# Copyright (C) 2016-2021 by the Free Software Foundation, Inc.
#
# This file is part of GNU Mailman.
#
# GNU Mailman 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.
#
# GNU Mailman 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
# GNU Mailman.  If not, see <https://www.gnu.org/licenses/>.

"""DMARC mitigation rule."""

import os
import re
import logging
import dns.resolver

from dns.exception import DNSException
from email.utils import parseaddr
from importlib_resources import read_binary
from lazr.config import as_timedelta
from mailman.config import config
from mailman.core.i18n import _
from mailman.interfaces.mailinglist import DMARCMitigateAction
from mailman.interfaces.rules import IRule
from mailman.utilities.datetime import now
from mailman.utilities.protocols import get
from mailman.utilities.string import wrap
from public import public
from requests.exceptions import HTTPError
from urllib.error import URLError
from zope.interface import implementer


elog = logging.getLogger('mailman.error')
vlog = logging.getLogger('mailman.vette')

DOT = '.'
EMPTYSTRING = ''
KEEP_LOOKING = object()
LOCAL_FILE_NAME = 'public_suffix_list.dat'

# Map organizational domain suffix rules to a boolean indicating whether the
# rule is an exception or not.
suffix_cache = dict()


def ensure_current_suffix_list():
    # Read and parse the organizational domain suffix list.  First look in the
    # cached directory to see if we already have a valid copy of it.
    cached_copy_path = os.path.join(config.VAR_DIR, LOCAL_FILE_NAME)
    lifetime = as_timedelta(config.dmarc.cache_lifetime)
    download = False
    try:
        mtime = os.stat(cached_copy_path).st_mtime
    except FileNotFoundError:
        vlog.info('No cached copy of the public suffix list found')
        download = True
        cache_found = False
    else:
        cache_found = True
        # Is the cached copy out-of-date?  Note that when we write a new cache
        # version we explicitly set its mtime to the time in the future when
        # the cache will expire.
        if mtime < now().timestamp():
            download = True
            vlog.info('Cached copy of public suffix list is out of date')
    if download:
        try:
            content = get(config.dmarc.org_domain_data_url)
        except (URLError, HTTPError) as error:
            elog.error('Unable to retrieve public suffix list from %s: %s',
                       config.dmarc.org_domain_data_url,
                       getattr(error, 'reason', str(error)))
            if cache_found:
                vlog.info('Using out of date public suffix list')
                content = None
            else:
                # We couldn't access the URL and didn't even have an out of
                # date suffix list cached.  Use the shipped version.
                content = read_binary('mailman.rules.data', LOCAL_FILE_NAME)
        if content is not None:
            # Content is either a string or UTF-8 encoded bytes.
            if isinstance(content, bytes):
                content = content.decode('utf-8')
            # Write the cache atomically.
            new_path = cached_copy_path + '.new'
            with open(new_path, 'w', encoding='utf-8') as fp:
                fp.write(content)
            # Set the expiry time to the future.
            mtime = (now() + lifetime).timestamp()
            os.utime(new_path, (mtime, mtime))
            # Flip the new file into the cached location.  This does not
            # modify the mtime.
            os.rename(new_path, cached_copy_path)
    return cached_copy_path


def parse_suffix_list(filename=None):
    # Parse the suffix list into a per process cache.
    if filename is None:
        filename = ensure_current_suffix_list()
    # At this point the cached copy must exist and is as valid as possible.
    # Read and return the contents as a UTF-8 string.
    with open(filename, 'r', encoding='utf-8') as fp:
        for line in fp:
            if not line.strip() or line.startswith('//'):
                continue
            line = re.sub(r'\s.*', '', line)
            if not line:
                continue
            parts = line.lower().split('.')
            if parts[0].startswith('!'):
                exception = True
                parts = [parts[0][1:]] + parts[1:]
            else:
                exception = False
            parts.reverse()
            key = DOT.join(parts)
            suffix_cache[key] = exception


def get_domain(parts, label):
    # A helper to get a domain name consisting of the first label+1 labels in
    # parts.
    domain = parts[:min(label+1, len(parts))]
    domain.reverse()
    return DOT.join(domain)


def get_organizational_domain(domain):
    # Given a domain name, this returns the corresponding Organizational
    # Domain which may be the same as the input.
    if len(suffix_cache) == 0:
        parse_suffix_list()
    hits = []
    parts = domain.lower().split('.')
    parts.reverse()
    for key in suffix_cache:
        key_parts = key.split('.')
        if len(parts) >= len(key_parts):
            for i in range(len(key_parts) - 1):
                if parts[i] != key_parts[i] and key_parts[i] != '*':
                    break
            else:
                if (parts[len(key_parts) - 1] == key_parts[-1] or
                        key_parts[-1] == '*'):
                    hits.append(key)
    if not hits:
        return get_domain(parts, 1)
    label = 0
    for key in hits:
        key_parts = key.split('.')
        if suffix_cache[key]:
            # It's an exception.
            return get_domain(parts, len(key_parts) - 1)
        if len(key_parts) > label:
            label = len(key_parts)
    return get_domain(parts, label)


def is_reject_or_quarantine(mlist, email, dmarc_domain, org=False):
    # This takes a mailing list, an email address as in the From: header, the
    # _dmarc host name for the domain in question, and a flag stating whether
    # we should check the organizational domains.  It returns one of three
    # values:
    # * True if the DMARC policy is reject or quarantine;
    # * False if is not;
    # * A special sentinel if we should continue looking
    resolver = dns.resolver.Resolver()
    resolver.timeout = as_timedelta(
        config.dmarc.resolver_timeout).total_seconds()
    resolver.lifetime = as_timedelta(
        config.dmarc.resolver_lifetime).total_seconds()
    try:
        txt_recs = resolver.query(dmarc_domain, dns.rdatatype.TXT)
    except (dns.resolver.NXDOMAIN, dns.resolver.NoAnswer):
        return KEEP_LOOKING
    except (dns.resolver.NoNameservers):
        elog.error(
            'DNSException: No Nameservers available for %s (%s).',
            email, dmarc_domain)
        # Typically this means a dnssec validation error.  Clients that don't
        # perform validation *may* successfully see a _dmarc RR whereas a
        # validating mailman server won't see the _dmarc RR.  We should
        # mitigate this email to be safe.
        return True
    except DNSException as error:
        elog.error(
            'DNSException: Unable to query DMARC policy for %s (%s). %s',
            email, dmarc_domain, error.__doc__)
        # While we can't be sure what caused the error, there is potentially
        # a DMARC policy record that we missed and that a receiver of the mail
        # might see.  Thus, we should err on the side of caution and mitigate.
        return True
    # Be as robust as possible in parsing the result.
    results_by_name = {}
    cnames = {}
    want_names = set([dmarc_domain + '.'])
    # Check all the TXT records returned by DNS.  Keep track of the CNAMEs for
    # checking later on.  Ignore any other non-TXT records.
    for txt_rec in txt_recs.response.answer:
        # Don't be fooled by an answer with uppercase in the name.
        name = txt_rec.name.to_text().lower()
        if txt_rec.rdtype == dns.rdatatype.CNAME:
            cnames[name] = (
                next(iter(txt_rec.items)).target.to_text())
        if txt_rec.rdtype != dns.rdatatype.TXT:
            continue
        result = EMPTYSTRING.join(
            str(record, encoding='utf-8')
            for record in next(iter(txt_rec.items)).strings)
        results_by_name.setdefault(name, []).append(result)
    expands = list(want_names)
    seen = set(expands)
    while expands:
        item = expands.pop(0)
        if item in cnames:
            if cnames[item] in seen:
                # CNAME loop.
                continue
            expands.append(cnames[item])
            seen.add(cnames[item])
            want_names.add(cnames[item])
            want_names.discard(item)
    assert len(want_names) == 1, (
        'Error in CNAME processing for {}; want_names != 1.'.format(
            dmarc_domain))
    for name in want_names:
        if name not in results_by_name:
            continue
        dmarcs = [
            record for record in results_by_name[name]
            if record.startswith('v=DMARC1;')
            ]
        if len(dmarcs) == 0:
            return KEEP_LOOKING
        if len(dmarcs) > 1:
            elog.error(
                'RRset of TXT records for %s has %d v=DMARC1 entries; '
                'testing them all',
                dmarc_domain, len(dmarcs))
        for entry in dmarcs:
            mo = re.search(r'\bsp=(\w*)\b', entry, re.IGNORECASE)
            if org and mo:
                policy = mo.group(1).lower()
            else:
                mo = re.search(r'\bp=(\w*)\b', entry, re.IGNORECASE)
                if mo:
                    policy = mo.group(1).lower()
                else:
                    # This continue does actually get covered by
                    # TestDMARCRules.test_domain_with_subdomain_policy() and
                    # TestDMARCRules.test_no_policy() but because of
                    # Coverage BitBucket issue #198 and
                    # https://bugs.python.org/issue2506 coverage cannot report
                    # it as such, so just pragma it away.
                    continue                        # pragma: missed
            if policy in ('reject', 'quarantine'):
                vlog.info(
                    '%s: DMARC lookup for %s (%s) found p=%s in %s = %s',
                    mlist.list_name,
                    email,
                    dmarc_domain,
                    policy,
                    name,
                    entry)
                return True
    return False


def maybe_mitigate(mlist, email):
    # This takes an email address, and returns True if DMARC policy is
    # p=reject or p=quarantine.
    email = email.lower()
    # Scan from the right in case quoted local part has an '@'.
    local, at, from_domain = email.rpartition('@')
    if at != '@':
        return False
    answer = is_reject_or_quarantine(
        mlist, email, '_dmarc.{}'.format(from_domain))
    if answer is not KEEP_LOOKING:
        return answer
    org_dom = get_organizational_domain(from_domain)
    if org_dom != from_domain:
        answer = is_reject_or_quarantine(
            mlist, email, '_dmarc.{}'.format(org_dom), org=True)
        if answer is not KEEP_LOOKING:
            return answer
    return False


@public
@implementer(IRule)
class DMARCMitigation:
    """The DMARC mitigation rule."""

    name = 'dmarc-mitigation'
    description = _('Find DMARC policy of From: domain.')
    record = True

    def check(self, mlist, msg, msgdata):
        """See `IRule`."""
        if mlist.dmarc_mitigate_action is DMARCMitigateAction.no_mitigation:
            # Don't bother to check if we're not going to do anything.
            return False
        display_name, address = parseaddr(str(msg.get('from', '')))
        if maybe_mitigate(mlist, address):
            # If dmarc_mitigate_action is discard or reject, this rule fires
            # and jumps to the 'moderation' chain to do the actual discard.
            # Otherwise, the rule misses but sets a flag for the dmarc handler
            # to do the appropriate action.
            msgdata['dmarc'] = True
            if mlist.dmarc_mitigate_action is DMARCMitigateAction.discard:
                msgdata['dmarc_action'] = 'discard'
                with _.defer_translation():
                    # This will be translated at the point of use.
                    msgdata.setdefault('moderation_reasons', []).append(
                        _('DMARC moderation'))
            elif mlist.dmarc_mitigate_action is DMARCMitigateAction.reject:
                listowner = mlist.owner_address       # noqa F841
                with _.defer_translation():
                    # This will be translated at the point of use.
                    reason = (mlist.dmarc_moderation_notice or _(
                        'You are not allowed to post to this mailing '
                        'list From: a domain which publishes a DMARC '
                        'policy of reject or quarantine, and your message'
                        ' has been automatically rejected.  If you think '
                        'that your messages are being rejected in error, '
                        'contact the mailing list owner at ${listowner}.'))
                msgdata.setdefault('moderation_reasons', []).append(
                    wrap(reason))
                msgdata['dmarc_action'] = 'reject'
            else:
                return False
            msgdata['moderation_sender'] = address
            return True
        return False
