What is reverse DNS lookup - Alibaba Cloud DNS - Alibaba Cloud Documentation Center

Reverse DNS Lookup finds the domain name associated with an IP address. This is the opposite of a forward Domain Name System (DNS) lookup, which finds the IP address for a domain name. It is commonly used to verify mail server identities, filter spam, and assess IP reputation. This process improves the credibility of your servers and domain names and improves email deliverability. The Reverse DNS Lookup feature in Alibaba Cloud DNS lets you configure PTR records for public IP addresses in your Alibaba Cloud account. These addresses include elastic IP addresses (EIPs) and static public IP addresses of ECS instances. These records enable reverse DNS lookups for the specified IP addresses.

Scenarios

Scenario	Description
Mail server verification	Many mail servers check if the sender's server IP address has a valid PTR record. If no PTR record exists, or if it does not match the HELO/EHLO domain name, the email may be classified as spam.
Security and identity verification	Some services, such as Secure Shell (SSH), FTP, and databases, may use reverse DNS lookups to authenticate client identities.

The verification process is as follows:

Query the PTR record of the source IP address to retrieve a domain name.
Query the A record of that domain name to retrieve an IP address.
If the IP address returned by the A record matches the source IP address, the verification is successful.

Example:

A mailbox receives an email from the IP address 47.92.*.*.
Run the dig -x 47.92.*.* +short command to query the reverse DNS lookup record. The command returns a domain name, such as mail.example.com.
Run the dig mail.example.com +short command to query the A record of the domain name. The command returns an IP address, such as 47.92.*.*.
Because the returned IP address matches the source IP address, the sender's identity is authenticated.

Billing

Billing method: The Reverse DNS Lookup feature uses the pay-as-you-go billing method.

Billable item: You are charged daily based on the number of your PTR records.

Billable item	Unit price	Description
PTR record	USD 0.15/record/day	You are charged daily based on the number of PTR records maintained in your account on the previous day. Note You are still charged for records even if they are disabled. To stop the billing, delete the record.

Cost estimation example

Usage: You maintain 5 PTR records, including enabled and disabled records.
Estimated monthly cost: 5 × 0.15 USD/record/day × 30 days/month = 22.5 USD/month.

Enable the service

Go to Alibaba Cloud DNS - Reverse DNS Lookup.
Click Enable Now.
1. If you have not used the Alibaba Cloud DNS (Pay-As-You-Go) service, you are prompted to activate it. You only need to activate the service once for each Alibaba Cloud account.
  1. Click Activate to go to the Alibaba Cloud DNS (pay-as-you-go) purchase page and complete the purchase.
    Important
    Note: The purchase step only activates the service. No fees are charged for this activation.
  2. Return to Alibaba Cloud DNS - Reverse DNS Lookup, and refresh the page.
  3. Click Enable Now again.
2. If you have used other Alibaba Cloud DNS (Pay-As-You-Go) services before, the service is enabled directly, and the PTR record list page is displayed.
  Important
  The service cannot be disabled after it is enabled. No fees are charged for enabling the service.