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Setting up a PTR record and reverse DNS zone

What is a PTR record for?

A PoinTeR or PTR record is an A record in reverse: it associates server IP with a domain name. A PTR could be used:

  • As an anti-spam measure in email delivery. The destination mail server compares the PTR record of the sender mail server IP address with the host/domain part of a sender email address. Today, there exist more convenient modes of authentication, like DKIM, DMARC, and SPF, and usually, these are used together with PTR records.
  • For the convenience of tracing dedicated subnets. This allows the naming of subnet sections with human-readable domains instead of IP addresses.

Note: A PTR record can be added only in the reverse DNS zone (RDNS) and not in a regular domain zone. Therefore, in the Gcore UI, there is no PTR record in the list of resource records that can be added for a forward domain zone.

What is a reverse DNS lookup zone?

A reverse DNS lookup is a special zone that is intended to define the hostname by its IPv4 or IPv6 address using a PTR record.

Note: You can add a PTR record to a reverse lookup zone only if your zone is under the top-level domain (TLD) in-addr.arpa (for IPv4) or ip6.arpa (for IPv6). After creating a zone under the TLD, you can then create a PTR record in the UI.

An IPv4 host address AAA.BBB.CCC.DDD in the reverse DNS zone is represented in the format: DDD.CCC.BBB.AAA.in-addr.arpa. For example, a Gcore site address 80.240.113.62 in a reverse DNS query would be 62.113.240.80.in-addr.arpa.

And the IPv6 in a reverse DNS lookup will work the same way, but with a slight difference. For example, the address 2a03:90c0:501:2801::62 will be rendered as 6.2.1.0.8.2.1.0.5.0.c.0.9.3.0.a.2.ip6.arpa in the reverse DNS zone.

Configure reverse DNS zone and PTR record

You need to have a dedicated IP space (IPv4 or IPv6) to configure the reverse DNS zone. Here’s how to do it:

1. Contact an organization with RIR status (that is, a technical registrar authorized to create and delegate reverse DNS zones) and ask them to delegate the zone of your subnet to Gcore NSs.

2. When you get a reverse DNS zone, add it in the Gcore control panel according to our guide Create a DNS zone. Note: You can add a subnet up to /24 bits for your IP address.

3. Open the added reverse zone xx.xx.xx.in-addr-arpa in the control panel and add the PTR records you need.

Configure reverse DNS zone and PTR record

4. Specify Gcore’s name servers ns1.gcorelabs.net and ns2.gcdn.services for the subnet xx.xx.xx/24 and wait up to 24 hours for DNS cache updating.

Configure reverse DNS zone and PTR record

That’s it! The PTR record is now added and will prevent your mail from ending up as spam.

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