No PTR records generated for headless service if hostname was not set.

[stroop23]

If there is no hostname entry on an endpoint of a headless service, a DNS record is created with a hostname part based on some sort of hash, ultimately derived from the pod ip:
https://github.com/kubernetes/dns/blob/master/pkg/dns/dns.go#L486

The PTR record is then explicitly not generated:
https://github.com/kubernetes/dns/blob/master/pkg/dns/dns.go#L503

The spec states:

Given a ready endpoint with hostname of <hostname> and IP address <a>.<b>.<c>.<d>, a PTR record of the following form must exist.

hostname is defined earlier as the value of the hostname field on the endpoint or a "unique, system-assigned identifier", which in this case is the hash generated on line 486.

I would expect either of the following:

• A PTR records is always generated, dropping the conditional on line 503, satisfying the spec.
• No DNS records are generated in these cases and the spec is updated to reflect this behaviour.

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[thockin]

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