Dynamic Host Configuration D. Lamparter Internet-Draft NetDEF, Inc. Intended status: Experimental T. Fiebig Expires: 24 January 2025 MPI-INF 23 July 2024 DHCPv4 Option for IPv4 routes with IPv6 nexthops draft-equinox-dhc-route4via6-00 Abstract // This draft lives at https://github.com/eqvinox/dhc-route4via6 As a result of the shortage of IPv4 addresses, installations are increasingly recovering IPv4 addresses from uses where they are not strictly necessary. One such situation is in establishing next hops for IPv4 routes, replacing this use with IPv6 addresses. This document describes how to provision DHCP-configured hosts with their routes in such a situation. 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 24 January 2025. Copyright Notice Copyright (c) 2024 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 Lamparter & Fiebig Expires 24 January 2025 [Page 1] Internet-Draft dhc-route4via6 July 2024 and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 3. Expected host behavior . . . . . . . . . . . . . . . . . . . 3 3.1. Overlapping routes . . . . . . . . . . . . . . . . . . . 3 3.2. Default route . . . . . . . . . . . . . . . . . . . . . . 4 3.3. Routes clashing with the connected subnet . . . . . . . . 4 4. DHCP Option encoding . . . . . . . . . . . . . . . . . . . . 4 4.1. Route encoding . . . . . . . . . . . . . . . . . . . . . 5 5. YANG model . . . . . . . . . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 9.1. Normative References . . . . . . . . . . . . . . . . . . 7 9.2. Informative References . . . . . . . . . . . . . . . . . 7 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 8 Example encoded options . . . . . . . . . . . . . . . . . . . . . 8 Editing notes (TO BE REMOVED) . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction // more intro TBA // DHCP is likely handing out individual IPv4 addresses, i.e. the // subnet mask is /32 when this mechanism is relevant. // To preempt any misunderstanding, this is an *IPv4 DHCP* option. // *NOT a DHCPv6 option*. Lamparter & Fiebig Expires 24 January 2025 [Page 2] Internet-Draft dhc-route4via6 July 2024 A particularly interesting scenario enabled by the extension described here is the complete removal of IPv4 addresses from first- hop routers acting as DHCP relays, while still providing IPv4 connectivity. In this scenario, the relay (assumed colocated with the router) has no IPv4 address to use to communicate with the client. An almost-working solution for this case is presented by [DHCPv6] with the [DHCP4o6] transport method. Since this mechanism encapsulates IPv4 DHCP messages, all related IPv4 configuration can be carried - but notably there is no way to encode an IPv6 default gateway or other route. If the router and relay are not co-located, the relay may have an IPv4 address while the router does not. In this case, the option described in this document could be carried in a plain IPv4 DHCP message. 2. 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. 3. Expected host behavior The option described in this document is intended to be implemented on hosts supporting IPv4 routes with IPv6 nexthops as described in [v4overv6]. Hosts that do not support this function MUST ignore the option described in this document and behave as before. Hosts that support [v4overv6] behavior and acquire their configuration from [DHCP] SHOULD implement the option described here. 3.1. Overlapping routes [RFC3442] documents a mechanism to communicate a set of routes and their nexthops over DHCP. The original DHCP "router" option (code 3) may communicate a default router. If either of these options is used, the routes communicated may overlap. To get consistent and unsurprising behavior, this document places the follwing expectations on the host: Lamparter & Fiebig Expires 24 January 2025 [Page 3] Internet-Draft dhc-route4via6 July 2024 * Routes that describe distinct destination prefixes MUST be handled independently. This includes routes that differ only in prefix length. As a result, the routing table MAY contain a mix of IPv4 routes with IPv4 nexthops as well as IPv6 nexthops. Standard longest prefix match behavior MUST be observed. * If routes with the same destination prefix are described both with previously existing methods as well as the options documented here, the route described by the latter MUST be used and the routes with IPv4 nexthops MUST be discarded. This notably includes "unreachable" routes described here; a route with an IPv4 nexthop for such a destination MUST still be discarded. * Multiple routes for the same destination prefix with different nexthops of the same address family SHOULD be combined into a single route for equal-cost multipath behavior, if the host supports this. If ECMP routes are not supported, the host MUST deterministically choose one of the routes. This MAY be done by using the first or last option as seen in DHCP packet order, or by choosing the numerically lowest or highest nexthop. 3.2. Default route The default route is expressed here as a route for 0.0.0.0/0. There is no distinct special encoding for a default gateway, any nexthop for 0.0.0.0/0 MUST be treated as if it were a default gateway. 3.3. Routes clashing with the connected subnet // TODO: determine what behavior is reasonable here. (The client is // likely to be given a /32 subnet mask anyway.) 4. DHCP Option encoding 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 (TBA1) | Length | Routes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : ... : Type TBA1 (field defined in [DHCP-OPT]) Length as defined in [DHCP-OPT] Routes One or more routes encoded as described below. DHCP Servers Lamparter & Fiebig Expires 24 January 2025 [Page 4] Internet-Draft dhc-route4via6 July 2024 MUST NOT emit this option with an empty payload. DHCP clients MUST ignore this option if its length is zero. It is easily possible for encoded routes and their nexthops to exceed the encodable size of 255 bytes. This is addressed by allowing multiple occurences of this option. DHCP clients MUST process all instances of this option additively. DHCP servers MAY encode configured routes in as many instances of this option as they choose; they SHOULD however ideally choose a space-efficient encoding. 4.1. Route encoding 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 +-+-+-+-+-+-+-+-+ | T | prefixlen | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv4 prefix (0 - 4 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv6 nexthop (0, 8 or 16 octets) | : ... : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ T Determines the function and nexthop encoding for this route. Valid values and their meanings are: // This encoding mechanism might be excessive. The original // motivation was to have a way to tell the client that it should use // the relay's link-local address as next-hop. Filling that in from // the DHCP server is annoying, since the server has otherwise no // need to know that address. It's not a problem if the operator // configures a fixed LL on all routers (e.g. fe80::1), but without // that each router would just be using its autoconf'd LL. // Having an encoding for 'unreachable' routes and a shorthand for // link-local addresses is really just "fluff" too. T = 0 - use DHCP packet source Indicates that this route should use the DHCP packet's source address as nexthop. When [DHCP4o6] is in use, hosts MUST retrieve the IPv6 source address of the DHCPv6 packet carrying the DHCPV4-RESPONSE message. TODO: does it really make sense to support IPv4 here? Maybe only allow this with DHCP4o6? Lamparter & Fiebig Expires 24 January 2025 [Page 5] Internet-Draft dhc-route4via6 July 2024 The IPv6 nexthop field for this route has zero length. T = 1 - unreachable destination Indicates that the destination prefix specified by this route is unreachable. More specific destination prefixes may still be reachable. TODO: specify fault mix (unreachable+reachable) The IPv6 nexthop field for this route has zero length. T = 2 - link-local IPv6 nexthop Indicates that this route uses a link-local IPv6 nexthop address, encoding only the latter 8 bytes of the address for space efficiency. The upper 8 bytes of the address are filled in with fe80::. The IPv6 nexthop field for this route is 8 octets long. T = 3 - generic IPv6 nexthop Indicates that this route uses the specified IPv6 nexthop, encoding the full address. The encoded address may be any type of unicast IPv4 address, including GUA, ULA and LL. The IPv6 nexthop field for this route is 16 octets long. prefixlen IPv4 prefix length, integer value from 0 to 32 (inclusive) IPv4 prefix The route's destination prefix, encoded in as few bytes necessary for the given prefixlen value. Calculate as ceil(prefixlen / 8). IPv6 nexthop encoding as determined by T 5. YANG model _TBD if needed_ 6. Security Considerations _TBD_ 7. Privacy Considerations _TBD_ Lamparter & Fiebig Expires 24 January 2025 [Page 6] Internet-Draft dhc-route4via6 July 2024 8. IANA Considerations A codepoint from the "BOOTP Vendor Extensions and DHCP Options" registry is requested for use with the option described in Section 4. // Editor note: 2 places of TBA1 9. References 9.1. 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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, DOI 10.17487/RFC2131, March 1997, . [DHCP-OPT] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997, . [v4overv6] Chroboczek, J., Kumari, W. A., and T. Høiland-Jørgensen, "IPv4 routes with an IPv6 next hop", Work in Progress, Internet-Draft, draft-chroboczek-intarea-v4-via-v6-01, 8 July 2024, . 9.2. Informative References [RFC3442] Lemon, T., Cheshire, S., and B. Volz, "The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4", RFC 3442, DOI 10.17487/RFC3442, December 2002, . [DHCPv6] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., Richardson, M., Jiang, S., Lemon, T., and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 8415, DOI 10.17487/RFC8415, November 2018, . Lamparter & Fiebig Expires 24 January 2025 [Page 7] Internet-Draft dhc-route4via6 July 2024 [DHCP4o6] Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I. Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport", RFC 7341, DOI 10.17487/RFC7341, August 2014, . Acknowledgements _TBD, FILL IN_ Example encoded options (TBA) 01 00 A single default route (0.0.0.0/0) using the DHCP packet's source address as nexthop. (TBA) 0a 88 0a 00 00 00 00 00 00 00 01 A single route for 10.0.0.0/8 via fe80::1. (TBA) 15 40 d8 c0 00 02 20 01 0d b8 12 34 56 78 00 00 00 00 00 00 00 00 A route marking 0.0.0.0/0 as unreachable, and another route for 192.0.2.0/24 via 2001:db8:1234:5678::. Editing notes (TO BE REMOVED) * -00: Authors' Addresses David 'equinox' Lamparter NetDEF, Inc. San Jose, United States of America Email: equinox@diac24.net, equinox@opensourcerouting.org Tobias Fiebig Max-Planck-Institut fuer Informatik Campus E14 66123 Saarbruecken Germany Phone: +49 681 9325 3527 Email: tfiebig@mpi-inf.mpg.de Lamparter & Fiebig Expires 24 January 2025 [Page 8]