Glossary · DNS

Reverse DNS (rDNS)

Reverse DNS maps an IP address back to a hostname by querying a PTR record in the in-addr.arpa or ip6.arpa tree, the opposite of a normal DNS lookup.

Updated October 6, 2026

Reverse DNS (rDNS) resolves an IP address to a name. The address is written in reverse order and looked up as a PTR record under in-addr.arpa for IPv4 or ip6.arpa for IPv6.

Why it matters

Mail servers commonly check that a sending IP has a PTR record, and that the name it returns resolves back to the same IP (forward-confirmed reverse DNS). Missing or generic rDNS is a frequent reason mail is rejected or filed as spam. Logs, traceroute output and security tools also use it to show readable hostnames. Control sits with whoever owns the IP block, usually your hosting provider or ISP, not with your domain registrar.

Example

$ dig -x 203.0.113.10 +short
mail.example.com.
$ dig mail.example.com A +short
203.0.113.10

The first query asks for 10.113.0.203.in-addr.arpa PTR. The match in the second confirms the pair.

Common mistakes

  • Trying to set a PTR record in your domain's DNS panel instead of asking the IP owner.
  • Using a hostname in PTR that does not resolve back to the same IP.
  • Assuming one IP can only have one name. It can have several, but mail systems expect one clear value.

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