DHCPv6 Dynamic DNS Reconfiguration

The default address selection rules prefers IPv6 over IPv4. If a dual-stack host is configured to use a DNS64 server, that DNS64 server will synthesize a AAAA response if there is an A record. Thus, the dual-stack host will always use IPv6 if a DNS lookup was involved, even if IPv4 could have been used more optimally. If NAT44 and NAT64 are deployed on the same network, it is preferable to use NAT44 over NAT64 because of scale, performance and application incompatibility issues (e.g., FTP) . At the same time, native IPv6 can still be preferred over IPv4. The DHCPv6 Relay Agent can observe host characteristics on a network to determine if the host is IPV4-only, dual-stack or IPV6-only and also determine transitions from single to dual-stack or vice-versa. In this document we propose a specification that allows the DHCPv6 Relay Agent to influence the DHCPv6 Server to send appropriately prioritized DNS Servers to the client as per host characteristics.

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 . DNS server : DNS server using an IPv4-mapped IPv6 address (that is, an IPv6 address starting with ::ffff:/96 IPv4-mapped prefix). Hosts can communicate with the DNS server only using IPv4 packets , section 4.2. DNS64 server : DNS server using an IPv6 address and synthesizes AAAA records from A records

This document describes a new DHCPv6 Option that can be used with the DHCPv6 RECONFIGURE_REQUEST by the DHCPv6 Relay Agent to indicate to the DHCPv6 Server of the priority of DNS servers to be provided to the specified host. The DHCPv6 Server then sends a Reconfigure message to the host providing updated/re-ordered DNS server list as suggested by the Relay Agent. The idea is for the DHCPv6 Relay Agent to dynamically send the reconfigure message based on host characteristics. IPv6-only transition to Dual-Stack : In case a host is IPv6-only to start off, it is provided a DNS64 Server. When transitioning to dual-stack, an IPv4 DNS Server is assigned as a consequence of obtaining an IPv4 Address. The DHCPv6 Relay Agent can detect this and send a RECONFIGURE_REQUEST message to the DHCPv6 Server indicating that the host needs to be provided with a regular DNS Server followed by DNS64 server. In lieu of this mechanism, the host would continue to use the DNS64 server until the host stack reinitializes. Dual-Stack to IPv6-only : In case a host is dual-stack, it is provided with a regular DNS server followed by DNS64 server. When transitioning to IPv6-only, the DHCPv6 Relay Agent can detect this and send a RECONFIGURE_REQUEST message to the DHCPv6 Server indicating that the host needs to be assigned a DNS64 server only. In lieu of this mechanism, the host would continue to use the regular DNS Server which is inaccessible and eventually time out to fail over to the DNS64 Server. The host will take additional time to fully initialize causing delays in connection. Dual-Stack to IPv4-only : In case a host is dual-stack, it is provided with a regualr DNS server followed by DNS64 server. When transitioning to IPv4-only, no change is required because the host continues to use regular DNS server.

The DNS_RECONFIG option is to be used only in a RECONFIGURE_REQUEST message and identifies the query being performed. The option includes a flag that determines the DNS server list to be provided by the DHCPv6 server to the respective client. The option is defined below:

DHCPv6 relay agents that implement this specification MUST be configurable for sending the RECONFIGURE_REQUEST message. The Relay Agent MUST set the "msg-type" field to RECONFIGURE_REQUEST. The Relay Agent detects host characteristics using mechanisms discussed in . For host transition from IPv6-only to dual-Stack or IPv4-only to dual-stack Relay Agent will set Info-flags with IPV6_HIG_PROI_NORM_SERV and for host transition from dual-stack to IPv6 only Relay-Agent will set Info-flags with IPV6_DNS64_SERV_ONLY.

Upon receiving RECONFIGURE_REQUEST message containing the DNS_RECONFIG Option, the DHCPv6 server processing is described below depending on the Info-flag values: IPV6_DNS64_SERV_ONLY : The DHCPv6 server will select only IPv6 address list of DNS64 recursive name servers to be sent to the client. The DHCPv6 server will send a reconfigure message to inform the client that the server has updated configuration information and the client initiates an Information-request with the server. The updated configuration will now be sent as part of Information-request reply by the DHCPv6 server. IPV6_HIGH_PROI_NORM_SERV : The DHCPv6 server will select DNS servers in this order, first is the regular DNS servers and then DNS64 servers. The DHCPv6 server will send a reconfigure message to inform the client that the server has updated configuration information and client initiates an Information-request with the server. The updated configuration will now be sent as part of Information-request reply by the DHCPv6 server. The order of DNS servers provided by option OPTION_DNS_SERVERS determines the preference for use by the DNS client resolver thus ensuring higher priority for regular DNS server list followed by DNS64 servers. DHCPv6 server will use the mechanism described in to create and send Reconfigure message. The server will remember this configuration for the life of the lease.

Relay Agents can actively keep track of all IPv4/IPv6 addresses and associated lease times assigned to hosts via the respective DHCP servers. Relay Agents can thus detect transitions from single to dual-stack and vice-versa efficiently. In addition to this technique, which is to be primarily used, transitions can also be detected using snooping mechanisms. Network devices today use mechanisms such as ARP and NDP snooping to determine host characteristics such as IPv4/IPv6 - MAC bindings. IPv4/IPv6 and MAC counters are also used to determine host liveliness. These mechanisms help determine if a particular IP address family is inactive, has reverted to using a single stack even though it initially had dual-stack capabilities and detect active dual-stack usage after long periods of single-stack activity.

Security considerations described in are applicable to this mechanism.

IANA is requested to assign new option codes for OPTION_DNS_RECONFIG from the option-code space as defined in section "DHCPv6 Options" of .