What is reverse DNS lookup-Alibaba Cloud DNS(DNS)-阿里云帮助中心

Reverse DNS Lookup maps an IP address to a domain name (the opposite of the standard process that maps a domain name to an IP address). It is commonly used for mail server identity verification, spam filtering, and IP reputation assessment. This enhances the trustworthiness of your server and domain and improves email delivery success rates. Alibaba Cloud DNS provides a Reverse DNS Lookup feature that lets you configure PTR records for public IP addresses under your Alibaba Cloud account (EIPs or ECS static public IP addresses) to enable reverse DNS lookup for those IPs.

Scenarios

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

The general verification flow is as follows:

Query the PTR record of the source IP to obtain 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, verification succeeds.

Example:

A mailbox receives an email from IP address 47.92.*.*.
Run the command dig -x 47.92.*.* +short to query the reverse DNS domain name. The result might be mail.example.com.
Query the A record for that domain name using dig mail.example.com +short. The result might be 47.92.*.*.
Because the resolved IP matches the original source IP, the sender's identity passes verification.

Billing details

Billing method: Reverse DNS Lookup uses a pay-as-you-go billing model.

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

Billable item	Unit price	Description
PTR record	CNY 1 per record per day	You are billed daily based on the number of PTR records held under your account the previous day. Note Even disabled PTR records incur charges. To stop billing, delete the record.

Cost estimation example

Usage: You hold 5 PTR records (including both enabled and disabled records).
Estimated monthly cost: 5 × CNY 1 per record per day × 30 days/month = CNY 150 per month.

Enable the service

Go to Alibaba Cloud DNS – Reverse DNS lookup.
Click Enable Now.
1. If you have never used the Alibaba Cloud DNS (pay-as-you-go) service, a prompt appears asking you to activate a service code. Each Alibaba Cloud account needs to do this only once.
  1. Click Activate to go to the Alibaba Cloud DNS (pay-as-you-go) purchase page.
    
    Important
    Note: This "purchase" only activates the service. The activation itself incurs no charge.
    
    The purchase page shows that the service includes public authoritative network traffic analysis and self-managed DNS. Charges are based on your actual configuration or DNS query volume. Read and accept the Product Service Agreement and Service Level Agreement, then click Buy now to complete activation.
  2. Return to Alibaba Cloud DNS – Reverse DNS lookup and refresh the page.
  3. Click Enable Now again.
2. If you have previously used another Alibaba Cloud DNS (pay-as-you-go) service, activation succeeds immediately and the PTR record list page appears.
  
  Important
  Once the service is activated, it cannot be shut down and will not incur any charges.