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.

A PTR record can be added only in the reverse DNS zone (RDNS) and not in a regular domain zone. Therefore, in the Gcore Customer Portal, there is no PTR record in the list of DNS 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.

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 Customer Portal according to the guide Getting started with Managed DNS.

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.

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.

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