Reverse DNS
Looking up the host name for an IP address through its PTR record, the reverse of the usual name-to-address lookup.
What it means
A reverse lookup turns 192.0.2.25 into a name by querying the PTR record at 25.2.0.192.in-addr.arpa. Mail receivers typically do it for each connecting server: an address with no reverse DNS, or with a generic name like 192-0-2-25.dynamic.isp.example, looks like a compromised home machine, not a mail server.
Gmail and Yahoo require valid forward and reverse DNS for sending IP addresses. RFC 1912 already advised in 1996 that every reachable host have matching PTR and A records.
Reverse DNS is set by the owner of the IP address, so on cloud servers and VPSs it is changed in the provider's control panel or through support.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 1912: Common DNS Operational and Configuration Errors, section 2.1rfc-editor.org
- Google: Email sender guidelinessupport.google.com
- Yahoo Sender Hub: Sender best practicessenders.yahooinc.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes