DNS Coach IPv6

Ipv6 Reverse Mapping

In honor of IPv6 Day, your DNS Made Easy team wants to help you with all your IPv6 needs.

In this article I will explain and demonstrate the process of reverse mapping an IPv6 address. At DNS Made Easy we often receive questions about IPv6 reverse mapping. While this subject can be intimidating, I assure you it is not as hard as it sounds. I hope that by the time you reach the end of this article, you will have a better understanding of the subject and be able to execute an IPv6 reverse mapping.

Why should you care?

IPv6 Reverse Mapping is constantly being used by mail servers. They use this technique regularly to check the reverse DNS of incoming mail servers to verify that the mail server is who it says it is. If there is no PTR record configured, then the receiving mail servers will reject the email.

Back to basics

An IPv6 address is 128 bits in size and is written as a series of eight hexadecimal strings separated by colons. An example of an IPv6 address looks like this:

2011:0df8:0001:0000:0000:0000:0000:0002

Writing out an IPv6 address can be daunting, especially if you have to deal with multiple addresses. In order to simplify the address IETF (Internet Engineering Task Force) created an abbreviated version. Here is how the previous IPv6 address would look in its abbreviated form:

2011:0df8:1::2

Since DNS Management is analyzed from right to left, and  IP addresses are read from left to right; the reverse zone for an IPv6 would be in reverse order (similar to IPv4).  So in order to create a PTR record for this address you will need to reverse the entire address and separate each character with a dot, and then end it with the domain (ip6.arpa) which is similar to the “in-addr.arpa” domain used in IPv4 reverse mapping.

A PTR Record for the above address would look like this:

2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.0.0.8.f.d.0.1.1.0.2.ip6.arpa

***You can also use an IPv6 Address Calculator (provided by Zytrax) here.

 

Ok, I get it… but how do you do it?

Here is an example of how you would set up a Reverse IPv6 address in DNS Made Easy.

  1. Contact the organization that is currently managing your reverse IPv6 address. Then have the reverse IPv6 block delegated to DNS Made Easy name servers. This organization is most likely going to be either your ISP or ARIN. If you are not sure what organization is currently managing your reverse IPv6 address you can run the following commands:

Linux/MAC users:

dig –x <and the IPv6 Address>

Example:

dig –x 2011:0df8:0001:0000:0000:0000:0000:0002

Windows Users:

nslookup <IPv6 address>

Example:

nslookup 2011:0df8:0001:0000:0000:0000:0000:0002

 

The output of either of these commands will give you a list of the name servers currently delegated to the Reverse IPv6 address: that would be the organization you would have to contact. If you are still not sure or are not familiar with command line tools you can contact DNS Made Easy support and we will be more than happy to assist you.

  1. For the rest of this example I will use the address 2011:0df8:1::/48. This gives the end user an 80-bit address for reverse mapping.
  1. Once the 2011:0df8:1::/48 netblock has been delegated to DNS Made Easy name servers you will create a domain that looks like this:

1.0.0.0.8.f.d.0.1.1.0.2.ip6.arpa

  1. Once you have created your domain you can set up PTR Records within that domain.
  • An example PTR record would look like this in the DNS Made Easy Control Panel:
PTR Record

PTR Record

 

***In addition here are two helpful tools you can use to help build a BIND IPv6 rDNS Zone.

http://rdns6.com/zone

http://www.zytrax.com/books/dns/ch3/#ipv6-tool

 

I hope you enjoyed this special holiday edition of DNS Coach (yes, we consider IPv6 Day a holiday, even if we still have to work).

Please keep checking our blog for more helpful hints.


Also published on Medium.