IPv6-Only-Preferred Option for DHCP
draft-link-dhc-v6only-00

Abstract

This document specifies a DHCP option to indicate that a host supports an IPv6-only mode and willing to forgo obtaining a IPv4 address if the network provides IPv6 access.

Status of This Memo

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

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This Internet-Draft will expire on June 6, 2020.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 1.2. Terminology
• 2. Reasons to Signal IPv6-Only Support in DHCPv4 Packets
• 3. IPv6-Only Preferred Option
• 3.1. Option format
• 3.2. DHCPv4 Client Behaviour
• 3.3. DHCPv4 Server Behaviour
• 3.4. Configuration Variables
• 4. IANA Considerations
• 5. Security Considerations
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Authors' Addresses

1. Introduction

One of the biggest challenges of deploying IPv6-only LANs is that such networks might contain rather heterogeneous collection of end hosts. Some of them are capable of operating in IPv6-only mode (either because the OS and all applications are IPv6-only capable or because the host has some form of 464XLAT [RFC6877] deployed). At the same time some devices might still have IPv4 dependencies and need IPv4 connectivity to operate properly. To incrementaly rollout IPv6-only, network operators need to provide IPv4-as-a-service when a host receives an IPv4 address if it needs it, while IPv6-only capable devices (such as modern mobile devices) are not allocated IPv4 addresses. Deploying separate LAN segments for IPv6-only and for dual-stack hosts (such as two WiFi SSIDs or two VLANs) is undesirable for a number of reasons, including but not limited to:

• Doubling number of network segments which leads to operational complexity and performance impact, for instance due to TCAM utilization increase from an increased number of ACL entries.
• Placing a host into correct network segment is problematic. For example, in the case of 802.11 Wi-Fi the user might select the wrong SSID. In the case of wired 802.1x authentication the authentication server might not have all information required to make the correct decision.

Therefore it would be beneficial for IPv6 deployment if operators could implement IPv4-mostly (or IPv4-as-a-Service) segments where IPv6-only hosts co-exist with legacy dual-stack devices. The trivial solution of disabling IPv4 stack on IPv6-only capable hosts is not feasible as those clients must be able to operate on IPv4-only networks as well. While IPv6-only capable devices might use a heuristic approach to learning if the network provdes IPv6-only functionality and stop using IPv4 if it does, it might be practically undesirable. One important reason is that when a host connects to a network, it does not know if the network is IPv4-only, dual-stack or IPv6-only. To ensure that the connectivity over whatever protocol is present becomes available as soon as possible the host usually starts configuring both IPv4 and IPv6 immidiately. If hosts were to delay requesting IPv4 until IPv6 reachability is confirmed, that would would penalize IPv4-only and dual-stack networks, which does not seem practical. Instead it would be useful to have a mechanism which would allow a host to indicate that IPv4 is optional and a network to signal that IPv6-only functionality (such as NAT64) is available. The proposed solution is to introduce a new DHCP option which a client uses to indicate that it does not need IPv4 if the network provides IPv6-only connectivty (as NAT64 and DNS64). If the particular network segment provides IPv4-as-a-service such clients would not be supplied with IPv4 addresses, while on IPv4-only or dual-stack segments without NAT64 services IPv4 addresses will be provided.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

1.2. Terminology

IPv6-only capable host: a host which does not require IPv4 and can operate on IPv6-only networks. Strictly speaking IPv6-only capability is specific to a given interface of the host: if some applocation on a host require IPv4 and 464XLAT clat [RFC6877] is only enabled on one interface, the host is IPv6-only capable if connected to a NAT64 network via that interface.

IPv4-as-a-Service: a deployment scenario when end hosts are expected to operate in IPv6-only mode by default and IPv4 addresses can be assigned to some hosts if those hosts explicitly opt-in to receiving IPv4 addresses;

IPv6-mostly network: a network which provides NAT64 (possibly with DNS64) service as well as IPv4 connectivity. Such deployment scenario allows operators to incrementaly turn off IPv4 on end hosts, while still providing IPv4 to devices which require IPv4 to operate. But, IPv6-only capable devices need not be assigned IPv4 addresses.

IPv6-Only network: a network which does not provide routing functionality for IPv4 packets. Such network may or may not allow intra-LAN IPv4 connectivity. IPv6-Only network usually provide access to IPv4-only resources via NAT64 [RFC6147].

NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers [RFC6146];

RA: Router Advertisement, a message used by IPv6 routers to advertise their presence together with various link and Internet parameters [RFC4861];

DNS64: a mechanism for synthesizing AAAA records from A records [RFC6147];

2. Reasons to Signal IPv6-Only Support in DHCPv4 Packets

For networks which contains both IPv6-capable and IPv4-requiring devices and utilizes DHCP for configuring IPv4 network stack on hosts, it seems only natural to leverage the same protocol to signal that IPv4 is discretional on the given segment. Such approach limits the attack surface to DHCP-related attacks without introducing new vulnerable elements.

Another benefit of using DHCPv4 for signalling is that IPv4 will be disabled only if both the client and the server indicate IPv6-only capability. It allows IPv6-only capable clients to turn off IPv6 only upon receiving an explicit signal from the network and operate in dual-stack or IPv4-only mode otherwise.

Coexistence of IPv6-only, dual-stack and even IPv4-only hosts on the same LAN would not only allow network administrators to preserve scarce IPv4 addresses but would also drastically simplify incremental deployment of IPv6-only networks, positively impacting IPv6 adoption.

3. IPv6-Only Preferred Option

3.1. Option format

  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |   Length      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

				      

Figure 1: IPv6-Only Preferred Option Format

Fields:

3.2. DHCPv4 Client Behaviour

A DHCP client SHOULD allow a device administrator to configure IPv6-only preferred mode either for a specific interface (to indicate that the device is IPv6-only capable if connected to a NAT64 network via that interface) or for all interfaces. If only a specific interface is configured as IPv6-only capable the DHCP client MUST NOT be considered as an IPv6-capable for the purpose of sending/receiving DHCP packets over any other interfaces.

Clients not capable of operating in an IPv6-only NAT64 environment MUST NOT include the IPv6-only Preferred option in the Parameter Request List of any DHCP packets and MUST ignore that option in packets received from DHCP servers.

IPv6-only capable clients SHOULD include the IPv6-only Preferred option in the Parameter Request List in DHCPDISCOVER and DHCPREQUEST messages.

If the client did not include the IPv6-only Preferred option in the DHCPDISCOVER or DHCPREQUEST message it MUST ignore the the IPv6-only Preferred option in any messages received from the server.

If the client includes the IPv6-only Preferred option in the Parameter Request List and the DHCPOFFER message from the server contains a valid IPv6-only Preferred option, the client MUST NOT configure the IPv4 address provided in the DHCPOFFER. The client SHOULD stop the DHCP configuration process for at least V6ONLY_WAIT seconds or until a network attachement event happens. The host MAY disable IPv4 stack completely for V6ONLY_WAIT seconds or until the network disconnection event heppens.

The client SHOULD include the IPv6-only Preferred option in DHCPREQUEST messages (after receiving a DHCPOFFER without this option, for a INIT-REBOOT, or when renewing or rebinding a leased address). If the DHCP server responds with a DHCPACK that includes the IPv6-only Preferred option, the client MAY send a DHCPRELEASE message and MAY either stop the DHCP configuration process or disable IPv4 stack completely for V6ONLY_WAIT seconds or until the network disconnection event heppens. Alternatively the client MAY continue to use the assigned IPv4 address until further DHCP reconfiguration events.

If the client includes the IPv6-only Preferred option in the Parameter Request List and the server responds with DHCPOFFER message without a valid IPv6-only Preferred option, the client MUST proceed as normal with a DHCPREQUEST.

If the client waits for multiple responses and the server the client sends the DHCPREQUEST to did not include the IPv6-only Preferred option in the DHCPOFFER, the client MUST NOT stop the DHCP configuration process or disable IPv4 stack even if other servers include the IPv6-only Preferred option in their responses.

When an IPv6-only capable client receives the IPv6-Only Preferred option from the server, The client MAY configure IPv4 link-local address [RFC3927]. In that case IPv6-Only capable devices might still be able to communicate over IPv4 to other devices on the link.

3.3. DHCPv4 Server Behaviour

The DHCP server SHOULD have a configuration option to mark the given DHCP pool as belonging to an IPv6-mostly network segment.

The server MUST NOT include the IPv6-only Preferred option in the DHCPOFFER or DHCPACK message if the YIADDR field in the message does not belong to a pool configured as IPv6-mostly. The server MUST NOT include the IPv6-only Preferred option in the DHCPOFFER or DHCPACK message if the option was not present in the Parameter Request List sent by the client.

If the IPv6-only Preferred option is present in the Parameter Request List received from the client and the corresponding DHCP pool is explicitly configured as belonging to an IPv6-mostly network segment, the server MUST include respond with the DHPCOFFER or DHCPACK message. If the pool is explicitly configured with a dedicated IPv4 address to be returned to IPv6-only capable clients the server MUST specify that address as the client's network address and MUST NOT verify its uniqueness. Otherwise the server SHOULD follow the recommendations in [RFC2131]. The client is not expected to use that IPv4 address so if the client responds with the DHCPREQUEST message for that address the server SHOULD respond with DHCPNAK.

If a client includes both a Rapid-Commit option [RFC4039] and IPv6-Only Preferred option in the DHCPDISCOVER message the server SHOULD NOT honor the Rapid-Commit option if the response woul contain the IPv6-only Preferred option to the client. It SHOULD instead respond with a DHCPOFFER so that the IP address does not need to be reserved for the client until the lease expires.

3.4. Configuration Variables

4. IANA Considerations

The IANA is requested to assign a new DHCP Option code for the IPv6-Only Preferred option from the BOOTP Vendor Extensions and DHCP Options registry, located at https://www.iana.org/assignments/bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml#options . If possible, please assign option code 108.

5. Security Considerations

The proposed mechanism is not introducing any new security implications. While clients using the IPv6-only Preferred option are vunerable to attacks related to a rogue DHCP server, enabling IPv6-only Preferred option does not provide an attacker with any additional mechanisms.

It should be noted that disabling IPv4 on a host upon receiving the IPv6-only Preferred option from the DHCP server protects the host from IPv4-related attacks and therefore could be considered a security feature.

6. Acknowledgements

Thanks to the following people (in alphabetical order) for their review and feedback: Bernie Volz.

7. References

7.1. Normative References

7.2. Informative References

Authors' Addresses