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Reverse DNS (RDNS) Lookup

Look up the hostname(s) an IP address resolves back to via its PTR record.

What is reverse DNS?

Reviewed by Prabath Thalangama · Last updated

Reverse DNS maps an IP address back to a host name through a PTR record under in-addr.arpa (IPv4) or ip6.arpa (IPv6). Mail servers rely on it to judge senders. This lookup takes an IPv4 or IPv6 address and returns every PTR name published for it.

PTR records are controlled by whoever owns the IP block, usually your hosting or cloud provider, not by your domain's DNS.

Mail receivers usually also want forward confirmation: the PTR name must resolve back to the same IP. This lookup shows the PTR name; check the forward direction with an A or AAAA lookup on that name.

How do I check a PTR record?

  1. Enter the IP address, for example the public IP of your mail server.
  2. Check the name, then look up its A or AAAA record and confirm it returns the same IP.

How to read the results

value
Each PTR name for the address. One is normal; several confuse some receivers.
No PTR records found
The address has no reverse DNS. Fine for most servers, a problem for mail servers.

Common reverse DNS problems

Mail rejected: no PTR or generic PTR

Ask the IP owner to set the PTR to your mail server's host name, such as mail.example.com, and make sure that name's A record points back to the IP.

550 5.7.25 The IP address sending this message does not have a PTR record setup

PTR doesn't match the HELO name

Configure the mail server to announce the same name in HELO/EHLO that the PTR record returns.

Examples and command-line equivalents

Check reverse and forward DNS

dig +short -x 203.0.113.25
dig +short A mail.example.com

Frequently asked questions

How do I set a PTR record?

Ask the provider that owns the IP address. On AWS, Azure, Google Cloud and most VPS hosts there is a setting or support request for reverse DNS on an elastic or static IP.

What is forward-confirmed reverse DNS?

The PTR name resolves back to the same IP address. Receivers treat a match as a sign the sender is properly configured, and many reject mail without it.

Further reading

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