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-rw-r--r--synapse/api/urls.py1
-rw-r--r--synapse/config/tls.py40
-rw-r--r--synapse/crypto/context_factory.py6
-rw-r--r--synapse/crypto/keyclient.py149
-rw-r--r--synapse/crypto/keyring.py30
-rw-r--r--synapse/federation/federation_server.py4
-rw-r--r--synapse/federation/transport/server.py42
-rw-r--r--synapse/http/endpoint.py75
-rw-r--r--synapse/http/federation/__init__.py14
-rw-r--r--synapse/http/federation/srv_resolver.py124
10 files changed, 193 insertions, 292 deletions
diff --git a/synapse/api/urls.py b/synapse/api/urls.py
index ba019001ff..8102176653 100644
--- a/synapse/api/urls.py
+++ b/synapse/api/urls.py
@@ -26,6 +26,7 @@ CLIENT_PREFIX = "/_matrix/client/api/v1"
 CLIENT_V2_ALPHA_PREFIX = "/_matrix/client/v2_alpha"
 FEDERATION_PREFIX = "/_matrix/federation"
 FEDERATION_V1_PREFIX = FEDERATION_PREFIX + "/v1"
+FEDERATION_V2_PREFIX = FEDERATION_PREFIX + "/v2"
 STATIC_PREFIX = "/_matrix/static"
 WEB_CLIENT_PREFIX = "/_matrix/client"
 CONTENT_REPO_PREFIX = "/_matrix/content"
diff --git a/synapse/config/tls.py b/synapse/config/tls.py
index fef1ea99cb..bb8952c672 100644
--- a/synapse/config/tls.py
+++ b/synapse/config/tls.py
@@ -14,7 +14,6 @@
 # limitations under the License.
 
 import os
-import subprocess
 from hashlib import sha256
 
 from unpaddedbase64 import encode_base64
@@ -23,8 +22,6 @@ from OpenSSL import crypto
 
 from ._base import Config
 
-GENERATE_DH_PARAMS = False
-
 
 class TlsConfig(Config):
     def read_config(self, config):
@@ -42,10 +39,6 @@ class TlsConfig(Config):
                 config.get("tls_private_key_path")
             )
 
-        self.tls_dh_params_path = self.check_file(
-            config.get("tls_dh_params_path"), "tls_dh_params"
-        )
-
         self.tls_fingerprints = config["tls_fingerprints"]
 
         # Check that our own certificate is included in the list of fingerprints
@@ -72,7 +65,6 @@ class TlsConfig(Config):
 
         tls_certificate_path = base_key_name + ".tls.crt"
         tls_private_key_path = base_key_name + ".tls.key"
-        tls_dh_params_path = base_key_name + ".tls.dh"
 
         return """\
         # PEM encoded X509 certificate for TLS.
@@ -85,9 +77,6 @@ class TlsConfig(Config):
         # PEM encoded private key for TLS
         tls_private_key_path: "%(tls_private_key_path)s"
 
-        # PEM dh parameters for ephemeral keys
-        tls_dh_params_path: "%(tls_dh_params_path)s"
-
         # Don't bind to the https port
         no_tls: False
 
@@ -131,7 +120,6 @@ class TlsConfig(Config):
     def generate_files(self, config):
         tls_certificate_path = config["tls_certificate_path"]
         tls_private_key_path = config["tls_private_key_path"]
-        tls_dh_params_path = config["tls_dh_params_path"]
 
         if not self.path_exists(tls_private_key_path):
             with open(tls_private_key_path, "wb") as private_key_file:
@@ -165,31 +153,3 @@ class TlsConfig(Config):
                 cert_pem = crypto.dump_certificate(crypto.FILETYPE_PEM, cert)
 
                 certificate_file.write(cert_pem)
-
-        if not self.path_exists(tls_dh_params_path):
-            if GENERATE_DH_PARAMS:
-                subprocess.check_call([
-                    "openssl", "dhparam",
-                    "-outform", "PEM",
-                    "-out", tls_dh_params_path,
-                    "2048"
-                ])
-            else:
-                with open(tls_dh_params_path, "w") as dh_params_file:
-                    dh_params_file.write(
-                        "2048-bit DH parameters taken from rfc3526\n"
-                        "-----BEGIN DH PARAMETERS-----\n"
-                        "MIIBCAKCAQEA///////////JD9qiIWjC"
-                        "NMTGYouA3BzRKQJOCIpnzHQCC76mOxOb\n"
-                        "IlFKCHmONATd75UZs806QxswKwpt8l8U"
-                        "N0/hNW1tUcJF5IW1dmJefsb0TELppjft\n"
-                        "awv/XLb0Brft7jhr+1qJn6WunyQRfEsf"
-                        "5kkoZlHs5Fs9wgB8uKFjvwWY2kg2HFXT\n"
-                        "mmkWP6j9JM9fg2VdI9yjrZYcYvNWIIVS"
-                        "u57VKQdwlpZtZww1Tkq8mATxdGwIyhgh\n"
-                        "fDKQXkYuNs474553LBgOhgObJ4Oi7Aei"
-                        "j7XFXfBvTFLJ3ivL9pVYFxg5lUl86pVq\n"
-                        "5RXSJhiY+gUQFXKOWoqsqmj/////////"
-                        "/wIBAg==\n"
-                        "-----END DH PARAMETERS-----\n"
-                    )
diff --git a/synapse/crypto/context_factory.py b/synapse/crypto/context_factory.py
index 02b76dfcfb..6ba3eca7b2 100644
--- a/synapse/crypto/context_factory.py
+++ b/synapse/crypto/context_factory.py
@@ -46,8 +46,10 @@ class ServerContextFactory(ContextFactory):
         if not config.no_tls:
             context.use_privatekey(config.tls_private_key)
 
-        context.load_tmp_dh(config.tls_dh_params_path)
-        context.set_cipher_list("!ADH:HIGH+kEDH:!AECDH:HIGH+kEECDH")
+        # https://hynek.me/articles/hardening-your-web-servers-ssl-ciphers/
+        context.set_cipher_list(
+            "ECDH+AESGCM:ECDH+CHACHA20:ECDH+AES256:ECDH+AES128:!aNULL:!SHA1"
+        )
 
     def getContext(self):
         return self._context
diff --git a/synapse/crypto/keyclient.py b/synapse/crypto/keyclient.py
deleted file mode 100644
index d40e4b8591..0000000000
--- a/synapse/crypto/keyclient.py
+++ /dev/null
@@ -1,149 +0,0 @@
-# -*- coding: utf-8 -*-
-# Copyright 2014-2016 OpenMarket Ltd
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-import logging
-
-from six.moves import urllib
-
-from canonicaljson import json
-
-from twisted.internet import defer, reactor
-from twisted.internet.error import ConnectError
-from twisted.internet.protocol import Factory
-from twisted.names.error import DomainError
-from twisted.web.http import HTTPClient
-
-from synapse.http.endpoint import matrix_federation_endpoint
-from synapse.util import logcontext
-
-logger = logging.getLogger(__name__)
-
-KEY_API_V2 = "/_matrix/key/v2/server/%s"
-
-
-@defer.inlineCallbacks
-def fetch_server_key(server_name, tls_client_options_factory, key_id):
-    """Fetch the keys for a remote server."""
-
-    factory = SynapseKeyClientFactory()
-    factory.path = KEY_API_V2 % (urllib.parse.quote(key_id), )
-    factory.host = server_name
-    endpoint = matrix_federation_endpoint(
-        reactor, server_name, tls_client_options_factory, timeout=30
-    )
-
-    for i in range(5):
-        try:
-            with logcontext.PreserveLoggingContext():
-                protocol = yield endpoint.connect(factory)
-                server_response, server_certificate = yield protocol.remote_key
-                defer.returnValue((server_response, server_certificate))
-        except SynapseKeyClientError as e:
-            logger.warn("Error getting key for %r: %s", server_name, e)
-            if e.status.startswith(b"4"):
-                # Don't retry for 4xx responses.
-                raise IOError("Cannot get key for %r" % server_name)
-        except (ConnectError, DomainError) as e:
-            logger.warn("Error getting key for %r: %s", server_name, e)
-        except Exception:
-            logger.exception("Error getting key for %r", server_name)
-    raise IOError("Cannot get key for %r" % server_name)
-
-
-class SynapseKeyClientError(Exception):
-    """The key wasn't retrieved from the remote server."""
-    status = None
-    pass
-
-
-class SynapseKeyClientProtocol(HTTPClient):
-    """Low level HTTPS client which retrieves an application/json response from
-    the server and extracts the X.509 certificate for the remote peer from the
-    SSL connection."""
-
-    timeout = 30
-
-    def __init__(self):
-        self.remote_key = defer.Deferred()
-        self.host = None
-        self._peer = None
-
-    def connectionMade(self):
-        self._peer = self.transport.getPeer()
-        logger.debug("Connected to %s", self._peer)
-
-        if not isinstance(self.path, bytes):
-            self.path = self.path.encode('ascii')
-
-        if not isinstance(self.host, bytes):
-            self.host = self.host.encode('ascii')
-
-        self.sendCommand(b"GET", self.path)
-        if self.host:
-            self.sendHeader(b"Host", self.host)
-        self.endHeaders()
-        self.timer = reactor.callLater(
-            self.timeout,
-            self.on_timeout
-        )
-
-    def errback(self, error):
-        if not self.remote_key.called:
-            self.remote_key.errback(error)
-
-    def callback(self, result):
-        if not self.remote_key.called:
-            self.remote_key.callback(result)
-
-    def handleStatus(self, version, status, message):
-        if status != b"200":
-            # logger.info("Non-200 response from %s: %s %s",
-            #            self.transport.getHost(), status, message)
-            error = SynapseKeyClientError(
-                "Non-200 response %r from %r" % (status, self.host)
-            )
-            error.status = status
-            self.errback(error)
-            self.transport.abortConnection()
-
-    def handleResponse(self, response_body_bytes):
-        try:
-            json_response = json.loads(response_body_bytes)
-        except ValueError:
-            # logger.info("Invalid JSON response from %s",
-            #            self.transport.getHost())
-            self.transport.abortConnection()
-            return
-
-        certificate = self.transport.getPeerCertificate()
-        self.callback((json_response, certificate))
-        self.transport.abortConnection()
-        self.timer.cancel()
-
-    def on_timeout(self):
-        logger.debug(
-            "Timeout waiting for response from %s: %s",
-            self.host, self._peer,
-        )
-        self.errback(IOError("Timeout waiting for response"))
-        self.transport.abortConnection()
-
-
-class SynapseKeyClientFactory(Factory):
-    def protocol(self):
-        protocol = SynapseKeyClientProtocol()
-        protocol.path = self.path
-        protocol.host = self.host
-        return protocol
diff --git a/synapse/crypto/keyring.py b/synapse/crypto/keyring.py
index 515ebbc148..3a96980bed 100644
--- a/synapse/crypto/keyring.py
+++ b/synapse/crypto/keyring.py
@@ -14,10 +14,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import hashlib
 import logging
 from collections import namedtuple
 
+from six.moves import urllib
+
 from signedjson.key import (
     decode_verify_key_bytes,
     encode_verify_key_base64,
@@ -30,13 +31,11 @@ from signedjson.sign import (
     signature_ids,
     verify_signed_json,
 )
-from unpaddedbase64 import decode_base64, encode_base64
+from unpaddedbase64 import decode_base64
 
-from OpenSSL import crypto
 from twisted.internet import defer
 
 from synapse.api.errors import Codes, SynapseError
-from synapse.crypto.keyclient import fetch_server_key
 from synapse.util import logcontext, unwrapFirstError
 from synapse.util.logcontext import (
     LoggingContext,
@@ -503,31 +502,16 @@ class Keyring(object):
             if requested_key_id in keys:
                 continue
 
-            (response, tls_certificate) = yield fetch_server_key(
-                server_name, self.hs.tls_client_options_factory, requested_key_id
+            response = yield self.client.get_json(
+                destination=server_name,
+                path="/_matrix/key/v2/server/" + urllib.parse.quote(requested_key_id),
+                ignore_backoff=True,
             )
 
             if (u"signatures" not in response
                     or server_name not in response[u"signatures"]):
                 raise KeyLookupError("Key response not signed by remote server")
 
-            if "tls_fingerprints" not in response:
-                raise KeyLookupError("Key response missing TLS fingerprints")
-
-            certificate_bytes = crypto.dump_certificate(
-                crypto.FILETYPE_ASN1, tls_certificate
-            )
-            sha256_fingerprint = hashlib.sha256(certificate_bytes).digest()
-            sha256_fingerprint_b64 = encode_base64(sha256_fingerprint)
-
-            response_sha256_fingerprints = set()
-            for fingerprint in response[u"tls_fingerprints"]:
-                if u"sha256" in fingerprint:
-                    response_sha256_fingerprints.add(fingerprint[u"sha256"])
-
-            if sha256_fingerprint_b64 not in response_sha256_fingerprints:
-                raise KeyLookupError("TLS certificate not allowed by fingerprints")
-
             response_keys = yield self.process_v2_response(
                 from_server=server_name,
                 requested_ids=[requested_key_id],
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index 98722ae543..37d29e7027 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -369,13 +369,13 @@ class FederationServer(FederationBase):
         })
 
     @defer.inlineCallbacks
-    def on_invite_request(self, origin, content):
+    def on_invite_request(self, origin, content, room_version):
         pdu = event_from_pdu_json(content)
         origin_host, _ = parse_server_name(origin)
         yield self.check_server_matches_acl(origin_host, pdu.room_id)
         ret_pdu = yield self.handler.on_invite_request(origin, pdu)
         time_now = self._clock.time_msec()
-        defer.returnValue((200, {"event": ret_pdu.get_pdu_json(time_now)}))
+        defer.returnValue({"event": ret_pdu.get_pdu_json(time_now)})
 
     @defer.inlineCallbacks
     def on_send_join_request(self, origin, content):
diff --git a/synapse/federation/transport/server.py b/synapse/federation/transport/server.py
index e592269cf4..4557a9e66e 100644
--- a/synapse/federation/transport/server.py
+++ b/synapse/federation/transport/server.py
@@ -21,8 +21,9 @@ import re
 from twisted.internet import defer
 
 import synapse
+from synapse.api.constants import RoomVersions
 from synapse.api.errors import Codes, FederationDeniedError, SynapseError
-from synapse.api.urls import FEDERATION_V1_PREFIX
+from synapse.api.urls import FEDERATION_V1_PREFIX, FEDERATION_V2_PREFIX
 from synapse.http.endpoint import parse_and_validate_server_name
 from synapse.http.server import JsonResource
 from synapse.http.servlet import (
@@ -490,14 +491,46 @@ class FederationSendJoinServlet(BaseFederationServlet):
         defer.returnValue((200, content))
 
 
-class FederationInviteServlet(BaseFederationServlet):
+class FederationV1InviteServlet(BaseFederationServlet):
     PATH = "/invite/(?P<context>[^/]*)/(?P<event_id>[^/]*)"
 
     @defer.inlineCallbacks
     def on_PUT(self, origin, content, query, context, event_id):
+        # We don't get a room version, so we have to assume its EITHER v1 or
+        # v2. This is "fine" as the only difference between V1 and V2 is the
+        # state resolution algorithm, and we don't use that for processing
+        # invites
+        content = yield self.handler.on_invite_request(
+            origin, content, room_version=RoomVersions.V1,
+        )
+
+        # V1 federation API is defined to return a content of `[200, {...}]`
+        # due to a historical bug.
+        defer.returnValue((200, (200, content)))
+
+
+class FederationV2InviteServlet(BaseFederationServlet):
+    PATH = "/invite/(?P<context>[^/]*)/(?P<event_id>[^/]*)"
+
+    PREFIX = FEDERATION_V2_PREFIX
+
+    @defer.inlineCallbacks
+    def on_PUT(self, origin, content, query, context, event_id):
         # TODO(paul): assert that context/event_id parsed from path actually
         #   match those given in content
-        content = yield self.handler.on_invite_request(origin, content)
+
+        room_version = content["room_version"]
+        event = content["event"]
+        invite_room_state = content["invite_room_state"]
+
+        # Synapse expects invite_room_state to be in unsigned, as it is in v1
+        # API
+
+        event.setdefault("unsigned", {})["invite_room_state"] = invite_room_state
+
+        content = yield self.handler.on_invite_request(
+            origin, event, room_version=room_version,
+        )
         defer.returnValue((200, content))
 
 
@@ -1265,7 +1298,8 @@ FEDERATION_SERVLET_CLASSES = (
     FederationEventServlet,
     FederationSendJoinServlet,
     FederationSendLeaveServlet,
-    FederationInviteServlet,
+    FederationV1InviteServlet,
+    FederationV2InviteServlet,
     FederationQueryAuthServlet,
     FederationGetMissingEventsServlet,
     FederationEventAuthServlet,
diff --git a/synapse/http/endpoint.py b/synapse/http/endpoint.py
index 1c3b7ea28a..815f8ff2f7 100644
--- a/synapse/http/endpoint.py
+++ b/synapse/http/endpoint.py
@@ -12,29 +12,17 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-import collections
 import logging
 import random
 import re
-import time
 
 from twisted.internet import defer
 from twisted.internet.endpoints import HostnameEndpoint, wrapClientTLS
 from twisted.internet.error import ConnectError
-from twisted.names import client, dns
-from twisted.names.error import DNSNameError, DomainError
 
-logger = logging.getLogger(__name__)
-
-SERVER_CACHE = {}
+from synapse.http.federation.srv_resolver import Server, resolve_service
 
-# our record of an individual server which can be tried to reach a destination.
-#
-# "host" is the hostname acquired from the SRV record. Except when there's
-# no SRV record, in which case it is the original hostname.
-_Server = collections.namedtuple(
-    "_Server", "priority weight host port expires"
-)
+logger = logging.getLogger(__name__)
 
 
 def parse_server_name(server_name):
@@ -165,12 +153,9 @@ class SRVClientEndpoint(object):
         self.service_name = "_%s._%s.%s" % (service, protocol, domain)
 
         if default_port is not None:
-            self.default_server = _Server(
+            self.default_server = Server(
                 host=domain,
                 port=default_port,
-                priority=0,
-                weight=0,
-                expires=0,
             )
         else:
             self.default_server = None
@@ -240,57 +225,3 @@ class SRVClientEndpoint(object):
         )
         connection = yield endpoint.connect(protocolFactory)
         defer.returnValue(connection)
-
-
-@defer.inlineCallbacks
-def resolve_service(service_name, dns_client=client, cache=SERVER_CACHE, clock=time):
-    cache_entry = cache.get(service_name, None)
-    if cache_entry:
-        if all(s.expires > int(clock.time()) for s in cache_entry):
-            servers = list(cache_entry)
-            defer.returnValue(servers)
-
-    servers = []
-
-    try:
-        try:
-            answers, _, _ = yield dns_client.lookupService(service_name)
-        except DNSNameError:
-            defer.returnValue([])
-
-        if (len(answers) == 1
-                and answers[0].type == dns.SRV
-                and answers[0].payload
-                and answers[0].payload.target == dns.Name(b'.')):
-            raise ConnectError("Service %s unavailable" % service_name)
-
-        for answer in answers:
-            if answer.type != dns.SRV or not answer.payload:
-                continue
-
-            payload = answer.payload
-
-            servers.append(_Server(
-                host=str(payload.target),
-                port=int(payload.port),
-                priority=int(payload.priority),
-                weight=int(payload.weight),
-                expires=int(clock.time()) + answer.ttl,
-            ))
-
-        servers.sort()
-        cache[service_name] = list(servers)
-    except DomainError as e:
-        # We failed to resolve the name (other than a NameError)
-        # Try something in the cache, else rereaise
-        cache_entry = cache.get(service_name, None)
-        if cache_entry:
-            logger.warn(
-                "Failed to resolve %r, falling back to cache. %r",
-                service_name, e
-            )
-            servers = list(cache_entry)
-        else:
-            raise e
-
-    defer.returnValue(servers)
diff --git a/synapse/http/federation/__init__.py b/synapse/http/federation/__init__.py
new file mode 100644
index 0000000000..1453d04571
--- /dev/null
+++ b/synapse/http/federation/__init__.py
@@ -0,0 +1,14 @@
+# -*- coding: utf-8 -*-
+# Copyright 2019 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
diff --git a/synapse/http/federation/srv_resolver.py b/synapse/http/federation/srv_resolver.py
new file mode 100644
index 0000000000..c49b82c394
--- /dev/null
+++ b/synapse/http/federation/srv_resolver.py
@@ -0,0 +1,124 @@
+# -*- coding: utf-8 -*-
+# Copyright 2014-2016 OpenMarket Ltd
+# Copyright 2019 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import logging
+import time
+
+import attr
+
+from twisted.internet import defer
+from twisted.internet.error import ConnectError
+from twisted.names import client, dns
+from twisted.names.error import DNSNameError, DomainError
+
+from synapse.util.logcontext import make_deferred_yieldable
+
+logger = logging.getLogger(__name__)
+
+SERVER_CACHE = {}
+
+
+@attr.s
+class Server(object):
+    """
+    Our record of an individual server which can be tried to reach a destination.
+
+    Attributes:
+        host (bytes): target hostname
+        port (int):
+        priority (int):
+        weight (int):
+        expires (int): when the cache should expire this record - in *seconds* since
+            the epoch
+    """
+    host = attr.ib()
+    port = attr.ib()
+    priority = attr.ib(default=0)
+    weight = attr.ib(default=0)
+    expires = attr.ib(default=0)
+
+
+@defer.inlineCallbacks
+def resolve_service(service_name, dns_client=client, cache=SERVER_CACHE, clock=time):
+    """Look up a SRV record, with caching
+
+    The default resolver in twisted.names doesn't do any caching (it has a CacheResolver,
+    but the cache never gets populated), so we add our own caching layer here.
+
+    Args:
+        service_name (unicode|bytes): record to look up
+        dns_client (twisted.internet.interfaces.IResolver): twisted resolver impl
+        cache (dict): cache object
+        clock (object): clock implementation. must provide a time() method.
+
+    Returns:
+        Deferred[list[Server]]: a list of the SRV records, or an empty list if none found
+    """
+    # TODO: the dns client handles both unicode names (encoding via idna) and pre-encoded
+    # byteses; however they will obviously end up as separate entries in the cache. We
+    # should pick one form and stick with it.
+    cache_entry = cache.get(service_name, None)
+    if cache_entry:
+        if all(s.expires > int(clock.time()) for s in cache_entry):
+            servers = list(cache_entry)
+            defer.returnValue(servers)
+
+    try:
+        answers, _, _ = yield make_deferred_yieldable(
+            dns_client.lookupService(service_name),
+        )
+    except DNSNameError:
+        # TODO: cache this. We can get the SOA out of the exception, and use
+        # the negative-TTL value.
+        defer.returnValue([])
+    except DomainError as e:
+        # We failed to resolve the name (other than a NameError)
+        # Try something in the cache, else rereaise
+        cache_entry = cache.get(service_name, None)
+        if cache_entry:
+            logger.warn(
+                "Failed to resolve %r, falling back to cache. %r",
+                service_name, e
+            )
+            defer.returnValue(list(cache_entry))
+        else:
+            raise e
+
+    if (len(answers) == 1
+            and answers[0].type == dns.SRV
+            and answers[0].payload
+            and answers[0].payload.target == dns.Name(b'.')):
+        raise ConnectError("Service %s unavailable" % service_name)
+
+    servers = []
+
+    for answer in answers:
+        if answer.type != dns.SRV or not answer.payload:
+            continue
+
+        payload = answer.payload
+
+        servers.append(Server(
+            host=payload.target.name,
+            port=payload.port,
+            priority=payload.priority,
+            weight=payload.weight,
+            expires=int(clock.time()) + answer.ttl,
+        ))
+
+    servers.sort()  # FIXME: get rid of this (it's broken by the attrs change)
+    cache[service_name] = list(servers)
+    defer.returnValue(servers)