"""
This addon determines the next protocol layer in our proxy stack.
Whenever a protocol layer in the proxy wants to pass a connection to a child layer and isn't sure which protocol comes
next, it calls the `next_layer` hook, which ends up here.
For example, if mitmproxy runs as a regular proxy, we first need to determine if
new clients start with a TLS handshake right away (Secure Web Proxy) or send a plaintext HTTP CONNECT request.
This addon here peeks at the incoming bytes and then makes a decision based on proxy mode, mitmproxy options, etc.

For a typical HTTPS request, this addon is called a couple of times: First to determine that we start with an HTTP layer
which processes the `CONNECT` request, a second time to determine that the client then starts negotiating TLS, and a
third time when we check if the protocol within that TLS stream is actually HTTP or something else.

Sometimes it's useful to hardcode specific logic in next_layer when one wants to do fancy things.
In that case it's not necessary to modify mitmproxy's source, adding a custom addon with a next_layer event hook
that sets nextlayer.layer works just as well.
"""

from __future__ import annotations

import logging
import re
import sys
from collections.abc import Iterable
from collections.abc import Sequence
from typing import Any
from typing import cast

from mitmproxy import ctx
from mitmproxy.connection import Address
from mitmproxy.net.tls import starts_like_dtls_record
from mitmproxy.net.tls import starts_like_tls_record
from mitmproxy.proxy import layer
from mitmproxy.proxy import layers
from mitmproxy.proxy import mode_specs
from mitmproxy.proxy import tunnel
from mitmproxy.proxy.context import Context
from mitmproxy.proxy.layer import Layer
from mitmproxy.proxy.layers import ClientQuicLayer
from mitmproxy.proxy.layers import ClientTLSLayer
from mitmproxy.proxy.layers import DNSLayer
from mitmproxy.proxy.layers import HttpLayer
from mitmproxy.proxy.layers import modes
from mitmproxy.proxy.layers import RawQuicLayer
from mitmproxy.proxy.layers import ServerQuicLayer
from mitmproxy.proxy.layers import ServerTLSLayer
from mitmproxy.proxy.layers import TCPLayer
from mitmproxy.proxy.layers import UDPLayer
from mitmproxy.proxy.layers.http import HTTPMode
from mitmproxy.proxy.layers.quic import quic_parse_client_hello_from_datagrams
from mitmproxy.proxy.layers.tls import dtls_parse_client_hello
from mitmproxy.proxy.layers.tls import HTTP_ALPNS
from mitmproxy.proxy.layers.tls import parse_client_hello
from mitmproxy.tls import ClientHello

if sys.version_info < (3, 11):
    from typing_extensions import assert_never
else:
    from typing import assert_never

logger = logging.getLogger(__name__)


def stack_match(
    context: Context, layers: Sequence[type[Layer] | tuple[type[Layer], ...]]
) -> bool:
    if len(context.layers) != len(layers):
        return False
    return all(
        expected is Any or isinstance(actual, expected)
        for actual, expected in zip(context.layers, layers)
    )


class NeedsMoreData(Exception):
    """Signal that the decision on which layer to put next needs to be deferred within the NextLayer addon."""


class NextLayer:
    ignore_hosts: Sequence[re.Pattern] = ()
    allow_hosts: Sequence[re.Pattern] = ()
    tcp_hosts: Sequence[re.Pattern] = ()
    udp_hosts: Sequence[re.Pattern] = ()

    def configure(self, updated):
        if "tcp_hosts" in updated:
            self.tcp_hosts = [
                re.compile(x, re.IGNORECASE) for x in ctx.options.tcp_hosts
            ]
        if "udp_hosts" in updated:
            self.udp_hosts = [
                re.compile(x, re.IGNORECASE) for x in ctx.options.udp_hosts
            ]
        if "allow_hosts" in updated or "ignore_hosts" in updated:
            self.ignore_hosts = [
                re.compile(x, re.IGNORECASE) for x in ctx.options.ignore_hosts
            ]
            self.allow_hosts = [
                re.compile(x, re.IGNORECASE) for x in ctx.options.allow_hosts
            ]

    def next_layer(self, nextlayer: layer.NextLayer):
        if nextlayer.layer:
            return  # do not override something another addon has set.
        try:
            nextlayer.layer = self._next_layer(
                nextlayer.context,
                nextlayer.data_client(),
                nextlayer.data_server(),
            )
        except NeedsMoreData:
            logger.debug(
                f"Deferring layer decision, not enough data: {nextlayer.data_client().hex()!r}"
            )

    def _next_layer(
        self, context: Context, data_client: bytes, data_server: bytes
    ) -> Layer | None:
        assert context.layers

        def s(*layers):
            return stack_match(context, layers)

        tcp_based = context.client.transport_protocol == "tcp"
        udp_based = context.client.transport_protocol == "udp"

        # 1)  check for --ignore/--allow
        if self._ignore_connection(context, data_client, data_server):
            return (
                layers.TCPLayer(context, ignore=not ctx.options.show_ignored_hosts)
                if tcp_based
                else layers.UDPLayer(context, ignore=not ctx.options.show_ignored_hosts)
            )

        # 2)  Handle proxy modes with well-defined next protocol
        # 2a) Reverse proxy: derive from spec
        if s(modes.ReverseProxy):
            return self._setup_reverse_proxy(context, data_client)
        # 2b) Explicit HTTP proxies
        if s((modes.HttpProxy, modes.HttpUpstreamProxy)):
            return self._setup_explicit_http_proxy(context, data_client)

        # 3)  Handle security protocols
        # 3a) TLS/DTLS
        is_tls_or_dtls = (
            tcp_based
            and starts_like_tls_record(data_client)
            or udp_based
            and starts_like_dtls_record(data_client)
        )
        if is_tls_or_dtls:
            server_tls = ServerTLSLayer(context)
            server_tls.child_layer = ClientTLSLayer(context)
            return server_tls
        # 3b) QUIC
        if udp_based and _starts_like_quic(data_client, context.server.address):
            server_quic = ServerQuicLayer(context)
            server_quic.child_layer = ClientQuicLayer(context)
            return server_quic

        # 4)  Check for --tcp/--udp
        if tcp_based and self._is_destination_in_hosts(context, self.tcp_hosts):
            return layers.TCPLayer(context)
        if udp_based and self._is_destination_in_hosts(context, self.udp_hosts):
            return layers.UDPLayer(context)

        # 5)  Handle application protocol
        # 5a) Do we have a known ALPN negotiation?
        if context.client.alpn:
            if context.client.alpn in HTTP_ALPNS:
                return layers.HttpLayer(context, HTTPMode.transparent)
            elif context.client.tls_version == "QUICv1":
                # TODO: Once we support more QUIC-based protocols, relax force_raw here.
                return layers.RawQuicLayer(context, force_raw=True)
        # 5b) Is it DNS?
        if context.server.address and context.server.address[1] in (53, 5353):
            return layers.DNSLayer(context)
        # 5c) We have no other specialized layers for UDP, so we fall back to raw forwarding.
        if udp_based:
            return layers.UDPLayer(context)
        # 5d) Check for raw tcp mode.
        probably_no_http = (
            # the first three bytes should be the HTTP verb, so A-Za-z is expected.
            len(data_client) < 3
            # HTTP would require whitespace...
            or b" " not in data_client
            # ...and that whitespace needs to be in the first line.
            or (data_client.find(b" ") > data_client.find(b"\n"))
            or not data_client[:3].isalpha()
            # a server greeting would be uncharacteristic.
            or data_server
            or data_client.startswith(b"SSH")
        )
        if ctx.options.rawtcp and probably_no_http:
            return layers.TCPLayer(context)
        # 5c) Assume HTTP by default.
        return layers.HttpLayer(context, HTTPMode.transparent)

    def _ignore_connection(
        self,
        context: Context,
        data_client: bytes,
        data_server: bytes,
    ) -> bool | None:
        """
        Returns:
            True, if the connection should be ignored.
            False, if it should not be ignored.

        Raises:
            NeedsMoreData, if we need to wait for more input data.
        """
        if not ctx.options.ignore_hosts and not ctx.options.allow_hosts:
            return False
        # Special handling for wireguard mode: if the hostname is "10.0.0.53", do not ignore the connection
        if isinstance(
            context.client.proxy_mode, mode_specs.WireGuardMode
        ) and context.server.address == ("10.0.0.53", 53):
            return False
        hostnames: list[str] = []
        if context.server.peername:
            host, port, *_ = context.server.peername
            hostnames.append(f"{host}:{port}")
        if context.server.address:
            host, port, *_ = context.server.address
            hostnames.append(f"{host}:{port}")

            # We also want to check for TLS SNI and HTTP host headers, but in order to ignore connections based on that
            # they must have a destination address. If they don't, we don't know how to establish an upstream connection
            # if we ignore.
            if host_header := self._get_host_header(context, data_client, data_server):
                if not re.search(r":\d+$", host_header):
                    host_header = f"{host_header}:{port}"
                hostnames.append(host_header)
            if (
                client_hello := self._get_client_hello(context, data_client)
            ) and client_hello.sni:
                hostnames.append(f"{client_hello.sni}:{port}")
            if context.client.sni:
                # Hostname may be allowed, TLS is already established, and we have another next layer decision.
                hostnames.append(f"{context.client.sni}:{port}")

        if not hostnames:
            return False

        if ctx.options.allow_hosts:
            not_allowed = not any(
                re.search(rex, host, re.IGNORECASE)
                for host in hostnames
                for rex in ctx.options.allow_hosts
            )
            if not_allowed:
                return True

        if ctx.options.ignore_hosts:
            ignored = any(
                re.search(rex, host, re.IGNORECASE)
                for host in hostnames
                for rex in ctx.options.ignore_hosts
            )
            if ignored:
                return True

        return False

    @staticmethod
    def _get_host_header(
        context: Context,
        data_client: bytes,
        data_server: bytes,
    ) -> str | None:
        """
        Try to read a host header from data_client.

        Returns:
            The host header value, or None, if no host header was found.

        Raises:
            NeedsMoreData, if the HTTP request is incomplete.
        """
        if context.client.transport_protocol != "tcp" or data_server:
            return None

        host_header_expected = re.match(
            rb"[A-Z]{3,}.+HTTP/", data_client, re.IGNORECASE
        )
        if host_header_expected:
            if m := re.search(
                rb"\r\n(?:Host:\s+(.+?)\s*)?\r\n", data_client, re.IGNORECASE
            ):
                if host := m.group(1):
                    return host.decode("utf-8", "surrogateescape")
                else:
                    return None  # \r\n\r\n - header end came first.
            else:
                raise NeedsMoreData
        else:
            return None

    @staticmethod
    def _get_client_hello(context: Context, data_client: bytes) -> ClientHello | None:
        """
        Try to read a TLS/DTLS/QUIC ClientHello from data_client.

        Returns:
            A complete ClientHello, or None, if no ClientHello was found.

        Raises:
            NeedsMoreData, if the ClientHello is incomplete.
        """
        match context.client.transport_protocol:
            case "tcp":
                if starts_like_tls_record(data_client):
                    try:
                        ch = parse_client_hello(data_client)
                    except ValueError:
                        pass
                    else:
                        if ch is None:
                            raise NeedsMoreData
                        return ch
                return None
            case "udp":
                try:
                    return quic_parse_client_hello_from_datagrams([data_client])
                except ValueError:
                    pass

                try:
                    ch = dtls_parse_client_hello(data_client)
                except ValueError:
                    pass
                else:
                    if ch is None:
                        raise NeedsMoreData
                    return ch
                return None
            case _:  # pragma: no cover
                assert_never(context.client.transport_protocol)

    @staticmethod
    def _setup_reverse_proxy(context: Context, data_client: bytes) -> Layer:
        spec = cast(mode_specs.ReverseMode, context.client.proxy_mode)
        stack = tunnel.LayerStack()

        match spec.scheme:
            case "http":
                if starts_like_tls_record(data_client):
                    stack /= ClientTLSLayer(context)
                stack /= HttpLayer(context, HTTPMode.transparent)
            case "https":
                if context.client.transport_protocol == "udp":
                    stack /= ServerQuicLayer(context)
                    stack /= ClientQuicLayer(context)
                    stack /= HttpLayer(context, HTTPMode.transparent)
                else:
                    stack /= ServerTLSLayer(context)
                    if starts_like_tls_record(data_client):
                        stack /= ClientTLSLayer(context)
                    stack /= HttpLayer(context, HTTPMode.transparent)

            case "tcp":
                if starts_like_tls_record(data_client):
                    stack /= ClientTLSLayer(context)
                stack /= TCPLayer(context)
            case "tls":
                stack /= ServerTLSLayer(context)
                if starts_like_tls_record(data_client):
                    stack /= ClientTLSLayer(context)
                stack /= TCPLayer(context)

            case "udp":
                if starts_like_dtls_record(data_client):
                    stack /= ClientTLSLayer(context)
                stack /= UDPLayer(context)
            case "dtls":
                stack /= ServerTLSLayer(context)
                if starts_like_dtls_record(data_client):
                    stack /= ClientTLSLayer(context)
                stack /= UDPLayer(context)

            case "dns":
                # TODO: DNS-over-TLS / DNS-over-DTLS
                # is_tls_or_dtls = (
                #     context.client.transport_protocol == "tcp" and starts_like_tls_record(data_client)
                #     or
                #     context.client.transport_protocol == "udp" and starts_like_dtls_record(data_client)
                # )
                # if is_tls_or_dtls:
                #     stack /= ClientTLSLayer(context)
                stack /= DNSLayer(context)

            case "http3":
                stack /= ServerQuicLayer(context)
                stack /= ClientQuicLayer(context)
                stack /= HttpLayer(context, HTTPMode.transparent)
            case "quic":
                stack /= ServerQuicLayer(context)
                stack /= ClientQuicLayer(context)
                stack /= RawQuicLayer(context, force_raw=True)

            case _:  # pragma: no cover
                assert_never(spec.scheme)

        return stack[0]

    @staticmethod
    def _setup_explicit_http_proxy(context: Context, data_client: bytes) -> Layer:
        stack = tunnel.LayerStack()

        if context.client.transport_protocol == "udp":
            stack /= layers.ClientQuicLayer(context)
        elif starts_like_tls_record(data_client):
            stack /= layers.ClientTLSLayer(context)

        if isinstance(context.layers[0], modes.HttpUpstreamProxy):
            stack /= layers.HttpLayer(context, HTTPMode.upstream)
        else:
            stack /= layers.HttpLayer(context, HTTPMode.regular)

        return stack[0]

    @staticmethod
    def _is_destination_in_hosts(context: Context, hosts: Iterable[re.Pattern]) -> bool:
        return any(
            (context.server.address and rex.search(context.server.address[0]))
            or (context.client.sni and rex.search(context.client.sni))
            for rex in hosts
        )


# https://www.iana.org/assignments/quic/quic.xhtml
KNOWN_QUIC_VERSIONS = {
    0x00000001,  # QUIC v1
    0x51303433,  # Google QUIC Q043
    0x51303436,  # Google QUIC Q046
    0x51303530,  # Google QUIC Q050
    0x6B3343CF,  # QUIC v2
    0x709A50C4,  # QUIC v2 draft codepoint
}

TYPICAL_QUIC_PORTS = {80, 443, 8443}


def _starts_like_quic(data_client: bytes, server_address: Address | None) -> bool:
    """
    Make an educated guess on whether this could be QUIC.
    This turns out to be quite hard in practice as 1-RTT packets are hardly distinguishable from noise.

    Returns:
        True, if the passed bytes could be the start of a QUIC packet.
        False, otherwise.
    """
    # Minimum size: 1 flag byte + 1+ packet number bytes + 16+ bytes encrypted payload
    if len(data_client) < 18:
        return False
    if starts_like_dtls_record(data_client):
        return False
    # TODO: Add more checks here to detect true negatives.

    # Long Header Packets
    if data_client[0] & 0x80:
        version = int.from_bytes(data_client[1:5], "big")
        if version in KNOWN_QUIC_VERSIONS:
            return True
        # https://www.rfc-editor.org/rfc/rfc9000.html#name-versions
        # Versions that follow the pattern 0x?a?a?a?a are reserved for use in forcing version negotiation
        if version & 0x0F0F0F0F == 0x0A0A0A0A:
            return True
    else:
        # ¯\_(ツ)_/¯
        # We can't even rely on the QUIC bit, see https://datatracker.ietf.org/doc/rfc9287/.
        pass

    return bool(server_address and server_address[1] in TYPICAL_QUIC_PORTS)