IPv6-only Capable Resolvers Utilising NAT64

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Table of Contents

1. Introduction

This document describes how an IPv6-only iterative resolver can use stateful NAT64 [NAT64] to connect to an IPv4-only authoritative server by performing IPv4-to-IPv6 address translation [RFC6052] when generating a query. When a specific DNS zone is only served by an IPv4-only authoritative server (which has only an A record), an IPv6-only iterative resolver cannot resolve that zone due to having no access to an IPv4 network. However, by performing IPv4-to-IPv6 address translation and utilising the stateful NAT64, it will be possible to access an IPv4-only authoritative server.¶

This document is meant to exemplify how existing tools can be used to allow IPv6-only resolvers to reach IPv4-only resolvers. DNS is thus seen as an application that uses NAT64.¶

The document focuses on the exchanges between iterative resolvers and authoritative resolvers but can be generalized to cover communications between IPv6-only recursive resolvers and IPv4-only resolvers.¶

2. Terminology

• Iterative resolver: A DNS server that repeatedly makes non-recursive queries and follows referrals and/or aliases, as defined in DNS Terminology [RFC8499]¶
• IPv6-only iterative resolvers: Iterative resolvers with only IPv6 connectivity.¶
• IPv6/IPv4 translator: A function that translates IPv6 packets to IPv4 packets and vice versa. It is only required that the communication initiated from the IPv6 side be supported.¶
• IPv4-only authoritative server: An authoritative server that either has only IPv4 connectivity or is registered with only an A record, making it accessible solely via IPv4.¶

3. Motivation and Problem Solved

An iterative resolver is one of the applications that require IPv4 connectivity. As stated in BCP91 [RFC3901], “every recursive name server SHOULD be either IPv4-only or dual stack.” This is because some authoritative servers do not support IPv6. As of 2023, even some of the most frequently queried authoritative servers cannot be accessed via IPv6. Without the utilization of an IPv6/IPv4 translation mechanism, IPv6-only resolvers need to forward queries to a dual-stack recursive name server performing the iterative queries.¶

The current situation where an iterative resolver cannot operate without IPv4 reachability may hinder the operation of a network's own iterative resolver in an IPv6-only network. Therefore, this document describes how iterative resolvers can be used without issues in IPv6-only networks by utilising NAT64 as an IPv6/IPv4 translation mechanism.¶

The NAT64/DNS64 mechanisms enable IPv6-only clients in an IPv6-only network to communicate with remote IPv4-only nodes. However, applications that rely upon IPv4 address literals instead of DNS names will fail (unless 464XLAT [RFC6877] is used). An iterative resolver is a service that uses literal IP addresses, so it cannot use the DNS64. This problem can be solved by the iterative resolver converting IPv4 addresses to IPv6 addresses using the Pref64::/n NAT64 prefix and following the address translation algorithm in [RFC6052]. In doing so, an IPv6 packet conveying the query is directed to a stateful NAT64 function that converts the IPv6 packet to an IPv4 packet. With this implementation, an iterative resolver can be operated even inside an IPv6-only network.¶

            +---------------------+         +---------------+
            |IPv6 network         |         |    IPv4       |
            |              +-------------+  |  Internet     |
            |           |--| Name server |--|               |
            |           |  | with DNS64  |  |  +----+       |
            |  +----+   |  +-------------+  |  | H2 |       |
            |  | H1 |---|         |         |  +----+       |
            |  +----+   |   +------------+  |  192.0.2.1    |
            |           |---| IPv6/IPv4  |--|               |
            |           |   | Translator |  |               |
            |           |   +------------+  |               |
            |           |         |         |               |
            +---------------------+         +---------------+

            +---------------------+         +---------------+
            |IPv6 network         |         |    IPv4       |
            |                 +--------+    |  Internet     |
            |           |-----| Name   |----|               |
            | +-----+   |     | server |    |  +----+       |
            | | H1  |   |     +--------+    |  | H2 |       |
            | |with |---|         |         |  +----+       |
            | |DNS64|   |   +------------+  |  192.0.2.1    |
            | +----+    |---| IPv6/IPv4  |--|               |
            |           |   | Translator |  |               |
            |           |   +------------+  |               |
            |           |         |         |               |
            +---------------------+         +---------------+

      +--------------------------+         +----------------------+
      | IPv6 network             |         |    IPv4              |
      |                          |         |  Internet            |
      |                |         |         |                      |
      | +----------+   |         |         |  +--------------+    |
      | |IPv6-only |   |         |         |  |Authoritative |    |
      | |Iterative |   |         |         |  |server        |    |
      | |resolver  |---|   +------------+  |  +--------------+    |
      | +----------+   |---| IPv6/IPv4  |--|  192.0.2.1           |
      |                |   | Translator |  |                      |
      |                    +------------+  |                      |
      |                          |         |                      |
      +--------------------------+         +----------------------+

4. Solution with Existing Protocols

This section describes the mechanism of an IPv6-only capable resolver utilising stateful NAT64. We assume that one or more IPv6/IPv4 translators [NAT64] are connecting an IPv6 network to an IPv4 network. The stateful NAT64 provides translation service and bridges the two networks, allowing communication between IPv6-only hosts and IPv4-only hosts. The IPv6-only capable resolver proposed in this document performs the IPv4 to IPv6 synthesis for the resolver to communicate with IPv4-only servers via stateful NAT64. By using stateful NAT64, this IPv6-only iterative resolver aligns with the dual-stack requirements of BCP91 [RFC3901].¶

5. Security Considerations

The use of NAT64 for address translation does not affect DNSSEC operations as no part of the DNS message is altered.¶

6. IANA Considerations

This document has no IANA actions.¶

7. Implementation Status

BIND has a work-in-progress branch available at:¶

https://gitlab.isc.org/isc-projects/bind9/-/merge_requests/6334/commits¶

Unbound has an implementation in source, available from below PR:¶

https://github.com/NLnetLabs/unbound/pull/722¶

Building from source or using a version after Version 1.19 will allow to use this function.¶

Acknowledgments

TODO: acknowledge people.¶

Thank you for reading this draft.¶

Authors' Addresses