Reverse DNS (rDNS)
Looking up the hostname associated with an IP address, the inverse of normal DNS. Used heavily by mail servers and security tools.
Reverse DNS (rDNS) is the lookup that goes from IP back to hostname. Normal DNS: example.com → 198.51.100.42. Reverse: 198.51.100.42 → example.com.
It works via PTR records in special domains (in-addr.arpa for IPv4, ip6.arpa for IPv6).
Where rDNS is checked
Email. Receiving mail servers do a reverse lookup on the sending IP and compare it to the EHLO/HELO hostname the sender claims. Mismatch or missing PTR → mail is suspect, often quarantined or rejected.
Logging. Web servers, firewalls, and security tools log inbound IPs. Some configurations resolve them to hostnames via PTR.
SSH. OpenSSH defaults to a reverse lookup on the client IP for logging. This adds 1–2 seconds to connection time if PTR fails. Disable with UseDNS no in sshd_config.
Spam blocklists. Most blocklists consider missing or generic PTR records as a signal (residential-looking PTRs like ip-203-0-113-42.isp.net get penalized for sending mail).
Who controls the PTR
The owner of the IP, not the owner of the domain. If you rent a server from DigitalOcean, DigitalOcean owns the IP block; you can request a PTR through their console. If you buy IPs from a registry (ARIN, RIPE), you control the PTR directly.
This is why a "vanity rDNS" requires your hosting provider to support it. Most consumer providers do; some basic VPS providers don't.
When SaaS engineers care
- Running your own outbound mail: yes, set PTR matching your HELO domain.
- Running a public-facing web server: no, PTR doesn't matter for browsers.
- Running anything mail-adjacent (transactional mail relay, etc.): yes, get PTR right.