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FCrDNS should only be required when the end user needs to properly identify itself with a domain or organization, commonly used for email where the from address header should match the PTR of the sending IP (IPv4 or IPv6 Address). Actually Tom, if you notice Google requires IPv6 initiated email connections to have a proper RDNS, so that's out. The fact is ...


Set up your own DNS recursive resolver with DNSSEC and set an outgoing firewall policy blocking DNS requests to anything else. ISPs tend to abuse DNS, you don't want to use their servers, even if they are configured correctly.

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