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# -*- test-case-name: twisted.names.test.test_srvconnect -*-
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
import random
from zope.interface import implementer
from twisted.internet import error, interfaces
from twisted.names import client, dns
from twisted.names.error import DNSNameError
from twisted.python.compat import nativeString
class _SRVConnector_ClientFactoryWrapper:
def __init__(self, connector, wrappedFactory):
self.__connector = connector
self.__wrappedFactory = wrappedFactory
def startedConnecting(self, connector):
self.__wrappedFactory.startedConnecting(self.__connector)
def clientConnectionFailed(self, connector, reason):
self.__connector.connectionFailed(reason)
def clientConnectionLost(self, connector, reason):
self.__connector.connectionLost(reason)
def __getattr__(self, key):
return getattr(self.__wrappedFactory, key)
@implementer(interfaces.IConnector)
class SRVConnector:
"""
A connector that looks up DNS SRV records.
RFC 2782 details how SRV records should be interpreted and selected
for subsequent connection attempts. The algorithm for using the records'
priority and weight is implemented in L{pickServer}.
@ivar servers: List of candidate server records for future connection
attempts.
@type servers: L{list} of L{dns.Record_SRV}
@ivar orderedServers: List of server records that have already been tried
in this round of connection attempts.
@type orderedServers: L{list} of L{dns.Record_SRV}
"""
stopAfterDNS = 0
def __init__(
self,
reactor,
service,
domain,
factory,
protocol="tcp",
connectFuncName="connectTCP",
connectFuncArgs=(),
connectFuncKwArgs={},
defaultPort=None,
):
"""
@param domain: The domain to connect to. If passed as a text
string, it will be encoded using C{idna} encoding.
@type domain: L{bytes} or L{str}
@param defaultPort: Optional default port number to be used when SRV
lookup fails and the service name is unknown. This should be the
port number associated with the service name as defined by the IANA
registry.
@type defaultPort: L{int}
"""
self.reactor = reactor
self.service = service
self.domain = None if domain is None else dns.domainString(domain)
self.factory = factory
self.protocol = protocol
self.connectFuncName = connectFuncName
self.connectFuncArgs = connectFuncArgs
self.connectFuncKwArgs = connectFuncKwArgs
self._defaultPort = defaultPort
self.connector = None
self.servers = None
# list of servers already used in this round:
self.orderedServers = None
def connect(self):
"""Start connection to remote server."""
self.factory.doStart()
self.factory.startedConnecting(self)
if not self.servers:
if self.domain is None:
self.connectionFailed(
error.DNSLookupError("Domain is not defined."),
)
return
d = client.lookupService(
"_%s._%s.%s"
% (
nativeString(self.service),
nativeString(self.protocol),
nativeString(self.domain),
),
)
d.addCallbacks(self._cbGotServers, self._ebGotServers)
d.addCallback(lambda x, self=self: self._reallyConnect())
if self._defaultPort:
d.addErrback(self._ebServiceUnknown)
d.addErrback(self.connectionFailed)
elif self.connector is None:
self._reallyConnect()
else:
self.connector.connect()
def _ebGotServers(self, failure):
failure.trap(DNSNameError)
# Some DNS servers reply with NXDOMAIN when in fact there are
# just no SRV records for that domain. Act as if we just got an
# empty response and use fallback.
self.servers = []
self.orderedServers = []
def _cbGotServers(self, result):
answers, auth, add = result
if (
len(answers) == 1
and answers[0].type == dns.SRV
and answers[0].payload
and answers[0].payload.target == dns.Name(b".")
):
# decidedly not available
raise error.DNSLookupError(
"Service %s not available for domain %s."
% (repr(self.service), repr(self.domain))
)
self.servers = []
self.orderedServers = []
for a in answers:
if a.type != dns.SRV or not a.payload:
continue
self.orderedServers.append(a.payload)
def _ebServiceUnknown(self, failure):
"""
Connect to the default port when the service name is unknown.
If no SRV records were found, the service name will be passed as the
port. If resolving the name fails with
L{error.ServiceNameUnknownError}, a final attempt is done using the
default port.
"""
failure.trap(error.ServiceNameUnknownError)
self.servers = [dns.Record_SRV(0, 0, self._defaultPort, self.domain)]
self.orderedServers = []
self.connect()
def pickServer(self):
"""
Pick the next server.
This selects the next server from the list of SRV records according
to their priority and weight values, as set out by the default
algorithm specified in RFC 2782.
At the beginning of a round, L{servers} is populated with
L{orderedServers}, and the latter is made empty. L{servers}
is the list of candidates, and L{orderedServers} is the list of servers
that have already been tried.
First, all records are ordered by priority and weight in ascending
order. Then for each priority level, a running sum is calculated
over the sorted list of records for that priority. Then a random value
between 0 and the final sum is compared to each record in order. The
first record that is greater than or equal to that random value is
chosen and removed from the list of candidates for this round.
@return: A tuple of target hostname and port from the chosen DNS SRV
record.
@rtype: L{tuple} of native L{str} and L{int}
"""
assert self.servers is not None
assert self.orderedServers is not None
if not self.servers and not self.orderedServers:
# no SRV record, fall back..
return nativeString(self.domain), self.service
if not self.servers and self.orderedServers:
# start new round
self.servers = self.orderedServers
self.orderedServers = []
assert self.servers
self.servers.sort(key=lambda record: (record.priority, record.weight))
minPriority = self.servers[0].priority
index = 0
weightSum = 0
weightIndex = []
for x in self.servers:
if x.priority == minPriority:
weightSum += x.weight
weightIndex.append((index, weightSum))
index += 1
rand = random.randint(0, weightSum)
for index, weight in weightIndex:
if weight >= rand:
chosen = self.servers[index]
del self.servers[index]
self.orderedServers.append(chosen)
return str(chosen.target), chosen.port
raise RuntimeError(f"Impossible {self.__class__.__name__} pickServer result.")
def _reallyConnect(self):
if self.stopAfterDNS:
self.stopAfterDNS = 0
return
self.host, self.port = self.pickServer()
assert self.host is not None, "Must have a host to connect to."
assert self.port is not None, "Must have a port to connect to."
connectFunc = getattr(self.reactor, self.connectFuncName)
self.connector = connectFunc(
self.host,
self.port,
_SRVConnector_ClientFactoryWrapper(self, self.factory),
*self.connectFuncArgs,
**self.connectFuncKwArgs,
)
def stopConnecting(self):
"""Stop attempting to connect."""
if self.connector:
self.connector.stopConnecting()
else:
self.stopAfterDNS = 1
def disconnect(self):
"""Disconnect whatever our are state is."""
if self.connector is not None:
self.connector.disconnect()
else:
self.stopConnecting()
def getDestination(self):
assert self.connector
return self.connector.getDestination()
def connectionFailed(self, reason):
self.factory.clientConnectionFailed(self, reason)
self.factory.doStop()
def connectionLost(self, reason):
self.factory.clientConnectionLost(self, reason)
self.factory.doStop()