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Other option considered :

Changing default behaviour.

Modifying the existing code path to perform reverse dns lookup will break SSL authentication if kafka users use brokers IP addresses in bootstrap.servers and in the SubjectAlternativeName field in their certificates.

In this case, parseAndValidateAddresses() will perform the lookup and replace the IP addresses with hostnames, which will be matched against the IPs in the certificates, so the SSL handshake will fail.
This mismatch won't be obvious to users, as both bootstrap.servers and the certificates are consistent.
Changing default behaviour means breaking a valid use case.