Reverse DNS lookups identify what IP address is associated with a particular domain or hostname. This works as the name implies the opposite of forward DNS lookups. Rather than a domain name being returned from a standard DNS query, the IP address is resolved.
Reverse DNS lookups are most commonly used by email servers in order to verify the validity of servers they are receiving messages from. The PTR records that are necessary for reverse DNS tell other mail servers that the IP of your mail server is authoritative for sending and receiving mail for your domain.
In this process, the IP owner will provide you with a zone for your mail server’s IP address, which is a special reverse DNS domain that ends in “in-addr.arpa”. The numbers that precede the “in-addr.arpa” are your IP block with the octets reversed.
The reverse DNS for the 192.168.1 class C would be “1.168.192.in-addr.arpa”. In this example, this reverse DNS zone would handle the reverse DNS for IPs 192.168.1.0 to 192.168.1.255.
If the IP block is smaller than a class C, the zone might be “27/1.168.192.in-addr.arpa” or “0-188.8.131.52.in-addr.arpa”. The difference is the syntax.
Once logged into your Constellix DNS dashboard, the following steps will guide you through the process of creating a reverse DNS domain with the information previously provided by the ISP.
1. Select Add Domain Option
From the upper right section of the Constellix DNS dashboard, click on the Add Domain button.
2. Enter Zone Information
Enter the zone that was provided by the ISP (or owner of the mail server’s IP block) and click on the Save button.
For full class C IP blocks, the syntax of delegation that should be entered is 147.94.208.in-addr.arpa.
3. Note the Assigned Nameservers
After adding your reverse DNS domain into the Constellix system you are provided with a list of nameservers that your reverse zone is assigned. Click on the Close button.
Once the reverse DNS domain is set up, a PTR record will need to be created. For assistance with this step, see our Create a PTR Record tutorial.
After the PTR Record has been created, the reverse DNS setup should be complete.
When a mail server performs a reverse DNS lookup, it will look for the following three requirements:
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