diff --git a/synapse/http/federation/well_known_resolver.py b/synapse/http/federation/well_known_resolver.py
new file mode 100644
index 0000000000..bab4ab015e
--- /dev/null
+++ b/synapse/http/federation/well_known_resolver.py
@@ -0,0 +1,184 @@
+# -*- coding: utf-8 -*-
+# Copyright 2019 The Matrix.org Foundation C.I.C.
+#
+# 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 json
+import logging
+import random
+import time
+
+import attr
+
+from twisted.internet import defer
+from twisted.web.client import RedirectAgent, readBody
+from twisted.web.http import stringToDatetime
+
+from synapse.logging.context import make_deferred_yieldable
+from synapse.util import Clock
+from synapse.util.caches.ttlcache import TTLCache
+from synapse.util.metrics import Measure
+
+# period to cache .well-known results for by default
+WELL_KNOWN_DEFAULT_CACHE_PERIOD = 24 * 3600
+
+# jitter to add to the .well-known default cache ttl
+WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER = 10 * 60
+
+# period to cache failure to fetch .well-known for
+WELL_KNOWN_INVALID_CACHE_PERIOD = 1 * 3600
+
+# cap for .well-known cache period
+WELL_KNOWN_MAX_CACHE_PERIOD = 48 * 3600
+
+# lower bound for .well-known cache period
+WELL_KNOWN_MIN_CACHE_PERIOD = 5 * 60
+
+logger = logging.getLogger(__name__)
+
+
+@attr.s(slots=True, frozen=True)
+class WellKnownLookupResult(object):
+ delegated_server = attr.ib()
+
+
+class WellKnownResolver(object):
+ """Handles well-known lookups for matrix servers.
+ """
+
+ def __init__(self, reactor, agent, well_known_cache=None):
+ self._reactor = reactor
+ self._clock = Clock(reactor)
+
+ if well_known_cache is None:
+ well_known_cache = TTLCache("well-known")
+
+ self._well_known_cache = well_known_cache
+ self._well_known_agent = RedirectAgent(agent)
+
+ @defer.inlineCallbacks
+ def get_well_known(self, server_name):
+ """Attempt to fetch and parse a .well-known file for the given server
+
+ Args:
+ server_name (bytes): name of the server, from the requested url
+
+ Returns:
+ Deferred[WellKnownLookupResult]: The result of the lookup
+ """
+ try:
+ result = self._well_known_cache[server_name]
+ except KeyError:
+ # TODO: should we linearise so that we don't end up doing two .well-known
+ # requests for the same server in parallel?
+ with Measure(self._clock, "get_well_known"):
+ result, cache_period = yield self._do_get_well_known(server_name)
+
+ if cache_period > 0:
+ self._well_known_cache.set(server_name, result, cache_period)
+
+ return WellKnownLookupResult(delegated_server=result)
+
+ @defer.inlineCallbacks
+ def _do_get_well_known(self, server_name):
+ """Actually fetch and parse a .well-known, without checking the cache
+
+ Args:
+ server_name (bytes): name of the server, from the requested url
+
+ Returns:
+ Deferred[Tuple[bytes|None|object],int]:
+ result, cache period, where result is one of:
+ - the new server name from the .well-known (as a `bytes`)
+ - None if there was no .well-known file.
+ - INVALID_WELL_KNOWN if the .well-known was invalid
+ """
+ uri = b"https://%s/.well-known/matrix/server" % (server_name,)
+ uri_str = uri.decode("ascii")
+ logger.info("Fetching %s", uri_str)
+ try:
+ response = yield make_deferred_yieldable(
+ self._well_known_agent.request(b"GET", uri)
+ )
+ body = yield make_deferred_yieldable(readBody(response))
+ if response.code != 200:
+ raise Exception("Non-200 response %s" % (response.code,))
+
+ parsed_body = json.loads(body.decode("utf-8"))
+ logger.info("Response from .well-known: %s", parsed_body)
+ if not isinstance(parsed_body, dict):
+ raise Exception("not a dict")
+ if "m.server" not in parsed_body:
+ raise Exception("Missing key 'm.server'")
+ except Exception as e:
+ logger.info("Error fetching %s: %s", uri_str, e)
+
+ # add some randomness to the TTL to avoid a stampeding herd every hour
+ # after startup
+ cache_period = WELL_KNOWN_INVALID_CACHE_PERIOD
+ cache_period += random.uniform(0, WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER)
+ return (None, cache_period)
+
+ result = parsed_body["m.server"].encode("ascii")
+
+ cache_period = _cache_period_from_headers(
+ response.headers, time_now=self._reactor.seconds
+ )
+ if cache_period is None:
+ cache_period = WELL_KNOWN_DEFAULT_CACHE_PERIOD
+ # add some randomness to the TTL to avoid a stampeding herd every 24 hours
+ # after startup
+ cache_period += random.uniform(0, WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER)
+ else:
+ cache_period = min(cache_period, WELL_KNOWN_MAX_CACHE_PERIOD)
+ cache_period = max(cache_period, WELL_KNOWN_MIN_CACHE_PERIOD)
+
+ return (result, cache_period)
+
+
+def _cache_period_from_headers(headers, time_now=time.time):
+ cache_controls = _parse_cache_control(headers)
+
+ if b"no-store" in cache_controls:
+ return 0
+
+ if b"max-age" in cache_controls:
+ try:
+ max_age = int(cache_controls[b"max-age"])
+ return max_age
+ except ValueError:
+ pass
+
+ expires = headers.getRawHeaders(b"expires")
+ if expires is not None:
+ try:
+ expires_date = stringToDatetime(expires[-1])
+ return expires_date - time_now()
+ except ValueError:
+ # RFC7234 says 'A cache recipient MUST interpret invalid date formats,
+ # especially the value "0", as representing a time in the past (i.e.,
+ # "already expired").
+ return 0
+
+ return None
+
+
+def _parse_cache_control(headers):
+ cache_controls = {}
+ for hdr in headers.getRawHeaders(b"cache-control", []):
+ for directive in hdr.split(b","):
+ splits = [x.strip() for x in directive.split(b"=", 1)]
+ k = splits[0].lower()
+ v = splits[1] if len(splits) > 1 else None
+ cache_controls[k] = v
+ return cache_controls
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