tftp.py

# this file borrowed from https://github.com/m4tx/pyTFTP by Mateusz Maćkowski, released under MIT license

import errno
import logging
import socket
from pathlib import Path, PurePosixPath
from threading import Thread
from typing import List, NewType, Tuple, Union, Dict

logger = logging.getLogger('tftpd')

BLOCK_SIZE = 512
BUF_SIZE = 65536
TIMEOUT = 0.5
MAX_RETRIES = 10


class TFTPOpcodes:
    """Class containing all the opcodes used in TFTP."""
    RRQ = b'\x00\x01'
    WRQ = b'\x00\x02'
    DATA = b'\x00\x03'
    ACK = b'\x00\x04'
    ERROR = b'\x00\x05'
    OACK = b'\x00\x06'


class TFTPErrorCodes:
    """Class containing all the error codes and their messages used in TFTP."""
    UNKNOWN = 0
    FILE_NOT_FOUND = 1
    ACCESS_VIOLATION = 2
    DISK_FULL = 3
    ILLEGAL_OPERATION = 4
    UNKNOWN_TRANSFER_ID = 5
    FILE_EXISTS = 6
    NO_SUCH_USER = 7
    INVALID_OPTIONS = 8

    __MESSAGES = {
        UNKNOWN: '',
        FILE_NOT_FOUND: 'File not found',
        ACCESS_VIOLATION: 'Access violation',
        DISK_FULL: 'Disk full or allocation exceeded',
        ILLEGAL_OPERATION: 'Illegal TFTP operation',
        UNKNOWN_TRANSFER_ID: 'Unknown transfer ID',
        FILE_EXISTS: 'File already exists',
        NO_SUCH_USER: 'No such user',
        INVALID_OPTIONS: 'Invalid options specified',
    }

    @classmethod
    def get_message(cls, error_code: int) -> str:
        """Return an error message for given error code.

        :param error_code: error code to get the message for
        :return: error message
        """
        return cls.__MESSAGES[error_code]


class TFTPOptions:
    # RFC 2348
    BLKSIZE = b'blksize'
    # RFC 7440
    WINDOWSIZE = b'windowsize'


Address = NewType('Address', tuple)
Packet = NewType('Packet', Tuple[bytes, Address])


class TFTPException(Exception):
    """Generic TFTP exception."""
    pass


class TFTPError(TFTPException):
    """Exception meaning that a TFTP ERROR packet received."""

    def __init__(self, error_id: int, message: str) -> None:
        super(TFTPError, self).__init__(
            'Error {}: {}'.format(error_id, message))
        self.error_id = error_id
        self.message = message


class TFTPTerminatedError(TFTPException):
    """Exception meaning that the TFTP connection was terminated for the
    reason passed in `error_id` and `message` arguments."""

    def __init__(self, error_id: int, error_message: str,
                 message: str) -> None:
        super(TFTPTerminatedError, self).__init__(
            'Terminated with error {}: {}; cause: {}'.format(
                error_id, error_message, message))
        self.error_id = error_id
        self.error_message = message
        self.message = message


class TFTP:
    """
    Base class for writing TFTP clients and servers. Handles all the basic
    communication: generic method for sending and receiving packets, methods
    for transmitting specific packets and whole files, as well as error
    and timeout handling.
    """

    def __init__(self, sock: socket.socket, addr: Address,
                 block_size: int = BLOCK_SIZE, window_size: int = 1) -> None:
        """
        :param sock: socket to use to communicate
        :param addr: address (host + port) of the connected host
        """
        self._sock = sock
        self._sock.settimeout(TIMEOUT)
        self._addr = addr
        self._block_size = block_size  # RFC 2348
        self._window_size = window_size  # RFC 7440
        # Whether to check the TID of incoming packets. If set to False, the
        # next packet received will be used to set the new TID (and this will
        # set _check_addr back to True)
        self._check_addr = True
        self.__last_packet: Packet = None
        self.__packet_buffer: Packet = None

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self._sock.close()

    ###########################################################################
    # Error handling
    ###########################################################################
    def _check_error(self, data: bytes, expected_opcodes: List[bytes]) -> None:
        """Check if the packet received has valid opcode and terminate the
        connection if not or an ERROR packet was received.

        :param data: the packet received
        :param expected_opcodes: list of valid opcodes
        :raise: TFTPTerminatedError if the opcode was not valid
        :raise: TFTPError if an ERROR packet was received
        """
        opcode = data[0:2]
        if opcode == TFTPOpcodes.ERROR:
            raise TFTPError(
                int.from_bytes(data[2:4], byteorder='big'),
                data[4:-1].decode('utf-8'))
        elif opcode not in expected_opcodes:
            self._terminate(TFTPErrorCodes.ILLEGAL_OPERATION,
                            'Invalid packet: {}'.format(data))

    def _terminate(self, error_code: int, message: str,
                   error_message: str = None) -> None:
        """Send an ERROR packet, terminate the connection, and raise
        a TFTPTerminatedError

        :param error_code: error code to send
        :param message: message to use for the exception
        :param error_message: message to send with the ERROR packet. If None,
            a default message for the given error code is used.
        :raise: TFTPTerminatedError
        """
        error_message = self._error_occurred(error_code, error_message)
        self._sock.close()
        raise TFTPTerminatedError(error_code, error_message, message)

    def _error_occurred(self, error_code: int, error_message: str = None,
                        addr: Address = None) -> str:
        """Send an ERROR packet, auto-generating the message if necessary.

        :param error_code: error code to send
        :param error_message: message to send with the ERROR packet. If None,
            a default message for the given error code is used.
        :param addr: the address to send the packet to
        :return: the error message that was sent
        """
        if error_message is None:
            error_message = TFTPErrorCodes.get_message(error_code)
        self._send_err(error_code, error_message, addr)
        return error_message

    ###########################################################################
    # Receiving
    ###########################################################################
    def _set_packet_buffer(self, data: bytes, addr: Address) -> None:
        """Set given packet as the "packet buffer". Packets in the buffer have
        priority when trying to retrieve data using _recv(), giving a way to
        use data from a different source (e.g. recvfrom() executed in another
        function) when receiving a packets using a unified function.

        :param data: data to be set in the buffer
        :param addr: address to be set in the buffer
        """
        self.__packet_buffer = Packet((data, addr))

    def _recv(self, handle_timeout: bool = True) -> Packet:
        """Receive a packet, taking into account packets in the packet buffer,
        and retrying (by resending the last sent packet) if needed.

        :return: packet received
        :raise: TFTPException on timeout
        """
        if self.__packet_buffer is not None:
            rv = self.__packet_buffer
            self.__packet_buffer = None
            return rv

        if not handle_timeout:
            r = self._sock.recvfrom(BUF_SIZE)
            return r

        retries = 0
        while retries <= MAX_RETRIES:
            try:
                r = self._sock.recvfrom(BUF_SIZE)
                return r
            except socket.timeout:
                retries += 1
                if retries <= MAX_RETRIES:
                    self.__resend_last_packet()
        raise TFTPException('Timed out')

    def _recv_packet_mul(
            self, opcodes: List[bytes],
            min_data_length: int, handle_timeout: bool = True) -> Tuple[
        Address, bytes, bytes]:
        """Receive a packet and check if its opcode, length, and TID are valid.

        :param opcodes: list of valid opcodes
        :param min_data_length: minimum valid length of the data
        :param check_addr: True if TID validity should be checked; False
            otherwise
        :return: a 3-tuple containing: source packet address, opcode received
            and the data
        """
        while True:
            data, addr = self._recv(handle_timeout)
            if not self._check_addr or addr == self._addr:
                break
            logger.warning('Invalid TID: %s (expected: %s)', addr, self._addr)
            self._error_occurred(TFTPErrorCodes.UNKNOWN_TRANSFER_ID, addr=addr)

        if not self._check_addr:
            self._addr = addr
            self._check_addr = True

        self._check_error(data, opcodes)
        if len(data) < min_data_length + 2:
            self._terminate(TFTPErrorCodes.ILLEGAL_OPERATION,
                            'Packet too short: {}'.format(data))
        return addr, data[0:2], data[2:]

    def _recv_packet(self, opcode: bytes, min_data_length: int,
                     handle_timeout: bool = True) -> Tuple[Address, bytes]:
        """Receive a packet and check if its opcode, length, and TID are valid.

        :param opcode: valid opcode
        :param min_data_length: minimum valid length of the data
        :return: a pair containing: source packet address and the data received
        """
        addr, _, data = self._recv_packet_mul([opcode], min_data_length,
                                              handle_timeout)
        return addr, data

    def _recv_data(
            self, handle_timeout: bool = True) -> Tuple[Address, bytes, bytes]:
        """Receive a DATA packet and return the block ID and the data.

        :return: 3-tuple containing the source address, block ID, and the data
        """
        addr, data = self._recv_packet(TFTPOpcodes.DATA, 2, handle_timeout)
        return addr, data[0:2], data[2:]

    def _recv_ack(self, handle_timeout: bool = True) -> Tuple[Address, int]:
        """Receive an ACK packet and return the block ID.

        :return: pair containing the source address and the block ID
        """
        addr, data = self._recv_packet(TFTPOpcodes.ACK, 2, handle_timeout)
        return addr, int.from_bytes(data, byteorder='big')

    ###########################################################################
    # Sending
    ###########################################################################
    def _send(self, data: bytes, addr: Address = None) -> None:
        """Send a packet and store it as the last packet sent.

        :param data: data to be sent
        :param addr: the destionation address to send the packet to. If None,
            self._addr is used.
        """
        if addr is None:
            addr = self._addr
        self.__last_packet = Packet((data, addr))
        self._sock.sendto(data, addr)

    def __resend_last_packet(self) -> None:
        """Resend the last packet received (used for retries in _recv())."""
        self._sock.sendto(*self.__last_packet)

    def _send_ack(self, block_id: Union[bytes, int]) -> None:
        """Send an ACK packet.

        :param block_id: block ID to send
        """
        if isinstance(block_id, int):
            block_id = block_id.to_bytes(2, byteorder='big')
        self._send(TFTPOpcodes.ACK + block_id)

    def _send_data(self, block_id: int, data: bytes) -> None:
        """Send a DATA packet.

        :param block_id: block ID of the data
        :param data: the data to send
        """
        self._send(
            TFTPOpcodes.DATA + block_id.to_bytes(2, byteorder='big') + data)

    def _send_err(self, error_code: int, error_message: str = None,
                  addr: Address = None) -> None:
        """Send an ERROR packet.

        :param error_code: error code to send
        :param error_message: error message to send
        :param addr: the desitination address to send the packet to
        """
        error_code_bytes = error_code.to_bytes(2, byteorder='big')
        error_message_bytes = error_message.encode('utf-8')

        self._send(TFTPOpcodes.ERROR + error_code_bytes + error_message_bytes +
                   b'\x00', addr)

    ###########################################################################
    # Options (RFC 2347)
    ###########################################################################
    def _process_options(self, options: List[bytes]) -> Dict[bytes, bytes]:
        """Process the options received in RRQ/WRQ packet.

        This is an implementation of the RFC 2347 Options Extension.

        :param options: list of the option strings (null-separated in
            the original packet)
        :return: dictionary of the processed and accepted options
        """
        if options[-1] == b'':
            options.pop()

        if len(options) % 2 == 1:
            raise ValueError

        ret_val = {}
        vals = zip(options[::2], options[1::2])
        d = {k.lower(): (k, v) for k, v in vals}

        # Block size (RFC 2348)
        if TFTPOptions.BLKSIZE in d:
            orig_key, orig_val = d[TFTPOptions.BLKSIZE]
            blk_size = int(orig_val)
            if blk_size < 8 or blk_size > 65464:
                # Invalid according to RFC 2348
                raise ValueError
            self._block_size = blk_size
            ret_val[orig_key] = orig_val

        # Window size (RFC 7440)
        if TFTPOptions.WINDOWSIZE in d:
            orig_key, orig_val = d[TFTPOptions.WINDOWSIZE]
            window_size = int(orig_val)
            if window_size < 1 or window_size > 65535:
                # Invalid according to RFC 7440
                raise ValueError
            self._window_size = window_size
            ret_val[orig_key] = orig_val

        return ret_val

    def _format_options(self, options: Dict[bytes, bytes]):
        """Create single options bytes object out of the provided dictionary.

        :param options: dictionary to convert to bytes object
        :return: generated bytes object
        """
        return b''.join(b'%s\x00%s\x00' % option for option in options.items())

    ###########################################################################
    # Files
    ###########################################################################
    def _recv_file(self) -> bytes:
        """Receive a file by listening for DATA packets and responding
        with ACKs.

        :return: received file
        """
        last_id = 0
        parts = []

        retries = 0
        while retries <= MAX_RETRIES:
            start_last_id = last_id
            for _ in range(self._window_size):
                try:
                    addr, block_id, data = self._recv_data(
                        handle_timeout=False)

                    id_int = int.from_bytes(block_id, byteorder='big')
                    if id_int == last_id + 1:
                        parts.append(data)
                        last_id = id_int

                        if block_id == b'\xff\xff':
                            last_id = -1

                        if len(data) < self._block_size:
                            self._send_ack(last_id)
                            return b''.join(parts)
                except socket.timeout:
                    if last_id == start_last_id:
                        retries += 1
                        break
                    else:
                        retries = 0

            if retries <= MAX_RETRIES:
                self._send_ack((65535 if last_id == -1 else last_id))

        raise TFTPException('Timed out')

    def _send_file(self, data: bytes) -> None:
        """Send a file by sending DATA packets and listening for ACKs.

        :param data: data to be sent
        """
        outer_block_id = 0
        block_id = 0

        while True:
            retries = 0
            while retries <= MAX_RETRIES:
                try:
                    if not self.__send_blocks(data, outer_block_id, block_id):
                        return

                    _, ack_block_id = self._recv_ack(handle_timeout=False)
                    last_block_id = block_id + self._window_size
                    if ((last_block_id >= ack_block_id >= block_id) or
                            (ack_block_id <= last_block_id % 65536 and
                             ack_block_id < block_id)):
                        # If received ACK is a reply to one of the blocks sent
                        # sent the next batch of blocks, else re-send
                        if ack_block_id < block_id:
                            outer_block_id += 1
                        block_id = ack_block_id
                    break
                except socket.timeout:
                    retries += 1
            else:
                raise TFTPException('Timed out')

    def __send_blocks(
            self, data: bytes, outer_block_id: int, inner_block_id: int):
        """Send a single window of data.

        :param data: data to be sent
        :param outer_block_id: starting "outer" block ID (incremented by 1
            each time inner block ID overflows)
        :param inner_block_id: starting "inner" block ID in the range [0, 65535]
        :return: False if there is no data to be sent; True otherwise
        """
        blk_size = self._block_size

        for i in range(self._window_size):
            local_blkid = outer_block_id * 65536 + inner_block_id + i
            if local_blkid * self._block_size > len(data):
                if i == 0:
                    return False
                else:
                    break

            to_send = data[local_blkid * blk_size:
                           (local_blkid + 1) * blk_size]
            self._send_data((local_blkid + 1) % 65536, to_send)

        return True


class TFTPClient(TFTP):
    """
    Class that handles communication with a TFTP server and allows to download
    and upload files.
    """

    def __init__(self, host: str, port: int,
                 block_size: int = BLOCK_SIZE, window_size: int = 1) -> None:
        """
        :param host: hostname/IP of the server to connect to
        :param port: UDP port of the server to connect to
        :param block_size: block size, as in RFC 2347
        :param window_size: window size, as in RFC 7440
        """
        super(TFTPClient, self).__init__(
            socket.socket(socket.AF_INET, socket.SOCK_DGRAM),
            Address((host, port)), block_size, window_size)
        self.__options = self._format_options(self.__create_options_dict())

    def __create_options_dict(self) -> Dict[bytes, bytes]:
        """Create options dictionary to feed into TFTP._format_options method.

        The method omits the options that have default value.

        :return: generated dictionary
        """
        d = {}

        if self._block_size != BLOCK_SIZE:
            d[TFTPOptions.BLKSIZE] = str(self._block_size).encode('utf-8')
        if self._window_size != 1:
            d[TFTPOptions.WINDOWSIZE] = str(self._window_size).encode('utf-8')

        return d

    def __send_rq(self, opcode: bytes, file_name: str,
                  mode: str = 'octet') -> None:
        """Send an RRQ/WRQ packet.

        :param opcode: opcode to send (see TFTPOpcodes.RRQ and TFTPOpcodes.WRQ)
        :param file_name: name of the file requested
        :param mode: requested file transfer mode ('octet' by default)
        """
        self._send(b'%s%s\x00%s\x00%s' % (
            opcode, bytes(file_name, 'utf-8'), bytes(mode, 'utf-8'),
            self.__options))

    def __send_rrq(self, file_name: str, mode: str = 'octet') -> None:
        """Send an RRQ packet.

        :param file_name: name of the file requested
        :param mode: requested file transfer mode ('octet' by default)
        """
        self.__send_rq(TFTPOpcodes.RRQ, file_name, mode)

    def __send_wrq(self, file_name: str, mode: str = 'octet') -> None:
        """Send a WRQ packet.

        :param file_name: name of the uploaded file
        :param mode: requested file transfer mode ('octet' by default)
        """
        self.__send_rq(TFTPOpcodes.WRQ, file_name, mode)

    def get_file(self, file_name: str) -> bytes:
        """Retrieve a file from the connected server.

        :param file_name: name of the file to download
        :return: file data returned by the server
        """
        self.__send_rrq(file_name)
        self._check_addr = False
        self.__recv_first_rrq_packet()
        return self._recv_file()

    def __recv_first_rrq_packet(self):
        """Receive and respond (in case of OACK) to the first packet after
        sending RRQ - either OACK or DATA.
        """
        addr, opcode, data = self._recv_packet_mul(
            [TFTPOpcodes.OACK, TFTPOpcodes.DATA], 0)
        if opcode == TFTPOpcodes.DATA:
            self._set_packet_buffer(opcode + data, addr)
        else:
            self.__process_oack(data)
            self._send_ack(b'\x00\x00')

    def put_file(self, file_name: str, data: bytes) -> None:
        """Upload a file to the connected server.

        :param file_name: name of the uploaded file
        :param data: data to be sent
        """
        self.__send_wrq(file_name)
        self._check_addr = False
        self.__recv_first_wrq_packet()
        self._send_file(data)

    def __recv_first_wrq_packet(self):
        """Receive the first packet after sending WRQ - either OACK or ACK."""
        addr, opcode, data = self._recv_packet_mul(
            [TFTPOpcodes.OACK, TFTPOpcodes.ACK], 0)
        if opcode == TFTPOpcodes.OACK:
            self.__process_oack(data)

    def __process_oack(self, data: bytes):
        """Process and apply the options from the OACK packet.

        :param data: raw data got from the packet
        """
        self._process_options(data.split(b'\0'))


class TFTPClientHandler(TFTP):
    """
    Class that handles the communication with a single TFTP client on the
    server side.
    """

    def __init__(self, host: str, addr: Address, root_dir: Path,
                 allow_upload: bool, initial_buffer: bytes = None) -> None:
        """
        :param host: host of the server to bind to
        :param addr: address of the client to connect with
        :param root_dir: root directory of the files to serve
        :param allow_upload: whether or not allow to upload files
        :param initial_buffer: initial packet buffer; usually a `bytes` object
            containing the first (RRQ/WRQ) packet, or None, if there is no
            external server that catches the first packet.
        """
        super().__init__(
            socket.socket(socket.AF_INET, socket.SOCK_DGRAM), addr)

        if initial_buffer is not None:
            self._set_packet_buffer(initial_buffer, self._addr)

        self._sock.bind((host, 0))
        logger.info('Incoming connection from %s, binding at: %s',
                    self._addr, self._sock.getsockname())

        self.__root_dir = root_dir
        self.__allow_upload = allow_upload

    def handle_client(self) -> None:
        """Handle the request sent by the connected client."""
        opcode, file_name, mode = self.__recv_rq()
        try:
            path = self.__get_file_path(file_name)
            if opcode == TFTPOpcodes.RRQ:
                self.__handle_rrq(path)
            else:
                self.__handle_wrq(path)
        except OSError as e:
            self.__handle_file_error(e)

    def __exit__(self, exc_type, exc_val, exc_tb):
        logger.info('Closing connection to %s, bound at: %s',
                    self._addr, self._sock.getsockname())
        super(TFTPClientHandler, self).__exit__(exc_type, exc_val, exc_tb)

    def __recv_rq(self) -> Tuple[bytes, str, str]:
        """Receive an RRQ/WRQ packet and return received data.

        :return: 3-tuple containing: received opcode, file name and file
            transfer mode
        """
        _, opcode, data = self._recv_packet_mul(
            [TFTPOpcodes.RRQ, TFTPOpcodes.WRQ], 2)

        try:
            file_name_bytes, mode_bytes, *options = data.split(b'\0')
            try:
                new_options = self._process_options(options)
                if len(new_options):
                    self.__send_oack(new_options)
                    if opcode == TFTPOpcodes.RRQ:
                        self._recv_ack()
            except ValueError:
                self._terminate(TFTPErrorCodes.INVALID_OPTIONS,
                                'Invalid options received')

            file_name = file_name_bytes.decode('utf-8')
            mode = mode_bytes.decode('utf-8')
        except ValueError as e:
            self._terminate(TFTPErrorCodes.ILLEGAL_OPERATION, str(e))
        if mode != 'octet':
            self._terminate(TFTPErrorCodes.ILLEGAL_OPERATION,
                            'Mode is not "octet": {}'.format(mode))

        return opcode, file_name, mode

    def __send_oack(self, new_options: Dict[bytes, bytes]):
        """Send an OACK packet.

        :param new_options: dictionary of options to be included in
            the OACK packet.
        """
        msg = TFTPOpcodes.OACK + self._format_options(new_options)
        self._send(msg)

    def __get_file_path(self, file_name: str) -> Path:
        """Return file path inside server root directory, ignoring "evil"
        paths, like "../../secret_file", "/etc/fstab", etc.

        :param file_name: file name to get the path to
        :return: absolute path inside the server root directory
        """
        while PurePosixPath(file_name).is_absolute():
            file_name = file_name[1:]
        path = self.__root_dir.joinpath(file_name)

        try:
            path.relative_to(self.__root_dir)
        except ValueError:
            self._terminate(TFTPErrorCodes.ACCESS_VIOLATION,
                            'Invalid path: {}'.format(file_name))
        return path

    def __handle_rrq(self, path: Path) -> None:
        """Handle RRQ request: read and send the requested file.

        :param path: path to the requested file
        """
        self._send_file(path.read_bytes())

    def __handle_wrq(self, path: Path) -> None:
        """Handle WRQ request: download and save the file from the client,
        taking into account the `__allow_upload` setting.

        :param path: path to save the file as
        """
        if not self.__allow_upload:
            self._terminate(TFTPErrorCodes.ACCESS_VIOLATION,
                            'Upload not allowed')
        if path.exists():
            self._terminate(TFTPErrorCodes.FILE_EXISTS,
                            'File exists: {}'.format(path))

        self._send_ack(b'\x00\x00')
        path.write_bytes(self._recv_file())

    def __handle_file_error(self, e: OSError) -> None:
        """Handle given IO error, sending an appropriate ERROR packet and
        terminating the transmission.

        :param e: error raised when trying to open the file
        """
        error_message = None
        if e.errno == errno.ENOENT:
            error_code = TFTPErrorCodes.FILE_NOT_FOUND
        elif e.errno == errno.EPERM or e.errno == errno.EACCES:
            error_code = TFTPErrorCodes.ACCESS_VIOLATION
        elif e.errno == errno.EFBIG or e.errno == errno.ENOSPC:
            error_code = TFTPErrorCodes.DISK_FULL
        else:
            error_code = TFTPErrorCodes.UNKNOWN
            error_message = e.strerror
        self._terminate(error_code, e.strerror, error_message)


class TFTPServer:
    """
    Class that handles communication with multiple TFTP clients. Uses
    TFTPClientHandler for the communication with each single client, running
    one instance of this class in a separate thread for each client.
    """

    def __init__(self, host: str, port: int, root_dir: Union[str, Path],
                 allow_upload: bool) -> None:
        """
        :param host: host of the server to bind to
        :param port: port to bind to
        :param root_dir: the directory where the files should be served from
        :param allow_upload: whether or not allow uploading new files
        """
        self.sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
        addr = (host, port)
        logger.info('Starting TFTP server, listening on %s', addr)
        self.sock.bind(addr)

        self.host = host
        self.root_dir = Path(root_dir)
        self.allow_upload = allow_upload

    def __enter__(self):
        return self

    def serve(self) -> None:
        """Run the main server loop: wait for new connections and run
        TFTPClientHandler for each.
        """
        while True:
            data, addr = self.sock.recvfrom(BUF_SIZE)

            def handle_client() -> None:
                TFTPClientHandler(
                    self.host, addr, self.root_dir, self.allow_upload,
                    data).handle_client()

            Thread(target=handle_client).start()

    def __exit__(self, exc_type, exc_val, exc_tb):
        logger.info('Stopping TFTP server')
        self.sock.close()