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-rw-r--r--changelog.d/8941.feature1
-rw-r--r--changelog.d/8950.misc1
-rw-r--r--changelog.d/8954.feature1
-rw-r--r--docs/sample_config.yaml24
-rw-r--r--synapse/config/federation.py12
-rw-r--r--synapse/config/server.py12
-rw-r--r--synapse/handlers/oidc_handler.py62
-rw-r--r--synapse/handlers/saml_handler.py37
-rw-r--r--synapse/handlers/sso.py58
-rw-r--r--synapse/http/client.py32
-rw-r--r--synapse/http/federation/well_known_resolver.py25
-rw-r--r--synapse/http/matrixfederationclient.py13
-rw-r--r--tests/handlers/test_saml.py8
-rw-r--r--tests/http/federation/test_matrix_federation_agent.py27
14 files changed, 185 insertions, 128 deletions
diff --git a/changelog.d/8941.feature b/changelog.d/8941.feature
new file mode 100644

index 0000000000..d450ef4998 --- /dev/null +++ b/changelog.d/8941.feature
@@ -0,0 +1 @@ +Add support for allowing users to pick their own user ID during a single-sign-on login. diff --git a/changelog.d/8950.misc b/changelog.d/8950.misc new file mode 100644
index 0000000000..42e0335afc --- /dev/null +++ b/changelog.d/8950.misc
@@ -0,0 +1 @@ +Add a maximum size of 50 kilobytes to .well-known lookups. diff --git a/changelog.d/8954.feature b/changelog.d/8954.feature new file mode 100644
index 0000000000..39f53174ad --- /dev/null +++ b/changelog.d/8954.feature
@@ -0,0 +1 @@ +Apply an IP range blacklist to push and key revocation requests. diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index c86d45235e..1852ee98e5 100644 --- a/docs/sample_config.yaml +++ b/docs/sample_config.yaml
@@ -173,6 +173,18 @@ pid_file: DATADIR/homeserver.pid # - 'fe80::/10' # - 'fc00::/7' +# List of IP address CIDR ranges that should be allowed for federation, +# identity servers, push servers, and for checking key validity for +# third-party invite events. This is useful for specifying exceptions to +# wide-ranging blacklisted target IP ranges - e.g. for communication with +# a push server only visible in your network. +# +# This whitelist overrides ip_range_blacklist and defaults to an empty +# list. +# +#ip_range_whitelist: +# - '192.168.1.1' + # List of ports that Synapse should listen on, their purpose and their # configuration. # @@ -739,18 +751,6 @@ acme: # - nyc.example.com # - syd.example.com -# List of IP address CIDR ranges that should be allowed for federation, -# identity servers, push servers, and for checking key validity for -# third-party invite events. This is useful for specifying exceptions to -# wide-ranging blacklisted target IP ranges - e.g. for communication with -# a push server only visible in your network. -# -# This whitelist overrides ip_range_blacklist and defaults to an empty -# list. -# -#ip_range_whitelist: -# - '192.168.1.1' - # Report prometheus metrics on the age of PDUs being sent to and received from # the following domains. This can be used to give an idea of "delay" on inbound # and outbound federation, though be aware that any delay can be due to problems diff --git a/synapse/config/federation.py b/synapse/config/federation.py
index a03a419e23..9f3c57e6a1 100644 --- a/synapse/config/federation.py +++ b/synapse/config/federation.py
@@ -56,18 +56,6 @@ class FederationConfig(Config): # - nyc.example.com # - syd.example.com - # List of IP address CIDR ranges that should be allowed for federation, - # identity servers, push servers, and for checking key validity for - # third-party invite events. This is useful for specifying exceptions to - # wide-ranging blacklisted target IP ranges - e.g. for communication with - # a push server only visible in your network. - # - # This whitelist overrides ip_range_blacklist and defaults to an empty - # list. - # - #ip_range_whitelist: - # - '192.168.1.1' - # Report prometheus metrics on the age of PDUs being sent to and received from # the following domains. This can be used to give an idea of "delay" on inbound # and outbound federation, though be aware that any delay can be due to problems diff --git a/synapse/config/server.py b/synapse/config/server.py
index 09394e8b24..8e9dc54697 100644 --- a/synapse/config/server.py +++ b/synapse/config/server.py
@@ -838,6 +838,18 @@ class ServerConfig(Config): #ip_range_blacklist: %(ip_range_blacklist)s + # List of IP address CIDR ranges that should be allowed for federation, + # identity servers, push servers, and for checking key validity for + # third-party invite events. This is useful for specifying exceptions to + # wide-ranging blacklisted target IP ranges - e.g. for communication with + # a push server only visible in your network. + # + # This whitelist overrides ip_range_blacklist and defaults to an empty + # list. + # + #ip_range_whitelist: + # - '192.168.1.1' + # List of ports that Synapse should listen on, their purpose and their # configuration. # diff --git a/synapse/handlers/oidc_handler.py b/synapse/handlers/oidc_handler.py
index f626117f76..cbd11a1382 100644 --- a/synapse/handlers/oidc_handler.py +++ b/synapse/handlers/oidc_handler.py
@@ -115,8 +115,6 @@ class OidcHandler(BaseHandler): self._allow_existing_users = hs.config.oidc_allow_existing_users # type: bool self._http_client = hs.get_proxied_http_client() - self._auth_handler = hs.get_auth_handler() - self._registration_handler = hs.get_registration_handler() self._server_name = hs.config.server_name # type: str self._macaroon_secret_key = hs.config.macaroon_secret_key @@ -689,33 +687,14 @@ class OidcHandler(BaseHandler): # otherwise, it's a login - # Pull out the user-agent and IP from the request. - user_agent = request.get_user_agent("") - ip_address = self.hs.get_ip_from_request(request) - # Call the mapper to register/login the user try: - user_id = await self._map_userinfo_to_user( - userinfo, token, user_agent, ip_address + await self._complete_oidc_login( + userinfo, token, request, client_redirect_url ) except MappingException as e: logger.exception("Could not map user") self._sso_handler.render_error(request, "mapping_error", str(e)) - return - - # Mapping providers might not have get_extra_attributes: only call this - # method if it exists. - extra_attributes = None - get_extra_attributes = getattr( - self._user_mapping_provider, "get_extra_attributes", None - ) - if get_extra_attributes: - extra_attributes = await get_extra_attributes(userinfo, token) - - # and finally complete the login - await self._auth_handler.complete_sso_login( - user_id, request, client_redirect_url, extra_attributes - ) def _generate_oidc_session_token( self, @@ -838,10 +817,14 @@ class OidcHandler(BaseHandler): now = self.clock.time_msec() return now < expiry - async def _map_userinfo_to_user( - self, userinfo: UserInfo, token: Token, user_agent: str, ip_address: str - ) -> str: - """Maps a UserInfo object to a mxid. + async def _complete_oidc_login( + self, + userinfo: UserInfo, + token: Token, + request: SynapseRequest, + client_redirect_url: str, + ) -> None: + """Given a UserInfo response, complete the login flow UserInfo should have a claim that uniquely identifies users. This claim is usually `sub`, but can be configured with `oidc_config.subject_claim`. @@ -853,17 +836,16 @@ class OidcHandler(BaseHandler): If a user already exists with the mxid we've mapped and allow_existing_users is disabled, raise an exception. + Otherwise, render a redirect back to the client_redirect_url with a loginToken. + Args: userinfo: an object representing the user token: a dict with the tokens obtained from the provider - user_agent: The user agent of the client making the request. - ip_address: The IP address of the client making the request. + request: The request to respond to + client_redirect_url: The redirect URL passed in by the client. Raises: MappingException: if there was an error while mapping some properties - - Returns: - The mxid of the user """ try: remote_user_id = self._remote_id_from_userinfo(userinfo) @@ -931,13 +913,23 @@ class OidcHandler(BaseHandler): return None - return await self._sso_handler.get_mxid_from_sso( + # Mapping providers might not have get_extra_attributes: only call this + # method if it exists. + extra_attributes = None + get_extra_attributes = getattr( + self._user_mapping_provider, "get_extra_attributes", None + ) + if get_extra_attributes: + extra_attributes = await get_extra_attributes(userinfo, token) + + await self._sso_handler.complete_sso_login_request( self._auth_provider_id, remote_user_id, - user_agent, - ip_address, + request, + client_redirect_url, oidc_response_to_user_attributes, grandfather_existing_users, + extra_attributes, ) def _remote_id_from_userinfo(self, userinfo: UserInfo) -> str: diff --git a/synapse/handlers/saml_handler.py b/synapse/handlers/saml_handler.py
index 6001fe3e27..5fa7ab3f8b 100644 --- a/synapse/handlers/saml_handler.py +++ b/synapse/handlers/saml_handler.py
@@ -58,8 +58,6 @@ class SamlHandler(BaseHandler): super().__init__(hs) self._saml_client = Saml2Client(hs.config.saml2_sp_config) self._saml_idp_entityid = hs.config.saml2_idp_entityid - self._auth_handler = hs.get_auth_handler() - self._registration_handler = hs.get_registration_handler() self._saml2_session_lifetime = hs.config.saml2_session_lifetime self._grandfathered_mxid_source_attribute = ( @@ -229,40 +227,29 @@ class SamlHandler(BaseHandler): ) return - # Pull out the user-agent and IP from the request. - user_agent = request.get_user_agent("") - ip_address = self.hs.get_ip_from_request(request) - # Call the mapper to register/login the user try: - user_id = await self._map_saml_response_to_user( - saml2_auth, relay_state, user_agent, ip_address - ) + await self._complete_saml_login(saml2_auth, request, relay_state) except MappingException as e: logger.exception("Could not map user") self._sso_handler.render_error(request, "mapping_error", str(e)) - return - await self._auth_handler.complete_sso_login(user_id, request, relay_state) - - async def _map_saml_response_to_user( + async def _complete_saml_login( self, saml2_auth: saml2.response.AuthnResponse, + request: SynapseRequest, client_redirect_url: str, - user_agent: str, - ip_address: str, - ) -> str: + ) -> None: """ - Given a SAML response, retrieve the user ID for it and possibly register the user. + Given a SAML response, complete the login flow + + Retrieves the remote user ID, registers the user if necessary, and serves + a redirect back to the client with a login-token. Args: saml2_auth: The parsed SAML2 response. + request: The request to respond to client_redirect_url: The redirect URL passed in by the client. - user_agent: The user agent of the client making the request. - ip_address: The IP address of the client making the request. - - Returns: - The user ID associated with this response. Raises: MappingException if there was a problem mapping the response to a user. @@ -318,11 +305,11 @@ class SamlHandler(BaseHandler): return None - return await self._sso_handler.get_mxid_from_sso( + await self._sso_handler.complete_sso_login_request( self._auth_provider_id, remote_user_id, - user_agent, - ip_address, + request, + client_redirect_url, saml_response_to_remapped_user_attributes, grandfather_existing_users, ) diff --git a/synapse/handlers/sso.py b/synapse/handlers/sso.py
index 112a7d5b2c..f054b66a53 100644 --- a/synapse/handlers/sso.py +++ b/synapse/handlers/sso.py
@@ -21,7 +21,8 @@ from twisted.web.http import Request from synapse.api.errors import RedirectException from synapse.http.server import respond_with_html -from synapse.types import UserID, contains_invalid_mxid_characters +from synapse.http.site import SynapseRequest +from synapse.types import JsonDict, UserID, contains_invalid_mxid_characters from synapse.util.async_helpers import Linearizer if TYPE_CHECKING: @@ -119,15 +120,16 @@ class SsoHandler: # No match. return None - async def get_mxid_from_sso( + async def complete_sso_login_request( self, auth_provider_id: str, remote_user_id: str, - user_agent: str, - ip_address: str, + request: SynapseRequest, + client_redirect_url: str, sso_to_matrix_id_mapper: Callable[[int], Awaitable[UserAttributes]], grandfather_existing_users: Optional[Callable[[], Awaitable[Optional[str]]]], - ) -> str: + extra_login_attributes: Optional[JsonDict] = None, + ) -> None: """ Given an SSO ID, retrieve the user ID for it and possibly register the user. @@ -146,12 +148,18 @@ class SsoHandler: given user-agent and IP address and the SSO ID is linked to this matrix ID for subsequent calls. + Finally, we generate a redirect to the supplied redirect uri, with a login token + Args: auth_provider_id: A unique identifier for this SSO provider, e.g. "oidc" or "saml". + remote_user_id: The unique identifier from the SSO provider. - user_agent: The user agent of the client making the request. - ip_address: The IP address of the client making the request. + + request: The request to respond to + + client_redirect_url: The redirect URL passed in by the client. + sso_to_matrix_id_mapper: A callable to generate the user attributes. The only parameter is an integer which represents the amount of times the returned mxid localpart mapping has failed. @@ -163,12 +171,13 @@ class SsoHandler: to the user. RedirectException to redirect to an additional page (e.g. to prompt the user for more information). + grandfather_existing_users: A callable which can return an previously existing matrix ID. The SSO ID is then linked to the returned matrix ID. - Returns: - The user ID associated with the SSO response. + extra_login_attributes: An optional dictionary of extra + attributes to be provided to the client in the login response. Raises: MappingException if there was a problem mapping the response to a user. @@ -181,28 +190,33 @@ class SsoHandler: # interstitial pages. with await self._mapping_lock.queue(auth_provider_id): # first of all, check if we already have a mapping for this user - previously_registered_user_id = await self.get_sso_user_by_remote_user_id( + user_id = await self.get_sso_user_by_remote_user_id( auth_provider_id, remote_user_id, ) - if previously_registered_user_id: - return previously_registered_user_id # Check for grandfathering of users. - if grandfather_existing_users: - previously_registered_user_id = await grandfather_existing_users() - if previously_registered_user_id: + if not user_id and grandfather_existing_users: + user_id = await grandfather_existing_users() + if user_id: # Future logins should also match this user ID. await self._store.record_user_external_id( - auth_provider_id, remote_user_id, previously_registered_user_id + auth_provider_id, remote_user_id, user_id ) - return previously_registered_user_id # Otherwise, generate a new user. - attributes = await self._call_attribute_mapper(sso_to_matrix_id_mapper) - user_id = await self._register_mapped_user( - attributes, auth_provider_id, remote_user_id, user_agent, ip_address, - ) - return user_id + if not user_id: + attributes = await self._call_attribute_mapper(sso_to_matrix_id_mapper) + user_id = await self._register_mapped_user( + attributes, + auth_provider_id, + remote_user_id, + request.get_user_agent(""), + request.getClientIP(), + ) + + await self._auth_handler.complete_sso_login( + user_id, request, client_redirect_url, extra_login_attributes + ) async def _call_attribute_mapper( self, sso_to_matrix_id_mapper: Callable[[int], Awaitable[UserAttributes]], diff --git a/synapse/http/client.py b/synapse/http/client.py
index 960d57fb27..d5a8b21575 100644 --- a/synapse/http/client.py +++ b/synapse/http/client.py
@@ -719,11 +719,14 @@ class SimpleHttpClient: try: length = await make_deferred_yieldable( - readBodyToFile(response, output_stream, max_size) + read_body_with_max_size(response, output_stream, max_size) + ) + except BodyExceededMaxSize: + SynapseError( + 502, + "Requested file is too large > %r bytes" % (max_size,), + Codes.TOO_LARGE, ) - except SynapseError: - # This can happen e.g. because the body is too large. - raise except Exception as e: raise SynapseError(502, ("Failed to download remote body: %s" % e)) from e @@ -747,7 +750,11 @@ def _timeout_to_request_timed_out_error(f: Failure): return f -class _ReadBodyToFileProtocol(protocol.Protocol): +class BodyExceededMaxSize(Exception): + """The maximum allowed size of the HTTP body was exceeded.""" + + +class _ReadBodyWithMaxSizeProtocol(protocol.Protocol): def __init__( self, stream: BinaryIO, deferred: defer.Deferred, max_size: Optional[int] ): @@ -760,13 +767,7 @@ class _ReadBodyToFileProtocol(protocol.Protocol): self.stream.write(data) self.length += len(data) if self.max_size is not None and self.length >= self.max_size: - self.deferred.errback( - SynapseError( - 502, - "Requested file is too large > %r bytes" % (self.max_size,), - Codes.TOO_LARGE, - ) - ) + self.deferred.errback(BodyExceededMaxSize()) self.deferred = defer.Deferred() self.transport.loseConnection() @@ -781,12 +782,15 @@ class _ReadBodyToFileProtocol(protocol.Protocol): self.deferred.errback(reason) -def readBodyToFile( +def read_body_with_max_size( response: IResponse, stream: BinaryIO, max_size: Optional[int] ) -> defer.Deferred: """ Read a HTTP response body to a file-object. Optionally enforcing a maximum file size. + If the maximum file size is reached, the returned Deferred will resolve to a + Failure with a BodyExceededMaxSize exception. + Args: response: The HTTP response to read from. stream: The file-object to write to. @@ -797,7 +801,7 @@ def readBodyToFile( """ d = defer.Deferred() - response.deliverBody(_ReadBodyToFileProtocol(stream, d, max_size)) + response.deliverBody(_ReadBodyWithMaxSizeProtocol(stream, d, max_size)) return d diff --git a/synapse/http/federation/well_known_resolver.py b/synapse/http/federation/well_known_resolver.py
index 5e08ef1664..b3b6dbcab0 100644 --- a/synapse/http/federation/well_known_resolver.py +++ b/synapse/http/federation/well_known_resolver.py
@@ -15,17 +15,19 @@ import logging import random import time +from io import BytesIO from typing import Callable, Dict, Optional, Tuple import attr from twisted.internet import defer from twisted.internet.interfaces import IReactorTime -from twisted.web.client import RedirectAgent, readBody +from twisted.web.client import RedirectAgent from twisted.web.http import stringToDatetime from twisted.web.http_headers import Headers from twisted.web.iweb import IAgent, IResponse +from synapse.http.client import BodyExceededMaxSize, read_body_with_max_size from synapse.logging.context import make_deferred_yieldable from synapse.util import Clock, json_decoder from synapse.util.caches.ttlcache import TTLCache @@ -53,6 +55,9 @@ WELL_KNOWN_MAX_CACHE_PERIOD = 48 * 3600 # lower bound for .well-known cache period WELL_KNOWN_MIN_CACHE_PERIOD = 5 * 60 +# The maximum size (in bytes) to allow a well-known file to be. +WELL_KNOWN_MAX_SIZE = 50 * 1024 # 50 KiB + # Attempt to refetch a cached well-known N% of the TTL before it expires. # e.g. if set to 0.2 and we have a cached entry with a TTL of 5mins, then # we'll start trying to refetch 1 minute before it expires. @@ -229,6 +234,9 @@ class WellKnownResolver: server_name: name of the server, from the requested url retry: Whether to retry the request if it fails. + Raises: + _FetchWellKnownFailure if we fail to lookup a result + Returns: Returns the response object and body. Response may be a non-200 response. """ @@ -250,7 +258,11 @@ class WellKnownResolver: b"GET", uri, headers=Headers(headers) ) ) - body = await make_deferred_yieldable(readBody(response)) + body_stream = BytesIO() + await make_deferred_yieldable( + read_body_with_max_size(response, body_stream, WELL_KNOWN_MAX_SIZE) + ) + body = body_stream.getvalue() if 500 <= response.code < 600: raise Exception("Non-200 response %s" % (response.code,)) @@ -259,6 +271,15 @@ class WellKnownResolver: except defer.CancelledError: # Bail if we've been cancelled raise + except BodyExceededMaxSize: + # If the well-known file was too large, do not keep attempting + # to download it, but consider it a temporary error. + logger.warning( + "Requested .well-known file for %s is too large > %r bytes", + server_name.decode("ascii"), + WELL_KNOWN_MAX_SIZE, + ) + raise _FetchWellKnownFailure(temporary=True) except Exception as e: if not retry or i >= WELL_KNOWN_RETRY_ATTEMPTS: logger.info("Error fetching %s: %s", uri_str, e) diff --git a/synapse/http/matrixfederationclient.py b/synapse/http/matrixfederationclient.py
index c962994727..b261e078c4 100644 --- a/synapse/http/matrixfederationclient.py +++ b/synapse/http/matrixfederationclient.py
@@ -37,16 +37,19 @@ from twisted.web.iweb import IBodyProducer, IResponse import synapse.metrics import synapse.util.retryutils from synapse.api.errors import ( + Codes, FederationDeniedError, HttpResponseException, RequestSendFailed, + SynapseError, ) from synapse.http import QuieterFileBodyProducer from synapse.http.client import ( BlacklistingAgentWrapper, BlacklistingReactorWrapper, + BodyExceededMaxSize, encode_query_args, - readBodyToFile, + read_body_with_max_size, ) from synapse.http.federation.matrix_federation_agent import MatrixFederationAgent from synapse.logging.context import make_deferred_yieldable @@ -975,9 +978,15 @@ class MatrixFederationHttpClient: headers = dict(response.headers.getAllRawHeaders()) try: - d = readBodyToFile(response, output_stream, max_size) + d = read_body_with_max_size(response, output_stream, max_size) d.addTimeout(self.default_timeout, self.reactor) length = await make_deferred_yieldable(d) + except BodyExceededMaxSize: + msg = "Requested file is too large > %r bytes" % (max_size,) + logger.warning( + "{%s} [%s] %s", request.txn_id, request.destination, msg, + ) + SynapseError(502, msg, Codes.TOO_LARGE) except Exception as e: logger.warning( "{%s} [%s] Error reading response: %s", diff --git a/tests/handlers/test_saml.py b/tests/handlers/test_saml.py
index 69927cf6be..548038214b 100644 --- a/tests/handlers/test_saml.py +++ b/tests/handlers/test_saml.py
@@ -131,7 +131,7 @@ class SamlHandlerTestCase(HomeserverTestCase): # check that the auth handler got called as expected auth_handler.complete_sso_login.assert_called_once_with( - "@test_user:test", request, "redirect_uri" + "@test_user:test", request, "redirect_uri", None ) @override_config({"saml2_config": {"grandfathered_mxid_source_attribute": "mxid"}}) @@ -157,7 +157,7 @@ class SamlHandlerTestCase(HomeserverTestCase): # check that the auth handler got called as expected auth_handler.complete_sso_login.assert_called_once_with( - "@test_user:test", request, "" + "@test_user:test", request, "", None ) # Subsequent calls should map to the same mxid. @@ -166,7 +166,7 @@ class SamlHandlerTestCase(HomeserverTestCase): self.handler._handle_authn_response(request, saml_response, "") ) auth_handler.complete_sso_login.assert_called_once_with( - "@test_user:test", request, "" + "@test_user:test", request, "", None ) def test_map_saml_response_to_invalid_localpart(self): @@ -214,7 +214,7 @@ class SamlHandlerTestCase(HomeserverTestCase): # test_user is already taken, so test_user1 gets registered instead. auth_handler.complete_sso_login.assert_called_once_with( - "@test_user1:test", request, "" + "@test_user1:test", request, "", None ) auth_handler.complete_sso_login.reset_mock() diff --git a/tests/http/federation/test_matrix_federation_agent.py b/tests/http/federation/test_matrix_federation_agent.py
index aac8c7d623..8d0d97a53b 100644 --- a/tests/http/federation/test_matrix_federation_agent.py +++ b/tests/http/federation/test_matrix_federation_agent.py
@@ -36,6 +36,7 @@ from synapse.crypto.context_factory import FederationPolicyForHTTPS from synapse.http.federation.matrix_federation_agent import MatrixFederationAgent from synapse.http.federation.srv_resolver import Server from synapse.http.federation.well_known_resolver import ( + WELL_KNOWN_MAX_SIZE, WellKnownResolver, _cache_period_from_headers, ) @@ -1107,6 +1108,32 @@ class MatrixFederationAgentTests(unittest.TestCase): r = self.successResultOf(fetch_d) self.assertEqual(r.delegated_server, None) + def test_well_known_too_large(self): + """A well-known query that returns a result which is too large should be rejected.""" + self.reactor.lookups["testserv"] = "1.2.3.4" + + fetch_d = defer.ensureDeferred( + self.well_known_resolver.get_well_known(b"testserv") + ) + + # there should be an attempt to connect on port 443 for the .well-known + clients = self.reactor.tcpClients + self.assertEqual(len(clients), 1) + (host, port, client_factory, _timeout, _bindAddress) = clients.pop(0) + self.assertEqual(host, "1.2.3.4") + self.assertEqual(port, 443) + + self._handle_well_known_connection( + client_factory, + expected_sni=b"testserv", + response_headers={b"Cache-Control": b"max-age=1000"}, + content=b'{ "m.server": "' + (b"a" * WELL_KNOWN_MAX_SIZE) + b'" }', + ) + + # The result is sucessful, but disabled delegation. + r = self.successResultOf(fetch_d) + self.assertIsNone(r.delegated_server) + def test_srv_fallbacks(self): """Test that other SRV results are tried if the first one fails. """