On July 1st, ARIN (American Registry for Internet Numbers) sent out a notification that it may not be able to give out IPv4 blocks of larger than 512 addresses. This officially makes the decade old prediction of IPv4 running out a reality, and makes it necessary for web hosts to start using IPv6 addresses.

Major Linux vendors like CentOS and Ubuntu already has IPv6 support, and web hosts have started enabling IPv6 for their services. As with any new technology, IPv6 also has its share of support requests, the most common one being setting reverse DNS for IPv6 addresses. IPv6 RDNS is a part of anti-spam measures some carriers use to block mail. So, setting the right RDNS is a priority for customers with IP blocks assigned to their dedicated servers. Here we’ll see how to setup IPv6 reverse DNS for cPanel/WHM servers.

Step 1 : Changing your IPv6 to RDNS nibble format

Let’s say your IP is “0:0:0:0:0:ffff:80b1:641e”, which can also be represented as “::ffff:80b1:641e”. This can be changed to a series of numbers separated by periods (.). For this, first expand double colons to full zeroes. Eg., “::ffff:80b1:641e” will become “0000:0000:0000:0000:0000:ffff:80b1:641e”.

Now, reverse the order of the characters, and put a period between each character. Eg., “0000:0000:0000:0000:0000:ffff:80b1:641e” will become “e.1.4.6.1.b.0.8.f.f.f.f.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0”.

Now, just add “ip6.arpa.” at the end of it like below, and you have the reverse DNS for “::ffff:80b1:641e”.

“e.1.4.6.1.b.0.8.f.f.f.f.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa.”

Step 2 : Updating IPv6 RDNS settings in WHM

Login to WHM and go to Home >> DNS Functions >> Add a DNS Zone

Enter your IP in the “IP” field, and the zone name as we created in the “Domain” field. In our example it will be “::ffff:80b1:641e” and “e.1.4.6.1.b.0.8.f.f.f.f.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa.” respectively.

Now go to Home >> DNS Functions >> Edit DNS Zone, and edit the PTR records for the domain you want to assign the RDNS for. For example, to assign the IP to mydomain.com, input “e.1.4.6.1.b.0.8.f.f.f.f.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa.” under “Add New Entries Below this Line”, choose the record type as “PTR” and input “mydomain.com” in the corresponding domain field.

Note: This procedure is applicable only if this cPanel/WHM server is assigned as the authoritative name server for your IPv6 address. You can find the authoritative name server of an IP by using the “nssearch” option of “dig” like shown below:

dig -6 +nssearch e.1.4.6.1.b.0.8.f.f.f.f.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa.
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