exchangelib.autodiscover API documentation

# Module `exchangelib.autodiscover`

Expand source code

    from .cache import AutodiscoverCache, autodiscover_cache
    from .discovery import Autodiscovery, discover
    from .protocol import AutodiscoverProtocol

    def close_connections():
        with autodiscover_cache:
            autodiscover_cache.close()

    def clear_cache():
        with autodiscover_cache:
            autodiscover_cache.clear()

    __all__ = [
        'AutodiscoverCache', 'AutodiscoverProtocol', 'Autodiscovery', 'discover', 'autodiscover_cache',
        'close_connections', 'clear_cache'
    ]

## Sub-modules

`exchangelib.autodiscover.cache`
`exchangelib.autodiscover.discovery`
`exchangelib.autodiscover.properties`
`exchangelib.autodiscover.protocol`

## Functions

`def clear_cache()`
  Expand source code

      def clear_cache():
          with autodiscover_cache:
              autodiscover_cache.clear()

`def close_connections()`
  Expand source code

      def close_connections():
          with autodiscover_cache:
              autodiscover_cache.close()

`def discover(email, credentials=None, auth_type=None, retry_policy=None)`
  Expand source code

      def discover(email, credentials=None, auth_type=None, retry_policy=None):
          return Autodiscovery(
              email=email, credentials=credentials, auth_type=auth_type, retry_policy=retry_policy
          ).discover()

## Classes

`class AutodiscoverCache`

  Stores the translation from (email domain, credentials) -> AutodiscoverProtocol object so we can re-use TCP connections to an autodiscover server within the same process. Also persists the email domain -> (autodiscover endpoint URL, auth_type) translation to the filesystem so the cache can be shared between multiple processes.

  According to Microsoft, we may forever cache the (email domain -> autodiscover endpoint URL) mapping, or until it stops responding. My previous experience with Exchange products in mind, I'm not sure if I should trust that advice. But it could save some valuable seconds every time we start a new connection to a known server. In any case, the persistent storage must not contain any sensitive information since the cache could be readable by unprivileged users. Domain, endpoint and auth_type are OK to cache since this info is make publicly available on HTTP and DNS servers via the autodiscover protocol. Just don't persist any credentials info.

  If an autodiscover lookup fails for any reason, the corresponding cache entry must be purged.

  'shelve' is supposedly thread-safe and process-safe, which suits our needs.

  Expand source code

      class AutodiscoverCache:
          """Stores the translation from (email domain, credentials) -> AutodiscoverProtocol object so we can re-use TCP
          connections to an autodiscover server within the same process. Also persists the email domain -> (autodiscover
          endpoint URL, auth_type) translation to the filesystem so the cache can be shared between multiple processes.

          According to Microsoft, we may forever cache the (email domain -> autodiscover endpoint URL) mapping, or until
          it stops responding. My previous experience with Exchange products in mind, I'm not sure if I should trust that
          advice. But it could save some valuable seconds every time we start a new connection to a known server. In any
          case, the persistent storage must not contain any sensitive information since the cache could be readable by
          unprivileged users. Domain, endpoint and auth_type are OK to cache since this info is make publicly available on
          HTTP and DNS servers via the autodiscover protocol. Just don't persist any credentials info.

          If an autodiscover lookup fails for any reason, the corresponding cache entry must be purged.

          'shelve' is supposedly thread-safe and process-safe, which suits our needs.
          """

          def __init__(self):
              self._protocols = {}  # Mapping from (domain, credentials) to AutodiscoverProtocol
              self._lock = RLock()

          @property
          def _storage_file(self):
              return AUTODISCOVER_PERSISTENT_STORAGE

          def clear(self):
              # Wipe the entire cache
              with shelve_open_with_failover(self._storage_file) as db:
                  db.clear()
              self._protocols.clear()

          def __len__(self):
              return len(self._protocols)

          def __contains__(self, key):
              domain = key[0]
              with shelve_open_with_failover(self._storage_file) as db:
                  return str(domain) in db

          def __getitem__(self, key):
              protocol = self._protocols.get(key)
              if protocol:
                  return protocol
              domain, credentials = key
              with shelve_open_with_failover(self._storage_file) as db:
                  endpoint, auth_type, retry_policy = db[str(domain)]  # It's OK to fail with KeyError here
              protocol = AutodiscoverProtocol(config=Configuration(
                  service_endpoint=endpoint, credentials=credentials, auth_type=auth_type, retry_policy=retry_policy
              ))
              self._protocols[key] = protocol
              return protocol

          def __setitem__(self, key, protocol):
              # Populate both local and persistent cache
              domain = key[0]
              with shelve_open_with_failover(self._storage_file) as db:
                  # Don't change this payload without bumping the cache file version in shelve_filename()
                  db[str(domain)] = (protocol.service_endpoint, protocol.auth_type, protocol.retry_policy)
              self._protocols[key] = protocol

          def __delitem__(self, key):
              # Empty both local and persistent cache. Don't fail on non-existing entries because we could end here
              # multiple times due to race conditions.
              domain = key[0]
              with shelve_open_with_failover(self._storage_file) as db:
                  try:
                      del db[str(domain)]
                  except KeyError:
                      pass
              try:
                  del self._protocols[key]
              except KeyError:
                  pass

          def close(self):
              # Close all open connections
              for (domain, _), protocol in self._protocols.items():
                  log.debug('Domain %s: Closing sessions', domain)
                  protocol.close()
                  del protocol
              self._protocols.clear()

          def __enter__(self):
              self._lock.__enter__()

          def __exit__(self, *args, **kwargs):
              self._lock.__exit__(*args, **kwargs)

          def __del__(self):
              # pylint: disable=bare-except
              try:
                  self.close()
              except Exception:  # nosec
                  # __del__ should never fail
                  pass

          def __str__(self):
              return str(self._protocols)

  ### Methods

  `def clear(self)`
    Expand source code

        def clear(self):
            # Wipe the entire cache
            with shelve_open_with_failover(self._storage_file) as db:
                db.clear()
            self._protocols.clear()

  `def close(self)`
    Expand source code

        def close(self):
            # Close all open connections
            for (domain, _), protocol in self._protocols.items():
                log.debug('Domain %s: Closing sessions', domain)
                protocol.close()
                del protocol
            self._protocols.clear()

`class AutodiscoverProtocol (config)`

  Protocol which implements the bare essentials for autodiscover.

  Expand source code

      class AutodiscoverProtocol(BaseProtocol):
          """Protocol which implements the bare essentials for autodiscover."""

          TIMEOUT = 10  # Seconds

          def __str__(self):
              return '''\
      Autodiscover endpoint: %s
      Auth type: %s''' % (
                  self.service_endpoint,
                  self.auth_type,
              )

  ### Ancestors

  - BaseProtocol

  ### Class variables

  `var TIMEOUT`

  ### Inherited members

  - `BaseProtocol`:
    - `HTTP_ADAPTER_CLS`
    - `decrease_poolsize`
    - `increase_poolsize`
`class Autodiscovery (email, credentials=None, auth_type=None, retry_policy=None)`

  Autodiscover is a Microsoft protocol for automatically getting the endpoint of the Exchange server and other connection-related settings holding the email address using only the email address, and username and password of the user.

  For a description of the protocol implemented, see "Autodiscover for Exchange ActiveSync developers":

  https://docs.microsoft.com/en-us/previous-versions/office/developer/exchange-server-interoperability-guidance/hh352638%28v%3dexchg.140%29

  Descriptions of the steps from the article are provided in their respective methods in this class.

  For a description of how to handle autodiscover error messages, see:

  https://docs.microsoft.com/en-us/exchange/client-developer/exchange-web-services/handling-autodiscover-error-messages

  A tip from the article: The client can perform steps 1 through 4 in any order or in parallel to expedite the process, but it must wait for responses to finish at each step before proceeding. Given that many organizations prefer to use the URL in step 2 to set up the Autodiscover service, the client might try this step first.

  Another possibly newer resource which has not yet been attempted is "Outlook 2016 Implementation of Autodiscover": https://support.microsoft.com/en-us/help/3211279/outlook-2016-implementation-of-autodiscover

  WARNING: The autodiscover protocol is very complicated. If you have problems autodiscovering using this implementation, start by doing an official test at https://testconnectivity.microsoft.com

  :param email: The email address to autodiscover :param credentials: Credentials with authorization to make autodiscover lookups for this Account (Default value = None) :param auth_type: (Default value = None) :param retry_policy: (Default value = None)

  Expand source code

      class Autodiscovery:
          """Autodiscover is a Microsoft protocol for automatically getting the endpoint of the Exchange server and other
          connection-related settings holding the email address using only the email address, and username and password of the
          user.

          For a description of the protocol implemented, see "Autodiscover for Exchange ActiveSync developers":

          https://docs.microsoft.com/en-us/previous-versions/office/developer/exchange-server-interoperability-guidance/hh352638%28v%3dexchg.140%29

          Descriptions of the steps from the article are provided in their respective methods in this class.

          For a description of how to handle autodiscover error messages, see:

          https://docs.microsoft.com/en-us/exchange/client-developer/exchange-web-services/handling-autodiscover-error-messages

          A tip from the article:
          The client can perform steps 1 through 4 in any order or in parallel to expedite the process, but it must wait for
          responses to finish at each step before proceeding. Given that many organizations prefer to use the URL in step 2 to
          set up the Autodiscover service, the client might try this step first.

          Another possibly newer resource which has not yet been attempted is "Outlook 2016 Implementation of Autodiscover":
          https://support.microsoft.com/en-us/help/3211279/outlook-2016-implementation-of-autodiscover

          WARNING: The autodiscover protocol is very complicated. If you have problems autodiscovering using this
          implementation, start by doing an official test at https://testconnectivity.microsoft.com
          """

          # When connecting to servers that may not be serving the correct endpoint, we should use a retry policy that does
          # not leave us hanging for a long time on each step in the protocol.
          INITIAL_RETRY_POLICY = FailFast()
          RETRY_WAIT = 10  # Seconds to wait before retry on connection errors
          MAX_REDIRECTS = 10  # Maximum number of URL redirects before we give up
          DNS_RESOLVER_KWARGS = {}
          DNS_RESOLVER_ATTRS = {
              'timeout': AutodiscoverProtocol.TIMEOUT,
          }

          def __init__(self, email, credentials=None, auth_type=None, retry_policy=None):
              """

              :param email: The email address to autodiscover
              :param credentials: Credentials with authorization to make autodiscover lookups for this Account
                  (Default value = None)
              :param auth_type:  (Default value = None)
              :param retry_policy:  (Default value = None)
              """
              self.email = email
              self.credentials = credentials
              self.auth_type = auth_type  # The auth type that the resulting protocol instance should have
              self.retry_policy = retry_policy  # The retry policy that the resulting protocol instance should have
              self._urls_visited = []  # Collects HTTP and Autodiscover redirects
              self._redirect_count = 0
              self._emails_visited = []  # Collects Autodiscover email redirects

          def discover(self):
              self._emails_visited.append(self.email.lower())

              # Check the autodiscover cache to see if we already know the autodiscover service endpoint for this email
              # domain. Use a lock to guard against multiple threads competing to cache information.
              log.debug('Waiting for autodiscover_cache lock')
              with autodiscover_cache:
                  log.debug('autodiscover_cache lock acquired')
                  cache_key = self._cache_key
                  domain = get_domain(self.email)
                  if cache_key in autodiscover_cache:
                      ad_protocol = autodiscover_cache[cache_key]
                      log.debug('Cache hit for key %s: %s', cache_key, ad_protocol.service_endpoint)
                      try:
                          ad_response = self._quick(protocol=ad_protocol)
                      except AutoDiscoverFailed:
                          # Autodiscover no longer works with this domain. Clear cache and try again after releasing the lock
                          log.debug('AD request failure. Removing cache for key %s', cache_key)
                          del autodiscover_cache[cache_key]
                          ad_response = self._step_1(hostname=domain)
                  else:
                      # This will cache the result
                      ad_response = self._step_1(hostname=domain)

              log.debug('Released autodiscover_cache_lock')
              if ad_response.redirect_address:
                  log.debug('Got a redirect address: %s', ad_response.redirect_address)
                  if ad_response.redirect_address.lower() in self._emails_visited:
                      raise AutoDiscoverCircularRedirect('We were redirected to an email address we have already seen')

                  # Start over, but with the new email address
                  self.email = ad_response.redirect_address
                  return self.discover()

              # We successfully received a response. Clear the cache of seen emails etc.
              self.clear()
              return self._build_response(ad_response=ad_response)

          def clear(self):
              # This resets cached variables
              self._urls_visited = []
              self._redirect_count = 0
              self._emails_visited = []

          @property
          def _cache_key(self):
              # We may be using multiple different credentials and changing our minds on TLS verification. This key
              # combination should be safe for caching.
              domain = get_domain(self.email)
              return domain, self.credentials

          @threaded_cached_property
          def resolver(self):
              resolver = dns.resolver.Resolver(**self.DNS_RESOLVER_KWARGS)
              for k, v in self.DNS_RESOLVER_ATTRS.items():
                  setattr(resolver, k, v)
              return resolver

          def _build_response(self, ad_response):
              ews_url = ad_response.ews_url
              if not ews_url:
                  raise AutoDiscoverFailed("Response is missing an 'ews_url' value")
              if not ad_response.autodiscover_smtp_address:
                  # Autodiscover does not always return an email address. In that case, the requesting email should be used
                  ad_response.user.autodiscover_smtp_address = self.email

              # Get the server version. Not all protocol entries have a server version so we cheat a bit and also look at the
              # other ones that point to the same endpoint.
              for protocol in ad_response.account.protocols:
                  if not protocol.ews_url or not protocol.server_version:
                      continue
                  if protocol.ews_url.lower() == ews_url.lower():
                      version = Version(build=protocol.server_version)
                      break
              else:
                  version = None
              # We may not want to use the auth_package hints in the AD response. It could be incorrect and we can just guess.
              protocol = Protocol(
                  config=Configuration(
                      service_endpoint=ews_url,
                      credentials=self.credentials,
                      version=version,
                      auth_type=self.auth_type,
                      retry_policy=self.retry_policy,
                  )
              )
              return ad_response, protocol

          def _quick(self, protocol):
              # Reset auth type and retry policy if we requested non-default values
              if self.auth_type:
                  protocol.config.auth_type = self.auth_type
              if self.retry_policy:
                  protocol.config.retry_policy = self.retry_policy
              try:
                  r = self._get_authenticated_response(protocol=protocol)
              except TransportError as e:
                  raise AutoDiscoverFailed('Response error: %s' % e)
              if r.status_code == 200:
                  try:
                      ad = Autodiscover.from_bytes(bytes_content=r.content)
                      return self._step_5(ad=ad)
                  except ValueError as e:
                      raise AutoDiscoverFailed('Invalid response: %s' % e)
              raise AutoDiscoverFailed('Invalid response code: %s' % r.status_code)

          def _redirect_url_is_valid(self, url):
              """Three separate responses can be “Redirect responses”:
              * An HTTP status code (301, 302) with a new URL
              * An HTTP status code of 200, but with a payload XML containing a redirect to a different URL
              * An HTTP status code of 200, but with a payload XML containing a different SMTP address as the target address

              We only handle the HTTP 302 redirects here. We validate the URL received in the redirect response to ensure that
              it does not redirect to non-SSL endpoints or SSL endpoints with invalid certificates, and that the redirect is
              not circular. Finally, we should fail after 10 redirects.

              :param url:
              :return:
              """
              if url.lower() in self._urls_visited:
                  log.warning('We have already tried this URL: %s', url)
                  return False

              if self._redirect_count >= self.MAX_REDIRECTS:
                  log.warning('We reached max redirects at URL: %s', url)
                  return False

              # We require TLS endpoints
              if not url.startswith('https://'):
                  log.debug('Invalid scheme for URL: %s', url)
                  return False

              # Quick test that the endpoint responds and that TLS handshake is OK
              try:
                  self._get_unauthenticated_response(url, method='head')
              except TransportError as e:
                  log.debug('Response error on redirect URL %s: %s', url, e)
                  return False

              self._redirect_count += 1
              return True

          def _get_unauthenticated_response(self, url, method='post'):
              """Get auth type by tasting headers from the server. Do POST requests be default. HEAD is too error prone, and
              some servers are set up to redirect to OWA on all requests except POST to the autodiscover endpoint.

              :param url:
              :param method:  (Default value = 'post')
              :return:
              """
              # We are connecting to untrusted servers here, so take necessary precautions.
              hostname = urlparse(url).netloc
              if not self._is_valid_hostname(hostname):
                  # 'requests' is really bad at reporting that a hostname cannot be resolved. Let's check this separately.
                  # Don't retry on DNS errors. They will most likely be persistent.
                  raise TransportError('%r has no DNS entry' % hostname)

              kwargs = dict(
                  url=url, headers=DEFAULT_HEADERS.copy(), allow_redirects=False, timeout=AutodiscoverProtocol.TIMEOUT
              )
              if method == 'post':
                  kwargs['data'] = Autodiscover.payload(email=self.email)
              retry = 0
              t_start = time.monotonic()
              while True:
                  _back_off_if_needed(self.INITIAL_RETRY_POLICY.back_off_until)
                  log.debug('Trying to get response from %s', url)
                  with AutodiscoverProtocol.raw_session(url) as s:
                      try:
                          r = getattr(s, method)(**kwargs)
                          r.close()  # Release memory
                          break
                      except TLS_ERRORS as e:
                          # Don't retry on TLS errors. They will most likely be persistent.
                          raise TransportError(str(e))
                      except CONNECTION_ERRORS as e:
                          r = DummyResponse(url=url, headers={}, request_headers=kwargs['headers'])
                          total_wait = time.monotonic() - t_start
                          if self.INITIAL_RETRY_POLICY.may_retry_on_error(response=r, wait=total_wait):
                              log.debug("Connection error on URL %s (retry %s, error: %s). Cool down", url, retry, e)
                              # Don't respect the 'Retry-After' header. We don't know if this is a useful endpoint, and we
                              # want autodiscover to be reasonably fast.
                              self.INITIAL_RETRY_POLICY.back_off(self.RETRY_WAIT)
                              retry += 1
                              continue
                          log.debug("Connection error on URL %s: %s", url, e)
                          raise TransportError(str(e))
              try:
                  auth_type = get_auth_method_from_response(response=r)
              except UnauthorizedError:
                  # Failed to guess the auth type
                  auth_type = NOAUTH
              if r.status_code in (301, 302) and 'location' in r.headers:
                  # Make the redirect URL absolute
                  try:
                      r.headers['location'] = get_redirect_url(r)
                  except TransportError:
                      del r.headers['location']
              return auth_type, r

          def _get_authenticated_response(self, protocol):
              """Get a response by using the credentials provided. We guess the auth type along the way.

              :param protocol:
              :return:
              """
              # Redo the request with the correct auth
              data = Autodiscover.payload(email=self.email)
              headers = DEFAULT_HEADERS.copy()
              session = protocol.get_session()
              if GSSAPI in AUTH_TYPE_MAP and isinstance(session.auth, AUTH_TYPE_MAP[GSSAPI]):
                  # https://docs.microsoft.com/en-us/exchange/client-developer/web-service-reference/pox-autodiscover-request-for-exchange
                  headers['X-ClientCanHandle'] = 'Negotiate'
              try:
                  r, session = post_ratelimited(protocol=protocol, session=session, url=protocol.service_endpoint,
                                                headers=headers, data=data, allow_redirects=False, stream=False)
                  protocol.release_session(session)
              except UnauthorizedError as e:
                  # It's entirely possible for the endpoint to ask for login. We should continue if login fails because this
                  # isn't necessarily the right endpoint to use.
                  raise TransportError(str(e))
              except RedirectError as e:
                  r = DummyResponse(url=protocol.service_endpoint, headers={'location': e.url}, request_headers=None,
                                    status_code=302)
              return r

          def _attempt_response(self, url):
              """Return an (is_valid_response, response) tuple.

              :param url:
              :return:
              """
              self._urls_visited.append(url.lower())
              log.debug('Attempting to get a valid response from %s', url)
              try:
                  auth_type, r = self._get_unauthenticated_response(url=url)
                  if isinstance(self.credentials, OAuth2Credentials):
                      # This type of credentials *must* use the OAuth auth type
                      auth_type = OAUTH2
                  elif self.credentials is None and auth_type in CREDENTIALS_REQUIRED:
                      raise ValueError('Auth type %r was detected but no credentials were provided' % auth_type)
                  ad_protocol = AutodiscoverProtocol(
                      config=Configuration(
                          service_endpoint=url,
                          credentials=self.credentials,
                          auth_type=auth_type,
                          retry_policy=self.INITIAL_RETRY_POLICY,
                      )
                  )
                  if auth_type != NOAUTH:
                      r = self._get_authenticated_response(protocol=ad_protocol)
              except TransportError as e:
                  log.debug('Failed to get a response: %s', e)
                  return False, None
              if r.status_code in (301, 302) and 'location' in r.headers:
                  redirect_url = get_redirect_url(r)
                  if self._redirect_url_is_valid(url=redirect_url):
                      # The protocol does not specify this explicitly, but by looking at how testconnectivity.microsoft.com
                      # works, it seems that we should follow this URL now and try to get a valid response.
                      return self._attempt_response(url=redirect_url)
              if r.status_code == 200:
                  try:
                      ad = Autodiscover.from_bytes(bytes_content=r.content)
                      # We got a valid response. Unless this is a URL redirect response, we cache the result
                      if ad.response is None or not ad.response.redirect_url:
                          cache_key = self._cache_key
                          log.debug('Adding cache entry for key %s: %s', cache_key, ad_protocol.service_endpoint)
                          autodiscover_cache[cache_key] = ad_protocol
                      return True, ad
                  except ValueError as e:
                      log.debug('Invalid response: %s', e)
              return False, None

          def _is_valid_hostname(self, hostname):
              log.debug('Checking if %s can be looked up in DNS', hostname)
              try:
                  self.resolver.resolve(hostname)
              except (dns.resolver.NoNameservers, dns.resolver.NXDOMAIN, dns.resolver.NoAnswer):
                  return False
              return True

          def _get_srv_records(self, hostname):
              """Send a DNS query for SRV entries for the hostname.

              An SRV entry that has been formatted for autodiscovery will have the following format:

                  canonical name = mail.example.com.
                  service = 8 100 443 webmail.example.com.

              The first three numbers in the service line are: priority, weight, port

              :param hostname:
              :return:
              """
              log.debug('Attempting to get SRV records for %s', hostname)
              records = []
              try:
                  answers = self.resolver.resolve('%s.' % hostname, 'SRV')
              except (dns.resolver.NoNameservers, dns.resolver.NoAnswer, dns.resolver.NXDOMAIN) as e:
                  log.debug('DNS lookup failure: %s', e)
                  return records
              for rdata in answers:
                  try:
                      vals = rdata.to_text().strip().rstrip('.').split(' ')
                      # Raise ValueError if the first three are not ints, and IndexError if there are less than 4 values
                      priority, weight, port, srv = int(vals[0]), int(vals[1]), int(vals[2]), vals[3]
                      record = SrvRecord(priority=priority, weight=weight, port=port, srv=srv)
                      log.debug('Found SRV record %s ', record)
                      records.append(record)
                  except (ValueError, IndexError):
                      log.debug('Incompatible SRV record for %s (%s)', hostname, rdata.to_text())
              return records

          def _step_1(self, hostname):
              """Perform step 1, where the client sends an Autodiscover request to
              https://example.com/autodiscover/autodiscover.xml and then does one of the following:
                  * If the Autodiscover attempt succeeds, the client proceeds to step 5.
                  * If the Autodiscover attempt fails, the client proceeds to step 2.

              :param hostname:
              :return:
              """
              url = 'https://%s/Autodiscover/Autodiscover.xml' % hostname
              log.info('Step 1: Trying autodiscover on %r with email %r', url, self.email)
              is_valid_response, ad = self._attempt_response(url=url)
              if is_valid_response:
                  return self._step_5(ad=ad)
              return self._step_2(hostname=hostname)

          def _step_2(self, hostname):
              """Perform step 2, where the client sends an Autodiscover request to
              https://autodiscover.example.com/autodiscover/autodiscover.xml and then does one of the following:
                  * If the Autodiscover attempt succeeds, the client proceeds to step 5.
                  * If the Autodiscover attempt fails, the client proceeds to step 3.

              :param hostname:
              :return:
              """
              url = 'https://autodiscover.%s/Autodiscover/Autodiscover.xml' % hostname
              log.info('Step 2: Trying autodiscover on %r with email %r', url, self.email)
              is_valid_response, ad = self._attempt_response(url=url)
              if is_valid_response:
                  return self._step_5(ad=ad)
              return self._step_3(hostname=hostname)

          def _step_3(self, hostname):
              """Perform step 3, where the client sends an unauth'ed GET method request to
              http://autodiscover.example.com/autodiscover/autodiscover.xml (Note that this is a non-HTTPS endpoint). The
              client then does one of the following:
                  * If the GET request returns a 302 redirect response, it gets the redirection URL from the 'Location' HTTP
                  header and validates it as described in the "Redirect responses" section. The client then does one of the
                  following:
                      * If the redirection URL is valid, the client tries the URL and then does one of the following:
                          * If the attempt succeeds, the client proceeds to step 5.
                          * If the attempt fails, the client proceeds to step 4.
                      * If the redirection URL is not valid, the client proceeds to step 4.
                  * If the GET request does not return a 302 redirect response, the client proceeds to step 4.

              :param hostname:
              :return:
              """
              url = 'http://autodiscover.%s/Autodiscover/Autodiscover.xml' % hostname
              log.info('Step 3: Trying autodiscover on %r with email %r', url, self.email)
              try:
                  _, r = self._get_unauthenticated_response(url=url, method='get')
              except TransportError:
                  r = DummyResponse(url=url, headers={}, request_headers={})
              if r.status_code in (301, 302) and 'location' in r.headers:
                  redirect_url = get_redirect_url(r)
                  if self._redirect_url_is_valid(url=redirect_url):
                      is_valid_response, ad = self._attempt_response(url=redirect_url)
                      if is_valid_response:
                          return self._step_5(ad=ad)
                      return self._step_4(hostname=hostname)
                  return self._step_4(hostname=hostname)
              return self._step_4(hostname=hostname)

          def _step_4(self, hostname):
              """Perform step 4, where the client performs a Domain Name System (DNS) query for an SRV record for
              _autodiscover._tcp.example.com. The query might return multiple records. The client selects only records that
              point to an SSL endpoint and that have the highest priority and weight. One of the following actions then
              occurs:
                  * If no such records are returned, the client proceeds to step 6.
                  * If records are returned, the application randomly chooses a record in the list and validates the endpoint
                    that it points to by following the process described in the "Redirect Response" section. The client then
                    does one of the following:
                      * If the redirection URL is valid, the client tries the URL and then does one of the following:
                          * If the attempt succeeds, the client proceeds to step 5.
                          * If the attempt fails, the client proceeds to step 6.
                      * If the redirection URL is not valid, the client proceeds to step 6.

              :param hostname:
              :return:
              """
              dns_hostname = '_autodiscover._tcp.%s' % hostname
              log.info('Step 4: Trying autodiscover on %r with email %r', dns_hostname, self.email)
              srv_records = self._get_srv_records(dns_hostname)
              try:
                  srv_host = _select_srv_host(srv_records)
              except ValueError:
                  srv_host = None
              if not srv_host:
                  return self._step_6()
              redirect_url = 'https://%s/Autodiscover/Autodiscover.xml' % srv_host
              if self._redirect_url_is_valid(url=redirect_url):
                  is_valid_response, ad = self._attempt_response(url=redirect_url)
                  if is_valid_response:
                      return self._step_5(ad=ad)
                  return self._step_6()
              return self._step_6()

          def _step_5(self, ad):
              """Perform step 5. When a valid Autodiscover request succeeds, the following sequence occurs:
                  * If the server responds with an HTTP 302 redirect, the client validates the redirection URL according to
                    the process defined in the "Redirect responses" and then does one of the following:
                      * If the redirection URL is valid, the client tries the URL and then does one of the following:
                          * If the attempt succeeds, the client repeats step 5 from the beginning.
                          * If the attempt fails, the client proceeds to step 6.
                      * If the redirection URL is not valid, the client proceeds to step 6.
                  * If the server responds with a valid Autodiscover response, the client does one of the following:
                      * If the value of the Action element is "Redirect", the client gets the redirection email address from
                        the Redirect element and then returns to step 1, using this new email address.
                      * If the value of the Action element is "Settings", the client has successfully received the requested
                        configuration settings for the specified user. The client does not need to proceed to step 6.

              :param ad:
              :return:
              """
              log.info('Step 5: Checking response')
              if ad.response is None:
                  # This is not explicit in the protocol, but let's raise errors here
                  ad.raise_errors()

              ad_response = ad.response
              if ad_response.redirect_url:
                  log.debug('Got a redirect URL: %s', ad_response.redirect_url)
                  # We are diverging a bit from the protocol here. We will never get an HTTP 302 since earlier steps already
                  # followed the redirects where possible. Instead, we handle retirect responses here.
                  if self._redirect_url_is_valid(url=ad_response.redirect_url):
                      is_valid_response, ad = self._attempt_response(url=ad_response.redirect_url)
                      if is_valid_response:
                          return self._step_5(ad=ad)
                      return self._step_6()
                  log.debug('Invalid redirect URL: %s', ad_response.redirect_url)
                  return self._step_6()
              # This could be an email redirect. Let outer layer handle this
              return ad_response

          def _step_6(self):
              """Perform step 6. If the client cannot contact the Autodiscover service, the client should ask the user for
              the Exchange server name and use it to construct an Exchange EWS URL. The client should try to use this URL for
              future requests.
              """
              raise AutoDiscoverFailed(
                  'All steps in the autodiscover protocol failed for email %r. If you think this is an error, consider doing '
                  'an official test at https://testconnectivity.microsoft.com' % self.email)

  ### Class variables

  `var DNS_RESOLVER_ATTRS`
  `var DNS_RESOLVER_KWARGS`
  `var INITIAL_RETRY_POLICY`
  `var MAX_REDIRECTS`
  `var RETRY_WAIT`

  ### Instance variables

  `var resolver`
    Expand source code

        def __get__(self, obj, cls):
            if obj is None:
                return self

            obj_dict = obj.__dict__
            name = self.func.__name__
            with self.lock:
                try:
                    # check if the value was computed before the lock was acquired
                    return obj_dict[name]

                except KeyError:
                    # if not, do the calculation and release the lock
                    return obj_dict.setdefault(name, self.func(obj))

  ### Methods

  `def clear(self)`
    Expand source code

        def clear(self):
            # This resets cached variables
            self._urls_visited = []
            self._redirect_count = 0
            self._emails_visited = []

  `def discover(self)`
    Expand source code

        def discover(self):
            self._emails_visited.append(self.email.lower())

            # Check the autodiscover cache to see if we already know the autodiscover service endpoint for this email
            # domain. Use a lock to guard against multiple threads competing to cache information.
            log.debug('Waiting for autodiscover_cache lock')
            with autodiscover_cache:
                log.debug('autodiscover_cache lock acquired')
                cache_key = self._cache_key
                domain = get_domain(self.email)
                if cache_key in autodiscover_cache:
                    ad_protocol = autodiscover_cache[cache_key]
                    log.debug('Cache hit for key %s: %s', cache_key, ad_protocol.service_endpoint)
                    try:
                        ad_response = self._quick(protocol=ad_protocol)
                    except AutoDiscoverFailed:
                        # Autodiscover no longer works with this domain. Clear cache and try again after releasing the lock
                        log.debug('AD request failure. Removing cache for key %s', cache_key)
                        del autodiscover_cache[cache_key]
                        ad_response = self._step_1(hostname=domain)
                else:
                    # This will cache the result
                    ad_response = self._step_1(hostname=domain)

            log.debug('Released autodiscover_cache_lock')
            if ad_response.redirect_address:
                log.debug('Got a redirect address: %s', ad_response.redirect_address)
                if ad_response.redirect_address.lower() in self._emails_visited:
                    raise AutoDiscoverCircularRedirect('We were redirected to an email address we have already seen')

                # Start over, but with the new email address
                self.email = ad_response.redirect_address
                return self.discover()

            # We successfully received a response. Clear the cache of seen emails etc.
            self.clear()
            return self._build_response(ad_response=ad_response)