Discovering PREF64 in Router Advertisements

NAT64 with DNS64 is a widely-deployed mechanism to provide IPv4 access on IPv6-only networks. In order to support functions such as local validation of DNSSEC responses, 464xlat , and local IPv4 address synthesis , the host must be aware of the NAT64 prefix in use by the network. This document specifies a Router Advertisement option to communicate the NAT64 prefix to hosts.

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.

Fate sharing. NAT64 requires a routing to be configured. IPv6 routing configuration requires receiving an IPv6 Router Advertisement . Compared to currently-deployed NAT64 prefix discovery methods such as , including the NAT64 prefix in the Router Advertisement minimizes the number of packets required to configure a host. This speeds up the process of connecting to a network that supports NAT64/DNS64, and simplifies host implementation by removing the possibility that the a can have an incomplete layer 3 configuration (e.g., IPv6 addresses and prefixes, but no NAT64 prefix). Deployability. All IPv6 hosts and networks are required to support . Other options such as b require implementing other protocols.

This option specifies exactly one NAT64 prefix for all IPv4 addresses. Observation of current deployments suggest that they use RFC7050, which also only supports one prefix. This option only supports the NAT64 prefix length of 96 bits. In the unlikely event of use cases for shorter prefixes () emerging another option could be created.

Fields: Type 8-bit identifier of the RDNSS option type as assigned by IANA: TBD Length 8-bit unsigned integer. The length of the option (including the Type and Length fields) is in units of 8 octets. The only valid value for this filed is 2. A host MUST ignore the NAT64 prefix option if the length field value is not set to 2. Lifetime 16-bit unsigned integer. The maximum time in seconds over which this NAT64 prefix MAY be used. The value of Lifetime SHOULD by default be set to lesser of 3 x MaxRtrAdvInterval or 65535 seconds. A value of zero means that the prefix MUST no longer be used. Prefix The highest 96-bits of the NAT64 prefix.

The IANA is requested to assign a new IPv6 Neighbor Discovery Option type for the PREF64 option defined in this document. Option Name Type PREF64 option (TBD) The IANA registry for these options is: https://www.iana.org/assignments/icmpv6-parameters

Because Router Advertisements are required in all IPv6 configuration scenarios, on IPv6-only networks, Router Advertisements must already be secured, e.g., by deploying RA guard . Providing all configuration in Router Advertisements increases security by ensuring that no other protocols can be abused by malicious attackers to provide hosts with invalid configuration. The security measures that must already be in place to ensure that Router Advertisements are only received from legitimate sources eliminate the problem of PREF64 validation described in section 3.1 of .