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Reverse DNS Overview

Reverse DNS resolves an IP address back to a domain, as opposed to forward DNS that maps a domain to an IP address.

Since an IP may be used by multiple domains, it is necessary to verify if this is the case before reverse DNS. It is impossible to start from the IP to traverse the entire DNS system for verification due to the heavy workload. Therefore, RFC 1035 defines a pointer (PTR) record to point an IP address to a domain.

Note：

Private DNS refers to the resolution of domains in a private network, while Public DNS is oriented to internet users. Reverse DNS can be carried out in a public or private network, and the definitions and features involved are consistent in both cases.

Adding a PTR Record

Before adding a PTR record, you must create the reverse private domain of the IP range first.

1. If your target IP range is 192.168.0.1, the corresponding reverse private domain should be 1.0.168.192.in-addr.arpa. In this case, we recommend creating the reverse private primary domain 0.168.192.in-addr.arpa, and adding a subdomain with the host as 0 to point the reverse private domain to the target domain.
2. When you add the private domain 0.168.192.in-addr.arpa in Private DNS and add the PTR record with the host as 1 pointing to www.dnspod.com, the reverse DNS of the IP address 192.168.0.1 is www.dnspod.com.

Note：

The prefix of the combination of the host and the private domain (the prefix of in-addr.arpa) must comply with the IPV4 format specification. For example, if the private domain created is 10.255.123.in-addr.arpa, the host can only be an integer between 0 and 255.

Adding Reverse Private Domain

The following steps take the reverse private domain 0.168.192.in-addr.arpa as an example.

1. Log in to the Private DNS console.

2. On the Private Domain List page, click Create Private Domain.

3. On the Create Private Domain page, enter the reverse DNS domain and select a VPC.
   For example, enter 0.168.192.in-addr.arpa:
   

   Note：

   • For a better experience, we recommend creating a private domain and setting a DNS record for it first before associating a VPC.
   • If no VPCs are displayed for the currently selected region, create one in the VPC console.
   • If existing VPCs cannot meet your needs, modify them in the VPC console.

4. Click Confirm to add the reverse private domain. You can directly go to the DNS records page of the domain to add a PTR record:
   

Private IP Range

On the public network, reverse DNS cannot be provided by DNS service providers, because the management permissions of IP addresses are owned by ISPs. Therefore, it is necessary to apply to ISPs for adding reverse DNS records.

However, the case is different for a VPC, where you can use any of the following as your VPC range. VPC CIDR supports any of the following IP ranges. If you need to achieve communication between different VPCs, ensure the configurations of CIDR blocks at both ends do not overlap.

`10.0.0.0` - `10.255.255.255` (mask range: 12 to 28)
`172.16.0.0` - `172.31.255.255` (mask range: 12 to 28)
`192.168.0.0` - `192.168.255.255` (mask range: 16 to 28)

Correspondingly, you can also create reverse private domains within these three IP ranges.

Note：

• For more information about private IP ranges, see Network Planning.
• For more information about how to add PTR records in Private DNS, see PTR Record.