Configuring Reverse DNS for BIND in WHM

Reverse DNS uses pointer records (PTR) to convert IP addresses into domain names. This is the opposite of forward DNS, which uses A records to convert domain names into IP addresses.

PTR records require authoritative DNS nameservers

Before you start, please note that only the PTR records on authoritative DNS nameservers will work, and many hosting providers do not delegate authority for PTR records to their hosted customers.

To find the authoritative DNS nameservers of your server's main IP address, trace the SOA (Start Of Authority) with the dig command.

For example, if your server's main IP address is 192.0.32.10, you would run the following command:

dig +nssearch 32.0.192.in-addr.arpa.

If your server's DNS nameservers are not authoritative for your IP address, changes in your server's DNS nameservers will not take effect. You will need to contact your upstream provider either to delegate authority to your nameservers or to set up your PTR records for you.

Adding the reverse DNS zone file

To begin configuring reverse DNS, you will need to create the reverse DNS zone file.

To do so, you will use the Add a DNS Zone feature in the DNS Functions section of your WHM interface (Home >> DNS Functions >> Add a DNS Zone):

1. Enter the IP address in the IP field.
2. Enter the appropriate name for the new reverse DNS zone in the Domain field.
   • For information on naming the zone, read the instructions for naming an IPv4 zone or naming an IPv6 zone, below.
3. Click the Add Zone button.

The Domain Selection section of the Add DNS Zone screen.

Naming the IPv4 reverse DNS zone

To properly format the Domain field in the interface using IPv4:

1. Drop the last octet from the IP address.
2. Reverse the order of the remaining octets.
3. Append in-addr.arpa to the end of the remaining octets.
4. Enter this value in the Domain field of the Add a DNS Zone feature.

For example, the reverse DNS zone for 192.168.0.1 is 0.168.192.in-addr.arpa.

You will specify the last octet of the IP address within the zone file.

Naming the IPv6 reverse DNS zone

To properly format the Domain field in the interface using IPv6:

1. From the end of the IPv6 address, remove the nibbles that you are going to specify in the zone file.
2. Reverse the order of the remaining nibbles in the IPv6 address.
3. Append ip6.arpa to the end of the zone file's name.
4. Enter this value in the Domain field of the Add a DNS Zone feature.

Note: Nibble is the term for a four-bit aggregation. In this case, the word nibble represents each value between an IPv6 address' dots, whenever the address is not written in hexadecimal format.

For example, if you begin with the following IPv6 address:

0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.2.0.0.d.2.0.0.0.0.0.0.2.6.2

and you want to specify the last 2 nibbles (6.2 in this case) within the zone file, you would create the zone file with the name 2.0.0.0.0.0.0.2.d.0.0.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.ip6.arpa.

Editing the Reverse DNS Zone File

Once you have created your zone file, you need to create each PTR record. You can do this using the WHM Edit DNS Zone screen (Home >> DNS Functions >> Edit DNS Zone).

Note: Your zone file should already contain an NS entry for every authoritative nameserver associated with your server.

To create a PTR record:

1. Enter the octet(s) or nibble(s) in an available field under Add New Entries Below this Line.
2. Select PTR in the corresponding pull-down menu.
3. In the resulting Hostname field, enter the hostname that corresponds to the last octet(s) or nibble(s) in the IP address.

Creating a PTR record on the Edit DNS Zone screen.

Testing Your Records

After the reverse DNS information has propagated, run a test to ensure that reverse DNS is properly configured. To do so, you can use the following command:

IPv4:

host 192.168.0.1

IPv6:

host 0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.2.0.0.d.2.0.0.0.0.0.0.2.6.2

If you have properly configured your reverse DNS zone file, the output should resemble the following:

IPv4:

user@host: [~]# host 192.168.0.1
1.0.168.192.in-addr.arpa domain name pointer www.example.com.

IPv6:

user@host: [~]# host 0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.2.0.0.d.2.0.0.0.0.0.0.2.6.2
2.0.0.0.0.0.0.2.d.0.0.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.ip6.arpa domain name pointer www.example.com