What is RDNS and Why is it Important?

Reverse DNS is used to determine what hostname is associated with a specific IP address. A Reverse DNS record must reside on the name servers set as the owner of a given IP/Netblock. That is, Hosting.com cannot set a Reverse DNS record for an IP that is not part of our network.

Reverse DNS lookups for IPv4 addresses use the special domain in-addr.arpa. An IPv4 address is represented in the in-addr.arpa domain by a sequence of bytes in reverse order represented as decimal numbers separated by dots with the suffix .in-addr.arpa. For example; the reverse lookup domain name corresponding to the IPv4 address 1.2.3.4 is 4.3.2.1.in-addr.arpa. A host name for 1.2.3.4 can be obtained by issuing a DNS query for the PTR record for that special address 4.3.2.1.in-addr.arpa.

It is important to setup a RDNS zone if mail is being sent from the server as most ISP’s reject mail from a server if there is not a valid RDNS zone attached to the IP address. Some ISP’s take this a step further and ensure a Forward Confirmed reverse DNS (FCrDNS) zone exists. This is typically enough verification in applying for a whitelist of an IP address as spammers and phishers cannot pass verification since they use zombie computers at their disposal and thus cannot make those DNS changes. A typical FCrDNS lookup will look like the below and can be checked at a site such as this one.

Reverse DNS (PTR) exists and claims to be: domain.com.

Forward DNS for domain.com is: 1.2.3.4.

DNS is consistent.

Once a server’s hostname is set RDNS can be configured. Please contact support@hosting.com to have this setup for you.