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2. Overview

This section provides an introduction to the DNS64 mechanism.

We assume that we have one or more IPv6/IPv4 translator boxes 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 and hosts that can only use IPv6, as well as cases where only IPv6 connectivity is available to the client. By "IPv4-only servers", we mean servers running IPv4-only applications and servers that can only use IPv4, as well as cases where only IPv4 connectivity is available to the server). Each 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 cannot retrieve a AAAA record for the queried name. The DNS64 service appears as a regular DNS server or resolver to the IPv6 initiator. The DNS64 receives a AAAA DNS query generated by the IPv6 initiator. It first attempts a resolution for the requested AAAA records. If there are no AAAA records 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. For each A record discovered, DNS64 creates a synthetic AAAA RR from the information retrieved in the A RR.

The owner name 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. If not explicitly mentioned, all prefixes are treated equally, and the operations described in this document are performed using the prefixes available. So as to be general, we will call any of these prefixes Pref64::/n, and describe the operations made with the generic prefix Pref64::/n. The IPv6 addresses representing IPv4 addresses included in the AAAA RR synthesized by the DNS64 contain Pref64::/n, and they also embed the original IPv4 address.

The same algorithm and the same Pref64::/n prefix(es) must be configured both in the DNS64 device and the IPv6/IPv4 translator(s), 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 address that contains the Pref64::/n be delivered to an IPv6/IPv4 translator that has that particular Pref64::/n configured, so they can be translated into IPv4 packets.

Once the DNS64 has synthesized the AAAA RRs, the synthetic AAAA RRs are 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 an 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 [RFC6052]. 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 [RFC6052] 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 to create IPv6 representations of IPv4 addresses.

The main difference 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 any of three places. The terms below are more formally defined in Section 4.

The first option is to locate the DNS64 function in authoritative servers for a zone. In this case, the authoritative server provides synthetic AAAA RRs for an IPv4-only host in its zone. This is one type of DNS64 server.

Another 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 recursive-resolver mode". This is a second type of DNS64 server, and it is also one type of DNS64 resolver.

The last option is to place the DNS64 function in the end hosts, 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 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". This is the second type of DNS64 resolver.