firewall/firewall.py

#!/usr/bin/python3

# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program. If not, see <http://www.gnu.org/licenses/>.

# Copyright © 2017 David Sinquin <david.re2o@sinquin.eu>


"""
Module for nftables set management.
"""

# Dependencies: python3-netaddr, python3-requests, nftables, sudo (optionnal)
#
# For sudo configuration, create a file in /etc/sudoers.d/ with:
#    "<python_user> ALL = (root) NOPASSWD: /sbin/nftables"

# netaddr :
# - https://pypi.python.org/pypi/netaddr/
# - https://github.com/drkjam/netaddr/
# - https://netaddr.readthedocs.io/en/latest/


import logging
import subprocess

import netaddr # MAC, IPv4, IPv6
import requests

from collections import Iterable

from config import Config


class ExecError(Exception):
    """Simple class to indicate an error in a process execution."""
    pass

class CommandExec:
    """Simple class to start a command, logging and returning errors if any."""
    @staticmethod
    def run_check_output(command, allowed_return_codes=(0,), timeout=15):
        """
        Run a command, logging output in case of an error.

        Actual timeout may be twice the given value in seconds."""
        logging.debug("Command to be run: '%s'", "' '".join(command))
        process = subprocess.Popen(
            command,
            shell=False,
            stdout=subprocess.PIPE,
            stderr=subprocess.PIPE,
            universal_newlines=True)
        try:
            result = process.communicate(timeout=timeout)
            return_code = process.wait(timeout=timeout)
        except subprocess.TimeoutExpired as err:
            process.kill()
            raise ExecError from err
        if return_code not in allowed_return_codes:
            error_message = ('Error running command: "{}", return code: {}.\n'
                             'Stderr:\n{}\nStdout:\n{}'.format(
                                 '" "'.join(command), return_code, *result))
            logging.error(error_message)
            raise ExecError(error_message)
        return (return_code, *result)

    @classmethod
    def run(cls, *args, **kwargs):
        """Run a command without checking outputs."""
        returncode, _, _ = cls.run_check_output(*args, **kwargs)
        return returncode

class Parser:
    """Parsers for commonly used formats."""
    @staticmethod
    def MAC(mac):
        """Check a MAC validity."""
        return netaddr.EUI(mac, dialect=netaddr.mac_unix_expanded)
    @staticmethod
    def IPv4(ip):
        """Check an IPv4 validity.

        Args:
            ip: can either be a tuple (in this case returns an IPRange), a
            single IP address or a IP Network.
        """
        if isinstance(ip, tuple):
            begin, end = ip
            return netaddr.IPRange(begin, end, version=4)
        try:
            return netaddr.IPAddress(ip, version=4)
        except ValueError:
            return netaddr.IPNetwork(ip, version=4)
    @staticmethod
    def IPv6(ip):
        """Check a IPv6 validity.

        Args:
            ip: can either be a tuple (in this case returns an IPRange), a
            single IP address or a IP Network.
        """
        if isinstance(ip, tuple):
            begin, end = ip
            return netaddr.IPRange(begin, end, version=6)
        try:
            return netaddr.IPAddress(ip, version=6)
        except ValueError:
            return netaddr.IPNetwork(ip, version=6)

    @staticmethod
    def protocol(protocol):
        """Check a protocol validity."""
        if protocol in ('tcp', 'udp', 'icmp'):
            return protocol
        raise ValueError('Invalid protocol: "{}".'.format(protocol))
    @staticmethod
    def port_number(port):
        """Check a port validity."""
        port_number = int(port)
        if 0 <= port_number < 65536:
            return port_number
        raise ValueError('Invalid port number: "{}".'.format(port))

class NetfilterSet:
    """Manage a netfilter set using nftables."""

    TYPES = {'IPv4': 'ipv4_addr', 'IPv6': 'ipv6_addr', 'MAC': 'ether_addr',
             'protocol': 'inet_proto', 'port': 'inet_service'}

    FILTERS = {'IPv4': Parser.IPv4, 'IPv6': Parser.IPv6, 'MAC': Parser.MAC,
               'protocol': Parser.protocol, 'port': Parser.port_number}

    ADDRESS_FAMILIES = {'ip', 'ip6', 'inet', 'arp', 'bridge', 'netdev'}

    FLAGS = {'constant', 'interval', 'timeout'}

    def __init__(self,
                 name,
                 type_, # e.g.: ('MAC', 'IPv4')
                 target_content=None,
                 use_sudo=True,
                 address_family='inet', # Manage both IPv4 and IPv6.
                 table_name='filter',
                 flags = []
                ):
        self.name = name
        self.content = set()
        # self.type
        self.set_type(type_)
        self.filters = tuple(self.FILTERS[i] for i in self.type)
        # self.address_family
        self.set_address_family(address_family)
        self.table = table_name
        self.set_flags(flags)
        sudo = ["/usr/bin/sudo"] * int(bool(use_sudo))
        self.nft = [*sudo, "/usr/sbin/nft"]
        if target_content:
            self._target_content = self.validate_set_data(target_content)
        else:
            self._target_content = set()

    @property
    def target_content(self):
        return self._target_content.copy() # Forbid in-place modification

    @target_content.setter
    def target_content(self, target_content):
        self._target_content = self.validate_set_data(target_content)

    def filter(self, elements):
        return (self.filters[i](element) for i, element in enumerate(elements))

    def set_type(self, type_):
        """Check set type validity and store it along with a type checker."""
        for element_type in type_:
            if element_type not in self.TYPES:
                raise ValueError('Invalid type: "{}".'.format(element_type))
        self.type = type_

    def set_address_family(self, address_family='ip'):
        """Set set addres_family, defaulting to "ip" like nftables."""
        if address_family not in self.ADDRESS_FAMILIES:
            raise ValueError(
                'Invalid address_family: "{}".'.format(address_family))
        self.address_family = address_family

    def set_flags(self, flags_):
        """Check set flags validity before saving them."""
        for f in flags_:
            if f not in self.FLAGS:
                raise ValueError('Invalid flag: "{}".'.format(f))
        self.flags = _flags

    def create_in_kernel(self):
        """Create the set, removing existing set if needed."""
        # Delete set if it exists with wrong type
        current_set = self._get_raw_netfilter_set(parse_elements=False)
        logging.info(current_set)
        if current_set is None:
            self._create_new_set_in_kernel()
        elif not self.has_type(current_set['type']):
            self._delete_in_kernel()
            self._create_new_set_in_kernel()

    def _delete_in_kernel(self):
        """Delete the set, table and set must exist."""
        CommandExec.run([
            *self.nft,
            'delete set {addr_family} {table} {set_}'.format(
                addr_family=self.address_family, table=self.table,
                set_=self.name)
            ])

    def _create_new_set_in_kernel(self):
        """Create the non-existing set, creating table if needed."""
        create_set = [
            *self.nft,
            'add set {addr_family} {table} {set_} {{ type {type_} ; flags {flags}}}'.format(
                addr_family=self.address_family, table=self.table,
                set_=self.name,
                type_=' . '.join(self.TYPES[i] for i in self.type)),
                flags=', '.join(self.flags)
            ]
        return_code = CommandExec.run(create_set, allowed_return_codes=(0, 1))
        if return_code == 0:
            return # Set creation successful.
        # return_code was 1, one error was detected in the rules.
        # Attempt to create the table first.
        create_table = [*self.nft, 'add table {addr_family} {table}'.format(
            addr_family=self.address_family, table=self.table)]
        CommandExec.run(create_table)
        CommandExec.run(create_set)

    def validate_set_data(self, set_data):
        """
        Validate data, returning it or raising a ValueError.

        For MAC-IPv4 set, data must be an iterable of (MAC, IPv4) iterables.
        """
        set_ = set()
        errors = []
        for n_uplet in set_data:
            try:
                set_.add(tuple(self.filter(n_uplet)))
            except Exception as err:
                errors.append(err)
        if errors:
            raise ValueError(
                'Error parsing data, encountered the folowing {} errors.\n"{}"'
                .format(len(errors), '",\n"'.join(map(str, errors))))
        return set_

    def _apply_target_content(self):
        """Change netfilter set content to target set."""
        current_set = self.get_netfilter_set_content()
        if current_set is None:
            raise ValueError('Cannot change "{}" netfilter set content: set '
                             'do not exist in "{}" "{}".'.format(
                                 self.name, self.address_family, self.table))
        to_delete = current_set - self._target_content
        to_add = self._target_content - current_set
        self._change_set_content(delete=to_delete, add=to_add)

    def _change_set_content(self, delete=None, add=None):
        todo = [tuple_ for tuple_ in (('add', add), ('delete', delete))
                if tuple_[1]]
        for action, elements in todo:
            content = ', '.join(' . '.join(str(element) for element in tuple_)
                                for tuple_ in elements)
            command = [
                *self.nft,
                '{action} element {addr_family} {table} {set_} {{{content}}}' \
                    .format(action=action, addr_family=self.address_family,
                            table=self.table, set_=self.name, content=content)
            ]
            CommandExec.run(command)

    def _get_raw_netfilter_set(self, parse_elements=True):
        """Return a dict describing the netfilter set matching self or None."""
        _, stdout, _ = CommandExec.run_check_output(
            [*self.nft, '-nn', 'list set {addr_family} {table} {set_}'.format(
                addr_family=self.address_family, table=self.table,
                set_=self.name)],
            allowed_return_codes=(0, 1) # In case table do not exist
        )
        if not stdout:
            return None
        else:
            netfilter_set = self._parse_netfilter_set_string(stdout)
            if netfilter_set['name'] != self.name \
                    or netfilter_set['address_family'] != self.address_family \
                    or netfilter_set['table'] != self.table \
                    or netfilter_set['type'] != [
                            self.TYPES[type_] for type_ in self.type]:
                raise ValueError('Did not get the right set, too wrong to fix.')
            if parse_elements:
                if netfilter_set['raw_content']:
                    netfilter_set['content'] = self.validate_set_data((
                        (element.strip() for element in n_uplet.split(' . '))
                        for n_uplet in netfilter_set['raw_content'].split(',')))
                else:
                    netfilter_set['content'] = set()
            return netfilter_set

    @staticmethod
    def _parse_netfilter_set_string(set_string):
        """
        Parse netfilter set definition and return set as dict.

        Do not validate content type against detected set type.
        Return a dict with 'name', 'address_family', 'table', 'type',
        'raw_content' keys (all strings, 'raw_content' can be None).
        Raise ValueError in case of unexpected syntax.
        """
        # Fragile code since using lexer / parser would be quite heavy
        lines = [line.lstrip('\t ') for line in set_string.strip().splitlines()]
        errors = []
        # 5 lines when empty, 6 with elements = { … }
        if len(lines) not in (5, 6):
            errors.append('Error, expecting 5 or 6 lines for set definition, '
                          'got "{}".'.format(set_string))

        line_iterator = iter(lines)
        set_definition = {}

        line = next(line_iterator).split(' ') # line #1
        # 'table <address_family> <chain> {'
        if len(line) != 4 or line[0] != 'table' or line[3] != '{':
            errors.append(
                'Cannot parse table definition, expecting "type <addr_family> '
                '<table> {{", got "{}".'.format(' '.join(line)))
        else:
            set_definition['address_family'] = line[1]
            set_definition['table'] = line[2]

        line = next(line_iterator).split(' ') # line #2
        # 'set <name> {'
        if len(line) != 3 or line[0] != 'set' or line[2] != '{':
            errors.append('Cannot parse set definition, expecting "set <name> '
                          '{{", got "{}".' .format(' '.join(line)))
        else:
            set_definition['name'] = line[1]

        line = next(line_iterator).split(' ') # line #3
        # 'type <type> [. <type>]...'
        if len(line) < 2 or len(line) % 2 != 0 or line[0] != 'type' \
                or any(element != '.' for element in line[2::2]):
            errors.append(
                'Cannot parse type definition, expecting "type <type> '
                '[. <type>]...", got "{}".'.format(' '.join(line)))
        else:
            set_definition['type'] = line[1::2]

        if len(lines) == 6:
            # set is not empty, getting raw elements
            line = next(line_iterator) # Unsplit line #4
            if line[:13] != 'elements = { ' or line[-1] != '}':
                errors.append('Cannot parse set elements, expecting "elements '
                              '= {{ <…>}}", got "{}".'.format(line))
            else:
                set_definition['raw_content'] = line[13:-1].strip()
        else:
            set_definition['raw_content'] = None

        # last two lines
        for i in range(2):
            line = next(line_iterator).split(' ')
            if line != ['}']:
                errors.append(
                    'No normal end to set definition, expecting "}}" on line '
                    '{}, got "{}".'.format(i+5, ' '.join(line)))
        if errors:
            raise ValueError('The following error(s) were encountered while '
                             'parsing set.\n"{}"'.format('",\n"'.join(errors)))
        return set_definition

    def get_netfilter_set_content(self):
        """Return current set content from netfilter."""
        netfilter_set = self._get_raw_netfilter_set(parse_elements=True)
        if netfilter_set is None:
            return None
        else:
            return netfilter_set['content']

    def has_type(self, type_):
        """Check if some type match the set's one."""
        return tuple(self.TYPES[t] for t in self.type) == tuple(type_)

    def manage(self):
        """Create set if needed and populate it with target content."""
        self.create_in_kernel()
        self._apply_target_content()


class Firewall:
    """Manages the firewall using nftables."""

    @staticmethod
    def manage_sets(sets, address_family=None, table=None, use_sudo=None):
        CONFIG = Config()
        address_family = address_family or CONFIG['address_family'] or 'inet'
        table = table or CONFIG['table'] or 'filter'
        sudo = use_sudo or (use_sudo is None and CONFIG['use_sudo'])
        for set_ in sets:
            NetfilterSet(
                name=set_['name'],
                type_=set_['type'],
                target_content=set_['content'],
                use_sudo=sudo,
                address_family=address_family,
                table_name=table).manage()