Add security layer negotiation to the GSSAPI authentication. (#1283)

When trying to establish a connection with Kafka using SASL with the
GSSAPI authentication mechanism the connection was hanging an timing out
after 60 secons. On the Kafka broker side I noticed that the
SaslServerAuthenticator was going from the AUTHENTICATE to the FAILED state.

The GSSAPI auth implementation was missing the second handshake defined in
RFC 2222, which happens after the security context is established. This
handshake is used by the client and server to negotiate the security layer (QoP)
to be used for the connection.

Kafka currently only support the "auth" QoP, so the implementation in this commit
doesn't make it configurable, but this can be extended later.

With this change I was able to successfully connect to a Kerberos-enabled Kafka
broker using the SASL_PLAINTEXT protocol and the GSSAPI mechanism.

Author: asdaraujo

kafka/conn.py

@@ -3,6 +3,7 @@
 import collections
 import copy
 import errno
+import io
 import logging
 from random import shuffle, uniform
 
@@ -27,7 +28,7 @@
 from kafka.protocol.commit import OffsetFetchRequest
 from kafka.protocol.metadata import MetadataRequest
 from kafka.protocol.parser import KafkaProtocol
-from kafka.protocol.types import Int32
+from kafka.protocol.types import Int32, Int8
 from kafka.version import __version__
 
 
@@ -39,6 +40,10 @@
 
 DEFAULT_KAFKA_PORT = 9092
 
+SASL_QOP_AUTH = 1
+SASL_QOP_AUTH_INT = 2
+SASL_QOP_AUTH_CONF = 4
+
 try:
     import ssl
     ssl_available = True
@@ -517,43 +522,59 @@ def _try_authenticate_plain(self, future):
         return future.success(True)
 
     def _try_authenticate_gssapi(self, future):
+        auth_id = self.config['sasl_kerberos_service_name'] + '@' + self.hostname
         gssapi_name = gssapi.Name(
-            self.config['sasl_kerberos_service_name'] + '@' + self.hostname,
+            auth_id,
             name_type=gssapi.NameType.hostbased_service
         ).canonicalize(gssapi.MechType.kerberos)
         log.debug('%s: GSSAPI name: %s', self, gssapi_name)
 
-        # Exchange tokens until authentication either succeeds or fails
-        client_ctx = gssapi.SecurityContext(name=gssapi_name, usage='initiate')
-        received_token = None
+        # Establish security context and negotiate protection level
+        # For reference RFC 2222, section 7.2.1
         try:
+            # Exchange tokens until authentication either succeeds or fails
+            client_ctx = gssapi.SecurityContext(name=gssapi_name, usage='initiate')
+            received_token = None
             while not client_ctx.complete:
                 # calculate an output token from kafka token (or None if first iteration)
                 output_token = client_ctx.step(received_token)
 
+                # pass output token to kafka, or send empty response if the security
+                # context is complete (output token is None in that case)
                 if output_token is None:
-                    continue
-
-                # pass output token to kafka
-                try:
+                    self._send_bytes_blocking(Int32.encode(0))
+                else:
                     msg = output_token
                     size = Int32.encode(len(msg))
                     self._send_bytes_blocking(size + msg)
 
-                    # The server will send a token back. Processing of this token either
-                    # establishes a security context, or it needs further token exchange.
-                    # The gssapi will be able to identify the needed next step.
-                    # The connection is closed on failure.
-                    header = self._recv_bytes_blocking(4)
-                    (token_size,) = struct.unpack('>i', header)
-                    received_token = self._recv_bytes_blocking(token_size)
-
-                except ConnectionError as e:
-                    log.exception("%s: Error receiving reply from server",  self)
-                    error = Errors.ConnectionError("%s: %s" % (self, e))
-                    self.close(error=error)
-                    return future.failure(error)
+                # The server will send a token back. Processing of this token either
+                # establishes a security context, or it needs further token exchange.
+                # The gssapi will be able to identify the needed next step.
+                # The connection is closed on failure.
+                header = self._recv_bytes_blocking(4)
+                (token_size,) = struct.unpack('>i', header)
+                received_token = self._recv_bytes_blocking(token_size)
+
+            # Process the security layer negotiation token, sent by the server
+            # once the security context is established.
+
+            # unwraps message containing supported protection levels and msg size
+            msg = client_ctx.unwrap(received_token).message
+            # Kafka currently doesn't support integrity or confidentiality security layers, so we
+            # simply set QoP to 'auth' only (first octet). We reuse the max message size proposed
+            # by the server
+            msg = Int8.encode(SASL_QOP_AUTH & Int8.decode(io.BytesIO(msg[0]))) + msg[1:]
+            # add authorization identity to the response, GSS-wrap and send it
+            msg = client_ctx.wrap(msg + auth_id, False).message
+            size = Int32.encode(len(msg))
+            self._send_bytes_blocking(size + msg)
 
+        except ConnectionError as e:
+            log.exception("%s: Error receiving reply from server",  self)
+            error = Errors.ConnectionError("%s: %s" % (self, e))
+            self.close(error=error)
+            return future.failure(error)
         except Exception as e:
             return future.failure(e)