Dynamic Host Configuration L. Colitti Internet-Draft J. Linkova Intended status: Standards Track Google Expires: June 11, 2020 M. Richardson Sandelman T. Mrugalski ISC December 9, 2019 IPv6-Only-Preferred Option for DHCP draft-link-dhc-v6only-01 Abstract This document specifies a DHCP option to indicate that a host supports an IPv6-only mode and willing to forgo obtaining an IPv4 address if the network provides IPv6 connectivity. 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 https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on June 11, 2020. Copyright Notice Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of Colitti, et al. Expires June 11, 2020 [Page 1] Internet-Draft IPv6-Only-Preferred Option for DHCP December 2019 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Reasons to Signal IPv6-Only Support in DHCPv4 Packets . . . . 4 3. IPv6-Only Preferred Option . . . . . . . . . . . . . . . . . 5 3.1. Option format . . . . . . . . . . . . . . . . . . . . . . 5 3.2. DHCPv4 Client Behaviour . . . . . . . . . . . . . . . . . 5 3.3. DHCPv4 Server Behaviour . . . . . . . . . . . . . . . . . 7 3.4. Configuration Variables . . . . . . . . . . . . . . . . . 8 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 7.1. Normative References . . . . . . . . . . . . . . . . . . 9 7.2. Informative References . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 1. Introduction One of the biggest challenges of deploying IPv6-only LANs is that such networks might contain rather heterogeneous collection of 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 incrementally rollout IPv6-only, network operators need to provide IPv4-as-a-service whereby 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: o Doubling the 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. o Placing a host into the 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 the information required to make the correct decision. Colitti, et al. Expires June 11, 2020 [Page 2] Internet-Draft IPv6-Only-Preferred Option for DHCP December 2019 It would be beneficial for IPv6 deployment if operators could implement IPv6-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 provides 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 immediately. If hosts were to delay requesting IPv4 until IPv6 reachability is confirmed, that would penalize IPv4-only and dual-stack networks, which does not seem practical. Requesting IPv4 and then releasing it later, after IPv6 reachability is confirmed, might cause user-visible errors as it would be disruptive for applications which have started using the assigned IPv4 address already. 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 connectivity (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 application 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 Colitti, et al. Expires June 11, 2020 [Page 3] Internet-Draft IPv6-Only-Preferred Option for DHCP December 2019 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 incrementally 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 networks may or may not allow intra-LAN IPv4 connectivity. IPv6-Only network usually provides 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 contain both IPv6-capable and IPv4-requiring devices and utilize DHCP for configuring IPv4 network stack on hosts, it seems only natural to leverage the same protocol to signal that IPv4 is discretional on a given segment. Such an approach limits the attack surface to DHCP-related attacks without introducing new vulnerable elements. Another benefit of using DHCPv4 for signaling is that IPv4 will be disabled only if both the client and the server indicate IPv6-only capability. It allows IPv6-only capable hosts to turn off IPv4 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. Colitti, et al. Expires June 11, 2020 [Page 4] Internet-Draft IPv6-Only-Preferred Option for DHCP December 2019 3. IPv6-Only Preferred Option 3.1. Option format 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value (contd) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: IPv6-Only Preferred Option Format Fields: Type 8-bit identifier of the IPv6-Only Preferred option type as assigned by IANA: TBD Length 8-bit unsigned integer. The length of the option excluding the Type and Length Fields. The server MUST set the length field to 4. The receiver MUST ignore the IPv6-Only Preferred option if the length field value is not 4. Value 32-bit unsigned integer. The number of seconds the client should disable DHCPv4 for (V6ONLY_WAIT configuration variable). If the server pool is explicitly conifgured with V6ONLY_WAIT timer the server MUST set the field to that configured value. Otherwise the server MUST set it to zero. The client MUST ignore V6ONLY_WAIT timer received from the server if the value is less than 300 seconds. 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 Colitti, et al. Expires June 11, 2020 [Page 5] Internet-Draft IPv6-Only-Preferred Option for DHCP December 2019 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 for interfaces so enabled and follow the processing as described below on a per interface enabled basis. If the client did not include the IPv6-only Preferred option in the DHCPDISCOVER or DHCPREQUEST message it MUST ignore 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. If the IPv6-only Preferred option returned by the server contains non-zero value the client SHOULD set the V6ONLY_WAIT timer to that value. If the server returns zero value the client MUST use its own configuration for V6ONLY_WAIT timer. The client SHOULD stop the DHCP configuration process for at least V6ONLY_WAIT seconds or until a network attachment event happens. The host MAY disable IPv4 stack completely for V6ONLY_WAIT seconds or until the network disconnection event happens. 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 happens. 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 DHCPOFFER responses and selects one of them, it MUST follow the processing for the IPv6-only Preferred option based on the selected response. A client MAY use the presence of the IPv6-only Preferred option as a selection criteria. When an IPv6-only capable client receives the IPv6-Only Preferred option from the server, the client MAY configure IPv4 link-local Colitti, et al. Expires June 11, 2020 [Page 6] Internet-Draft IPv6-Only-Preferred Option for DHCP December 2019 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 configure the given DHCP pool with an IPv6-only preferred option. The DHCP server MAY have a configuration option to specify V6ONLY_WAIT timer for all or individual IPv6-mostly pools. 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 the IPv6-only Preferred option when responding with the DHCPOFFER or DHCPACK message. If the server responds with the IPv6-only Preferred option and the V6ONLY_WAIT timer is configured for the pool, the server MUST copy the configured value to the IPv6-only Preferred option value field. Otherwise it MUST set the field to zero. The server SHOULD include an available IPv4 address from the pool into the DHCPOFFER as per recommendations in [RFC2131] but SHOULD NOT reserve the address and SHOULD NOT verify its uniqueness. 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. As an optional optimization an IPv6-mostly pool MAY be configured with a dedicated IPv4 address to be returned to IPv6-only capable clients. In that case the server SHOULD specify that address as the client's network address and MUST NOT verify its uniqueness. 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 would 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. Colitti, et al. Expires June 11, 2020 [Page 7] Internet-Draft IPv6-Only-Preferred Option for DHCP December 2019 3.4. Configuration Variables V6ONLY_WAIT The minimum time the client SHOULD stop the DHCP configuration process for. MUST be no less than 300 seconds. Default: 1800 seconds 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. +----------------------------+-------+ | Option Name | Type | +----------------------------+-------+ | IPv6-only Preferred option | (TBD) | +----------------------------+-------+ Table 1 5. Security Considerations The proposed mechanism is not introducing any new security implications. While clients using the IPv6-only Preferred option are vulnerable 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: Mohamed Bboucadair, Bjorn Mork, Bernie Volz (AI: add more names here). Authors would like to thank Bob Hinden and Brian Carpenter for the initial idea of signaling IPv6-only capability to hosts. 7. References Colitti, et al. Expires June 11, 2020 [Page 8] Internet-Draft IPv6-Only-Preferred Option for DHCP December 2019 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, DOI 10.17487/RFC2131, March 1997, . [RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic Configuration of IPv4 Link-Local Addresses", RFC 3927, DOI 10.17487/RFC3927, May 2005, . [RFC4039] Park, S., Kim, P., and B. Volz, "Rapid Commit Option for the Dynamic Host Configuration Protocol version 4 (DHCPv4)", RFC 4039, DOI 10.17487/RFC4039, March 2005, . [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 7.2. Informative References [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146, April 2011, . [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, DOI 10.17487/RFC6147, April 2011, . [RFC6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT: Combination of Stateful and Stateless Translation", RFC 6877, DOI 10.17487/RFC6877, April 2013, . Colitti, et al. Expires June 11, 2020 [Page 9] Internet-Draft IPv6-Only-Preferred Option for DHCP December 2019 Authors' Addresses Lorenzo Colitti Google Shibuya 3-21-3 Shibuya, Tokyo 150-0002 JP Email: lorenzo@google.com Jen Linkova Google 1 Darling Island Rd Pyrmont, NSW 2009 AU Email: furry@google.com Michael C. Richardson Sandelman Software Works Email: mcr+ietf@sandelman.ca URI: http://www.sandelman.ca/ Tomek Mrugalski Internet Systems Consortium, Inc. 950 Charter Street Redwood City, CA 94063 USA Email: tomasz.mrugalski@gmail.com Colitti, et al. Expires June 11, 2020 [Page 10]