What Is Reverse DNS?
TL;DR
Reverse DNS is the lookup that turns an IP address into a hostname, using PTR records in the in-addr.arpa and ip6.arpa namespaces. Mail servers perform it on every incoming connection, which makes it one of the first things a receiver learns about a sender.
How does a reverse DNS lookup work?
The address is reversed and turned into a name. For IPv4 the four octets are written in reverse order with in-addr.arpa appended, so 198.51.100.7 becomes 7.100.51.198.in-addr.arpa, and an ordinary query for a PTR record at that name returns the hostname.
IPv6 uses the same idea at finer granularity, with each hexadecimal digit of the expanded address becoming its own label under ip6.arpa. Either way the query is plain DNS, answered by the nameservers for the address block rather than by the ones for your domain.
What is forward-confirmed reverse DNS?
It is the check that both directions agree. The receiver resolves the connecting address to a hostname with a PTR lookup, then resolves that hostname back with a normal A or AAAA lookup, and expects to arrive at the address it started from.
Only the pair means anything. A PTR record on its own can claim any hostname at all, since it is published by whoever holds the address block, so the forward lookup is what stops the claim from being free.
Why does reverse DNS affect deliverability?
It is evaluated at connection time, before authentication and before content. A missing record, or a hostname that does not confirm, gets the connection refused or heavily throttled by some receivers, with no bounce that explains why.
Google names it as a requirement for all senders rather than only for bulk senders. That makes it a prerequisite rather than an optimisation for anyone sending from their own mail server.
Frequently asked questions
Nearly. The PTR record is the data and reverse DNS is the lookup that retrieves it, so the terms get used interchangeably. When a provider asks you to set up reverse DNS, they mean the PTR record for your sending address.