exchangelib.autodiscover.discovery API documentation

# Module `exchangelib.autodiscover.discovery`

## Functions

`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):
          ad_response, protocol = Autodiscovery(email=email, credentials=credentials).discover()
          protocol.config.auth_typ = auth_type
          protocol.config.retry_policy = retry_policy
          return ad_response, protocol

## Classes

`class Autodiscovery (email, credentials=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()
          MAX_REDIRECTS = 10  # Maximum number of URL redirects before we give up
          DNS_RESOLVER_KWARGS = {}
          DNS_RESOLVER_ATTRS = {
              "timeout": AutodiscoverProtocol.TIMEOUT / 2.5,  # Timeout for query to a single nameserver
          }
          DNS_RESOLVER_LIFETIME = AutodiscoverProtocol.TIMEOUT  # Total timeout for a query in case of multiple nameservers
          URL_PATH = "autodiscover/autodiscover.svc"

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

              :param email: The email address to autodiscover
              :param credentials: Credentials with authorization to make autodiscover lookups for this Account
                  (Default value = None)
              """
              self.email = email
              self.credentials = credentials
              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 = 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 = self._step_1(hostname=domain)
                  else:
                      # This will cache the result
                      log.debug("Cache miss for key %s", cache_key)
                      ad = self._step_1(hostname=domain)

              log.debug("Released autodiscover_cache_lock")
              if ad.redirect_address:
                  log.debug("Got a redirect address: %s", ad.redirect_address)
                  if ad.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.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)

          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):
              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.autodiscover_smtp_address = self.email

              protocol = Protocol(
                  config=Configuration(
                      service_endpoint=ad_response.ews_url,
                      credentials=self.credentials,
                      version=ad_response.version,
                      # TODO: Detect EWS service auth type somehow
                  )
              )
              return ad_response, protocol

          def _quick(self, protocol):
              try:
                  user_response = protocol.get_user_settings(user=self.email)
              except TransportError as e:
                  raise AutoDiscoverFailed(f"Response error: {e}")
              return self._step_5(ad=user_response)

          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 response from server using the given HTTP method

              :param url:
              :return:
              """
              # We are connecting to untrusted servers here, so take necessary precautions.
              self._ensure_valid_hostname(url)

              protocol = AutodiscoverProtocol(
                  config=Configuration(
                      service_endpoint=url,
                      retry_policy=self.INITIAL_RETRY_POLICY,
                  )
              )
              return None, get_unauthenticated_autodiscover_response(protocol=protocol, method=method)

          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:
                  self._ensure_valid_hostname(url)
              except TransportError:
                  return False, None

              protocol = AutodiscoverProtocol(
                  config=Configuration(
                      service_endpoint=url,
                      credentials=self.credentials,
                      retry_policy=self.INITIAL_RETRY_POLICY,
                  )
              )
              try:
                  user_response = protocol.get_user_settings(user=self.email)
              except RedirectError as e:
                  if self._redirect_url_is_valid(url=e.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=e.url)
                  log.debug("Invalid redirect URL: %s", e.url)
                  return False, None
              except TransportError as e:
                  log.debug("Failed to get a response: %s", e)
                  return False, None
              except CONNECTION_ERRORS as e:
                  log.debug("Failed to get a response: %s", e)
                  return False, None

              # We got a valid response. Unless this is a URL redirect response, we cache the result
              if not user_response.redirect_url:
                  cache_key = self._cache_key
                  log.debug("Adding cache entry for key %s: %s", cache_key, protocol.service_endpoint)
                  autodiscover_cache[cache_key] = protocol
              return True, user_response

          def _ensure_valid_hostname(self, url):
              hostname = urlparse(url).netloc
              log.debug("Checking if %s can be looked up in DNS", hostname)
              try:
                  self.resolver.resolve(f"{hostname}.", "A", lifetime=self.DNS_RESOLVER_LIFETIME)
              except DNS_LOOKUP_ERRORS as e:
                  log.debug("DNS A lookup failure: %s", e)
                  # 'requests' is 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(f"{hostname!r} has no DNS entry")

          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(f"{hostname}.", "SRV", lifetime=self.DNS_RESOLVER_LIFETIME)
              except DNS_LOOKUP_ERRORS as e:
                  log.debug("DNS SRV 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/ 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 = f"https://{hostname}/{self.URL_PATH}"
              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/ 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 = f"https://autodiscover.{hostname}/{self.URL_PATH}"
              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 unauthenticated GET method request to
              http://autodiscover.example.com/ (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 = f"http://autodiscover.{hostname}/{self.URL_PATH}"
              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:
                  pass
              else:
                  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)
                          log.debug("Got invalid response")
                          return self._step_4(hostname=hostname)
                      log.debug("Got invalid redirect URL")
                      return self._step_4(hostname=hostname)
              log.debug("Got no redirect URL")
              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 = f"_autodiscover._tcp.{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 = f"https://{srv_host}/{self.URL_PATH}"
              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)
                  log.debug("Got invalid response")
                  return self._step_6()
              log.debug("Got invalid redirect URL")
              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")
              # This is not explicit in the protocol, but let's raise any errors here
              ad.raise_errors()

              if ad.redirect_url:
                  log.debug("Got a redirect URL: %s", ad.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 redirect responses here.
                  if self._redirect_url_is_valid(url=ad.redirect_url):
                      is_valid_response, ad = self._attempt_response(url=ad.redirect_url)
                      if is_valid_response:
                          return self._step_5(ad=ad)
                      log.debug("Got invalid response")
                      return self._step_6()
                  log.debug("Invalid redirect URL")
                  return self._step_6()
              # This could be an email redirect. Let outer layer handle this
              return ad

          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(
                  f"All steps in the autodiscover protocol failed for email {self.email}. If you think this is an error, "
                  f"consider doing an official test at https://testconnectivity.microsoft.com"
              )

  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)

  ### Class variables

  `var DNS_RESOLVER_ATTRS`
  `var DNS_RESOLVER_KWARGS`
  `var DNS_RESOLVER_LIFETIME`
  `var INITIAL_RETRY_POLICY`
  `var MAX_REDIRECTS`
  `var URL_PATH`

  ### 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 = 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 = self._step_1(hostname=domain)
                else:
                    # This will cache the result
                    log.debug("Cache miss for key %s", cache_key)
                    ad = self._step_1(hostname=domain)

            log.debug("Released autodiscover_cache_lock")
            if ad.redirect_address:
                log.debug("Got a redirect address: %s", ad.redirect_address)
                if ad.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.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)

`class SrvRecord (priority, weight, port, srv)`
  Expand source code

      class SrvRecord:
          """A container for autodiscover-related SRV records in DNS."""

          def __init__(self, priority, weight, port, srv):
              self.priority = priority
              self.weight = weight
              self.port = port
              self.srv = srv

          def __hash__(self):
              return hash((self.priority, self.weight, self.port, self.srv))

          def __eq__(self, other):
              return all(getattr(self, k) == getattr(other, k) for k in self.__dict__)

  A container for autodiscover-related SRV records in DNS.