DNS64: DNS extensions for Network Address Translation from IPv6 Clients to IPv4 Servers
draft-ietf-behave-dns64-05

Abstract

DNS64 is a mechanism for synthesizing AAAA records from A records. DNS64 is used with an IPv6/IPv4 translator to enable client-server communication between an IPv6-only client and an IPv4-only server, without requiring any changes to either the IPv6 or the IPv4 node, for the class of applications that work through NATs. This document specifies DNS64, and provides suggestions on how it should be deployed in conjunction with IPv6/IPv4 translators.

Status of this Memo

This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”

The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.

This Internet-Draft will expire on June 21, 2010.

Copyright Notice

Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the BSD License.

Table of Contents

1.  Introduction
2.  Overview
3.  Background to DNS64 - DNSSEC interaction
4.  Terminology
5.  DNS64 Normative Specification
    5.1.  Resolving AAAA queries and the answer section
        5.1.1.  The answer when there is AAAA data available
        5.1.2.  The answer when there is an error
        5.1.3.  Special exclusion set for AAAA records
        5.1.4.  Dealing with CNAME and DNAME
        5.1.5.  Data for the answer when performing synthesis
        5.1.6.  Performing the synthesis
        5.1.7.  Querying in parallel
    5.2.  Generation of the IPv6 representations of IPv4 addresses
    5.3.  Handling other RRs and the Additional Section
        5.3.1.  PTR queries
        5.3.2.  Handling the additional section
        5.3.3.  Other records
    5.4.  Assembling a synthesized response to a AAAA query
    5.5.  DNSSEC processing: DNS64 in recursive server mode
6.  Deployment notes
    6.1.  DNS resolvers and DNS64
    6.2.  DNSSEC validators and DNS64
    6.3.  DNS64 and multihomed and dual-stack hosts
        6.3.1.  IPv6 multihomed hosts
        6.3.2.  Accidental dual-stack DNS64 use
        6.3.3.  Intentional dual-stack DNS64 use
7.  Security Considerations
8.  IANA Considerations
9.  Contributors
10.  Acknowledgements
11.  References
    11.1.  Normative References
    11.2.  Informative References
Appendix A.  Deployment scenarios and examples
    A.1.  Example of IPv6/IPv4 address transformation algorithm
    A.2.  Example of An-IPv6-network-to-IPv4-Internet setup with DNS64 in DNS server mode
    A.3.  An example of an-IPv6-network-to-IPv4-Internet setup with DNS64 in stub-resolver mode
    A.4.  Example of IPv6-Internet-to-an-IPv4-network setup DNS64 in DNS server mode
Appendix B.  Motivations and Implications of synthesizing AAAA RR when real AAAA RR exists
§  Authors' Addresses

1.  Introduction

This document specifies DNS64, a mechanism that is part of the toolbox for IPv6-IPv4 transition and co-existence. DNS64, used together with an IPv6/IPv4 translator such as NAT64 [I‑D.ietf‑behave‑v6v4‑xlate‑stateful] (Bagnulo, M., Matthews, P., and I. Beijnum, “Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers,” March 2010.), allows an IPv6-only client to initiate communications by name to an IPv4-only server.

DNS64 is a mechanism for synthesizing AAAA resource records (RRs) from A RRs. A synthetic AAAA RR created by the DNS64 from an original A RR contains the same FQDN of the original A RR but it contains an IPv6 address instead of an IPv4 address. The IPv6 address is an IPv6 representation of the IPv4 address contained in the original A RR. The IPv6 representation of the IPv4 address is algorithmically generated from the IPv4 address returned in the A RR and a set of parameters configured in the DNS64 (typically, an IPv6 prefix used by IPv6 representations of IPv4 addresses and optionally other parameters).

Together with a IPv6/IPv4 translator, these two mechanisms allow an IPv6-only client to initiate communications to an IPv4-only server using the FQDN of the server.

These mechanisms are expected to play a critical role in the IPv4-IPv6 transition and co-existence. Due to IPv4 address depletion, it is likely that in the future, many IPv6-only clients will want to connect to IPv4-only servers. In the typical case, the approach only requires the deployment of IPv6/IPv4 translators that connect an IPv6-only network to an IPv4-only network, along with the deployment of one or more DNS64-enabled name servers. However, some advanced features require performing the DNS64 function directly by the end-hosts themselves.

2.  Overview

This section provides a non-normative introduction to the DNS64 mechanism.

We assume that we have an IPv6/IPv4 translator box connecting an IPv4 network and an IPv6 network. The IPv6/IPv4 translator device provides translation services between the two networks enabling communication between IPv4-only hosts and IPv6-only hosts. (NOTE: By IPv6-only hosts we mean hosts running IPv6-only applications, hosts that can only use IPv6, as well as the cases where only IPv6 connectivity is available to the client. By IPv4-only servers we mean servers running IPv4-only applications, servers that can only use IPv4, as well as the cases where only IPv4 connectivity is available to the server). The IPv6/IPv4 translator used in conjunction with DNS64 must allow communications initiated from the IPv6-only host to the IPv4-only host.

To allow an IPv6 initiator to do a standard AAAA RR DNS lookup to learn the address of the responder, DNS64 is used to synthesize a AAAA record from an A record containing a real IPv4 address of the responder, whenever the DNS64 service cannot retrieve a AAAA record for the requested host name. The DNS64 device appears as a regular recursive resolver for the IPv6 initiator. The DNS64 box receives an AAAA DNS query generated by the IPv6 initiator. It first attempts a recursive resolution for the requested AAAA records. If there is no AAAA record available for the target node (which is the normal case when the target node is an IPv4-only node), DNS64 performs a query for A records. If any A records are discovered, DNS64 creates a synthetic AAAA RR from the information retrieved in each A RR.

The FQDN of a synthetic AAAA RR is the same as that of the original A RR, but an IPv6 representation of the IPv4 address contained in the original A RR is included in the AAAA RR. The IPv6 representation of the IPv4 address is algorithmically generated from the IPv4 address and additional parameters configured in the DNS64. Among those parameters configured in the DNS64, there is at least one IPv6 prefix, called Pref64::/n. The IPv6 address representing IPv4 addresses included in the AAAA RR synthesized by the DNS64 function contain Pref64::/n and they also embed the original IPv4 address.

The same algorithm and the same Pref64::/n prefix or prefixes must be configured both in the DNS64 device and the IPv6/IPv4 translator, so that both can algorithmically generate the same IPv6 representation for a given IPv4 address. In addition, it is required that IPv6 packets addressed to an IPv6 destination that contains the Pref64::/n be delivered to the IPv6/IPv4 translator, so they can be translated into IPv4 packets.

Once the DNS64 has synthesized the AAAA RR, the synthetic AAAA RR is passed back to the IPv6 initiator, which will initiate an IPv6 communication with the IPv6 address associated with the IPv4 receiver. The packet will be routed to the IPv6/IPv4 translator which will forward it to the IPv4 network .

In general, the only shared state between the DNS64 and the IPv6/IPv4 translator is the Pref64::/n and an optional set of static parameters. The Pref64::/n and the set of static parameters must be configured to be the same on both; there is no communication between the DNS64 device and IPv6/IPv4 translator functions. The mechanism to be used for configuring the parameters of the DNS64 is beyond the scope of this memo.

The prefixes to be used as Pref64::/n and their applicability are discussed in [I‑D.ietf‑behave‑address‑format] (Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li, “IPv6 Addressing of IPv4/IPv6 Translators,” April 2010.). There are two types of prefixes that can be used as Pref64::/n.

The Pref64::/n can be the Well-Known prefix 64:FF9B::/96 reserved by [I‑D.ietf‑behave‑address‑format] (Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li, “IPv6 Addressing of IPv4/IPv6 Translators,” April 2010.) for the purpose of representing IPv4 addresses in IPv6 address space.

The Pref64::/n can be a Network Specific Prefix (NSP). An NSP is an IPv6 prefix assigned by an organization for to create IPv6 representations of IPv4 addresses.

The main different in the nature of the two types of prefixes is that the NSP is a locally assigned prefix that is under control of the organization that is providing the translation services, while the Well-Known prefix is a prefix that has a global meaning since it has been assigned for the specific purpose of representing IPv4 addresses in IPv6 address space.

The DNS64 function can be performed in two places.

One option is to locate the DNS64 function in recursive name servers serving end hosts. In this case, when an IPv6-only host queries the name server for AAAA RRs for an IPv4-only host, the name server can perform the synthesis of AAAA RRs and pass them back to the IPv6 only initiator. The main advantage of this mode is that current IPv6 nodes can use this mechanism without requiring any modification. This mode is called "DNS64 in DNS server mode".

The other option is to place the DNS64 function in the end hosts themselves, coupled to the local stub resolver. In this case, the stub resolver will try to obtain (real) AAAA RRs and in case they are not available, the DNS64 function will synthesize AAAA RRs for internal usage. This mode is compatible with some advanced functions like DNSSEC validation in the end host. The main drawback of this mode is its deployability, since it requires changes in the end hosts. This mode is called "DNS64 in stub-resolver mode"".

3.  Background to DNS64 - DNSSEC interaction

DNSSEC presents a special challenge for DNS64, because DNSSEC is designed to detect changes to DNS answers, and DNS64 may alter answers coming from an authoritative server.

A recursive resolver can be security-aware or security-oblivious. Moreover, a security-aware recursive name server can be validating or non-validating, according to operator policy. In the cases below, the recursive server is also performing DNS64, and has a local policy to validate. We call this general case vDNS64, but in all the cases below the DNS64 functionality should be assumed needed.

DNSSEC includes some signaling bits that offer some indicators of what the query originator understands.

If a query arrives at a vDNS64 device with the DO bit set, the query originator is signaling that it understands DNSSEC. The DO bit does not indicate that the query originator will validate the response. It only means that the query originator can understand responses containing DNSSEC data. Conversely, if the DO bit is clear, that is evidence that the querying agent is not aware of DNSSEC.

If a query arrives at a vDNS64 device with the CD bit set, it is an indication that the querying agent wants all the validation data so it can do checking itself. By local policy, vDNS64 could still validate, but it must return all data to the querying agent anyway.

Here are the possible cases:

1. A security-oblivious DNS64 node receives a query with the DO bit clear. In this case, DNSSEC is not a concern, because the querying agent does not understand DNSSEC responses.
2. A security-oblivious DNS64 node receives a query with the DO bit set, and the CD bit clear. This is just like the case of a non-DNS64 case: the server doesn't support it, so the querying agent is out of luck.
3. A security-aware and non-validating DNS64 node receives a query with the DO bit set and the CD bit clear. Such a resolver is not validating responses, likely due to local policy (see [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.), section 4.2). For that reason, this case amounts to the same as the previous case, and no validation happens.
4. A security-aware and non-validating DNS64 node receives a query with the DO bit set and the CD bit set. In this case, the resolver is supposed to pass on all the data it gets to the query initiator (see section 3.2.2 of [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.)). This case will be problematic with DNS64. If the DNS64 server modifies the record, the client will get the data back and try to validate it, and the data will be invalid as far as the client is concerned.
5. A security-aware and validating DNS64 node receives a query with the DO bit clear and CD clear. In this case, the resolver validates the data. If it fails, it returns RCODE 2 (SERVFAIL); otherwise, it returns the answer. This is the ideal case for vDNS64. The resolver validates the data, and then synthesizes the new record and passes that to the client. The client, which is presumably not validating (else it would have set DO and CD), cannot tell that DNS64 is involved.
6. A security-aware and validating DNS64 node receives a query with the DO bit set and CD clear. In principle, this ought to work like the previous case, except that the resolver should also set the AD bit on the response.
7. A security-aware and validating DNS64 node receives a query with the DO bit set and CD set. This is effectively the same as the case where a security-aware and non-validating recursive resolver receives a similar query, and the same thing will happen: the downstream validator will mark the data as invalid if DNS64 has performed synthesis.

4.  Terminology

This section provides definitions for the special terms used in the document.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

Authoritative server:

A DNS server that can answer authoritatively a given DNS question.

DNS64:

A logical function that synthesizes DNS resource records (e.g AAAA records containing IPv6 addresses) from DNS resource records actually contained in the global DNS (e.g. A records containing IPv4 addresses).

DNS64 recursor:

A recursive resolver that provides the DNS64 functionality as part of its operation.

Recursive resolver:

A DNS server that accepts requests from one resolver, and asks another resolver for the answer on behalf of the first resolver.

Synthetic RR:

A DNS resource record (RR) that is not contained in any zone data file, but has been synthesized from other RRs. An example is a synthetic AAAA record created from an A record.

IPv6/IPv4 translator:

A device that translates IPv6 packets to IPv4 packets and vice-versa. It is only required that the communication initiated from the IPv6 side be supported.

For a detailed understanding of this document, the reader should also be familiar with DNS terminology from [RFC1034] (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.),[RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) and current NAT terminology from [RFC4787] (Audet, F. and C. Jennings, “Network Address Translation (NAT) Behavioral Requirements for Unicast UDP,” January 2007.). Some parts of this document assume familiarity with the terminology of the DNS security extensions outlined in [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.).

5.  DNS64 Normative Specification

A DNS64 is a logical function that synthesizes AAAA records from A records. The DNS64 function may be implemented in a stub resolver, in a recursive resolver, or in an authoritative name server.

The implementation SHOULD support mapping of IPv4 address ranges to separate IPv6 prefixes for AAAA record synthesis. This allows handling of special use IPv4 addresses [I‑D.iana‑rfc3330bis] (Cotton, M. and L. Vegoda, “Special Use IPv4 Addresses,” August 2009.). Multicast address handling is further specified in [I‑D.venaas‑behave‑mcast46] (Venaas, S., Asaeda, H., SUZUKI, S., and T. Fujisaki, “An IPv4 - IPv6 multicast translator,” July 2009.).

5.1.  Resolving AAAA queries and the answer section

When the DNS64 receives a query for RRs of type AAAA and class IN, it first attempts to retrieve non-synthetic RRs of this type and class, either by performing a query or, in the case of an authoritative server, by examining its own results.

5.1.1.  The answer when there is AAAA data available

If the query results in one or more AAAA records in the answer section, the result is returned to the requesting client as per normal DNS semantics, except in the case where any of the AAAA records match a special exclusion set of prefixes, considered in Section 5.1.3 (Special exclusion set for AAAA records). If there is (non-excluded) AAAA data available, DNS64 SHOULD NOT include synthetic AAAA RRs in the response (see Appendix B (Motivations and Implications of synthesizing AAAA RR when real AAAA RR exists) for an analysis of the motivations for and the implications of not complying with this recommendation). By default DNS64 implementations MUST NOT synthesize AAAA RRs when real AAAA RRs exist.

5.1.2.  The answer when there is an error

If the query results in a response with RCODE other than 0, then there are two possibilities. A result with RCODE=3 is handled according to normal DNS operation (which is normally to return the error to the client). This stage is still prior to any synthesis having happened, so a response to be returned to the client does not need any special assembly than would usually happen in DNS operation.

Any other RCODE is treated as though the RCODE were 0 and the answer section were empty. This is because of the large number of different responses from deployed name servers when they receive AAAA queries without a AAAA record being available.

It is important to note that, as of this writing, some servers respond with RCODE=3 to a AAAA query even if there is an A record available for that owner name. Those servers are in clear violation of the meaning of RCODE 3, and it is expected that they will decline in use as IPv6 deployment increases.

5.1.3.  Special exclusion set for AAAA records

Some IPv6 addresses are not actually usable by IPv6-only hosts. If they are returned to IPv6-only querying agents as AAAA records, therefore, the goal of decreasing the number of failure modes will not be attained. Examples include AAAA records with addresses in the ::ffff/96 network, and possibly (depending on the context) AAAA records with the site's Pref::64/n or the Well-Known prefix (see below for more about the Well-Known prefix). A DNS64 implementation SHOULD provide a mechanism to specify IPv6 prefix ranges to be treated as though the AAAA containing them were an empty answer. An implementation SHOULD include the ::ffff/96 network in that range by default. Failure to provide this facility will mean that clients querying the DNS64 function may not be able to communicate with hosts that would be reachable from a dial-stack host.

When the DNS64 performs its initial AAAA query, if it receives an answer with only AAAA records containing addresses in the target range(s), then it MUST treat the answer as though it were an empty answer, and proceed accordingly. If it receives an answer with at least one AAAA record containing an address outside any of the target ranges, then it MAY build an answer section for a response including only the AAAA record(s) that do not contain any of the addresses inside the excluded ranges. That answer section is used in the assembly of a response as detailed in Section 5.4 (Assembling a synthesized response to a AAAA query). Alternatively, it MAY treat the answer as though it were an empty answer, and proceed accordingly. It MUST NOT return the offending AAAA records as part of a response.

5.1.4.  Dealing with CNAME and DNAME

If the response contains a CNAME or a DNAME, then the CNAME or DNAME chain is followed until the first terminating A or AAAA record is reached. This may require the DNS64 to ask for an A record, in case the response to the original AAAA query is a CNAME or DNAME without an AAAA record to follow. The resulting AAAA or A record is treated like any other AAAA or A case, as appropriate.

When assembling the answer section, the original CNAME or DNAME RR is included as part of the answer, and the synthetic AAAA, if appropriate, is included.

5.1.5.  Data for the answer when performing synthesis

If the query results in no error but an empty answer section in the response, the DNS64 resolver attempts to retrieve A records for the name in question. If this new A RR query results in an empty answer or in an error, then the empty result or error is used as the basis for the answer returned to the querying client. (Transient errors may result in retrying the query, depending on the operation of the resolver; this is just as in Section 5.1.2 (The answer when there is an error).) If instead the query results in one or more A RRs, the DNS64 synthesizes AAAA RRs based on the A RRs according to the procedure outlined in Section 5.1.6 (Performing the synthesis). The DNS64 resolver then returns the synthesized AAAA records in the answer section to the client, removing the A records that form the basis of the synthesis.

5.1.6.  Performing the synthesis

A synthetic AAAA record is created from an A record as follows:

• The NAME field is set to the NAME field from the A record
• The TYPE field is set to 28 (AAAA)
• The CLASS field is set to 1 (IN)
• The TTL field is set to the minimum of the TTL of the original A RR and the SOA RR for the queried domain. (Note that in order to obtain the TTL of the SOA RR the DNS64 does not need to perform a new query, but it can remember the TTL from the SOA RR in the negative response to the AAAA query).
• The RDLENGTH field is set to 16
• The RDATA field is set to the IPv6 representation of the IPv4 address from the RDATA field of the A record. The DNS64 SHOULD check each A RR against IPv4 address ranges and select the corresponding IPv6 prefix to use in synthesizing the AAAA RR. See Section 5.2 (Generation of the IPv6 representations of IPv4 addresses) for discussion of the algorithms to be used in effecting the transformation.

5.1.7.  Querying in parallel

DNS64 MAY perform the query for the AAAA RR and for the A RR in parallel, in order to minimize the delay. However, this would result in performing unnecessary A RR queries in the case no AAAA RR synthesis is required. A possible trade-off would be to perform them sequentially but with a very short interval between them, so if we obtain a fast reply, we avoid doing the additional query. (Note that this discussion is relevant only if the DNS64 function needs to perform external queries to fetch the RR. If the needed RR information is available locally, as in the case of an authoritative server, the issue is no longer relevant.)

5.2.  Generation of the IPv6 representations of IPv4 addresses

DNS64 supports multiple algorithms for the generation of the IPv6 representation of an IPv4 address. The constraints imposed on the generation algorithms are the following:

The same algorithm to create an IPv6 address from an IPv4 address MUST be used by both the DNS64 to create the IPv6 address to be returned in the synthetic AAAA RR from the IPv4 address contained in original A RR, and by the IPv6/IPv4 translator to create the IPv6 address to be included in the destination address field of the outgoing IPv6 packets from the IPv4 address included in the destination address field of the incoming IPv4 packet.

The algorithm MUST be reversible, i.e. it MUST be possible to extract the original IPv4 address from the IPv6 representation.

The input for the algorithm MUST be limited to the IPv4 address, the IPv6 prefix (denoted Pref64::/n) used in the IPv6 representations and optionally a set of stable parameters that are configured in the DNS64 (such as fixed string to be used as a suffix).

If we note n the length of the prefix Pref64::/n, then n MUST the less or equal than 96. If a Pref64::/n is configured through any means in the DNS64 (such as manually configured, or other automatic mean not specified in this document), the default algorithm MUST use this prefix. If no prefix is available, the algorithm MUST use the Well-Known prefix 64:FF9B::/96 defined in [I‑D.ietf‑behave‑address‑format] (Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li, “IPv6 Addressing of IPv4/IPv6 Translators,” April 2010.)

[anchor9] (Note in document: The value 64:FF9B::/96 is proposed as the value for the Well-Known prefix and needs to be confirmed whenis published as RFC.)[I‑D.ietf‑behave‑address‑format] (Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li, “IPv6 Addressing of IPv4/IPv6 Translators,” April 2010.)

DNS64 MUST support the algorithm for generating IPv6 representations of IPv4 addresses defined Section 2 of [I‑D.ietf‑behave‑address‑format] (Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li, “IPv6 Addressing of IPv4/IPv6 Translators,” April 2010.). Moreover, the aforementioned algorithm MUST be the default algorithm used by DNS64. While the normative description of the algorithm is provided in [I‑D.ietf‑behave‑address‑format] (Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li, “IPv6 Addressing of IPv4/IPv6 Translators,” April 2010.), an sample description of the algorithm and its application to different scenarios is provided in Appendix A (Deployment scenarios and examples) for illustration purposes.

5.3.  Handling other RRs and the Additional Section

5.3.1.  PTR queries

If a DNS64 nameserver receives a PTR query for a record in the IP6.ARPA domain, it MUST strip the IP6.ARPA labels from the QNAME, reverse the address portion of the QNAME according to the encoding scheme outlined in section 2.5 of [RFC3596] (Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, “DNS Extensions to Support IP Version 6,” October 2003.) , and examine the resulting address to see whether its prefix matches the locally-configured Pref64::/n. There are two alternatives for a DNS64 nameserver to respond to such PTR queries. A DNS64 node MUST provide one of these, and SHOULD NOT provide both at the same time unless different IP6.ARPA zones require answers of different sorts.

The first option is for the DNS64 nameserver to respond authoritatively for its prefixes. If the address prefix matches any Pref64::/n used in the site, either a NSP prefix or the Well-Known prefix used for NAT64 (i.e. 64:FF9B::/96) as defined in [I‑D.ietf‑behave‑address‑format] (Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li, “IPv6 Addressing of IPv4/IPv6 Translators,” April 2010.), then the DNS64 server MAY answer the query using locally-appropriate RDATA. The DNS64 server MAY use the same RDATA for all answers. Note that the requirement is to match any Pref64::/n used at the site, and not merely the locally-configured Pref64::/n. This is because end clients could ask for a PTR record matching an address received through a different (site-provided) DNS64, and if this strategy is in effect, those queries should never be sent to the global DNS. The advantage of this strategy is that it makes plain to the querying client that the prefix is one operated by the DNS64 site, and that the answers the client is getting are generated by the DNS64. The disadvantage is that any useful reverse-tree information that might be in the global DNS is unavailable to the clients querying the DNS64.

The second option is for the DNS64 nameserver to synthesize a CNAME mapping the IP6.ARPA namespace to the corresponding IN-ADDR.ARPA name. The rest of the response would be the normal DNS processing. The CNAME can be signed on the fly if need be. The advantage of this approach is that any useful information in the reverse tree is available to the querying client. The disadvantage is that it adds additional load to the DNS64 (because CNAMEs have to be synthesized for each PTR query that matches the Pref64::/n), and that it may require signing on the fly. In addition, the generated CNAME could correspond to an unpopulated in-addr.arpa zone, so the CNAME would provide a reference to a non-existent record.

If the address prefix does not match any of the Pref64::/n, then the DNS64 server MUST process the query as though it were any other query -- i.e. a recursive nameserver MUST attempt to resolve the query as though it were any other (non-A/AAAA) query, and an authoritative server MUST respond authoritatively or with a referral, as appropriate.

5.3.2.  Handling the additional section

DNS64 synthesis MUST NOT be performed on any records in the additional section of synthesized answers. The DNS64 MUST pass the additional section unchanged.

5.3.3.  Other records

If the DNS64 is in recursive resolver mode, then it SHOULD also serve the zones specified in [I‑D.ietf‑dnsop‑default‑local‑zones] (Andrews, M., “Locally-served DNS Zones,” April 2010.), rather than forwarding those queries elsewhere to be handled.

All other RRs MUST be returned unchanged.

5.4.  Assembling a synthesized response to a AAAA query

The DNS64 uses different pieces of data to build the response returned to the querying client.

The query that is used as the basis for synthesis results either in an error, an answer that can be used as a basis for synthesis, or an empty (authoritative) answer. If there is an empty answer, then the DNS64 responds to the original querying client with the answer the DNS64 received to the original AAAA query. Otherwise, the response is assembled as follows.

The header fields are set according to the usual rules for recursive or authoritative servers, depending on the role that the DNS64 is serving. The question section is copied from the original AAAA query. The answer section is populated according to the rules in Section 5.1.6 (Performing the synthesis). The authority and additional sections are copied from the response to the A query that the DNS64 performed.

5.5.  DNSSEC processing: DNS64 in recursive server mode

We consider the case where the recursive server that is performing DNS64 also has a local policy to validate the answers according to the procedures outlined in [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) Section 5. We call this general case vDNS64.

The vDNS64 uses the presence of the DO and CD bits to make some decisions about what the query originator needs, and can react accordingly:

1. If CD is not set and DO is not set, vDNS64 SHOULD perform validation and do synthesis as needed.
2. If CD is not set and DO is set, then vDNS64 SHOULD perform validation. Whenever vDNS64 performs validation, it MUST validate the negative answer for AAAA queries before proceeding to query for A records for the same name, in order to be sure that there is not a legitimate AAAA record on the Internet. Failing to observe this step would allow an attacker to use DNS64 as a mechanism to circumvent DNSSEC. If the negative response validates, and the response to the A query validates, then the vDNS64 MAY perform synthesis and SHOULD set the AD bit in the answer to the client. This is acceptable, because [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.), section 3.2.3 says that the AD bit is set by the name server side of a security-aware recursive name server if and only if it considers all the RRSets in the Answer and Authority sections to be authentic. In this case, the name server has reason to believe the RRSets are all authentic, so it SHOULD set the AD bit. If the data does not validate, the vDNS64 MUST respond with RCODE=2 (server failure).
   A security-aware end point might take the presence of the AD bit as an indication that the data is valid, and may pass the DNS (and DNSSEC) data to an application. If the application attempts to validate the synthesized data, of course, the validation will fail. One could argue therefore that this approach is not desirable. But security aware stub resolvers MUST NOT place any reliance on data received from resolvers and validated on their behalf without certain criteria established by [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.), section 4.9.3. An application that wants to perform validation on its own should use the CD bit.
3. If the CD bit is set and DO is set, then vDNS64 MAY perform validation, but MUST NOT perform synthesis. It MUST hand the data back to the query initiator, just like a regular recursive resolver, and depend on the client to do the validation and the synthesis itself.
   The disadvantage to this approach is that an end point that is translation-oblivious but security-aware and validating will not be able to use the DNS64 functionality. In this case, the end point will not have the desired benefit of NAT64. In effect, this strategy means that any end point that wishes to do validation in a NAT64 context must be upgraded to be translation-aware as well.

6.  Deployment notes

While DNS64 is intended to be part of a strategy for aiding IPv6 deployment in an internetworking environment with some IPv4-only and IPv6-only networks, it is important to realise that it is incompatible with some things that may be deployed in an IPv4-only or dual-stack context.

6.1.  DNS resolvers and DNS64

Full-service resolvers that are unaware of the DNS64 function can be (mis)configured to act as mixed-mode iterative and forwarding resolvers. In a native-IPv4 context, this sort of configuration may appear to work. It is impossible to make it work properly without it being aware of the DNS64 function, because it will likely at some point obtain IPv4-only glue records and attempt to use them for resolution. The result that is returned will contain only A records, and without the ability to perform the DNS64 function the resolver will simply be unable to answer the necessary AAAA queries.

6.2.  DNSSEC validators and DNS64

Existing DNSSEC validators (i.e. that are unaware of DNS64) will reject all the data that comes from the DNS64 as having been tampered with. If it is necessary to have validation behind the DNS64, then the validator must know how to perform the DNS64 function itself. Alternatively, the validating host may establish a trusted connection with the DNS64, and allow the DNS64 to do all validation on its behalf.

6.3.  DNS64 and multihomed and dual-stack hosts

6.3.1.  IPv6 multihomed hosts

Synthetic AAAA records may be constructed on the basis of the network context in which they were constructed. If a host sends DNS queries to resolvers in multiple networks, it is possible that some of them will receive answers from a DNS64 without all of them being connected via a NAT64. For instance, suppose a system has two interfaces, i1 and i2. Whereas i1 is connected to the IPv4 Internet via NAT64, i2 has native IPv6 connectivity only. I1 might receive a AAAA answer from a DNS64 that is configured for a particular NAT64; the IPv6 address contained in that AAAA answer will not connect with anything via i2.

This example illustrates why it is generally preferable that hosts treat DNS answers from one interface as local to that interface. The answer received on one interface will not work on the other interface. Hosts that attempt to use DNS answers globally may encounter surprising failures in these cases. For more discussion of this topic, see [I‑D.savolainen‑mif‑dns‑server‑selection] (Savolainen, T., “DNS Server Selection on Multi-Homed Hosts,” February 2010.).

Note that the issue is not that there are two interfaces, but that there are two networks involved. The same results could be achieved with a single interface routed to two different networks.

6.3.2.  Accidental dual-stack DNS64 use

Similarly, suppose that i1 has IPv6 connectivity and can connect to the IPv4 Internet through NAT64, but i2 has IPv4 native transit. In this case, i1 could receive an IPv6 address from a synthetic AAAA that would better be reached via native IPv4 transit. Again, it is worth emphasising that this arises because there are two networks involved.

Since it is most likely that the host will attempt AAAA resolution first, in this arrangement the host will often use the NAT64 when a native IPv4 would be preferable. For this reason, hosts with IPv4 connectivity to the Internet should avoid using DNS64. This can be partly resolved by ISPs when providing DNS resolvers to clients, but that is not a guarantee that the NAT64 will never be used when a native IPv4 connection should be used. There is no general-purpose mechanism to ensure that native IPv4 transit will always be preferred, because to a DNS64-oblivious host, the DNS64 looks just like an ordinary DNS server. Operators of a NAT64 should expect traffic to pass through the NAT64 even when it is not necessary.

6.3.3.  Intentional dual-stack DNS64 use

Finally, consider the case where the IPv4 connectivity on i2 is only to a LAN, with an IPv6-only connection on i1 to the Internet, connecting to the IPv4 Internet via NAT64. Traffic to the LAN may not be routable from the global Internet, as is often the case (for instance) with LANs using RFC1918 addresses. In this case, it is critical that the DNS64 not synthesize AAAA responses for hosts in the LAN, or else that the DNS64 be aware of hosts in the LAN and provide context-sensitive answers ("split view" DNS answers) for hosts inside the LAN. As with any split view DNS arrangement, operators must be prepared for data to leak from one context to another, and for failures to occur because nodes accessible from one context are not accessible from the other.

7.  Security Considerations

See the discussion on the usage of DNSSEC and DNS64 described in the document.

8.  IANA Considerations

This memo makes no request of IANA.

9.  Contributors

Dave Thaler

Microsoft

dthaler@windows.microsoft.com

10.  Acknowledgements

This draft contains the result of discussions involving many people, including the participants of the IETF BEHAVE Working Group. The following IETF participants made specific contributions to parts of the text, and their help is gratefully acknowledged: Mark Andrews, Jari Arkko, Rob Austein, Timothy Baldwin, Fred Baker, Marc Blanchet, Cameron Byrne, Brian Carpenter, Hui Deng, Francis Dupont, Ed Jankiewicz, Peter Koch, Suresh Krishnan, Ed Lewis, Xing Li, Matthijs Mekking, Hiroshi Miyata, Simon Perrault, Teemu Savolainen, Jyrki Soini, Dave Thaler, Mark Townsley, Stig Venaas, Magnus Westerlund, Florian Weimer, Dan Wing, Xu Xiaohu.

Marcelo Bagnulo and Iljitsch van Beijnum are partly funded by Trilogy, a research project supported by the European Commission under its Seventh Framework Program.

11.  References

11.1. Normative References

11.2. Informative References

Appendix A.  Deployment scenarios and examples

In this section, we first provide an example of the address transformation algorithm and then we walk through some sample scenarios that are expected to be common deployment cases. It should be noted that is provided for illustrative purposes and this section is not normative. The normative definition of DNS64 is provided in Section 5 (DNS64 Normative Specification) and the normative definition of the address transformation algorithm is provided in [I‑D.ietf‑behave‑address‑format] (Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li, “IPv6 Addressing of IPv4/IPv6 Translators,” April 2010.).

There are two main different setups where DNS64 is expected to be used (other setups are possible as well, but these two are the main ones identified at the time of this writing).

One possible setup that is expected to be common is the case of an end site or an ISP that is providing IPv6-only connectivity or connectivity to IPv6-only hosts that wants to allow the communication from these IPv6-only connected hosts to the IPv4 Internet. This case is called An-IPv6-network-to-IPv4-Internet. In this case, the IPv6/IPv4 Translator is used to connect the end site or the ISP to the IPv4 Internet and the DNS64 function is provided by the end site or the ISP.

The other possible setup that is expected is an IPv4 site that wants that its IPv4 servers to be reachable from the IPv6 Internet. This case is called IPv6-Internet-to-an-IPv4-network. It should be noted that the IPv4 addresses used in the IPv4 site can be either public or private. In this case, the IPv6/IPv4 Translator is used to connect the IPv4 end site to the IPv6 Internet and the DNS64 function is provided by the end site itself.

In this section we illustrate how the DNS64 behaves in the different scenarios that are expected to be common. We consider then 3 possible scenarios, namely:

1. An-IPv6-network-to-IPv4-Internet setup with DNS64 in DNS server mode
2. An-IPv6-network-to-IPv4-Internet setup with DNS64 in stub-resolver mode
3. IPv6-Internet-to-an-IPv4-network setup with DNS64 in DNS server mode

The notation used is the following: upper case letters are IPv4 addresses; upper case letters with a prime(') are IPv6 addresses; lower case letters are ports; prefixes are indicated by "P::X", which is an IPv6 address built from an IPv4 address X by adding the prefix P, mappings are indicated as "(X,x) <--> (Y',y)".

A.1.  Example of IPv6/IPv4 address transformation algorithm

In this section we describe the algorithm for the generation of IPv6 address from IPv4 address to be implemented in the DNS64 in the case of a Pref64::/n with n=96.

The only parameter required by the default algorithm is an IPv6 prefix. This prefix is used to map IPv4 addresses into IPv6 addresses, and is denoted Pref64::/96. If an Pref64::/96 is configured through any means in the DNS64 (such as manually configured, or other automatic mean not specified in this document), the default algorithm will use this prefix. If no prefix is available the algorithm will use the Well-Know prefix (64:FF9B::/96) defined in [I‑D.ietf‑behave‑address‑format] (Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li, “IPv6 Addressing of IPv4/IPv6 Translators,” April 2010.)

The input for the algorithm are:

The IPv4 address: X

The IPv6 prefix: Pref64::/96

The IPv6 address is generated by concatenating the prefix Pref64::/96, the IPv4 address X So, the resulting IPv6 address would be Pref64:X

Reverse algorithm

We next describe the reverse algorithm of the algorithm described in the previous section. This algorithm allows to generate and IPv4 address from an IPv6 address. This reverse algorithm is NOT implemented by the DNS64 but it is implemented in the IPv6/IPv4 translator that is serving the same domain the DNS64.

The only parameter required by the default algorithm is an IPv6 prefix. This prefix is the one originally used to map IPv4 addresses into IPv6 addresses, and is denoted Pref64::/96.

The input for the algorithm are:

The IPv6 address: X'

The IPv6 prefix: Pref64::/n

First, the algorithm checks that the fist 96 bits of the IPv6 address X' match with the prefix Pref64::/96 i.e. verifies that Pref64::/96 = X'/96.

If this is not the case, the algorithm ends and no IPv4 address is generated.

If the verification is successful, then last 32 bits of address X' are extracted to form the IPv4 address.

A.2.  Example of An-IPv6-network-to-IPv4-Internet setup with DNS64 in DNS server mode

In this example, we consider an IPv6 node located in an IPv6-only site that initiates a communication to an IPv4 node located in the IPv4 Internet. In this example, the Pref64::/n has n equal to 96.

The scenario for this case is depicted in the following figure:

+---------------------------------------+         +-----------+
|IPv6 site       +-------------+        |IP Addr: |           |
|  +----+        | Name server |   +-------+ T    |   IPv4    |
|  | H1 |        | with DNS64  |   |64Trans|------| Internet  |
|  +----+        +-------------+   +-------+      +-----------+
|    |IP addr: Y'     |              |  |            |IP addr: X
|    ---------------------------------  |          +----+
+---------------------------------------+          | H2 |
                                                   +----+

The figure shows an IPv6 node H1 which has an IPv6 address Y' and an IPv4 node H2 with IPv4 address X.

A IPv6/IPv4 Translator connects the IPv6 network to the IPv4 Internet. This IPv6/IPv4 Translator has a prefix (called Pref64::/96) an IPv4 address T assigned to its IPv4 interface.

The other element involved is the local name server. The name server is a dual-stack node, so that H1 can contact it via IPv6, while it can contact IPv4-only name servers via IPv4.

The local name server needs to know the prefix assigned to the local IPv6/IPv4 Translator (Pref64::/96). For the purpose of this example, we assume it learns this through manual configuration.

For this example, assume the typical DNS situation where IPv6 hosts have only stub resolvers, and always query a name server that performs recursive lookups (henceforth called "the recursive nameserver").

The steps by which H1 establishes communication with H2 are:

1. H1 does a DNS lookup for FQDN(H2). H1 does this by sending a DNS query for an AAAA record for H2 to the recursive name server. The recursive name server implements DNS64 functionality.
2. The recursive name server resolves the query, and discovers that there are no AAAA records for H2.
3. The recursive name server queries for an A record for H2 and gets back an A record containing the IPv4 address X. The name server then synthesizes an AAAA record. The IPv6 address in the AAAA record contains the prefix assigned to the IPv6/IPv4 Translator in the upper 96 bits then the IPv4 address X i.e. the resulting IPv6 address is Pref64:X
4. H1 receives the synthetic AAAA record and sends a packet towards H2. The packet is sent from a source transport address of (Y',y) to a destination transport address of (Pref64:X,x), where y and x are ports chosen by H2.
5. The packet is routed to the IPv6 interface of the IPv6/IPv4 Translator and the subsequent communication flows by means of the IPv6/IPv4 Translator mechanisms.

A.3.  An example of an-IPv6-network-to-IPv4-Internet setup with DNS64 in stub-resolver mode

The scenario also considers a Pref64::/n with n equal to 96. This case is depicted in the following figure:

+---------------------------------------+         +-----------+
|IPv6 site             +-------+        |IP addr: |           |
|  +---------------+   | Name  |   +-------+  T   |   IPv4    |
|  | H1 with DNS64 |   | Server|   |64Trans|------| Internet  |
|  +---------------+   +-------+   +-------+      +-----------+
|        |IP addr: Y'      |         |  |            |IP addr: X
|    ---------------------------------  |          +----+
+---------------------------------------+          | H2 |
                                                   +----+

The figure shows an IPv6 node H1 which has an IPv6 address Y' and an IPv4 node H2 with IPv4 address X. Node H1 is implementing the DNS64 function.

A IPv6/IPv4 Translator connects the IPv6 network to the IPv4 Internet. This IPv6/IPv4 Translator has a prefix (called Pref64::/96) and an IPv4 address T assigned to its IPv4 interface.

H1 needs to know the prefix assigned to the local IPv6/IPv4 Translator (Pref64::/96). For the purpose of this example, we assume it learns this through manual configuration.

Also shown is a name server. For the purpose of this example, we assume that the name server is a dual-stack node, so that H1 can contact it via IPv6, while it can contact IPv4-only name servers via IPv4.

For this example, assume the typical situation where IPv6 hosts have only stub resolvers and always query a name server that provides recursive lookups (henceforth called "the recursive name server"). The recursive name server does not perform the DNS64 function.

The steps by which H1 establishes communication with H2 are:

1. H1 does a DNS lookup for FQDN(H2). H1 does this by sending a DNS query for a AAAA record for H2 to the recursive name server.
2. The recursive DNS server resolves the query, and returns the answer to H1. Because there are no AAAA records in the global DNS for H2, the answer is empty.
3. The stub resolver at H1 then queries for an A record for H2 and gets back an A record containing the IPv4 address X. The DNS64 function within H1 then synthesizes a AAAA record. The IPv6 address in the AAAA record contains the prefix assigned to the IPv6/IPv4 Translator in the upper 96 bits, then the IPv4 address X i.e. the resulting IPv6 address is Pref64:X.
4. H1 sends a packet towards H2. The packet is sent from a source transport address of (Y',y) to a destination transport address of (Pref64:X,x), where y and x are ports chosen by H2.
5. The packet is routed to the IPv6 interface of the IPv6/IPv4 Translator and the subsequent communication flows using the IPv6/IPv4 Translator mechanisms.

A.4.  Example of IPv6-Internet-to-an-IPv4-network setup DNS64 in DNS server mode

In this example, we consider an IPv6 node located in the IPv6 Internet site that initiates a communication to a IPv4 node located in the IPv4 site. Once again, in this example, the Pref64::/n has n equal to 96.

This scenario can be addressed without using any form of DNS64 function. This is so because it is possible to assign a fixed IPv6 address to each of the IPv4 servers. Such an IPv6 address would be constructed as the Pref64::/96 concatenated with the IPv4 address of the IPv4 server. Note that the IPv4 address can be a public or a private address; the latter does not present any additional difficulty, since the NSP prefix must be used a Pref64::/96 (in this scenario the usage of the Well-Known prefix is not supported). Once these IPv6 addresses have been assigned to represent the IPv4 servers in the IPv6 Internet, real AAAA RRs containing these addresses can be published in the DNS under the site's domain. This is the recommended approach to handle this scenario, because it does not involve synthesizing AAAA records at the time of query. Such a configuration is easier to troubleshoot in the event of problems, because it always provides the same answer to every query.

However, there are some more dynamic scenarios, where synthesizing AAAA RRs in this setup may be needed. In particular, when DNS Update [RFC2136] (Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, “Dynamic Updates in the Domain Name System (DNS UPDATE),” April 1997.) is used in the IPv4 site to update the A RRs for the IPv4 servers, there are two options: One option is to modify the server that receives the dynamic DNS updates. That would normally be the authoritative server for the zone. So the authoritative zone would have normal AAAA RRs that are synthesized as dynamic updates occur. The other option is modify the authoritative server to generate synthetic AAAA records for a zone, possibly based on additional constraints, upon the receipt of a DNS query for the AAAA RR. The first option -- in which the AAAA is synthesized when the DNS update message is received, and the data published in the relevant zone -- is recommended over the second option (i.e. the synthesis upon receipt of the AAAA DNS query). This is because it is usually easier to solve problems of misconfiguration and so on when the DNS responses are not being generated dynamically. For completeness, the DNS64 behavior that we describe in this section covers the case of synthesizing the AAAA RR when the DNS query arrives. Nevertheless, such a configuration is not recommended. Troubleshooting configurations that change the data depending on the query they receive is notoriously hard, and the IPv4/IPv6 translation scenario is complicated enough without adding additional opportunities for possible malfunction.

The scenario for this case is depicted in the following figure:

+-----------+          +----------------------------------------+
|           |          |   IPv4 site            +-------------+ |
|   IPv6    |      +-------+      +----+        | Name server | |
| Internet  |------|64Trans|      | H2 |        | with DNS64  | |
+-----------+      +-------+      +----+        +-------------+ |
  |IP addr: Y'         |  |         |IP addr: X     |           |
+----+                 | -----------------------------------    |
| H1 |                 +----------------------------------------+
+----+

The figure shows an IPv6 node H1 which has an IPv6 address Y' and an IPv4 node H2 with IPv4 address X.

A IPv6/IPv4 Translator connects the IPv4 network to the IPv6 Internet. This IPv6/IPv4 Translator has a prefix (called Pref64::/96).

Also shown is the authoritative name server for the local domain with DNS64 functionality. For the purpose of this example, we assume that the name server is a dual-stack node, so that H1 or a recursive resolver acting on the request of H1 can contact it via IPv6, while it can be contacted by IPv4-only nodes to receive dynamic DNS updates via IPv4.

The local name server needs to know the prefix assigned to the local IPv6/IPv4 Translator (Pref64::/96). For the purpose of this example, we assume it learns this through manual configuration.

The steps by which H1 establishes communication with H2 are:

1. H1 does a DNS lookup for FQDN(H2). H1 does this by sending a DNS query for an AAAA record for H2. The query is eventually forwarded to the server in the IPv4 site.
2. The local DNS server resolves the query (locally), and discovers that there are no AAAA records for H2.
3. The name server verifies that FQDN(H2) and its A RR are among those that the local policy defines as allowed to generate a AAAA RR from. If that is the case, the name server synthesizes an AAAA record from the A RR and the relevant Pref64::/96. The IPv6 address in the AAAA record contains the prefix assigned to the IPv6/IPv4 Translator in the first 96 bits and the IPv4 address X.
4. H1 receives the synthetic AAAA record and sends a packet towards H2. The packet is sent from a source transport address of (Y',y) to a destination transport address of (Pref64:X,x), where y and x are ports chosen by H2.
5. The packet is routed through the IPv6 Internet to the IPv6 interface of the IPv6/IPv4 Translator and the communication flows using the IPv6/IPv4 Translator mechanisms.

Appendix B.  Motivations and Implications of synthesizing AAAA RR when real AAAA RR exists

The motivation for synthesizing AAAA RR when a real AAAA RR exists is to support the following scenario:

An IPv4-only server application (e.g. web server software) is running on a dual-stack host. There may also be dual-stack server applications also running on the same host. That host has fully routable IPv4 and IPv6 addresses and hence the authoritative DNS server has an A and a AAAA record as a result.

An IPv6-only client (regardless of whether the client application is IPv6-only, the client stack is IPv6-only, or it only has an IPv6 address) wants to access the above server.

The client issues a DNS query to a DNS64 recursor.

If the DNS64 only generates a synthetic AAAA if there's no real AAAA, then the communication will fail. Even though there's a real AAAA, the only way for communication to succeed is with the translated address. So, in order to support this scenario, the administrator of a DNS64 service may want to enable the synthesis of AAAA RR even when real AAAA RR exist.

The implication of including synthetic AAAA RR when real AAAA RR exist is that translated connectivity may be preferred over native connectivity in some cases where the DNS64 is operated in DNS server mode.

RFC3484 [RFC3484] (Draves, R., “Default Address Selection for Internet Protocol version 6 (IPv6),” February 2003.) rules use longest prefix match to select which is the preferred destination address to use. So, if the DNS64 recursor returns both the synthetic AAAA RR and the real AAAA RR, then if the DNS64 is operated by the same domain as the initiating host, and a global unicast prefix (called the NSP prefix as defined in [I‑D.ietf‑behave‑address‑format] (Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li, “IPv6 Addressing of IPv4/IPv6 Translators,” April 2010.)) is used, then the synthetic AAAA RR is likely to be preferred.

This means that without further configuration:

In the case of An IPv6 network to the IPv4 internet, the host will prefer translated connectivity if NSP prefix is used. If the Well-Known prefix defined in [I‑D.ietf‑behave‑address‑format] (Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li, “IPv6 Addressing of IPv4/IPv6 Translators,” April 2010.) is used, it will probably prefer native connectivity.

In the case of the IPv6 Internet to an IPv4 network, it is possible to bias the selection towards the real AAAA RR if the DNS64 recursor returns the real AAAA first in the DNS reply, when the NSP prefix is used (the Well-Known prefix usage is not recommended in this case)

In the case of the IPv6 to IPv4 in the same network, for local destinations (i.e., target hosts inside the local site), it is likely that the NSP prefix and the destination prefix are the same, so we can use the order of RR in the DNS reply to bias the selection through native connectivity. If a Well-Known prefix is used, the longest prefix match rule will select native connectivity.

So this option introduces problems in the following cases:

An IPv6 network to the IPv4 internet with the NSP prefix

IPv6 to IPv4 in the same network when reaching external destinations and the NSP prefix is used.

In any case, the problem can be solved by properly configuring the RFC3484 [RFC3484] (Draves, R., “Default Address Selection for Internet Protocol version 6 (IPv6),” February 2003.) policy table, but this requires effort on the part of the site operator.

Authors' Addresses