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)
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