What is rDNS?

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rDNS entry - Resource Record

rDNS (reverse DNS):

In computer networking, reverse DNS lookup or reverse DNS resolution (rDNS) is associated with an IP address via querying DNS. It is usually “reverse” DNS lookup of an IP for a domain name.

rDNS Record:

Generally, we have DNS record for the domain name. The rDNS record is also known as PTR record. Without a valid reverse DNS record, many ISP (Internet Service Provider) or hosting provider will block your email. Reverse DNS as the name, it reverses the normal DNS record’s task.  90% of legitimate mail servers will reject your mail without rDNS setup.

Example:

Forward A record: dig mail.example.com
Answer section: mail.example.com 681 IN A 192.168.12.2
Reverse PTR record: dig -x 192.168.12.2
Answer section: 2.12.168.192.in-addr.arpa. 3600 IN PTR mail.example.com

Reverse DNS setup:

PTR records are 1 to 1 mapping that means one domain is assigned to a PTR record. If multiple PTR records are assigned to a single server, it creates a problem.

To create the reverse DNS zone file:

  • Login to your WHM portal.
  • Home -> DNS Functions -> Add a DNS Zone.
  • In the IP text box, enter your IP address.
    Note: It does not recognize the IPv6 address. To add the IPv6 reverse DNS zone (ip6.arpa), you must enter an IPv4 address in the IP text box. In the domain text box, enter the appropriate domain name for the new reverse DNS zone.
  • Click the AddZone button.

Name the IPv4 reverse DNS zone:

The format for naming the IPv4 address is listed below

  • Enter the last octet from the IP address.
  • Reverse the order of the remaining octet.
  • Add in-addr.arpa to the end of the remaining octets.
  • Enter this value in the domain text box.

Example: The reverse DNS zone for 192.168.12.1 is 1.12.168.192.in-addr.arpa.

Name the IPv6 reverse DNS zone:

The format for naming the IPv6 address is listed below

  • Identify the nibble. You want to specify in the zone file which is usually at the end of the IPv6 address.
    Note: Nibble in terms of a four-bit aggregation. The word represents each value between an IPv6 address dots.
  • Remove the nibble that you plan to specify in the zone file.
  • Enter the IPv6 address in reverse order.
  • Add ip6.arpa to the end of the zone file name.
  • Enter this value in the Domain text box.

Example:

If you begin with this IPv6 address 
0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.2.2.e.0.d.2.0.0.c.0.0.0.2.8.a 

and you want to specify that two nibbles (8.a in this case) within the zone file. You would create the zone file with the name. 
2.0.0.0.c.0.0.2.d.0.e.2.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.ip6.arpa

To edit the Reverse DNS zone file. Go to WHM Home -> DNS Functions -> Edit DNS Zone.

To create a PTR record:

  • Enter the octet(s) or nibble(s) in available text box under ‘Add new entries’.
  • Select PTR in an appropriate menu.
  • In the Hostname text box, enter the hostname that corresponding to the last octet(s) or nibble(s) in the IP address.

To Know more about the rDNS

RDNS Policies: xfinity | craigslist | AOL

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