Status

Current state: Under Discussion

Discussion thread: kafka-dev

JIRA: Unable to render Jira issues macro, execution error. , Unable to render Jira issues macro, execution error.

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Motivation

Kafka 0.9.0.0 added SASL-based authentication for clients and inter-broker communication. SASL is a framework that enables authentication and data security via replaceable mechanisms. But at the moment, SASL implementation in Kafka supports only SASL/GSSAPI using Kerberos and does not allow other SASL mechanisms to be plugged in. Enabling other SASL mechanisms will allow better integration with existing non-Kerberos authentication servers. In this KIP, we discuss a proposal for enabling other SASL mechanisms.

This KIP addresses the following extensions to the existing implementation:

1. Configurable SASL implementation to enable integration with existing authentication servers
2. Support for additional SASL mechanisms. This enables standard mechanisms that do not have a default implementation in Kafka (eg. DIGEST-MD5) as well as custom mechanisms
3. Configurable callback handlers to provide mechanism-specific input
4. SASL/PLAIN implementation to enable simple username/password authentication

This KIP does not address:

1. Support for multiple SASL mechanisms within a broker. This would require multiple login contexts to support each mechanism and a means of choosing the mechanism for authentication in the server. It would be better to address this in another KIP if there is a requirement.

Public Interfaces

Configuration options

The following options will be added to SaslConfigs.java and can be configured as properties for Kafka clients and server:

1. sasl.mechanism (String) Specifies the SASL mechanism. This may be any mechanism for which a security provider is available in the JVM. Default value is GSSAPI.
2. sasl.callback.handler.class  (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 mechanism. Default implementation supports callback handlers for GSSAPI and PLAIN.

Callback handler interface

package org.apache.kafka.common.security.auth;
import org.apache.kafka.common.annotation.InterfaceStability;
import org.apache.kafka.common.Configurable;
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, Configurable {
    /**
     * Closes this instance.
     */
    void close() throws KafkaException;
}

Proposed Changes

The changes will be implemented under two JIRAs. KAFKA-xxx implements the changes to enable new mechanisms. KAFKA-2658 will be rebased after the review of this KIP to include a simple default implementation for SASL/PLAIN in Kafka. This section gives a summary of the changes and the rationale behind them.

SASL configuration

SASL in Kafka is configured using the standard JAAS configuration. SASL configuration consists of:

1. LoginContext that specifies the login module class and properties for the login module, specified using JAAS configuration
2. SASL mechanism and other properties specific to the mechanism configured as Kafka client or server properties. Additional properties and selection policies handled by the SASL implementation may also be specified when the SaslClient or SaslServer is constructed.
3. Additional input required by the SASL implementation obtained using CallbackHandlers
   
4. SaslServer or SaslClient implementation for the configured mechanism. These are installed as security providers in the JVM

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 configuration for 2) and 3) to enable any SASL mechanism to be supported in Kafka clients and servers.

SASL mechanism

Kafka server and client will have a new configuration option sasl.mechanism to specify the mechanism. In order to plug in any SASL mechanism including custom mechanisms, this property will be a String parameter rather than an enum with a restricted set of values.

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.

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.

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.

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 to test an additional mechanism using the extension points.
• System tests will be added for SASL/PLAIN.

Compatibility, Deprecation, and Migration Plan

Impact on existing clients

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 with change in SASL mechanism

Since this KIP does not address support for multiple SASL mechanisms within a broker, rolling upgrade to switch SASL mechanisms needs to be done in two stages. This involves a switch from SASL to non-SASL, followed another switch from non-SASL to SASL with the new mechanism.

1. In the first stage, perform rolling restart of Kafka brokers to enable a non-SASL port (eg. SSL).
   1. Enable SSL port for the brokers and perform rolling restart of all brokers
   2. Restart all clients with SSL as protocol
   3. If using SASL for inter-broker communication, switch to SSL and perform another rolling restart of brokers.
2. In the second stage when SASL is no longer in use, perform rolling restart of Kafka brokers to update SASL mechanism with the updated JAAS configuration and properties.
   1. Update SASL mechanism, JAAS configuration and other properties for the brokers and perform rolling restart of all brokers
   2. Restart all clients with SASL_SSL/SASL_PLAINTEXT as required
   3. If using SASL for inter-broker communication, switch to SASL_SSL/SASL_PLAINTEXT and perform rolling restart of brokers again.

Rejected Alternatives

Enable a small set of SASL mechanisms with a default implementation in Kafka

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 the restriction that a SASL mechanism without a default implementation in Kafka cannot be used at all.

Make Authenticator configurable

This would provide additional flexibility, but would require users to implement more code. This would be more suitable if there is a requirement to implement authentication using protocols other than SASL.