset(0x0A0A + i * 0x1010 for i in range(16))

_TLS_RECORD_HANDSHAKE: int = 0x16
_TLS_HT_CLIENT_HELLO: int = 0x01
_TLS_HT_SERVER_HELLO: int = 0x02
_TLS_HT_CERTIFICATE: int = 0x0B

_EXT_SNI: int = 0x0000
_EXT_SUPPORTED_GROUPS: int = 0x000A
_EXT_EC_POINT_FORMATS: int = 0x000B
_EXT_SIGNATURE_ALGORITHMS: int = 0x000D
_EXT_ALPN: int = 0x0010
_EXT_SESSION_TICKET: int = 0x0023
_EXT_SUPPORTED_VERSIONS: int = 0x002B
_EXT_PRE_SHARED_KEY: int = 0x0029
_EXT_EARLY_DATA: int = 0x002A

_TCP_SYN: int = 0x02
_TCP_ACK: int = 0x10


# ─── GREASE helpers ──────────────────────────────────────────────────────────

def _is_grease(value: int) -> bool:
    return value in _GREASE


def _filter_grease(values: list[int]) -> list[int]:
    return [v for v in values if not _is_grease(v)]


# ─── TLS parsers ─────────────────────────────────────────────────────────────

def _parse_client_hello(data: bytes) -> dict[str, Any] | None:
    try:
        if len(data) < 6:
            return None
        if data[0] != _TLS_RECORD_HANDSHAKE:
            return None
        record_len = struct.unpack_from("!H", data, 3)[0]
        if len(data) < 5 + record_len:
            return None

        hs = data[5:]
        if hs[0] != _TLS_HT_CLIENT_HELLO:
            return None

        hs_len = struct.unpack_from("!I", b"\x00" + hs[1:4])[0]
        body = hs[4: 4 + hs_len]
        if len(body) < 34:
            return None

        pos = 0
        tls_version = struct.unpack_from("!H", body, pos)[0]
        pos += 2
        pos += 32  # Random

        session_id_len = body[pos]
        session_id = body[pos + 1: pos + 1 + session_id_len]
        pos += 1 + session_id_len

        cs_len = struct.unpack_from("!H", body, pos)[0]
        pos += 2
        cipher_suites = [
            struct.unpack_from("!H", body, pos + i * 2)[0]
            for i in range(cs_len // 2)
        ]
        pos += cs_len

        comp_len = body[pos]
        pos += 1 + comp_len

        extensions: list[int] = []
        supported_groups: list[int] = []
        ec_point_formats: list[int] = []
        signature_algorithms: list[int] = []
        supported_versions: list[int] = []
        sni: str = ""
        alpn: list[str] = []
        has_session_ticket_data: bool = False
        has_pre_shared_key: bool = False
        has_early_data: bool = False

        if pos + 2 <= len(body):
            ext_total = struct.unpack_from("!H", body, pos)[0]
            pos += 2
            ext_end = pos + ext_total

            while pos + 4 <= ext_end:
                ext_type = struct.unpack_from("!H", body, pos)[0]
                ext_len = struct.unpack_from("!H", body, pos + 2)[0]
                ext_data = body[pos + 4: pos + 4 + ext_len]
                pos += 4 + ext_len

                if not _is_grease(ext_type):
                    extensions.append(ext_type)

                if ext_type == _EXT_SNI and len(ext_data) > 5:
                    sni = ext_data[5:].decode("ascii", errors="replace")

                elif ext_type == _EXT_SUPPORTED_GROUPS and len(ext_data) >= 2:
                    grp_len = struct.unpack_from("!H", ext_data, 0)[0]
                    supported_groups = [
                        struct.unpack_from("!H", ext_data, 2 + i * 2)[0]
                        for i in range(grp_len // 2)
                    ]

                elif ext_type == _EXT_EC_POINT_FORMATS and len(ext_data) >= 1:
                    pf_len = ext_data[0]
                    ec_point_formats = list(ext_data[1: 1 + pf_len])

                elif ext_type == _EXT_ALPN and len(ext_data) >= 2:
                    proto_list_len = struct.unpack_from("!H", ext_data, 0)[0]
                    ap = 2
                    while ap < 2 + proto_list_len:
                        plen = ext_data[ap]
                        alpn.append(ext_data[ap + 1: ap + 1 + plen].decode("ascii", errors="replace"))
                        ap += 1 + plen

                elif ext_type == _EXT_SIGNATURE_ALGORITHMS and len(ext_data) >= 2:
                    sa_len = struct.unpack_from("!H", ext_data, 0)[0]
                    signature_algorithms = [
                        struct.unpack_from("!H", ext_data, 2 + i * 2)[0]
                        for i in range(sa_len // 2)
                    ]

                elif ext_type == _EXT_SUPPORTED_VERSIONS and len(ext_data) >= 1:
                    sv_len = ext_data[0]
                    supported_versions = [
                        struct.unpack_from("!H", ext_data, 1 + i * 2)[0]
                        for i in range(sv_len // 2)
                    ]

                elif ext_type == _EXT_SESSION_TICKET:
                    has_session_ticket_data = len(ext_data) > 0

                elif ext_type == _EXT_PRE_SHARED_KEY:
                    has_pre_shared_key = True

                elif ext_type == _EXT_EARLY_DATA:
                    has_early_data = True

        filtered_ciphers = _filter_grease(cipher_suites)
        filtered_groups = _filter_grease(supported_groups)
        filtered_sig_algs = _filter_grease(signature_algorithms)
        filtered_versions = _filter_grease(supported_versions)

        return {
            "tls_version": tls_version,
            "cipher_suites": filtered_ciphers,
            "extensions": extensions,
            "supported_groups": filtered_groups,
            "ec_point_formats": ec_point_formats,
            "signature_algorithms": filtered_sig_algs,
            "supported_versions": filtered_versions,
            "sni": sni,
            "alpn": alpn,
            "session_id": session_id,
            "has_session_ticket_data": has_session_ticket_data,
            "has_pre_shared_key": has_pre_shared_key,
            "has_early_data": has_early_data,
        }

    except Exception:
        return None


def _parse_server_hello(data: bytes) -> dict[str, Any] | None:
    try:
        if len(data) < 6 or data[0] != _TLS_RECORD_HANDSHAKE:
            return None

        hs = data[5:]
        if hs[0] != _TLS_HT_SERVER_HELLO:
            return None

        hs_len = struct.unpack_from("!I", b"\x00" + hs[1:4])[0]
        body = hs[4: 4 + hs_len]
        if len(body) < 35:
            return None

        pos = 0
        tls_version = struct.unpack_from("!H", body, pos)[0]
        pos += 2
        pos += 32  # Random

        session_id_len = body[pos]
        pos += 1 + session_id_len

        if pos + 2 > len(body):
            return None

        cipher_suite = struct.unpack_from("!H", body, pos)[0]
        pos += 2
        pos += 1  # Compression method

        extensions: list[int] = []
        selected_version: int | None = None
        alpn: str = ""

        if pos + 2 <= len(body):
            ext_total = struct.unpack_from("!H", body, pos)[0]
            pos += 2
            ext_end = pos + ext_total
            while pos + 4 <= ext_end:
                ext_type = struct.unpack_from("!H", body, pos)[0]
                ext_len = struct.unpack_from("!H", body, pos + 2)[0]
                ext_data = body[pos + 4: pos + 4 + ext_len]
                pos += 4 + ext_len
                if not _is_grease(ext_type):
                    extensions.append(ext_type)

                if ext_type == _EXT_SUPPORTED_VERSIONS and len(ext_data) >= 2:
                    selected_version = struct.unpack_from("!H", ext_data, 0)[0]

                elif ext_type == _EXT_ALPN and len(ext_data) >= 2:
                    proto_list_len = struct.unpack_from("!H", ext_data, 0)[0]
                    if proto_list_len > 0 and len(ext_data) >= 4:
                        plen = ext_data[2]
                        alpn = ext_data[3: 3 + plen].decode("ascii", errors="replace")

        return {
            "tls_version": tls_version,
            "cipher_suite": cipher_suite,
            "extensions": extensions,
            "selected_version": selected_version,
            "alpn": alpn,
        }

    except Exception:
        return None


def _parse_certificate(data: bytes) -> dict[str, Any] | None:
    try:
        if len(data) < 6 or data[0] != _TLS_RECORD_HANDSHAKE:
            return None

        hs = data[5:]
        if hs[0] != _TLS_HT_CERTIFICATE:
            return None

        hs_len = struct.unpack_from("!I", b"\x00" + hs[1:4])[0]
        body = hs[4: 4 + hs_len]
        if len(body) < 3:
            return None

        certs_len = struct.unpack_from("!I", b"\x00" + body[0:3])[0]
        if certs_len == 0:
            return None

        pos = 3
        if pos + 3 > len(body):
            return None
        cert_len = struct.unpack_from("!I", b"\x00" + body[pos:pos + 3])[0]
        pos += 3
        if pos + cert_len > len(body):
            return None

        cert_der = body[pos: pos + cert_len]
        return _parse_x509_der(cert_der)

    except Exception:
        return None


# ─── Minimal DER/ASN.1 X.509 parser ─────────────────────────────────────────

def _der_read_tag_len(data: bytes, pos: int) -> tuple[int, int, int]:
    tag = data[pos]
    pos += 1
    length_byte = data[pos]
    pos += 1
    if length_byte & 0x80:
        num_bytes = length_byte & 0x7F
        length = int.from_bytes(data[pos: pos + num_bytes], "big")
        pos += num_bytes
    else:
        length = length_byte
    return tag, pos, length


def _der_read_sequence(data: bytes, pos: int) -> tuple[int, int]:
    tag, content_start, length = _der_read_tag_len(data, pos)
    return content_start, length


def _der_read_oid(data: bytes, pos: int, length: int) -> str:
    if length < 1:
        return ""
    first = data[pos]
    oid_parts = [str(first // 40), str(first % 40)]
    val = 0
    for i in range(1, length):
        b = data[pos + i]
        val = (val << 7) | (b & 0x7F)
        if not (b & 0x80):
            oid_parts.append(str(val))
            val = 0
    return ".".join(oid_parts)


def _der_extract_cn(data: bytes, start: int, length: int) -> str:
    pos = start
    end = start + length
    while pos < end:
        set_tag, set_start, set_len = _der_read_tag_len(data, pos)
        if set_tag != 0x31:
            break
        set_end = set_start + set_len
        attr_pos = set_start
        while attr_pos < set_end:
            seq_tag, seq_start, seq_len = _der_read_tag_len(data, attr_pos)
            if seq_tag != 0x30:
                break
            oid_tag, oid_start, oid_len = _der_read_tag_len(data, seq_start)
            if oid_tag == 0x06:
                oid = _der_read_oid(data, oid_start, oid_len)
                if oid == "2.5.4.3":
                    val_tag, val_start, val_len = _der_read_tag_len(data, oid_start + oid_len)
                    return data[val_start: val_start + val_len].decode("utf-8", errors="replace")
            attr_pos = seq_start + seq_len
        pos = set_end
    return ""


def _der_extract_name_str(data: bytes, start: int, length: int) -> str:
    parts: list[str] = []
    pos = start
    end = start + length
    oid_names = {
        "2.5.4.3": "CN",
        "2.5.4.6": "C",
        "2.5.4.7": "L",
        "2.5.4.8": "ST",
        "2.5.4.10": "O",
        "2.5.4.11": "OU",
    }
    while pos < end:
        set_tag, set_start, set_len = _der_read_tag_len(data, pos)
        if set_tag != 0x31:
            break
        set_end = set_start + set_len
        attr_pos = set_start
        while attr_pos < set_end:
            seq_tag, seq_start, seq_len = _der_read_tag_len(data, attr_pos)
            if seq_tag != 0x30:
                break
            oid_tag, oid_start, oid_len = _der_read_tag_len(data, seq_start)
            if oid_tag == 0x06:
                oid = _der_read_oid(data, oid_start, oid_len)
                val_tag, val_start, val_len = _der_read_tag_len(data, oid_start + oid_len)
                val = data[val_start: val_start + val_len].decode("utf-8", errors="replace")
                name = oid_names.get(oid, oid)
                parts.append(f"{name}={val}")
            attr_pos = seq_start + seq_len
        pos = set_end
    return ", ".join(parts)


def _parse_x509_der(cert_der: bytes) -> dict[str, Any] | None:
    try:
        outer_start, outer_len = _der_read_sequence(cert_der, 0)
        tbs_tag, tbs_start, tbs_len = _der_read_tag_len(cert_der, outer_start)
        tbs_end = tbs_start + tbs_len
        pos = tbs_start

        if cert_der[pos] == 0xA0:
            _, v_start, v_len = _der_read_tag_len(cert_der, pos)
            pos = v_start + v_len

        _, sn_start, sn_len = _der_read_tag_len(cert_der, pos)
        pos = sn_start + sn_len

        _, sa_start, sa_len = _der_read_tag_len(cert_der, pos)
        pos = sa_start + sa_len

        issuer_tag, issuer_start, issuer_len = _der_read_tag_len(cert_der, pos)
        issuer_str = _der_extract_name_str(cert_der, issuer_start, issuer_len)
        issuer_cn = _der_extract_cn(cert_der, issuer_start, issuer_len)
        pos = issuer_start + issuer_len

        val_tag, val_start, val_len = _der_read_tag_len(cert_der, pos)
        nb_tag, nb_start, nb_len = _der_read_tag_len(cert_der, val_start)
        not_before = cert_der[nb_start: nb_start + nb_len].decode("ascii", errors="replace")
        na_tag, na_start, na_len = _der_read_tag_len(cert_der, nb_start + nb_len)
        not_after = cert_der[na_start: na_start + na_len].decode("ascii", errors="replace")
        pos = val_start + val_len

        subj_tag, subj_start, subj_len = _der_read_tag_len(cert_der, pos)
        subject_cn = _der_extract_cn(cert_der, subj_start, subj_len)
        subject_str = _der_extract_name_str(cert_der, subj_start, subj_len)

        self_signed = (issuer_cn == subject_cn) and subject_cn != ""

        pos = subj_start + subj_len
        sans: list[str] = _extract_sans(cert_der, pos, tbs_end)

        return {
            "subject_cn": subject_cn,
            "subject": subject_str,
            "issuer": issuer_str,
            "issuer_cn": issuer_cn,
            "not_before": not_before,
            "not_after": not_after,
            "self_signed": self_signed,
            "sans": sans,
        }

    except Exception:
        return None


def _extract_sans(cert_der: bytes, pos: int, end: int) -> list[str]:
    sans: list[str] = []
    try:
        if pos >= end:
            return sans
        spki_tag, spki_start, spki_len = _der_read_tag_len(cert_der, pos)
        pos = spki_start + spki_len

        while pos < end:
            tag = cert_der[pos]
            if tag == 0xA3:
                _, ext_wrap_start, ext_wrap_len = _der_read_tag_len(cert_der, pos)
                _, exts_start, exts_len = _der_read_tag_len(cert_der, ext_wrap_start)
                epos = exts_start
                eend = exts_start + exts_len
                while epos < eend:
                    ext_tag, ext_start, ext_len = _der_read_tag_len(cert_der, epos)
                    ext_end = ext_start + ext_len
                    oid_tag, oid_start, oid_len = _der_read_tag_len(cert_der, ext_start)
                    if oid_tag == 0x06:
                        oid = _der_read_oid(cert_der, oid_start, oid_len)
                        if oid == "2.5.29.17":
                            vpos = oid_start + oid_len
                            if vpos < ext_end and cert_der[vpos] == 0x01:
                                _, bs, bl = _der_read_tag_len(cert_der, vpos)
                                vpos = bs + bl
                            if vpos < ext_end:
                                os_tag, os_start, os_len = _der_read_tag_len(cert_der, vpos)
                                if os_tag == 0x04:
                                    sans = _parse_san_sequence(cert_der, os_start, os_len)
                    epos = ext_end
                break
            else:
                _, skip_start, skip_len = _der_read_tag_len(cert_der, pos)
                pos = skip_start + skip_len
    except Exception:
        pass
    return sans


def _parse_san_sequence(data: bytes, start: int, length: int) -> list[str]:
    names: list[str] = []
    try:
        seq_tag, seq_start, seq_len = _der_read_tag_len(data, start)
        pos = seq_start
        end = seq_start + seq_len
        while pos < end:
            tag = data[pos]
            _, val_start, val_len = _der_read_tag_len(data, pos)
            context_tag = tag & 0x1F
            if context_tag == 2:
                names.append(data[val_start: val_start + val_len].decode("ascii", errors="replace"))
            elif context_tag == 7 and val_len == 4:
                names.append(".".join(str(b) for b in data[val_start: val_start + val_len]))
            pos = val_start + val_len
    except Exception:
        pass
    return names


# ─── JA3 / JA3S ─────────────────────────────────────────────────────────────

def _tls_version_str(version: int) -> str:
    return {
        0x0301: "TLS 1.0",
        0x0302: "TLS 1.1",
        0x0303: "TLS 1.2",
        0x0304: "TLS 1.3",
        0x0200: "SSL 2.0",
        0x0300: "SSL 3.0",
    }.get(version, f"0x{version:04x}")


def _ja3(ch: dict[str, Any]) -> tuple[str, str]:
    parts = [
        str(ch["tls_version"]),
        "-".join(str(c) for c in ch["cipher_suites"]),
        "-".join(str(e) for e in ch["extensions"]),
        "-".join(str(g) for g in ch["supported_groups"]),
        "-".join(str(p) for p in ch["ec_point_formats"]),
    ]
    ja3_str = ",".join(parts)
    return ja3_str, hashlib.md5(ja3_str.encode()).hexdigest()


def _ja3s(sh: dict[str, Any]) -> tuple[str, str]:
    parts = [
        str(sh["tls_version"]),
        str(sh["cipher_suite"]),
        "-".join(str(e) for e in sh["extensions"]),
    ]
    ja3s_str = ",".join(parts)
    return ja3s_str, hashlib.md5(ja3s_str.encode()).hexdigest()


# ─── JA4 / JA4S ─────────────────────────────────────────────────────────────

def _ja4_version(ch: dict[str, Any]) -> str:
    versions = ch.get("supported_versions", [])
    if versions:
        best = max(versions)
    else:
        best = ch["tls_version"]
    return {
        0x0304: "13",
        0x0303: "12",
        0x0302: "11",
        0x0301: "10",
        0x0300: "s3",
        0x0200: "s2",
    }.get(best, "00")


def _ja4_alpn_tag(alpn_list: list[str] | str) -> str:
    if isinstance(alpn_list, str):
        proto = alpn_list
    elif alpn_list:
        proto = alpn_list[0]
    else:
        return "00"
    if not proto:
        return "00"
    if len(proto) == 1:
        return proto[0] + proto[0]
    return proto[0] + proto[-1]


def _sha256_12(text: str) -> str:
    return hashlib.sha256(text.encode()).hexdigest()[:12]


def _ja4(ch: dict[str, Any]) -> str:
    proto = "t"
    ver = _ja4_version(ch)
    sni_flag = "d" if ch.get("sni") else "i"
    cs_count = min(len(ch["cipher_suites"]), 99)
    ext_count = min(len(ch["extensions"]), 99)
    alpn_tag = _ja4_alpn_tag(ch.get("alpn", []))
    section_a = f"{proto}{ver}{sni_flag}{cs_count:02d}{ext_count:02d}{alpn_tag}"
    sorted_cs = sorted(ch["cipher_suites"])
    section_b = _sha256_12(",".join(str(c) for c in sorted_cs))
    sorted_ext = sorted(ch["extensions"])
    sorted_sa = sorted(ch.get("signature_algorithms", []))
    ext_str = ",".join(str(e) for e in sorted_ext)
    sa_str = ",".join(str(s) for s in sorted_sa)
    combined = f"{ext_str}_{sa_str}" if sa_str else ext_str
    section_c = _sha256_12(combined)
    return f"{section_a}_{section_b}_{section_c}"


def _ja4s(sh: dict[str, Any]) -> str:
    proto = "t"
    selected = sh.get("selected_version")
    if selected:
        ver = {0x0304: "13", 0x0303: "12", 0x0302: "11", 0x0301: "10",
               0x0300: "s3", 0x0200: "s2"}.get(selected, "00")
    else:
        ver = {0x0304: "13", 0x0303: "12", 0x0302: "11", 0x0301: "10",
               0x0300: "s3", 0x0200: "s2"}.get(sh["tls_version"], "00")
    ext_count = min(len(sh["extensions"]), 99)
    alpn_tag = _ja4_alpn_tag(sh.get("alpn", ""))
    section_a = f"{proto}{ver}{ext_count:02d}{alpn_tag}"
    sorted_ext = sorted(sh["extensions"])
    inner = f"{sh['cipher_suite']},{','.join(str(e) for e in sorted_ext)}"
    section_b = _sha256_12(inner)
    return f"{section_a}_{section_b}"


# ─── JA4L (latency) ─────────────────────────────────────────────────────────

def _ja4l(
    key: tuple[str, int, str, int],
    tcp_rtt: dict[tuple[str, int, str, int], dict[str, Any]],
) -> dict[str, Any] | None:
    return tcp_rtt.get(key)


# ─── Session resumption ─────────────────────────────────────────────────────

def _session_resumption_info(ch: dict[str, Any]) -> dict[str, Any]:
    mechanisms: list[str] = []
    if ch.get("has_session_ticket_data"):
        mechanisms.append("session_ticket")
    if ch.get("has_pre_shared_key"):
        mechanisms.append("psk")
    if ch.get("has_early_data"):
        mechanisms.append("early_data_0rtt")
    if ch.get("session_id") and len(ch["session_id"]) > 0:
        mechanisms.append("session_id")
    return {
        "resumption_attempted": len(mechanisms) > 0,
        "mechanisms": mechanisms,
    }


# ─── Sniffer engine (stateful, one instance per worker) ─────────────────────

class SnifferEngine:
    """
    Stateful TLS fingerprinting engine. Tracks sessions, TCP RTTs,
    and dedup state. Thread-safe only when called from a single thread
    (the scapy sniff thread).
    """

    def __init__(
        self,
        ip_to_decky: dict[str, str],
        write_fn: Callable[[str], None],
        dedup_ttl: float = 300.0,
    ):
        self._ip_to_decky = ip_to_decky
        self._write_fn = write_fn
        self._dedup_ttl = dedup_ttl

        self._sessions: dict[tuple[str, int, str, int], dict[str, Any]] = {}
        self._session_ts: dict[tuple[str, int, str, int], float] = {}
        self._tcp_syn: dict[tuple[str, int, str, int], dict[str, Any]] = {}
        self._tcp_rtt: dict[tuple[str, int, str, int], dict[str, Any]] = {}

        self._dedup_cache: dict[tuple[str, str, str], float] = {}
        self._dedup_last_cleanup: float = 0.0
        self._DEDUP_CLEANUP_INTERVAL: float = 60.0

    def update_ip_map(self, ip_to_decky: dict[str, str]) -> None:
        self._ip_to_decky = ip_to_decky

    def _resolve_decky(self, src_ip: str, dst_ip: str) -> str | None:
        """Map a packet to a decky name. Returns None if neither IP is a known decky."""
        if dst_ip in self._ip_to_decky:
            return self._ip_to_decky[dst_ip]
        if src_ip in self._ip_to_decky:
            return self._ip_to_decky[src_ip]
        return None

    def _cleanup_sessions(self) -> None:
        now = time.monotonic()
        stale = [k for k, ts in self._session_ts.items() if now - ts > _SESSION_TTL]
        for k in stale:
            self._sessions.pop(k, None)
            self._session_ts.pop(k, None)
        stale_syn = [k for k, v in self._tcp_syn.items()
                     if now - v.get("time", 0) > _SESSION_TTL]
        for k in stale_syn:
            self._tcp_syn.pop(k, None)
        stale_rtt = [k for k, _ in self._tcp_rtt.items()
                     if k not in self._sessions and k not in self._session_ts]
        for k in stale_rtt:
            self._tcp_rtt.pop(k, None)

    def _dedup_key_for(self, event_type: str, fields: dict[str, Any]) -> str:
        if event_type == "tls_client_hello":
            return fields.get("ja3", "") + "|" + fields.get("ja4", "")
        if event_type == "tls_session":
            return (fields.get("ja3", "") + "|" + fields.get("ja3s", "") +
                    "|" + fields.get("ja4", "") + "|" + fields.get("ja4s", ""))
        if event_type == "tls_certificate":
            return fields.get("subject_cn", "") + "|" + fields.get("issuer", "")
        return fields.get("mechanisms", fields.get("resumption", ""))

    def _is_duplicate(self, event_type: str, fields: dict[str, Any]) -> bool:
        if self._dedup_ttl <= 0:
            return False
        now = time.monotonic()
        if now - self._dedup_last_cleanup > self._DEDUP_CLEANUP_INTERVAL:
            stale = [k for k, ts in self._dedup_cache.items() if now - ts > self._dedup_ttl]
            for k in stale:
                del self._dedup_cache[k]
            self._dedup_last_cleanup = now
        src_ip = fields.get("src_ip", "")
        fp = self._dedup_key_for(event_type, fields)
        cache_key = (src_ip, event_type, fp)
        last_seen = self._dedup_cache.get(cache_key)
        if last_seen is not None and now - last_seen < self._dedup_ttl:
            return True
        self._dedup_cache[cache_key] = now
        return False

    def _log(self, node_name: str, event_type: str, severity: int = SEVERITY_INFO, **fields: Any) -> None:
        if self._is_duplicate(event_type, fields):
            return
        line = syslog_line(SERVICE_NAME, node_name, event_type, severity=severity, **fields)
        self._write_fn(line)

    def on_packet(self, pkt: Any) -> None:
        """Process a single scapy packet. Called from the sniff thread."""
        try:
            from scapy.layers.inet import IP, TCP
        except ImportError:
            return

        if not (pkt.haslayer(IP) and pkt.haslayer(TCP)):
            return

        ip = pkt[IP]
        tcp = pkt[TCP]

        src_ip: str = ip.src
        dst_ip: str = ip.dst
        src_port: int = tcp.sport
        dst_port: int = tcp.dport
        flags: int = tcp.flags.value if hasattr(tcp.flags, 'value') else int(tcp.flags)

        # Skip traffic not involving any decky
        node_name = self._resolve_decky(src_ip, dst_ip)
        if node_name is None:
            return

        # TCP SYN tracking for JA4L
        if flags & _TCP_SYN and not (flags & _TCP_ACK):
            key = (src_ip, src_port, dst_ip, dst_port)
            self._tcp_syn[key] = {"time": time.monotonic(), "ttl": ip.ttl}

        elif flags & _TCP_SYN and flags & _TCP_ACK:
            rev_key = (dst_ip, dst_port, src_ip, src_port)
            syn_data = self._tcp_syn.pop(rev_key, None)
            if syn_data:
                rtt_ms = round((time.monotonic() - syn_data["time"]) * 1000, 2)
                self._tcp_rtt[rev_key] = {
                    "rtt_ms": rtt_ms,
                    "client_ttl": syn_data["ttl"],
                }

        payload = bytes(tcp.payload)
        if not payload:
            return

        if payload[0] != _TLS_RECORD_HANDSHAKE:
            return

        # ClientHello
        ch = _parse_client_hello(payload)
        if ch is not None:
            self._cleanup_sessions()

            key = (src_ip, src_port, dst_ip, dst_port)
            ja3_str, ja3_hash = _ja3(ch)
            ja4_hash = _ja4(ch)
            resumption = _session_resumption_info(ch)
            rtt_data = _ja4l(key, self._tcp_rtt)

            self._sessions[key] = {
                "ja3": ja3_hash,
                "ja3_str": ja3_str,
                "ja4": ja4_hash,
                "tls_version": ch["tls_version"],
                "cipher_suites": ch["cipher_suites"],
                "extensions": ch["extensions"],
                "signature_algorithms": ch.get("signature_algorithms", []),
                "supported_versions": ch.get("supported_versions", []),
                "sni": ch["sni"],
                "alpn": ch["alpn"],
                "resumption": resumption,
            }
            self._session_ts[key] = time.monotonic()

            log_fields: dict[str, Any] = {
                "src_ip": src_ip,
                "src_port": str(src_port),
                "dst_ip": dst_ip,
                "dst_port": str(dst_port),
                "ja3": ja3_hash,
                "ja4": ja4_hash,
                "tls_version": _tls_version_str(ch["tls_version"]),
                "sni": ch["sni"] or "",
                "alpn": ",".join(ch["alpn"]),
                "raw_ciphers": "-".join(str(c) for c in ch["cipher_suites"]),
                "raw_extensions": "-".join(str(e) for e in ch["extensions"]),
            }

            if resumption["resumption_attempted"]:
                log_fields["resumption"] = ",".join(resumption["mechanisms"])

            if rtt_data:
                log_fields["ja4l_rtt_ms"] = str(rtt_data["rtt_ms"])
                log_fields["ja4l_client_ttl"] = str(rtt_data["client_ttl"])

            # Resolve node for the *destination* (the decky being attacked)
            target_node = self._ip_to_decky.get(dst_ip, node_name)
            self._log(target_node, "tls_client_hello", **log_fields)
            return

        # ServerHello
        sh = _parse_server_hello(payload)
        if sh is not None:
            rev_key = (dst_ip, dst_port, src_ip, src_port)
            ch_data = self._sessions.pop(rev_key, None)
            self._session_ts.pop(rev_key, None)

            ja3s_str, ja3s_hash = _ja3s(sh)
            ja4s_hash = _ja4s(sh)

            fields: dict[str, Any] = {
                "src_ip": dst_ip,
                "src_port": str(dst_port),
                "dst_ip": src_ip,
                "dst_port": str(src_port),
                "ja3s": ja3s_hash,
                "ja4s": ja4s_hash,
                "tls_version": _tls_version_str(sh["tls_version"]),
            }

            if ch_data:
                fields["ja3"] = ch_data["ja3"]
                fields["ja4"] = ch_data.get("ja4", "")
                fields["sni"] = ch_data["sni"] or ""
                fields["alpn"] = ",".join(ch_data["alpn"])
                fields["raw_ciphers"] = "-".join(str(c) for c in ch_data["cipher_suites"])
                fields["raw_extensions"] = "-".join(str(e) for e in ch_data["extensions"])
                if ch_data.get("resumption", {}).get("resumption_attempted"):
                    fields["resumption"] = ",".join(ch_data["resumption"]["mechanisms"])

            rtt_data = self._tcp_rtt.pop(rev_key, None)
            if rtt_data:
                fields["ja4l_rtt_ms"] = str(rtt_data["rtt_ms"])
                fields["ja4l_client_ttl"] = str(rtt_data["client_ttl"])

            # Server response — resolve by src_ip (the decky responding)
            target_node = self._ip_to_decky.get(src_ip, node_name)
            self._log(target_node, "tls_session", severity=SEVERITY_WARNING, **fields)
            return

        # Certificate (TLS 1.2 only)
        cert = _parse_certificate(payload)
        if cert is not None:
            rev_key = (dst_ip, dst_port, src_ip, src_port)
            ch_data = self._sessions.get(rev_key)

            cert_fields: dict[str, Any] = {
                "src_ip": dst_ip,
                "src_port": str(dst_port),
                "dst_ip": src_ip,
                "dst_port": str(src_port),
                "subject_cn": cert["subject_cn"],
                "issuer": cert["issuer"],
                "self_signed": str(cert["self_signed"]).lower(),
                "not_before": cert["not_before"],
                "not_after": cert["not_after"],
            }
            if cert["sans"]:
                cert_fields["sans"] = ",".join(cert["sans"])
            if ch_data:
                cert_fields["sni"] = ch_data.get("sni", "")

            target_node = self._ip_to_decky.get(src_ip, node_name)
            self._log(target_node, "tls_certificate", **cert_fields)