DNS Resolver Information Key for DNS64

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Table of Contents

1. Introduction

DNS64 [RFC6147] performed by a DNS resolver together with NAT64 [RFC6146] allows an IPv6-only client to initiate communications by name to an IPv4-only server. To achieve this, the DNS resolver synthesizes AAAA records from A records. However, this breaks DNSSEC [RFC4033] [RFC4034] [RFC4035] validation for DNS clients of the resolver if they perform their own validation. [RFC6147] Section 3 discusses this in detail. In general, a validating DNS client has to be aware that a resolver is performing DNS64.¶

[RFC9606] specifies a DNS resource record (RR) type RESINFO to allow resolvers to publish information about their capabilities and policies. This can be used to inform DNS clients that DNS64 is performed by the DNS resolver.¶

2. Relation to RFC 7050

[RFC7050] describes a heuristic to learn the IPv6 prefix used by a NAT64 gateway. Nodes can use this information to perform local address synthesis, for example using 464XLAT [RFC6877].¶

This has security implications, making [RFC7050] difficult to deploy. A modern mechanism, like PREF64 [RFC8781] is easier to deploy, leading to a desire to deprecate [RFC7050] entirely.¶

A validating DNS client on the other hand still needs to know about the presence of DNS64 on the DNS resolver it uses, as noted in the introduction. Using the [RFC9606] RESINFO mechanism and not using the learned information for address synthesis avoids the security problems of [RFC7050].¶

3. dns64 Resolver Information Key/Value

dns64:

The presence of this key indicates that the DNS resolver performs address synthesis.¶

Note that, per the rules for the keys defined in Section 6.4 of [RFC6763], if there is no '=' in a key, then it is a boolean attribute, simply identified as being present, with no value.¶

The DNS resolver MAY return an IPv6 prefix or a comma separated list of prefixes to indicate not just that DNS64 is being performed but also to let the client know which prefix or prefixes are in use. The prefixes MUST be represented using the canonical representation format of [RFC5952].¶

resolver.example.net. 7200 IN RESINFO dns64

resolver.example.net. 7200 IN RESINFO dns64=64:ff9b::/96

resolver.example.net. 7200 IN RESINFO
  dns64=64:ff9b::/96,2001:db8:64::/56

4. Security Considerations

The security considerations of [RFC9606] apply.¶

5. IANA Considerations

The IANA is requested to add the key "dns64", with the description of "The presence of the key indicates that DNS64 address synthesis is performed", and a reference to this document.¶

6. References

Author's Address