Reverse Resolution

What Is Reverse Resolution?

A PTR record provides the domain name associated with an IP address. It is the opposite of a regular DNS lookup. PTR records are used in many network applications. For example, email servers use reverse resolution to verify the sender's IP address to reduce spam and network fraud.

After a recipient server receives an email, it checks whether the IP address and domain name of the sender server are trustworthy and determines whether the email is spam. If the recipient server fails to obtain the domain name mapped to the sender's IP address, it concludes that the email is sent by a malicious host and rejects it. It is necessary to configure pointer records (PTR) to point the IP addresses of your email servers to domain names.

In the following figure, an ECS serves as an email server, and a PTR record is configured to map the EIP of the ECS to the domain name configured for accessing the email server.

Figure 1 Reverse resolution
Click to enlarge

Figure 1 shows only the process for reverse resolution. Information about how an email server checks the credibility of the sender's IP address and whether the domain name is available on the Internet is not provided here.

If no PTR records are configured, the recipient server will treat emails from the email server as spam or malicious and discard them.

Scenarios

Reverse resolution is mainly used for email server verification in the following scenario's:

Anti-spam: Email servers usually use reverse resolution to verify the sender's IP address. If the IP address cannot be resolved to a valid domain name or the resolution result does not match the sender information in the email, the email may be marked as spam or rejected.
SPF record verification: An SPF record specifies the IP addresses authorized to send emails from a domain name. Reverse resolution checks whether the sender's IP address is in the SPF record.

Product Advantages

Improved email delivery rate
You can configure correct reverse DNS records to improve the email delivery rate and reduce the risk of being identified as spam.
Enhanced network security
Reverse resolution helps network administrators quickly locate and identify devices on the network and respond to and handle security events. For example, when a DDoS attack occurs, you can use reverse resolution to quickly locate the attack source.
Simplified troubleshooting
Reverse resolution can translate an IP address into a domain name that is easier to understand. This helps technical personnel quickly locate faults.

Functions

**Table 1** Common functions of reverse resolution
Function	Description
PTR record	DNS allows you to configure PTR records for EIPs and global EIPs. You can create, modify, delete, and view PTR records.
TTL	TTL is short for time to live, which specifies how long a record record is cached on a local DNS server. It is measured in seconds. The TTL value ranges from 1 to 2147483647.