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PTR record

A PTR record maps an IP address back to a hostname. How in-addr.arpa works, who can set it, why mail servers need forward-confirmed reverse DNS, and how to check it.

What it is

A PTR record (pointer) answers the reverse question: given an IP address, what is its hostname? It lives in a special tree. For IPv4, the address is written backwards under in-addr.arpa; for IPv6, every hex digit (nibble) is reversed under ip6.arpa (RFC 1035, RFC 3596).

10.113.0.203.in-addr.arpa.  3600  IN  PTR  mail.example.com.

That record says 203.0.113.10 is mail.example.com. It's independent of the forward A record: neither creates the other.

The fields

Field Example Meaning
Name 10.113.0.203.in-addr.arpa. The IP address, reversed, under the arpa tree
TTL 3600 Seconds resolvers may cache it
Type PTR Pointer
Data mail.example.com. The hostname for that address

An address normally has one PTR. Several are allowed, but many programs read only the first.

Who can set it

Whoever controls the IP range, not whoever owns the domain. Reverse zones are delegated by the regional internet registries to the ISP, cloud or hosting provider holding the block. So you set a PTR in your provider's panel (AWS Elastic IPs, Hetzner, DigitalOcean droplet names and OVH all offer it), through a support ticket, or, for a delegated block, in your own reverse zone. Your domain's DNS provider can't do it, and a PTR record added to your forward zone does nothing. Home and office connections usually can't set one at all.

Why it matters for email

Receiving mail servers check the sending IP's reverse DNS. Gmail's sender guidelines require sending IPs to have a PTR whose hostname resolves back to the same IP. This is forward-confirmed reverse DNS (FCrDNS):

  1. 203.0.113.10 has a PTR of mail.example.com.
  2. mail.example.com has an A record of 203.0.113.10.

A generic name like 203-0-113-10.static.provider.net passes the check but looks like a residential or bulk host to spam filters; a name in your own domain is better. If the server also sends over IPv6, its IPv6 address needs the same.

How to check it

Enter an IP address in our DNS Lookup; 1.1.1.1 returns one.one.one.one. The IP Lookup shows the PTR along with the network owner, and the Blacklist Checker tells you whether a mail IP is listed. From a terminal:

dig -x 1.1.1.1 +short
dig -x 2606:4700:4700::1111 +short
nslookup 1.1.1.1

dig -x builds the in-addr.arpa or ip6.arpa name for you.

Common mistakes

  • Asking your DNS provider to add it. Only the IP's provider can.
  • PTR without a matching A record (or the A record pointing elsewhere), so the forward-confirmed check fails.
  • Keeping the provider's default generic hostname on a mail server.
  • Forgetting IPv6: a mail server with an AAAA record and no IPv6 PTR gets rejected over IPv6 while IPv4 works.
  • Expecting PTR to matter for websites. Browsers never look it up; it's for mail, logs and diagnostics.