DHC Working Group D. Wing
Internet-Draft T. Reddy
Intended status: Standards Track P. Patil
Expires: December 09, 2013 Cisco
M. Boucadair
France Telecom
June 07, 2013

DHCPv6 Dynamic DNS Reconfiguration
draft-wing-dhc-dns-reconfigure-01

Abstract

Some networks are expected to support IPv4-only, dual-stack, and IPV6-only hosts at the same time. This makes prioritizing the DNS servers for hosts tricky due to a heterogeneous mix of protocol stacks causing optimal behavior to occur only when the host stack re-initializes. The networks infrastructure is usually well equipped to be aware of single/dual-stack nature of hosts. This specification extends DHCPv6 so that a DHCPv6 Relay Agent can dynamically influence the priority of DNS servers provided to the host, so that the host can use an optimal DNS server for resolution.

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on December 09, 2013.

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Table of Contents

1. Introduction

The default address selection rules [RFC6724] 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) [RFC6384]. 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.

2. Terminology

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 [RFC2119].

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 [RFC6052], section 4.2.

DNS64 server : DNS server using an IPv6 address and synthesizes AAAA records from A records [RFC6147]

3. Mechanism

This document describes a new DHCPv6 Option that can be used with the DHCPv6 RECONFIGURE_REQUEST [I-D.ietf-dhc-triggered-reconfigure] 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.

  1. 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.
  2. 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.
  3. 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.

4. DNS_RECONFIG option

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:

 0                   1                   2                     3

 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  OPTION_DNS_RECONFIG          | Option-len                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Info-flags   |                                               |
-----------------------------------------------------------------


 OPTION_DNS_RECONFIG : 16-bit option code
 Option-length       : 16-bit unsigned integer indicating length
                       in octets of this option

 Info-flags          : Flag indicating the type of DNS Server
 +------+--------------------------------
 |Value | Name                          |
 +------+--------------------------------
 | 0x01 | IPV6_DNS64_SERV_ONLY          |
 | 0x02 | IPV6_HIGH_PROI_NORM_SERV      | 
 +------+--------------------------------

 IPV6_DNS64_SERV_ONLY - Provide only DNS64 address list to the client.

 IPV6_HIGH_PROI_NORM_SERV - Provide DNS address list in this order to the client,
                            first regular DNS servers and then DNS64 servers.

Figure 1: DNS Reconfigure option message format

5. DHCPv6 Relay Agent Behaviour

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 Section 7. 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.

6. DHCPv6 Server Behaviour

Upon receiving RECONFIGURE_REQUEST message containing the DNS_RECONFIG Option, the DHCPv6 server processing is described below depending on the Info-flag values:

[I-D.ietf-dhc-triggered-reconfigure] to create and send Reconfigure message. The server will remember this configuration for the life of the lease.

DHCPv6 server will use the mechanism described in

7. Host Tracking

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.

8. Security Considerations

Security considerations described in [I-D.ietf-dhc-triggered-reconfigure]are applicable to this mechanism.

9. IANA Considerations

IANA is requested to assign new option codes for OPTION_DNS_RECONFIG from the option-code space as defined in section "DHCPv6 Options" of [RFC3315].

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.
[I-D.ietf-dhc-triggered-reconfigure] Boucadair, M. and X. Pougnard, "RECONFIGURE Triggered by DHCPv6 Relay Agents", Internet-Draft draft-ietf-dhc-triggered-reconfigure-03, January 2013.
[RFC6724] Thaler, D., Draves, R., Matsumoto, A. and T. Chown, "Default Address Selection for Internet Protocol Version 6 (IPv6)", RFC 6724, September 2012.
[RFC3646] Droms, R., "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, December 2003.
[RFC6147] Bagnulo, M., Sullivan, A., Matthews, P. and I. van Beijnum, "DNS64: DNS Extensions for Network Address Translation from IPv6 Clients to IPv4 Servers", RFC 6147, April 2011.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M. and X. Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052, October 2010.
[RFC6384] van Beijnum, I., "An FTP Application Layer Gateway (ALG) for IPv6-to-IPv4 Translation", RFC 6384, October 2011.

10.2. Informative References

[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6", RFC 3633, December 2003.
[RFC5007] Brzozowski, J., Kinnear, K., Volz, B. and S. Zeng, "DHCPv6 Leasequery", RFC 5007, September 2007.
[I-D.wing-behave-dns64-config] Wing, D., "IPv6-only and Dual Stack Hosts on the Same Network with DNS64", Internet-Draft draft-wing-behave-dns64-config-03, February 2011.

Authors' Addresses

Dan Wing Cisco Systems, Inc. 170 West Tasman Drive San Jose, California 95134 USA EMail: dwing@cisco.com
Tirumaleswar Reddy Cisco Systems, Inc. Cessna Business Park, Varthur Hobli Sarjapur Marathalli Outer Ring Road Bangalore, Karnataka 560103 India EMail: tireddy@cisco.com
Prashanth Patil Cisco Systems, Inc. Bangalore, India EMail: praspati@cisco.com
Mohamed Boucadair France Telecom Rennes, 35000 France EMail: mohamed.boucadair@orange.com