DECNET/decnet/templates/mssql/server.py

#!/usr/bin/env python3
"""
MSSQL (TDS)server.
Reads TDS pre-login and login7 packets, extracts username, responds with
a login failed error. Logs auth attempts as JSON.
"""

import asyncio
import os
import struct
from syslog_bridge import syslog_line, write_syslog_file, forward_syslog

NODE_NAME = os.environ.get("NODE_NAME", "dbserver")
SERVICE_NAME   = "mssql"
LOG_TARGET = os.environ.get("LOG_TARGET", "")

_PRELOGIN_RESP = bytes([
    0x04, 0x01, 0x00, 0x2f, 0x00, 0x00, 0x01, 0x00,  # TDS header type=4, status=1, len=47
    # 0. VERSION option
    0x00, 0x00, 0x1a, 0x00, 0x06,
    # 1. ENCRYPTION option
    0x01, 0x00, 0x20, 0x00, 0x01,
    # 2. INSTOPT
    0x02, 0x00, 0x21, 0x00, 0x01,
    # 3. THREADID
    0x03, 0x00, 0x22, 0x00, 0x04,
    # 4. MARS
    0x04, 0x00, 0x26, 0x00, 0x01,
    # TERMINATOR
    0xff,
    # version data: 14.0.2000
    0x0e, 0x00, 0x07, 0xd0, 0x00, 0x00,
    # encryption: NOT_SUP
    0x02,
    # instopt
    0x00,
    # thread id
    0x00, 0x00, 0x00, 0x00,
    # mars
    0x00,
])




def _log(event_type: str, severity: int = 6, **kwargs) -> None:
    line = syslog_line(SERVICE_NAME, NODE_NAME, event_type, severity, **kwargs)
    write_syslog_file(line)
    forward_syslog(line, LOG_TARGET)


def _tds_error_packet(message: str) -> bytes:
    msg_enc = message.encode("utf-16-le")
    # Token type 0xAA = ERROR, followed by length, error number, state, class, msg_len, msg
    token = (
        b"\xaa"
        + struct.pack("<H", 4 + 1 + 1 + 2 + len(msg_enc) + 1 + 1 + 1 + 1 + 4)
        + struct.pack("<I", 18456)   # SQL error number: login failed
        + b"\x01"                    # state
        + b"\x0e"                    # class
        + struct.pack("<H", len(message))
        + msg_enc
        + b"\x00"                    # server name length
        + b"\x00"                    # proc name length
        + struct.pack("<I", 1)       # line number
    )
    done = b"\xfd\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
    payload = token + done
    header = struct.pack(">BBHBBBB", 0x04, 0x01, len(payload) + 8, 0x00, 0x00, 0x01, 0x00)
    return header + payload


class MSSQLProtocol(asyncio.Protocol):
    def __init__(self):
        self._transport = None
        self._peer = None
        self._buf = b""
        self._prelogin_done = False

    def connection_made(self, transport):
        self._transport = transport
        self._peer = transport.get_extra_info("peername", ("?", 0))
        _log("connect", src=self._peer[0], src_port=self._peer[1])

    def data_received(self, data):
        self._buf += data
        while len(self._buf) >= 8:
            pkt_type = self._buf[0]
            pkt_len = struct.unpack(">H", self._buf[2:4])[0]
            if pkt_len < 8:
                _log("unknown_packet", src=self._peer[0], pkt_type=hex(pkt_type))
                self._transport.close()
                self._buf = b""
                return
            if len(self._buf) < pkt_len:
                break
            payload = self._buf[8:pkt_len]
            self._buf = self._buf[pkt_len:]
            self._handle_packet(pkt_type, payload)
            if self._transport.is_closing():
                self._buf = b""
                break

    def _handle_packet(self, pkt_type: int, payload: bytes):
        if pkt_type == 0x12:  # Pre-login
            self._transport.write(_PRELOGIN_RESP)
            self._prelogin_done = True
        elif pkt_type == 0x10:  # Login7
            username = self._parse_login7_username(payload)
            _log("auth", src=self._peer[0], username=username)
            self._transport.write(_tds_error_packet("Login failed for user."))
            self._transport.close()
        else:
            _log("unknown_packet", src=self._peer[0], pkt_type=hex(pkt_type))
            self._transport.close()

    def _parse_login7_username(self, payload: bytes) -> str:
        try:
            # Login7 layout: fixed header 36 bytes, then offsets
            # Username offset at bytes 36-37, length at 38-39
            if len(payload) < 40:
                return "<short_packet>"
            offset = struct.unpack("<H", payload[36:38])[0]
            length = struct.unpack("<H", payload[38:40])[0]
            username = payload[offset:offset + length * 2].decode("utf-16-le", errors="replace")
            return username
        except Exception:
            return "<parse_error>"

    def connection_lost(self, exc):
        _log("disconnect", src=self._peer[0] if self._peer else "?")


async def main():
    _log("startup", msg=f"MSSQL server starting as {NODE_NAME}")
    loop = asyncio.get_running_loop()
    server = await loop.create_server(MSSQLProtocol, "0.0.0.0", 1433)  # nosec B104
    async with server:
        await server.serve_forever()


if __name__ == "__main__":
    asyncio.run(main())