Status

Current state: Under Discussion Accepted

Discussion thread: kafka-dev

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This KIP addresses the following extensions to the existing implementation:

1. Configurable SASL implementation mechanism to enable integration with existing authentication servers
2. Support for additional standard SASL mechanisms . This enables standard mechanisms that do not have a default implementation in Kafka (eg. DIGEST-MD5) as well as custom mechanismsConfigurable callback handlers to provide mechanism-specific inputwhich do not require custom callbacks or login interfaces to support token refresh
3. SASL/PLAIN implementation to enable simple username/password authentication without complex infrastructure
4. Support for multiple SASL mechanisms within the same server. This will be useful, for example, in organizations where internal and external users require different authentication mechanisms.

This KIP does not address the following extensions which be considered in a follow-on KIP:

1. Support for custom mechanisms
2. Configurable callback handlers to provide mechanism-specific input
3. Configurable login interface that manages the login process and the lifecyle of login context related resources to support custom mechanisms that require periodic ticket refresh

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1. sasl.mechanism (String) SASL mechanism used for client connections. This may be any mechanism for which a security provider is available in the JVM. Default value is GSSAPI.
2. sasl.enabled.mechanisms (List<String>) The list of SASL mechanisms enabled in the Kafka server. This may include any mechanism for which a security provider is available in the JVM. Default value is GSSAPI.
3. sasl.mechanism.callbackinter.handlerbroker.class  protocol (Class). The fully qualified name of a class that implements the AuthCallbackHandler interface. This class should have a default constructor and implement the callback handlers required for the configured mechanisms. Default implementation supports callback handlers for GSSAPI and PLAIN.
4. sasl.login.class  (Class). The fully qualified name of a class that implements the Login interface. This class should have a default constructor.

Callback handler interface

package org.apache.kafka.common.security.auth;
import org.apache.kafka.common.annotation.InterfaceStability;
import org.apache.kafka.common.network.Mode;
import org.apache.kafka.common.KafkaException;
import javax.security.auth.callback.CallbackHandler;
/*
 * Callback handler for SASL-based authentication
 */
@InterfaceStability.Unstable
public interface AuthCallbackHandler extends CallbackHandler {
    
    /**
     * Configures this callback handler.
     * 
     * @param configs Configuration
     * @param mode The mode that indicates if this is a client or server connection
     * @param subject Subject from login context
     * @param saslMechanism Negotiated SASL mechanism
     */
    void configure(Map<String, ?> configs, Mode mode, Subject subject, String saslMechanism) throws KafkaException;
    /**
     * Closes this instance.
     */
    void close() throws KafkaException;
}

Login interface

package org.apache.kafka.common.security.auth;
import java.util.Map;
import javax.security.auth.Subject;
import javax.security.auth.login.LoginContext;
import javax.security.auth.login.LoginException;
/**
 * Login interface for authentication
 */
public interface Login {    
    /**
     * Configures this login instance.
     */
    void configure(Map<String, ?> configs, String loginContextName);
    /**
     * Performs login for each login module specified for the login context of this instance.
     */
    LoginContext login() throws LoginException;    
    /**
     * Returns the authenticated subject of this login context.
     */
    Subject subject();    
    /**
     * Returns the service name
     */
    String serviceName();
    /**
     * Closes this instance.
     */
    void close();
}

1. String) SASL mechanism used for inter-broker connections. Default value is GSSAPI.

Protocol changes

A new Kafka SaslHandshakeRequest and response type will be defined to add a handshake request flow to enable clients to communicate their chosen SASL mechanism to the broker. The formats of the request and response are shown below:

SaslHandshakeRequest => Mechanism
  Mechanism => string

SaslHandshakeResponse => ErrorCode EnabledMechanisms
  ErrorCode => int16
  EnabledMechanisms => [string]

Successful Kafka SaslHandshakeRequest/Response flow should be immediately followed by the actual SASL authentication packets using the selected SASL mechanism. SASL authentication exchange consists of opaque client and server tokens as defined by the SASL mechanism and are typically obtained using a standard SASL library. These packets are not prefixed with Kafka request/response headers. No further Kafka requests may be sent until SASL authentication exchange is completed. For interoperability with 0.9.0.x, the Kafka handshake request flow is omitted for inter-broker communication using SASL for connections to older brokers. 

Proposed Changes

The changes will be implemented under KAFKA-3149. The implementation will include changes to support additional mechanisms and enable multiple mechanisms in the broker. A simple default implementation for SASL/PLAIN in Kafka that is currently in KAFKA-2658 will also be included. This section gives a summary of the changes and the rationale behind them.

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All the four types of configuration above need to be configured consistently for SASL authentication to operate correctly. 1) and 4) are JVM configuration options. 2) and 3) are currently hard-coded in Kafka. The proposed changes enable flexible enable  configuration for 2) and 3)   to enable any more SASL mechanism mechanisms to be supported in Kafka clients and servers. 3) will be addressed in a follow-on KIP to support any mechanism including custom mechanisms.

SASL mechanism

Client connections will use the property sasl.mechanism to specify the mechanism to be used for SASL authentication. Kafka servers may also specify the configuration option sasl.enabled.mechanisms to provide the list of enabled mechanisms when multiple mechanisms are enabled in the server. In order to plug in any SASL mechanism including custom mechanisms, mechanism will be specified as String rather than an enum with a restricted set of values.  GSSAPI will be used as the default mechanism for interoperability of new clients with 0.9.0.0 brokers. x. If sasl.enabled.mechanisms is not specified, only GSSAPI will be enabled in the server. If a list of values is specified, GSSAPI will be enabled only if included in the list, allowing servers to be run with SASL without complex Kerberbos setup if required.

Clients may enable only one mechanism and the mechanism name is sent to the server before any SASL authentication packets are sent, if the mechanism is not GSSAPI. Server fails the authentication if the client mechanism is not enabled in the broker.

SASL callback handler

Kafka server and client will have a new configuration option sasl.callback.handler.class to provide a callback handler class. Default callbacks are included in Kafka for the mechanisms which have an implementation in Kafka (GSSAPI and PLAIN). Default client callback handler obtains authentication id and password from the public and private credentials of the Subject respectively as String values. Configurable callbacks will enable other mechanisms to be used with Kafka without any changes to Kafka code. The callback handler interface has been derived from the implementation for GSSAPI and PLAIN mechanisms. Apart from Subject, configuration properties and mechanism are provided to the callback object to enable custom mechanisms to be added without any changes to Kafka.

SASL properties

The current implementation does not specify any properties when the SaslClient or SaslServer is constructed. To make the Kafka implementation flexible with pluggable mechanisms, all properties specified for Kafka client/server will be passed to the SaslClient/SaslServer. These include all properties specified by the user including properties not defined in Kafka, so that additional properties can be added without changes to Kafka.

Login interface

Kafka server and client will have a new configuration option sasl.login.class to manage the login process. This is required to plugin custom mechanisms which require login tokens to be refreshed periodically. Default login implementation will support GSSAPI and PLAIN and will be suitable for most of the standard mechanisms where mechanism-specific code can reside in the login module implementation. This interface is targeted at protocols similar to Kerberos which require a background thread to handle ticket refreshFor inter-broker communication, sasl.mechanism.inter.broker.protocol configuration on the broker is used by the client-mode connection to choose the SASL mechanism.

Support for multiple mechanisms in a broker

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To keep the Kafka protocol simple, only a single SASL mechanism is allowed for Kafka clients. For protocols other than GSSAPI, client sends the protocol Clients send the mechanism name to the server in a handshake request before any SASL authentication packets are sent. Kafka server checks the first packet and if it is one of the mechanism names that it recognizes including custom protocol mechanism names, it switches to that mechanism and starts SASL authentication using subsequent packets. If not, the server defaults to GSSAPI mechanism, treating the first packet as the first GSSAPI authentication packet from the client. The Kafka Java client implementation skips handshake request flow for GSSAPI for inter-broker connections if inter.broker.protocol.version is 0.9.0.x, enabling rolling upgrade of 0.9.0.x clusters which use SASL for replication. Handshake requests are sent by other clients even for GSSAPI from 0.10.0.0 onwards.

With GSSAPI, the first context establishment packet starts with byte 0x60 (APPLICATION-0 tag) followed by a variable-length encoded size. The first Kafka handshake request packet containing client mechanism starts with a two-byte API key of 0x0011, making it easy to distinguish from a GSSAPI packet.

Client flow:

1. If sasl.mechanism is not GSSAPI, send a Kafka handshake request packet with the mechanism name to the server. Otherwise go to Step 3.

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1. • Request Format: |

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1. • Kafka RequestHeader | Kafka SaslHandshakeRequest |
2. Wait for response from the server. If the error code in the response is non-zero, indicating failure, report the error and fail authentication.
3. Perform SASL authentication with the configured client mechanism. SASL authentication packets do not contain a Kafka RequestHeader.
   • Client token Format: | Size (int32) | SASL client authentication token |

Server flow:

1. Wait for first authentication packet from client
2. If this packet is a not valid mechanism Kafka handshake request, go to Step 4 and process this packet as the first GSSAPI client token
3. If the client mechanism in the Kafka handshake request received in Step 2 is enabled in the broker, send a response with error code zero and start authentication using the specified mechanism. Otherwise, send an error response including the list of enabled mechanisms and fail authentication.
   
   • Packet Response Format: | Version (Int16) | ErrorCode (Int16)| EnabledMechanisms (ArrayOf(String)) Kafka ResponseHeader | Kafka SaslHandshakeResponse |
4. Perform SASL authentication with the selected mechanism. If mechanism exchange was skipped, process the initial packet that was received from the client first. SASL authentication packets are expected without a Kafka RequestHeader until SASL authentication exchange completes. SASL server authentication packets are sent back without a Kafka response header.
   
   • Server token Format: | Size (int32) | SASL server authentication token |

Step 3 in the client flow and Step 4 in the server flow correspond to the start of the actual SASL authentication exchange when authentication tokens are exchanged for the selected SASL mechanism. These are treated as opaque tokens and are processed by the SASL provider of the mechanism. No further Kafka requests may be sent until the authentication exchange completes.

SASL/PLAIN implementation

SASL/PLAIN is a simple username/password authentication mechanism that is typically used with TLS for encryption to implement secure authentication. Unlike Kerberos, PLAIN does not require complex authentication infrastructure. Adding a default implementation for PLAIN in Kafka enables a simpler authentication mechanism for organizations which do not already use Kerberos. SASL/PLAIN protocol and its uses are described in https://tools.ietf.org/html/rfc4616. .
The PR in KAFKA-2658 will be rebased on the extensible interface from this KIP for this implementation. For the default SASL/PLAIN implementation included in Kafka, the username specified as authentication ID will be used as the authorization ID and principal.

Testing

• Due to the complexity of setting up Kerberos, limited unit testing has been implemented for SASL in the clients project. Along with the implementation for SASL/PLAIN, comprehensive unit tests will be added for the existing SASL implementation as well as the new interfaces. 

• End-to-end tests will be added in the core project along with the existing SASL/Kerberos tests for SASL/PLAIN

  ,

  and multi-mechanism configuration

  and to test an additional mechanism using the extension points.

  
  

• System tests will be added for SASL/PLAIN and for multi-mechanism support.
  

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Existing clients will continue to use GSSAPI as the SASL mechanism and will not be impacted by the changes. Since default callback handlers can be used for SASL mechanisms that are implemented in Kafka, no configuration changes are required.

Rolling upgrade from 0.9.0.

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x

Rolling upgrade with GSSAPI as the SASL mechanism can be performed using a simple standard rolling restart with no change in properties inter.broker.protocol.version set to 0.9.0.x in the first sequence of the upgrade. By default, if sasl.mechanism property is not specified, GSSAPI will be used without any exchange of mechanisms. Handshake requests are not sent for GSSAPI when inter.broker.protocol.version is 0.9.0.x. Once the cluster is upgraded, inter.broker.protocol.version can be set to 0.10.0, enabling handshake requests for all SASL connections. If the mechanism is to be changed, this rolling restart can be followed by the addition of the new mechanism as described below.

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Since the security requirements and infrastructure used in different organizations vary, default implementation of login modules and security providers and unlikely to be sufficient for all users. Unlike Kerberos, where most users are likely to use the Kerberos module provided in the JDK, other mechanisms are likely to be customized by users to enable integration with existing authentication providers. The proposed implementation removes along with a follow-on KIP to configure callbacks will remove the restriction that a SASL mechanism without a default implementation in Kafka cannot be used at all.

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