rDNS

rDNS (reverse DNS) is a DNS lookup that works the opposite way to a standard one: instead of resolving a domain name into an IP address, it maps an IP address back to its associated hostname. Where forward DNS answers "what address does nodea.pl have?", rDNS answers "what name is behind the address 203.0.113.10?".

How rDNS works

Reverse lookups use a special namespace, in-addr.arpa (for IPv4) and ip6.arpa (for IPv6). The key elements are:

• PTR record — this is what performs the IP-to-name mapping; it is the reverse counterpart of the A record (see the PTR record).
• Reversed address notation — the IP is written with its octets in reverse order plus the .in-addr.arpa suffix, e.g. 10.113.0.203.in-addr.arpa for 203.0.113.10.
• Zone delegation — control over the reverse zone belongs to whoever manages the address block (the IP provider), not the domain owner.

For rDNS to be considered trustworthy, forward-confirmed reverse DNS (FCrDNS) should hold — the name from the PTR record, when checked forward, must resolve back to the same IP.

rDNS in practice

Reverse DNS has several important uses:

1. Email — SMTP servers verify the sender's PTR record; correct rDNS is a condition for good deliverability and avoiding a spam classification.
2. Logs and diagnostics — network tools show hostnames instead of raw addresses, easing traffic analysis.
3. Security — some systems only accept connections from hosts with valid, consistent rDNS.

When running your own mail server on a VPS, setting the PTR record with the IP operator is one of the first steps that determine whether messages get delivered.