Reverse DNS: From IP to Hostname with PTR Records

Reverse DNS: DNS in reverse

While traditional DNS (forward DNS) converts a hostname into an IP address, Reverse DNS does the opposite: given an IP address, it returns the associated hostname. This translation happens through PTR (Pointer) records, stored in special DNS zones: in-addr.arpa for IPv4 and ip6.arpa for IPv6. The IP address is reversed and concatenated to the arpa zone: to resolve 192.168.1.100, the resolver queries 100.1.168.192.in-addr.arpa for the PTR record.

Reverse DNS might seem like a secondary feature, but it has enormous practical implications, especially for email. Most mail servers (Gmail, Outlook, Yahoo) verify the reverse DNS of the sender's IP. If the IP has no PTR record, or if the PTR does not match the forward DNS (the so-called FCrDNS — Forward Confirmed Reverse DNS), emails are penalized or rejected. This check verifies that the server is legitimately configured and is not a bot or compromised server sending spam.

Configuring Reverse DNS

# Forward DNS (record A)
mail.esempio.com.  IN  A  203.0.113.50

# Reverse DNS (record PTR)
50.113.0.203.in-addr.arpa.  IN  PTR  mail.esempio.com.

# Verifica FCrDNS (Forward Confirmed Reverse DNS):
# 1. PTR di 203.0.113.50 → mail.esempio.com ✓
# 2. A di mail.esempio.com → 203.0.113.50 ✓
# FCrDNS valido: il PTR e l'A corrispondono

Unlike A and MX records that you manage in your domain's DNS, PTR records are managed by the owner of the IP block — typically your ISP or hosting provider. To configure or modify a PTR record, you need to contact the provider that assigned the IP and request creation or update. Many cloud providers (AWS, DigitalOcean, Hetzner) offer self-service interfaces to manage PTR records for their IPs.

Reverse DNS and email deliverability

Reverse DNS is a critical factor for email deliverability. Verify the PTR of your mail servers with our Reverse DNS and make sure FCrDNS is valid. If the PTR is missing or does not match, your emails have a high probability of landing in spam or being rejected. Beyond the PTR, verify that the IP is not blacklisted with Blacklist Check — a missing PTR is often correlated with blacklisted IPs because both are indicators of poorly configured servers.

For email servers, the best practice is to have a PTR that points to a meaningful hostname (like mail.example.com, not 50-113-0-203.generic.provider.com) and that this hostname has an A record resolving to the same IP. Avoid generic PTRs assigned by the provider and multiple PTRs for the same IP. Verify the complete configuration of your mail server with SMTP Diagnostics which includes a reverse DNS check in its report.

A special case is servers behind NAT or load balancers: make sure the PTR is configured on the actual public IP from which emails are sent, not on the internal IP. If you use an email relay service (like Amazon SES or SendGrid), the PTR is managed by the service provider and you don't need to configure it yourself. But if you manage a dedicated mail server, reverse DNS is your responsibility and one of the most frequently overlooked aspects.