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What are the DNS extensions for IPv6

General

Since DNS is so “architecturally distant” from IP down there at layer three, the changes required are not extensive. RFC 1886, entitled IPv6 DNS Extensions and published in December 1995, was the IETF's first formalized attempt to describe the changes needed in DNS to support IPv6. It defines three specific modifications to DNS for IPv6.

New Resource Record Type—AAAA (IPv6 Address)

The regular DNS Address resource record is defined for a 32-bit IPv4 address, so a new one was created to allow a domain name to be associated with a 128-bit IPv6 address. The four “A”s (“AAAA”) are a mnemonic to indicate that the IPv6 address is four times the size of the IPv4 address. The AAAA record is structured in very much the same way as the A record in both binary and master file formats; it is just much larger. The DNS resource record Type value for AAAA is 28.

Example: www IN AAAA 3ffe:8280:10:a10::1

New Reverse Resolution Hierarchy:

A new hierarchical structure similar to IN-ADDR.ARPA is defined for IPv6 reverse lookups, but the IETF put it in a different top-level domain. The new domain is IP6.INT, and is used in a way similar to how IN-ADDR.ARPA works. However, since IPv6 addresses are expressed in hexadecimal instead of dotted-decimal, IP6.INT has sixteen subdomains “0” through “F”, and each of those has sixteen subdomains “0” through “F”, and so on, sixteen layers deep. Yes, this leads to a potentially frightfully large reverse resolution database!

Example:
Imagine your prefix is: 3ffe:8280:10:a10/60, then you write it in the complete form with all the zeros: 3ffe:8280:0010:0a10/60.
Next you write the prefix in reverse form with dots between every number: 0.1.a.0.0.1.0.0.0.8.2.8.e.f.f.3.
In the end you add the domain ip6.arpa at the end and like this you get the name of the reverse zone: 0.1.a.0.0.1.0.0.0.8.2.8.e.f.f.3.ip6.arpa.

Changes To Query Types And Resolution Procedure:

All query types that work with A records or result in A records being included in the Additional section of a reply must be changed to also handle AAAA records. Also, queries that would normally result in A records being returned in the Additional section must return the corresponding AAAA records only in the Answer section, not the Additional section.

Reverse DNS requests via dig

To lookup a Reverse DNS record you have to add the options "-n -x" as in the following example:

server:~$ dig -n -x 2a02:578:8000:2:212:71:0:79
[...]
9.7.0.0.0.0.0.0.1.7.0.0.2.1.2.0.2.0.0.0.0.0.0.8.8.7.5.0.2.0.a.2.ip6.arpa. IN PTR
[...]

Proposed Changes to the IPv6 DNS Extensions

In 2000, the IETF published RFC 2874, DNS Extensions to Support IPv6 Address Aggregation and Renumbering. This standard proposed a replacement for the IPv6 support introduced in RFC 1886, using a new record type, A6, instead of 1886's AAAA. The main difference between AAAA and A6 records is that the former are just whole addresses like A records, while A6 records can contain either a whole or partial address.

The idea behind RFC 2874 was that A6 records could be set up in a manner that complements the IPv6 format for unicast addresses. Then, name resolution would involve a technique called chaining to determine a full address for a name from a set of partially-specified address components. In essence, this would make the addresses behave much the way hierarchical names themselves work, providing some potential flexibility benefits.

For a couple of years, both RFC 1886 and RFC 2874 were proposed standards, and this led to considerable confusion. In August 2002, RFCs 3363 and 3364 were published, which clarified the situation with these two proposals. RFC 3363 represents the “Supreme Court decision”, which was that RFC 2874 and the A6 record be changed to experimental status and the AAAA record of RFC 1886 be kept as the DNS IPv6 standard.

The full explanation for the decision can be found in RFC 3364. In a nutshell, it boiled down to the IETF believing that there were significant potential risks in the successful implementation of RFC 2874. While the capabilities of the A6 record were “interesting”, it was not clear that they were needed, and given those risks, they felt sticking with RFC 1886 was the better move.

Links

(Reverse) DNS lookup IPv6
IPv6 Network tools

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