Reverse DNS Lookup

What is Reverse DNS?

Reverse DNS (rDNS) maps an IP address back to a hostname, the opposite of a standard forward DNS lookup. This mapping is stored as a PTR (Pointer) record in a special DNS zone. For IPv4, the zone is in-addr.arpa. For IPv6, the zone is ip6.arpa. The IP address is reversed and appended to the zone to form the query name.

For example, the reverse DNS name for 1.2.3.4 is 4.3.2.1.in-addr.arpa. A PTR record at that name points back to the hostname associated with the IP.

Why PTR Records Matter for Email

Many receiving mail servers check reverse DNS as part of their spam filtering. An IP address without a PTR record, or with a PTR that does not match any forward DNS record, is often considered suspicious. Several major email providers (including Gmail and Microsoft 365) may reject or penalize messages from IPs that fail reverse DNS checks.

Best practice is to ensure every IP address used for sending email has a PTR record that resolves to a meaningful hostname, and that the hostname resolves back to the same IP (forward-confirmed reverse DNS).

FCrDNS Explained

Forward-Confirmed reverse DNS (FCrDNS) is a validation check where the PTR hostname is looked up again via a forward DNS query (A record). If the forward lookup returns the original IP address, FCrDNS passes. This confirms that the IP owner legitimately controls the hostname and has not simply set an arbitrary PTR record.