track embedded python distribution

1 files changed, 714 insertions, 0 deletions

diff --git a/python/gevent/_sslgte279.py b/python/gevent/_sslgte279.py
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+# Wrapper module for _ssl. Written by Bill Janssen.
+# Ported to gevent by Denis Bilenko.
+"""SSL wrapper for socket objects on Python 2.7.9 and above.
+
+For the documentation, refer to :mod:`ssl` module manual.
+
+This module implements cooperative SSL socket wrappers.
+"""
+
+from __future__ import absolute_import
+# Our import magic sadly makes this warning useless
+# pylint: disable=undefined-variable
+# pylint: disable=too-many-instance-attributes,too-many-locals,too-many-statements,too-many-branches
+# pylint: disable=arguments-differ,too-many-public-methods
+
+import ssl as __ssl__
+
+_ssl = __ssl__._ssl # pylint:disable=no-member
+
+import errno
+from gevent._socket2 import socket
+from gevent.socket import timeout_default
+from gevent.socket import create_connection
+from gevent.socket import error as socket_error
+from gevent.socket import timeout as _socket_timeout
+from gevent._compat import PYPY
+from gevent._util import copy_globals
+
+__implements__ = [
+    'SSLContext',
+    'SSLSocket',
+    'wrap_socket',
+    'get_server_certificate',
+    'create_default_context',
+    '_create_unverified_context',
+    '_create_default_https_context',
+    '_create_stdlib_context',
+]
+
+# Import all symbols from Python's ssl.py, except those that we are implementing
+# and "private" symbols.
+__imports__ = copy_globals(__ssl__, globals(),
+                           # SSLSocket *must* subclass gevent.socket.socket; see issue 597 and 801
+                           names_to_ignore=__implements__ + ['socket', 'create_connection'],
+                           dunder_names_to_keep=())
+
+try:
+    _delegate_methods
+except NameError: # PyPy doesn't expose this detail
+    _delegate_methods = ('recv', 'recvfrom', 'recv_into', 'recvfrom_into', 'send', 'sendto')
+
+__all__ = __implements__ + __imports__
+if 'namedtuple' in __all__:
+    __all__.remove('namedtuple')
+
+orig_SSLContext = __ssl__.SSLContext # pylint: disable=no-member
+
+
+class SSLContext(orig_SSLContext):
+    def wrap_socket(self, sock, server_side=False,
+                    do_handshake_on_connect=True,
+                    suppress_ragged_eofs=True,
+                    server_hostname=None):
+        return SSLSocket(sock=sock, server_side=server_side,
+                         do_handshake_on_connect=do_handshake_on_connect,
+                         suppress_ragged_eofs=suppress_ragged_eofs,
+                         server_hostname=server_hostname,
+                         _context=self)
+
+
+def create_default_context(purpose=Purpose.SERVER_AUTH, cafile=None,
+                           capath=None, cadata=None):
+    """Create a SSLContext object with default settings.
+
+    NOTE: The protocol and settings may change anytime without prior
+          deprecation. The values represent a fair balance between maximum
+          compatibility and security.
+    """
+    if not isinstance(purpose, _ASN1Object):
+        raise TypeError(purpose)
+
+    context = SSLContext(PROTOCOL_SSLv23)
+
+    # SSLv2 considered harmful.
+    context.options |= OP_NO_SSLv2
+
+    # SSLv3 has problematic security and is only required for really old
+    # clients such as IE6 on Windows XP
+    context.options |= OP_NO_SSLv3
+
+    # disable compression to prevent CRIME attacks (OpenSSL 1.0+)
+    context.options |= getattr(_ssl, "OP_NO_COMPRESSION", 0)
+
+    if purpose == Purpose.SERVER_AUTH:
+        # verify certs and host name in client mode
+        context.verify_mode = CERT_REQUIRED
+        context.check_hostname = True # pylint: disable=attribute-defined-outside-init
+    elif purpose == Purpose.CLIENT_AUTH:
+        # Prefer the server's ciphers by default so that we get stronger
+        # encryption
+        context.options |= getattr(_ssl, "OP_CIPHER_SERVER_PREFERENCE", 0)
+
+        # Use single use keys in order to improve forward secrecy
+        context.options |= getattr(_ssl, "OP_SINGLE_DH_USE", 0)
+        context.options |= getattr(_ssl, "OP_SINGLE_ECDH_USE", 0)
+
+        # disallow ciphers with known vulnerabilities
+        context.set_ciphers(_RESTRICTED_SERVER_CIPHERS)
+
+    if cafile or capath or cadata:
+        context.load_verify_locations(cafile, capath, cadata)
+    elif context.verify_mode != CERT_NONE:
+        # no explicit cafile, capath or cadata but the verify mode is
+        # CERT_OPTIONAL or CERT_REQUIRED. Let's try to load default system
+        # root CA certificates for the given purpose. This may fail silently.
+        context.load_default_certs(purpose)
+    return context
+
+def _create_unverified_context(protocol=PROTOCOL_SSLv23, cert_reqs=None,
+                               check_hostname=False, purpose=Purpose.SERVER_AUTH,
+                               certfile=None, keyfile=None,
+                               cafile=None, capath=None, cadata=None):
+    """Create a SSLContext object for Python stdlib modules
+
+    All Python stdlib modules shall use this function to create SSLContext
+    objects in order to keep common settings in one place. The configuration
+    is less restrict than create_default_context()'s to increase backward
+    compatibility.
+    """
+    if not isinstance(purpose, _ASN1Object):
+        raise TypeError(purpose)
+
+    context = SSLContext(protocol)
+    # SSLv2 considered harmful.
+    context.options |= OP_NO_SSLv2
+    # SSLv3 has problematic security and is only required for really old
+    # clients such as IE6 on Windows XP
+    context.options |= OP_NO_SSLv3
+
+    if cert_reqs is not None:
+        context.verify_mode = cert_reqs
+    context.check_hostname = check_hostname # pylint: disable=attribute-defined-outside-init
+
+    if keyfile and not certfile:
+        raise ValueError("certfile must be specified")
+    if certfile or keyfile:
+        context.load_cert_chain(certfile, keyfile)
+
+    # load CA root certs
+    if cafile or capath or cadata:
+        context.load_verify_locations(cafile, capath, cadata)
+    elif context.verify_mode != CERT_NONE:
+        # no explicit cafile, capath or cadata but the verify mode is
+        # CERT_OPTIONAL or CERT_REQUIRED. Let's try to load default system
+        # root CA certificates for the given purpose. This may fail silently.
+        context.load_default_certs(purpose)
+
+    return context
+
+# Used by http.client if no context is explicitly passed.
+_create_default_https_context = create_default_context
+
+
+# Backwards compatibility alias, even though it's not a public name.
+_create_stdlib_context = _create_unverified_context
+
+class SSLSocket(socket):
+    """
+    gevent `ssl.SSLSocket <https://docs.python.org/2/library/ssl.html#ssl-sockets>`_
+    for Pythons >= 2.7.9 but less than 3.
+    """
+
+    def __init__(self, sock=None, keyfile=None, certfile=None,
+                 server_side=False, cert_reqs=CERT_NONE,
+                 ssl_version=PROTOCOL_SSLv23, ca_certs=None,
+                 do_handshake_on_connect=True,
+                 family=AF_INET, type=SOCK_STREAM, proto=0, fileno=None,
+                 suppress_ragged_eofs=True, npn_protocols=None, ciphers=None,
+                 server_hostname=None,
+                 _context=None):
+        # fileno is ignored
+        # pylint: disable=unused-argument
+        if _context:
+            self._context = _context
+        else:
+            if server_side and not certfile:
+                raise ValueError("certfile must be specified for server-side "
+                                 "operations")
+            if keyfile and not certfile:
+                raise ValueError("certfile must be specified")
+            if certfile and not keyfile:
+                keyfile = certfile
+            self._context = SSLContext(ssl_version)
+            self._context.verify_mode = cert_reqs
+            if ca_certs:
+                self._context.load_verify_locations(ca_certs)
+            if certfile:
+                self._context.load_cert_chain(certfile, keyfile)
+            if npn_protocols:
+                self._context.set_npn_protocols(npn_protocols)
+            if ciphers:
+                self._context.set_ciphers(ciphers)
+            self.keyfile = keyfile
+            self.certfile = certfile
+            self.cert_reqs = cert_reqs
+            self.ssl_version = ssl_version
+            self.ca_certs = ca_certs
+            self.ciphers = ciphers
+        # Can't use sock.type as other flags (such as SOCK_NONBLOCK) get
+        # mixed in.
+        if sock.getsockopt(SOL_SOCKET, SO_TYPE) != SOCK_STREAM:
+            raise NotImplementedError("only stream sockets are supported")
+
+        if PYPY:
+            socket.__init__(self, _sock=sock)
+            sock._drop()
+        else:
+            # CPython: XXX: Must pass the underlying socket, not our
+            # potential wrapper; test___example_servers fails the SSL test
+            # with a client-side EOF error. (Why?)
+            socket.__init__(self, _sock=sock._sock)
+
+        # The initializer for socket overrides the methods send(), recv(), etc.
+        # in the instance, which we don't need -- but we want to provide the
+        # methods defined in SSLSocket.
+        for attr in _delegate_methods:
+            try:
+                delattr(self, attr)
+            except AttributeError:
+                pass
+        if server_side and server_hostname:
+            raise ValueError("server_hostname can only be specified "
+                             "in client mode")
+        if self._context.check_hostname and not server_hostname:
+            raise ValueError("check_hostname requires server_hostname")
+        self.server_side = server_side
+        self.server_hostname = server_hostname
+        self.do_handshake_on_connect = do_handshake_on_connect
+        self.suppress_ragged_eofs = suppress_ragged_eofs
+        self.settimeout(sock.gettimeout())
+
+        # See if we are connected
+        try:
+            self.getpeername()
+        except socket_error as e:
+            if e.errno != errno.ENOTCONN:
+                raise
+            connected = False
+        else:
+            connected = True
+
+        self._makefile_refs = 0
+        self._closed = False
+        self._sslobj = None
+        self._connected = connected
+        if connected:
+            # create the SSL object
+            try:
+                self._sslobj = self._context._wrap_socket(self._sock, server_side,
+                                                          server_hostname, ssl_sock=self)
+                if do_handshake_on_connect:
+                    timeout = self.gettimeout()
+                    if timeout == 0.0:
+                        # non-blocking
+                        raise ValueError("do_handshake_on_connect should not be specified for non-blocking sockets")
+                    self.do_handshake()
+
+            except socket_error as x:
+                self.close()
+                raise x
+
+
+    @property
+    def context(self):
+        return self._context
+
+    @context.setter
+    def context(self, ctx):
+        self._context = ctx
+        self._sslobj.context = ctx
+
+    def dup(self):
+        raise NotImplementedError("Can't dup() %s instances" %
+                                  self.__class__.__name__)
+
+    def _checkClosed(self, msg=None):
+        # raise an exception here if you wish to check for spurious closes
+        pass
+
+    def _check_connected(self):
+        if not self._connected:
+            # getpeername() will raise ENOTCONN if the socket is really
+            # not connected; note that we can be connected even without
+            # _connected being set, e.g. if connect() first returned
+            # EAGAIN.
+            self.getpeername()
+
+    def read(self, len=1024, buffer=None):
+        """Read up to LEN bytes and return them.
+        Return zero-length string on EOF."""
+        self._checkClosed()
+
+        while 1:
+            if not self._sslobj:
+                raise ValueError("Read on closed or unwrapped SSL socket.")
+            if len == 0:
+                return b'' if buffer is None else 0
+            if len < 0 and buffer is None:
+                # This is handled natively in python 2.7.12+
+                raise ValueError("Negative read length")
+            try:
+                if buffer is not None:
+                    return self._sslobj.read(len, buffer)
+                return self._sslobj.read(len or 1024)
+            except SSLWantReadError:
+                if self.timeout == 0.0:
+                    raise
+                self._wait(self._read_event, timeout_exc=_SSLErrorReadTimeout)
+            except SSLWantWriteError:
+                if self.timeout == 0.0:
+                    raise
+                # note: using _SSLErrorReadTimeout rather than _SSLErrorWriteTimeout below is intentional
+                self._wait(self._write_event, timeout_exc=_SSLErrorReadTimeout)
+            except SSLError as ex:
+                if ex.args[0] == SSL_ERROR_EOF and self.suppress_ragged_eofs:
+                    if buffer is not None:
+                        return 0
+                    return b''
+                else:
+                    raise
+
+    def write(self, data):
+        """Write DATA to the underlying SSL channel.  Returns
+        number of bytes of DATA actually transmitted."""
+        self._checkClosed()
+
+        while 1:
+            if not self._sslobj:
+                raise ValueError("Write on closed or unwrapped SSL socket.")
+
+            try:
+                return self._sslobj.write(data)
+            except SSLError as ex:
+                if ex.args[0] == SSL_ERROR_WANT_READ:
+                    if self.timeout == 0.0:
+                        raise
+                    self._wait(self._read_event, timeout_exc=_SSLErrorWriteTimeout)
+                elif ex.args[0] == SSL_ERROR_WANT_WRITE:
+                    if self.timeout == 0.0:
+                        raise
+                    self._wait(self._write_event, timeout_exc=_SSLErrorWriteTimeout)
+                else:
+                    raise
+
+    def getpeercert(self, binary_form=False):
+        """Returns a formatted version of the data in the
+        certificate provided by the other end of the SSL channel.
+        Return None if no certificate was provided, {} if a
+        certificate was provided, but not validated."""
+
+        self._checkClosed()
+        self._check_connected()
+        return self._sslobj.peer_certificate(binary_form)
+
+    def selected_npn_protocol(self):
+        self._checkClosed()
+        if not self._sslobj or not _ssl.HAS_NPN:
+            return None
+        return self._sslobj.selected_npn_protocol()
+
+    if hasattr(_ssl, 'HAS_ALPN'):
+        # 2.7.10+
+        def selected_alpn_protocol(self):
+            self._checkClosed()
+            if not self._sslobj or not _ssl.HAS_ALPN: # pylint:disable=no-member
+                return None
+            return self._sslobj.selected_alpn_protocol()
+
+    def cipher(self):
+        self._checkClosed()
+        if not self._sslobj:
+            return None
+        return self._sslobj.cipher()
+
+    def compression(self):
+        self._checkClosed()
+        if not self._sslobj:
+            return None
+        return self._sslobj.compression()
+
+    def __check_flags(self, meth, flags):
+        if flags != 0:
+            raise ValueError(
+                "non-zero flags not allowed in calls to %s on %s" %
+                (meth, self.__class__))
+
+    def send(self, data, flags=0, timeout=timeout_default):
+        self._checkClosed()
+        self.__check_flags('send', flags)
+
+        if timeout is timeout_default:
+            timeout = self.timeout
+
+        if not self._sslobj:
+            return socket.send(self, data, flags, timeout)
+
+        while True:
+            try:
+                return self._sslobj.write(data)
+            except SSLWantReadError:
+                if self.timeout == 0.0:
+                    return 0
+                self._wait(self._read_event)
+            except SSLWantWriteError:
+                if self.timeout == 0.0:
+                    return 0
+                self._wait(self._write_event)
+
+    def sendto(self, data, flags_or_addr, addr=None):
+        self._checkClosed()
+        if self._sslobj:
+            raise ValueError("sendto not allowed on instances of %s" %
+                             self.__class__)
+        elif addr is None:
+            return socket.sendto(self, data, flags_or_addr)
+        else:
+            return socket.sendto(self, data, flags_or_addr, addr)
+
+    def sendmsg(self, *args, **kwargs):
+        # Ensure programs don't send data unencrypted if they try to
+        # use this method.
+        raise NotImplementedError("sendmsg not allowed on instances of %s" %
+                                  self.__class__)
+
+    def sendall(self, data, flags=0):
+        self._checkClosed()
+        self.__check_flags('sendall', flags)
+
+        try:
+            socket.sendall(self, data)
+        except _socket_timeout as ex:
+            if self.timeout == 0.0:
+                # Python 2 simply *hangs* in this case, which is bad, but
+                # Python 3 raises SSLWantWriteError. We do the same.
+                raise SSLWantWriteError("The operation did not complete (write)")
+            # Convert the socket.timeout back to the sslerror
+            raise SSLError(*ex.args)
+
+    def recv(self, buflen=1024, flags=0):
+        self._checkClosed()
+        if self._sslobj:
+            if flags != 0:
+                raise ValueError(
+                    "non-zero flags not allowed in calls to recv() on %s" %
+                    self.__class__)
+            if buflen == 0:
+                return b''
+            return self.read(buflen)
+        else:
+            return socket.recv(self, buflen, flags)
+
+    def recv_into(self, buffer, nbytes=None, flags=0):
+        self._checkClosed()
+        if buffer is not None and (nbytes is None):
+            # Fix for python bug #23804: bool(bytearray()) is False,
+            # but we should read 0 bytes.
+            nbytes = len(buffer)
+        elif nbytes is None:
+            nbytes = 1024
+        if self._sslobj:
+            if flags != 0:
+                raise ValueError(
+                    "non-zero flags not allowed in calls to recv_into() on %s" %
+                    self.__class__)
+            return self.read(nbytes, buffer)
+        else:
+            return socket.recv_into(self, buffer, nbytes, flags)
+
+    def recvfrom(self, buflen=1024, flags=0):
+        self._checkClosed()
+        if self._sslobj:
+            raise ValueError("recvfrom not allowed on instances of %s" %
+                             self.__class__)
+        else:
+            return socket.recvfrom(self, buflen, flags)
+
+    def recvfrom_into(self, buffer, nbytes=None, flags=0):
+        self._checkClosed()
+        if self._sslobj:
+            raise ValueError("recvfrom_into not allowed on instances of %s" %
+                             self.__class__)
+        else:
+            return socket.recvfrom_into(self, buffer, nbytes, flags)
+
+    def recvmsg(self, *args, **kwargs):
+        raise NotImplementedError("recvmsg not allowed on instances of %s" %
+                                  self.__class__)
+
+    def recvmsg_into(self, *args, **kwargs):
+        raise NotImplementedError("recvmsg_into not allowed on instances of "
+                                  "%s" % self.__class__)
+
+    def pending(self):
+        self._checkClosed()
+        if self._sslobj:
+            return self._sslobj.pending()
+        return 0
+
+    def shutdown(self, how):
+        self._checkClosed()
+        self._sslobj = None
+        socket.shutdown(self, how)
+
+    def close(self):
+        if self._makefile_refs < 1:
+            self._sslobj = None
+            socket.close(self)
+        else:
+            self._makefile_refs -= 1
+
+    if PYPY:
+
+        def _reuse(self):
+            self._makefile_refs += 1
+
+        def _drop(self):
+            if self._makefile_refs < 1:
+                self.close()
+            else:
+                self._makefile_refs -= 1
+
+    def _sslobj_shutdown(self):
+        while True:
+            try:
+                return self._sslobj.shutdown()
+            except SSLError as ex:
+                if ex.args[0] == SSL_ERROR_EOF and self.suppress_ragged_eofs:
+                    return ''
+                elif ex.args[0] == SSL_ERROR_WANT_READ:
+                    if self.timeout == 0.0:
+                        raise
+                    sys.exc_clear()
+                    self._wait(self._read_event, timeout_exc=_SSLErrorReadTimeout)
+                elif ex.args[0] == SSL_ERROR_WANT_WRITE:
+                    if self.timeout == 0.0:
+                        raise
+                    sys.exc_clear()
+                    self._wait(self._write_event, timeout_exc=_SSLErrorWriteTimeout)
+                else:
+                    raise
+
+    def unwrap(self):
+        if self._sslobj:
+            s = self._sslobj_shutdown()
+            self._sslobj = None
+            return socket(_sock=s) # match _ssl2; critical to drop/reuse here on PyPy
+        else:
+            raise ValueError("No SSL wrapper around " + str(self))
+
+    def _real_close(self):
+        self._sslobj = None
+        socket._real_close(self) # pylint: disable=no-member
+
+    def do_handshake(self):
+        """Perform a TLS/SSL handshake."""
+        self._check_connected()
+        while True:
+            try:
+                self._sslobj.do_handshake()
+                break
+            except SSLWantReadError:
+                if self.timeout == 0.0:
+                    raise
+                self._wait(self._read_event, timeout_exc=_SSLErrorHandshakeTimeout)
+            except SSLWantWriteError:
+                if self.timeout == 0.0:
+                    raise
+                self._wait(self._write_event, timeout_exc=_SSLErrorHandshakeTimeout)
+
+        if self._context.check_hostname:
+            if not self.server_hostname:
+                raise ValueError("check_hostname needs server_hostname "
+                                 "argument")
+            match_hostname(self.getpeercert(), self.server_hostname)
+
+    def _real_connect(self, addr, connect_ex):
+        if self.server_side:
+            raise ValueError("can't connect in server-side mode")
+        # Here we assume that the socket is client-side, and not
+        # connected at the time of the call.  We connect it, then wrap it.
+        if self._connected:
+            raise ValueError("attempt to connect already-connected SSLSocket!")
+        self._sslobj = self._context._wrap_socket(self._sock, False, self.server_hostname, ssl_sock=self)
+        try:
+            if connect_ex:
+                rc = socket.connect_ex(self, addr)
+            else:
+                rc = None
+                socket.connect(self, addr)
+            if not rc:
+                self._connected = True
+                if self.do_handshake_on_connect:
+                    self.do_handshake()
+            return rc
+        except socket_error:
+            self._sslobj = None
+            raise
+
+    def connect(self, addr):
+        """Connects to remote ADDR, and then wraps the connection in
+        an SSL channel."""
+        self._real_connect(addr, False)
+
+    def connect_ex(self, addr):
+        """Connects to remote ADDR, and then wraps the connection in
+        an SSL channel."""
+        return self._real_connect(addr, True)
+
+    def accept(self):
+        """Accepts a new connection from a remote client, and returns
+        a tuple containing that new connection wrapped with a server-side
+        SSL channel, and the address of the remote client."""
+
+        newsock, addr = socket.accept(self)
+        newsock = self._context.wrap_socket(newsock,
+                                            do_handshake_on_connect=self.do_handshake_on_connect,
+                                            suppress_ragged_eofs=self.suppress_ragged_eofs,
+                                            server_side=True)
+        return newsock, addr
+
+    def makefile(self, mode='r', bufsize=-1):
+
+        """Make and return a file-like object that
+        works with the SSL connection.  Just use the code
+        from the socket module."""
+        if not PYPY:
+            self._makefile_refs += 1
+        # close=True so as to decrement the reference count when done with
+        # the file-like object.
+        return _fileobject(self, mode, bufsize, close=True)
+
+    def get_channel_binding(self, cb_type="tls-unique"):
+        """Get channel binding data for current connection.  Raise ValueError
+        if the requested `cb_type` is not supported.  Return bytes of the data
+        or None if the data is not available (e.g. before the handshake).
+        """
+        if cb_type not in CHANNEL_BINDING_TYPES:
+            raise ValueError("Unsupported channel binding type")
+        if cb_type != "tls-unique":
+            raise NotImplementedError(
+                "{0} channel binding type not implemented"
+                .format(cb_type))
+        if self._sslobj is None:
+            return None
+        return self._sslobj.tls_unique_cb()
+
+    def version(self):
+        """
+        Return a string identifying the protocol version used by the
+        current SSL channel, or None if there is no established channel.
+        """
+        if self._sslobj is None:
+            return None
+        return self._sslobj.version()
+
+if PYPY or not hasattr(SSLSocket, 'timeout'):
+    # PyPy (and certain versions of CPython) doesn't have a direct
+    # 'timeout' property on raw sockets, because that's not part of
+    # the documented specification. We may wind up wrapping a raw
+    # socket (when ssl is used with PyWSGI) or a gevent socket, which
+    # does have a read/write timeout property as an alias for
+    # get/settimeout, so make sure that's always the case because
+    # pywsgi can depend on that.
+    SSLSocket.timeout = property(lambda self: self.gettimeout(),
+                                 lambda self, value: self.settimeout(value))
+
+
+
+_SSLErrorReadTimeout = SSLError('The read operation timed out')
+_SSLErrorWriteTimeout = SSLError('The write operation timed out')
+_SSLErrorHandshakeTimeout = SSLError('The handshake operation timed out')
+
+def wrap_socket(sock, keyfile=None, certfile=None,
+                server_side=False, cert_reqs=CERT_NONE,
+                ssl_version=PROTOCOL_SSLv23, ca_certs=None,
+                do_handshake_on_connect=True,
+                suppress_ragged_eofs=True,
+                ciphers=None):
+
+    return SSLSocket(sock=sock, keyfile=keyfile, certfile=certfile,
+                     server_side=server_side, cert_reqs=cert_reqs,
+                     ssl_version=ssl_version, ca_certs=ca_certs,
+                     do_handshake_on_connect=do_handshake_on_connect,
+                     suppress_ragged_eofs=suppress_ragged_eofs,
+                     ciphers=ciphers)
+
+def get_server_certificate(addr, ssl_version=PROTOCOL_SSLv23, ca_certs=None):
+    """Retrieve the certificate from the server at the specified address,
+    and return it as a PEM-encoded string.
+    If 'ca_certs' is specified, validate the server cert against it.
+    If 'ssl_version' is specified, use it in the connection attempt."""
+
+    _, _ = addr
+    if ca_certs is not None:
+        cert_reqs = CERT_REQUIRED
+    else:
+        cert_reqs = CERT_NONE
+    context = _create_stdlib_context(ssl_version,
+                                     cert_reqs=cert_reqs,
+                                     cafile=ca_certs)
+    with closing(create_connection(addr)) as sock:
+        with closing(context.wrap_socket(sock)) as sslsock:
+            dercert = sslsock.getpeercert(True)
+    return DER_cert_to_PEM_cert(dercert)