Reverse DNS lookup

Enter an IP address to see its reverse DNS name, the PTR record. The tool then looks the name up again to check that it points back to the same address, the check mail servers make before they accept your email.

An IPv4 address such as 8.8.4.4 or an IPv6 address such as 2001:4860:4860::8888.
Reverse DNS Enter an address above, or use your own. You get its PTR record and whether that name points back to the address.

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The address you enter is sent to our server, which asks Cloudflare's DNS resolver for its PTR record and for the addresses of the name it returns. Nothing is stored or logged.

How reverse DNS works

A normal DNS lookup turns a name into an address. Reverse DNS goes the other way: it asks for the PTR record of a special name built from the address. For IPv4 the four numbers are written backwards under in-addr.arpa, so 8.8.4.4 becomes 4.4.8.8.in-addr.arpa. For IPv6 the address is written out in full, all 32 hex digits including the zeros that "::" hides, and each digit becomes one label, backwards, under ip6.arpa.

Anyone who controls the reverse zone can put any name in a PTR record, so a PTR on its own proves little. That is why the tool looks the name up forward again (A records for IPv4, AAAA for IPv6). When one of the answers is the address you started with, the reverse DNS is forward-confirmed (FCrDNS): whoever controls the name agrees.

Mail servers care because spam often comes from home connections and hijacked machines with generic or missing reverse DNS. Gmail, Outlook and most other providers check that a sending server has a PTR record that resolves back to its IP, and they reject or spam-folder mail that fails. For a mail server, set the PTR to the same name the server uses in its SMTP greeting (HELO).

Example

8.8.4.4 is one of Google's public DNS servers. The tool asks for the PTR record of 4.4.8.8.in-addr.arpa and gets dns.google. It then asks for the A records of dns.google, which are 8.8.4.4 and 8.8.8.8: 8.8.4.4 is among them, so the reverse DNS is forward-confirmed. For an IPv6 address such as 2001:db8::1, the name asked for is 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa.

Questions

What is a reverse DNS lookup?

It finds the host name for an IP address, the opposite of a normal DNS lookup. The name is stored in a PTR record. For example, the PTR record of 1.1.1.1 is one.one.one.one, Cloudflare's public DNS resolver.

What is a PTR record?

A DNS record that points from an address to a name. It lives in the reverse zones in-addr.arpa (IPv4) and ip6.arpa (IPv6), which are run by whoever holds the address block: your hosting or internet provider, not your domain registrar.

How do I set up reverse DNS for my server?

In your hosting provider's control panel: most VPS providers have a "reverse DNS" or "PTR" field next to each IP address. First create an A (or AAAA) record for the name, such as mail.example.com pointing to the server's IP, then set the PTR to that name. Some providers only set it on request through a support ticket.

Why do mail servers check reverse DNS?

It is a cheap first filter against spam. A server without reverse DNS, or with a PTR name that does not resolve back to its address, looks like a home connection or a hijacked machine. Gmail and Outlook require a valid PTR record for senders, so mail from a server that fails this check is often rejected or lands in spam.

Why does my IP address have no reverse DNS?

Nobody has set a PTR record for it. Many providers leave new server addresses without one, or give them a generic name such as 203-0-113-5.static.example.net. Home connections usually get the internet provider's generic name, which you cannot change.