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Do an AAAA lookup on SRV record targets (#2462)
Support SRV records which point at AAAA records, as well as A records. Fixes https://github.com/matrix-org/synapse/issues/2405
This commit is contained in:
Richard van der Hoff
GitHub
parent
f496399ac4
commit
f65e31d22f
@ -12,6 +12,7 @@
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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import socket
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from twisted.internet.endpoints import HostnameEndpoint, wrapClientTLS
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from twisted.internet import defer, reactor
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@ -30,7 +31,10 @@ logger = logging.getLogger(__name__)
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SERVER_CACHE = {}
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# our record of an individual server which can be tried to reach a destination.
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#
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# "host" is actually a dotted-quad or ipv6 address string. Except when there's
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# no SRV record, in which case it is the original hostname.
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_Server = collections.namedtuple(
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"_Server", "priority weight host port expires"
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)
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@ -219,9 +223,10 @@ class SRVClientEndpoint(object):
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return self.default_server
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else:
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raise ConnectError(
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"Not server available for %s" % self.service_name
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"No server available for %s" % self.service_name
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)
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# look for all servers with the same priority
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min_priority = self.servers[0].priority
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weight_indexes = list(
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(index, server.weight + 1)
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@ -231,11 +236,22 @@ class SRVClientEndpoint(object):
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total_weight = sum(weight for index, weight in weight_indexes)
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target_weight = random.randint(0, total_weight)
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for index, weight in weight_indexes:
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target_weight -= weight
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if target_weight <= 0:
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server = self.servers[index]
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# XXX: this looks totally dubious:
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#
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# (a) we never reuse a server until we have been through
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# all of the servers at the same priority, so if the
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# weights are A: 100, B:1, we always do ABABAB instead of
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# AAAA...AAAB (approximately).
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#
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# (b) After using all the servers at the lowest priority,
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# we move onto the next priority. We should only use the
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# second priority if servers at the top priority are
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# unreachable.
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#
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del self.servers[index]
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self.used_servers.append(server)
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return server
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@ -280,26 +296,21 @@ def resolve_service(service_name, dns_client=client, cache=SERVER_CACHE, clock=t
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continue
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payload = answer.payload
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host = str(payload.target)
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srv_ttl = answer.ttl
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try:
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answers, _, _ = yield dns_client.lookupAddress(host)
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except DNSNameError:
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continue
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hosts = yield _get_hosts_for_srv_record(
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dns_client, str(payload.target)
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)
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for answer in answers:
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if answer.type == dns.A and answer.payload:
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ip = answer.payload.dottedQuad()
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host_ttl = min(srv_ttl, answer.ttl)
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for (ip, ttl) in hosts:
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host_ttl = min(answer.ttl, ttl)
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servers.append(_Server(
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host=ip,
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port=int(payload.port),
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priority=int(payload.priority),
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weight=int(payload.weight),
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expires=int(clock.time()) + host_ttl,
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))
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servers.append(_Server(
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host=ip,
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port=int(payload.port),
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priority=int(payload.priority),
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weight=int(payload.weight),
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expires=int(clock.time()) + host_ttl,
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))
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servers.sort()
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cache[service_name] = list(servers)
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@ -317,3 +328,68 @@ def resolve_service(service_name, dns_client=client, cache=SERVER_CACHE, clock=t
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raise e
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defer.returnValue(servers)
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@defer.inlineCallbacks
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def _get_hosts_for_srv_record(dns_client, host):
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"""Look up each of the hosts in a SRV record
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Args:
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dns_client (twisted.names.dns.IResolver):
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host (basestring): host to look up
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Returns:
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Deferred[list[(str, int)]]: a list of (host, ttl) pairs
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"""
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ip4_servers = []
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ip6_servers = []
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def cb(res):
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# lookupAddress and lookupIP6Address return a three-tuple
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# giving the answer, authority, and additional sections of the
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# response.
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#
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# we only care about the answers.
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return res[0]
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def eb(res):
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res.trap(DNSNameError)
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return []
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# no logcontexts here, so we can safely fire these off and gatherResults
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d1 = dns_client.lookupAddress(host).addCallbacks(cb, eb)
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d2 = dns_client.lookupIPV6Address(host).addCallbacks(cb, eb)
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results = yield defer.gatherResults([d1, d2], consumeErrors=True)
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for result in results:
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for answer in result:
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if not answer.payload:
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continue
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try:
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if answer.type == dns.A:
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ip = answer.payload.dottedQuad()
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ip4_servers.append((ip, answer.ttl))
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elif answer.type == dns.AAAA:
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ip = socket.inet_ntop(
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socket.AF_INET6, answer.payload.address,
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)
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ip6_servers.append((ip, answer.ttl))
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else:
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# the most likely candidate here is a CNAME record.
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# rfc2782 says srvs may not point to aliases.
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logger.warn(
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"Ignoring unexpected DNS record type %s for %s",
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answer.type, host,
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)
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continue
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except Exception as e:
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logger.warn("Ignoring invalid DNS response for %s: %s",
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host, e)
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continue
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# keep the ipv4 results before the ipv6 results, mostly to match historical
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# behaviour.
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defer.returnValue(ip4_servers + ip6_servers)
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@ -24,15 +24,17 @@ from synapse.http.endpoint import resolve_service
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from tests.utils import MockClock
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@unittest.DEBUG
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class DnsTestCase(unittest.TestCase):
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@defer.inlineCallbacks
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def test_resolve(self):
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dns_client_mock = Mock()
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service_name = "test_service.examle.com"
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service_name = "test_service.example.com"
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host_name = "example.com"
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ip_address = "127.0.0.1"
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ip6_address = "::1"
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answer_srv = dns.RRHeader(
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type=dns.SRV,
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@ -48,8 +50,22 @@ class DnsTestCase(unittest.TestCase):
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)
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)
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dns_client_mock.lookupService.return_value = ([answer_srv], None, None)
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dns_client_mock.lookupAddress.return_value = ([answer_a], None, None)
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answer_aaaa = dns.RRHeader(
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type=dns.AAAA,
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payload=dns.Record_AAAA(
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address=ip6_address,
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)
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)
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dns_client_mock.lookupService.return_value = defer.succeed(
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([answer_srv], None, None),
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)
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dns_client_mock.lookupAddress.return_value = defer.succeed(
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([answer_a], None, None),
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)
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dns_client_mock.lookupIPV6Address.return_value = defer.succeed(
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([answer_aaaa], None, None),
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)
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cache = {}
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@ -59,10 +75,12 @@ class DnsTestCase(unittest.TestCase):
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dns_client_mock.lookupService.assert_called_once_with(service_name)
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dns_client_mock.lookupAddress.assert_called_once_with(host_name)
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dns_client_mock.lookupIPV6Address.assert_called_once_with(host_name)
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self.assertEquals(len(servers), 1)
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self.assertEquals(len(servers), 2)
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self.assertEquals(servers, cache[service_name])
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self.assertEquals(servers[0].host, ip_address)
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self.assertEquals(servers[1].host, ip6_address)
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@defer.inlineCallbacks
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def test_from_cache_expired_and_dns_fail(self):
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