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"""
Copyright (c) 2023 Proton AG
This file is part of Proton.
Proton 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.
Proton 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 ProtonVPN. If not, see <https://www.gnu.org/licenses/>.
"""
import ipaddress
import logging
import re
import struct
import typing
import random
class DNSResponseError(Exception):
pass
class DNSParsingException(DNSResponseError):
pass
class DNSParser:
"""Parse response from any DNS resolvers"""
STRUCT_REPLY_COUNTS = struct.Struct('>HHHH')
STRUCT_REC_FORMAT = struct.Struct('>HHIH')
_MINIMUM_RECORD_LENGTH = 12 # => Transaction ID/Flags/#Questions/#Answers/#AuthorRRs/#AdditRRs
# Regular expression used in _is_valid_hostname.
# Each hostname segment (the strings in between the period characters) is only valid if:
# - it has a minimum of 1 character and maximum of 63,
# - it only contains alphanumeric characters or the hyphen but
# - it does not start or end with a hyphen.
_VALID_HOSTNAME_SEGMENT = re.compile(r"(?!-)[A-Z\d-]{1,63}(?<!-)$", re.IGNORECASE)
# type definitions
IPvxAddress = typing.Union[ipaddress.IPv4Address, ipaddress.IPv6Address]
ParsedData = typing.Union[str, IPvxAddress]
@classmethod
def parse(cls, reply_data) -> typing.Optional[typing.List[typing.Tuple[int, ParsedData]]]:
""" parse DNS reply and returns list of : TTL, address(IP or CNAME)"""
if len(reply_data) < cls._MINIMUM_RECORD_LENGTH:
raise DNSParsingException(f"(truncated reply)")
# Match reply code (0x0 = OK)
dns_rcode = reply_data[3] & 0xf
# ensure we have something to parse
if dns_rcode == 0x3:
#NXDOMAIN, this is fatal
raise DNSResponseError("No alternative routing exists for this environment (NXDOMAIN)")
elif dns_rcode != 0x0:
raise DNSResponseError(f"DNS response error code: {dns_rcode}")
# Get counts
offset = 4
dns_qdcount, dns_ancount, dns_nscount, dns_arcount = cls.STRUCT_REPLY_COUNTS.unpack_from(reply_data[offset:])
offset += cls.STRUCT_REPLY_COUNTS.size
# skip questions
for dns_qd_idx in range(dns_qdcount):
length, data = cls._get_name(reply_data, offset)
# We ignore QTYPE/QCLASS
offset += length + 4
# tuples (TTL, data)
answers = []
# answers
for dns_an_idx in range(dns_ancount):
length, data = cls._get_name(reply_data, offset)
offset += length
try:
rec_type, rec_class, rec_ttl, rec_dlen = cls.STRUCT_REC_FORMAT.unpack_from(reply_data[offset:])
except struct.error:
raise DNSParsingException(f"(truncated record headers)")
offset += cls.STRUCT_REC_FORMAT.size
record = reply_data[offset:offset + rec_dlen]
if offset + rec_dlen > len(reply_data):
raise DNSParsingException(f"(truncated reply while parsing record)")
offset += rec_dlen
if rec_type == 0x10 and rec_class == 0x01: # IN TXT
if record[0] != rec_dlen - 1:
raise DNSParsingException(f"(length of TXT record doesn't match REC_DLEN)")
if record[0] != len(record) - 1:
raise DNSParsingException(f"(length of TXT record doesn't actual record data)")
try:
hostname = record[1:].decode('ascii')
# Only hostnames with a valid format are accepted in TXT records.
# This is to avoid possible security exploits where the TXT record contains
# a full URL (hostname + path), which e.g. could trigger SSO redirects.
if not cls._is_valid_hostname(hostname):
raise DNSParsingException(f"Invalid hostname in TXT record: {hostname}")
answers.append((int(rec_ttl), hostname))
except UnicodeDecodeError:
raise DNSParsingException(f"(UnicodeDecodeError in TXT record)")
elif rec_type == 0x01 and rec_class == 0x01: # IN A
if len(record) != 4:
raise DNSParsingException(f"(length of A record doesn't match)")
try:
ipv4_address = ipaddress.IPv4Address(record)
except ValueError as exc:
raise DNSParsingException(f"Invalid IP address in A record: {record}") from exc
answers.append((int(rec_ttl), ipv4_address))
else:
logging.debug(f"record type currently not supported: {rec_type}... skip")
return answers
@staticmethod
def _get_name(buffer: bytes, offset=0):
# Length that we've parsed (for that specific record)
parsed_length = 0
# Have we jumped to somewhere else?
has_jumped = False
# Parts we've seen until now
parts = []
# While we're in the buffer, and we are not on a null byte (terminator)
while offset < len(buffer) and buffer[offset] != 0:
# Read the length of the part, in one byte
length = buffer[offset]
# If the length starts with two one bytes, then it's a pointer
if length & 0b1100_0000 == 0b1100_0000:
offset = ((buffer[offset] & 0b0011_1111) << 8) + buffer[offset + 1]
# Pointers have length 2
if not has_jumped:
parsed_length += 2
# We're not any more in the current record, stop counting
has_jumped = True
else:
# Real entry
# Add the part
if offset + 1 + length > len(buffer):
raise DNSParsingException(f"DNS resolution failed (non-parsable value)")
parts.append(buffer[offset + 1:offset + 1 + length])
# Add length, and the length byte
if not has_jumped:
parsed_length += length + 1
offset += length + 1
# This is for the 0-byte that terminates a name
if not has_jumped:
parsed_length += 1
return parsed_length, parts
@classmethod
def _build_simple_query(cls, domain: bytes, qtype: int, qclass: int):
"""internal utility to build the simplest DNS request we need"""
id: bytes = struct.pack('!H', random.randint(0, 65535))
qtype: bytes = struct.pack('!H', qtype)
qclass: bytes = struct.pack('!H', qclass)
# it's a query with a single question, no AN, no RR, no AR
return id + b"\x01\x20\x00\x01\x00\x00\x00\x00\x00\x00" + domain + qtype + qclass
@classmethod
def build_query(cls, fqdn: typing.Union[str, bytes], qtype, qclass):
"""Build a very simple dns request that is just a query with no AN, no RR, no AR, and a single question"""
if type(fqdn) == str:
domain = b''.join([bytes([len(el)]) + el.encode('ascii') for el in fqdn.split('.')]) + b'\x00'
elif type(fqdn) == bytes:
domain = b''.join([bytes([len(el)]) + el for el in fqdn.split(b'.')]) + b'\x00'
else:
raise TypeError("fqdn can only be str or bytes")
query = cls._build_simple_query(domain, qtype, qclass)
return query
@classmethod
def _is_valid_hostname(cls, hostname):
if len(hostname) > 255:
return False
# Strip exactly one dot from the right, if present.
if hostname[-1] == ".":
hostname = hostname[:-1]
# The hostname is valid if all its segments are valid.
return all(cls._VALID_HOSTNAME_SEGMENT.match(segment) for segment in hostname.split("."))
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