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Describe the problems you are trying to solve.

From Kafka documentation 

By default, the SASL user name will be the primary part of the Kerberos principal. One can change that by
setting sasl.kerberos.principal.to.local.rules to a customized rule in server.properties. The format of
sasl.kerberos.principal.to.local.rules is a list where each rule works in the same way as
the auth_to_local in Kerberos configuration file (krb5.conf) (http://web.mit.edu/Kerberos/krb5-latest/doc/admin/conf_files/krb5_conf.html).

Each rules starts with RULE: and contains an expression in the format [n:string](regexp)s/pattern/replacement/g.

Public Interfaces

Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed. The purpose of this section is to concisely call out the public contract that will come along with this feature.

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Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

1. https://community.hortonworks.com/articles/14463/auth-to-local-rules-syntax.html
2. 
   

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

  This is an extension to the existing sasl.kerberos.principal.to.local.rules format and there will be no impact on existing users.

Rejected Alternatives

If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.