Principal name mapping

Kafka can be configured to translate certificate subject names into short names. This is done by adding mapping rules to Kafka's configuration. These short names can be used as the unique identifier of the user. Compared to subject names, short names are much easier to manage.

When a client authenticates using a TLS/SSL keystore, by default Kafka assumes that the username for that client is the certificate's subject name, which is usually a Distinguished Name such as the following:

Working with these long names is difficult. Security policies and group mappings are usually defined in terms of the user's short name (alice) rather than the full Distinguished Name. Kafka can be configured to translate the certificate's subject into a short name that can be used as the unique identifier of the user.

This can be done by adding the necessary mapping rules to the ssl.principal.mapping.rules Kafka property. However, this property is not directly configurable in Cloudera Manager. As a result, you need to use the Kafka Broker Advanced Configuration Snippet (Safety Valve) for property to add it to your configuration.

The rule takes the form of a regular expression to match the subject name of the certificate and the transformation to apply to the match. The property accepts multiple rules. Each rule has to be separated by a comma. The last rule is usually the DEFAULT rule, which uses the full subject name.

For example, consider the following setting:
This configuration has two rules which are processed in the following order:
  1. RULE:^[Cc][Nn]=([a-zA-Z0-9.]*).*$/$1/L

The first rule to match the certificate's subject name is used, later ones are ignored. The DEFAULT rule is a "catch all" rule. It always matches and does not do any replacement if none of the previous ones were matched.

The regular expression of the first rule,^[Cc][Nn]=([a-zA-Z0-9.]*).*$, matches any subject that starts with CN=,cn=, Cn=, or cN=, followed by the user's short name, that contains characters ranging between a-z,A-Z, and 0-9, followed by any string. It then replaces the matched string with the user's short name. The short name is the content matched inside the parenthesis and is referenced in the second part of the rule as $1. The L at the end of the rule converts the resulting string to lowercase.

For more information and examples on principal mapping rules, see the Apache Kafka documentation.