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PTR Hostname Format

Catches malformed PTRs that won't survive scrutiny by strict receivers, including single-label hostnames, non-alpha TLDs, and other format issues.

This is independent of the Generic PTR Pattern check; a hostname can be a valid format and still look generic.

Applies to: IPv4 and IPv6 hosts.

How It Works​

For each PTR returned, we verify:

  • the hostname has at least 2 labels (e.g. mail.example.com, not just mail)
  • the rightmost label is alphabetic
  • the rightmost label is between 2 and 24 characters long

Fails On​

  • PTR has fewer than 2 labels.
  • PTR's TLD is non-alphabetic.
  • PTR's TLD length is outside 2-24 characters.
  • No PTR record published for the IP.