Principal Propagation

Principal propagation takes the principal from authentication mechanism configured for client to the REST proxy and propagates that same principal when making requests to the Kafka broker.

Today, Kafka supports two different authentication mechanism; SSL and SASL based on the client’s security.protocol config. Principal propagation is supported for the supported kafka security mechanisms by setting up the client appropriately.

For further reference on Kafka Security, please refer to Kafka Security

SSL

SSL propagation is picked by the plugin when the security.protocol is set to SSL. For SSL propagation to work, it is required to all load all the certificates corresponding to the required principal in a single client KeyStore file. Once this is done, the plugin would pick the appropriate certificate alias based on the logged on principal while making requests to Kafka . Currently, the logged on principal must exactly match the X500 Principal of the certificate. This is straight forward when SSL is used as the authentication mechanism.

For example, if there were two clients integrated to Kafka REST the setup could be as simple as below

  • Client A authenticates to Kafka REST using its key store which contains Certificate-A
  • Client B authenticates to Kafka REST using its key store which contains Certificate-B
  • Kafka REST’s key store client.ssl.keystore.location is loaded with Certificate-A and Certificate-B. The certificate is then chosen by the plugin based on who the client is.

If the ACL’s need to be setup with different principal other than the default X500 on the Kafka broker, you can provide a custom principal builder for Kafka using the broker server.properties

principal.builder.class=CustomizedPrincipalBuilderClass

SASL

SASL propagation is picked by the plugin when the security.protocol is set to either of SASL_PLAIN or SASL_SSL. Security plugin supports all the sasl.mechanism supported by Kafka clients. Just like a regular Kafka client, the plugin also expects a JAAS config file to be configured through -Djava.security.auth.login.config. It is required for all the principals to be specified in the JAAS config file under the section KafkaClient. Refer to SASL Configuration for details about configuring SASL for Kafka client and broker.

In the JAAS config file all the principals need to be explicitly specified. The plugin supports specifying principals using following supported mechanisms: GSSAPI, PLAIN, SCRAM-SHA-256 and SCRAM-SHA-512. Also, the plugin ignores any configured sasl.mechanism and picks it automatically based on the LoginModule specified for the principal.

KafkaClient {
  com.sun.security.auth.module.Krb5LoginModule required
  useKeyTab=true
  storeKey=true
  keyTab="/etc/security/keytabs/kafka_client.keytab"
  principal="kafka-client-1@EXAMPLE.COM";

  com.sun.security.auth.module.Krb5LoginModule required
  useKeyTab=true
  storeKey=true
  keyTab="/etc/security/keytabs/kafka_client_2.keytab"
  principal="kafka-client-2@EXAMPLE.COM";

  org.apache.kafka.common.security.plain.PlainLoginModule required
  username="alice-plain"
  password="alice-secret";

  org.apache.kafka.common.security.scram.ScramLoginModule required
  username="alice-scram"
  password="alice-secret";

  org.apache.kafka.common.security.scram.ScramLoginModule required
  username="alice-scram-256"
  password="alice-secret"
  mechanism="SCRAM-SHA-256";
};

Below is the mapping of sasl.mechanism for the configured login modules

Principal’s Login Module SASL Mechanism
com.sun.security.auth.module.Krb5LoginModule
GSSAPI
org.apache.kafka.common.security.plain.PlainLoginModule
PLAIN
org.apache.kafka.common.security.scram.ScramLoginModule
SCRAM-SHA-512
For SCRAM-SHA-256 set
mechanism=SCRAM-SHA-256
as an option in ScramLoginModule

All the mechanisms except SCRAM-SHA-256 would be automatically detected by the plugin and SCRAM-SHA-256 can be explicitly mentioned as an option in the ScramLoginModule.