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Versions: (draft-link-dhc-v6only) 00 08

Dynamic Host Configuration                                    L. Colitti
Internet-Draft                                                J. Linkova
Updates: 2563 (if approved)                                       Google
Intended status: Standards Track                           M. Richardson
Expires: February 14, 2021                                     Sandelman
                                                            T. Mrugalski
                                                                     ISC
                                                         August 13, 2020


                 IPv6-Only-Preferred Option for DHCPv4
                        draft-ietf-dhc-v6only-08

Abstract

   This document specifies a DHCPv4 option to indicate that a host
   supports an IPv6-only mode and is willing to forgo obtaining an IPv4
   address if the network provides IPv6 connectivity.  It also updates
   RFC2563 to specify the DHCPv4 server behavior when the server
   receives a DHCPDISCOVER not containing the Auto-Configure option but
   containing the new option defined in this document.

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 February 14, 2021.

Copyright Notice

   Copyright (c) 2020 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



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   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
   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 . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Reasons to Signal IPv6-Only Support in DHCPv4 Packets . . . .   5
   3.  IPv6-Only Preferred Option  . . . . . . . . . . . . . . . . .   5
     3.1.  Option format . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  DHCPv4 Client Behavior  . . . . . . . . . . . . . . . . .   6
     3.3.  DHCPv4 Server Behavior  . . . . . . . . . . . . . . . . .   8
       3.3.1.  Interaction with RFC2563  . . . . . . . . . . . . . .   9
     3.4.  Constants and Configuration Variables . . . . . . . . . .  10
   4.  IPv6-Only Transition Technologies Considerations  . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   One of the biggest challenges of deploying IPv6-only LANs is that
   such networks might contain rather heterogeneous collection of hosts.
   While some hosts 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), others might
   still have IPv4 dependencies and need IPv4 addresses to operate
   properly.  To incrementally rollout IPv6-only, network operators
   might need to provide IPv4 on demand whereby a host receives an IPv4
   address if it needs it, while IPv6-only capable hosts (such as modern
   mobile devices) are not allocated IPv4 addresses.  Traditionally that
   goal is achieved by placing IPv6-only capable devices into a
   dedicated IPv6-only network segment or WiFi SSID, while dual-stack
   devices reside in another network with IPv4 and DHCPv4 enabled.
   However such an approach has a number of drawbacks, including but not
   limited to:

   o  Doubling the number of network segments leads to operational
      complexity and performance impact, for instance due to high memory
      utilization caused by an increased number of ACL entries.



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   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 and choose between an IPv6-only and a
      dual-stack VLAN.

   It would be beneficial for IPv6 deployment if operators could
   implement IPv6-mostly (or IPv4-on-demand) 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, such an approach 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 its request for
   an IPv4 address is optional and a network to signal that IPv6-only
   functionality (such as NAT64, [RFC6146]) is available.  The proposed
   solution is to introduce a new DHCPv4 option which a client uses to
   indicate that it does not need an IPv4 address if the network
   provides IPv6-only connectivity (as NAT64 and DNS64).  If the
   particular network segment provides IPv4-on-demand 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.

   [RFC2563] introduces the Auto-Configure DHCPv4 option and describes
   DHCPv4 servers behavior if no address is chosen for a host.  This
   document updates [RFC2563] to modify the server behavior if the
   DHCPOFFER contains the IPv6-only Preferred option.

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.



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1.2.  Terminology

   Dual-stack network or device: a network or device which has both
   versions of the Internet Protocol (IPv4 and IPv6) enabled and
   operational.

   IPv6-only capable host: a host which does not require an IPv4 address
   and can operate on IPv6-only networks.  More precisely, 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.  This document implies that
   IPv6-only capable hosts reach IPv4-only destinations via a NAT64
   service provided by the network.  Section 4 discusses hypothetical
   scenarios of other transition technologies being used.

   IPv4-requiring host: a host which is not IPv6-only capable and can
   not operate in an IPv6-only network providing NAT64 service.

   IPv4-on-demand: a deployment scenario where 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 and allows coexistence of
   IPv6-only, dual-stack and IPv4-only hosts on the same segment.  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 mode: a mode of operation when a host acts as an IPv6-only
   capable host and does not have IPv4 addresses assigned (except that
   IPv4 link-local addresses [RFC3927] may have been configured).

   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 [RFC6146].

   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].




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

   Network attachment event: A Link Up event, as described by [RFC4957]
   which results in a host detecting an available network.

   Disabling IPv4 stack on the host interface: the host behavior when
   the host:

   o  does not send any IPv4 packets from that interface,

   o  drops all IPv4 packets received on that interface and

   o  does not forward any IPv4 packets to that interface.

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

   For networks which contain a mix of both IPv6-only capable hosts and
   IPv4-requiring hosts, and which utilize DHCPv4 for configuring the
   IPv4 network stack on hosts, it seems natural to leverage the same
   protocol to signal that IPv4 is discretional on a given segment.  An
   ability to remotely disable IPv4 on a host can be seen as a new
   denial-of-service attack vector.  The proposed approach limits the
   attack surface to DHCPv4-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.  In addition, the proposed
   mechanism does not introduce any additional delays to the process of
   configuring IP stack on hosts.  If the network does not support IPv6-
   only/IPv4-on-demand mode, an IPv6-only capable host would configure
   an IPv4 address as quickly as on any other host.

   Being a client/server protocol, DHCPv4 allows IPv4 to be selectively
   disabled on a per-host basis on a given network segment.  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







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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Code      |   Length      |           Value               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Value (contd)         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                Figure 1: IPv6-Only Preferred Option Format

   Fields:

   Code:    8-bit identifier of the IPv6-Only Preferred option code as
           assigned by IANA: TBD.
           The client includes the Code in the Parameter Request List in
           DHCPDISCOVER and DHCPREQUEST messages as described in
           Section 3.2.
   Length:  8-bit unsigned integer.  The length of the option excluding
           the Code and Length Fields.  The server MUST set the length
           field to 4. The client 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 configured with a
           V6ONLY_WAIT timer the server MUST set the field to that
           configured value. Otherwise the server MUST set it to zero.
           The client MUST process that field as described in
           Section 3.2.
           The client never sets this field as it never sends the full
           option but includes the option code in the Parameter Request
           List as described in Section 3.2.

3.2.  DHCPv4 Client Behavior

   A DHCPv4 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 DHCPv4 client MUST
   NOT consider the host to be an IPv6-only capable for the purpose of
   sending/receiving DHCPv4 packets over any other interfaces.




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   The DHCPv4 client on an IPv4-requiring host MUST NOT include the
   IPv6-only Preferred option in the Parameter Request List of any
   DHCPv4 packets and MUST ignore that option in packets received from
   DHCPv4 servers.

   DHCPv4 clients running on IPv6-only capable hosts SHOULD include the
   IPv6-only Preferred option code in the Parameter Request List in
   DHCPDISCOVER and DHCPREQUEST messages for interfaces so enabled and
   follow the processing as described below on a per enabled interface
   basis.

   If the client did not include the IPv6-only Preferred option code in
   the Parameter Request List 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 SHOULD NOT
   request the IPv4 address provided in the DHCPOFFER.  If the IPv6-only
   Preferred option returned by the server contains a value greater or
   equal to MIN_V6ONLY_WAIT, the client SHOULD set the V6ONLY_WAIT timer
   to that value.  Otherwise, the client SHOULD set the V6ONLY_WAIT
   timer to MIN_V6ONLY_WAIT.  The client SHOULD stop the DHCPv4
   configuration process for V6ONLY_WAIT seconds or until a network
   attachment event, whichever happens first.  The host MAY disable the
   IPv4 stack completely on the affected interface for V6ONLY_WAIT
   seconds or until the network attachment event, whichever happens
   first.

   The IPv6-only Preferred option SHOULD be included in the Parameter
   Request List 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 DHCPv4 server responds with a
   DHCPACK that includes the IPv6-only Preferred option, the client
   behaviour depends on the client's state.  If the client is in the
   INIT-REBOOT state it SHOULD stop the DHCPv4 configuration process or
   disable IPv4 stack completely for V6ONLY_WAIT seconds or until the
   network event, whichever happens first.  It also MAY send a
   DHCPRELEASE message.  If the client is in any other state it SHOULD
   continue to use the assigned IPv4 address until further DHCPv4
   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.




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   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 configurean 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.
   The Auto-Configure Option [RFC2563] can be used to control IPv4 link-
   local addresses autoconfiguration.  Section 3.3.1 discusses the
   interaction between the IPv6-only Preferred and the Auto-Configure
   options.

3.3.  DHCPv4 Server Behavior

   The DHCPv4 server SHOULD be able to configure all or individual pools
   to include the IPv6-only preferred option in DHCPv4 responses if the
   client included the option code in the Parameter Request List option.
   The DHCPv4 server MAY have a configuration option to specify the
   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 DHCPv4 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 NOT assign an address
   from the pool.  Instead it SHOULD return 0.0.0.0 as the offered
   address.  Alternatively, if offering 0.0.0.0 is not feasible, for
   example due to some limitations of the server or the network
   infrastructure, the server MAY include an available IPv4 address from
   the pool into the DHCPOFFER as per recommendations in [RFC2131].  In
   this case, the offered address MUST be a valid address that is not
   committed to any other client.  Because the client is not expected
   ever to request this address, the server SHOULD NOT reserve the
   address and SHOULD NOT verify its uniqueness.  If the client then
   issues a DHCPREQUEST for the address, the server MUST process it per



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   [RFC2131], including replying with a DHCPACK for the address if in
   the meantime it has not been committed to another client.

   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 as indicated above.

   If the server receives a DHCPREQUEST containing the IPv6-only
   Preferred option for the address from a pool configured as
   IPv6-mostly, the server MUST process it per [RFC2131].

3.3.1.  Interaction with RFC2563

   [RFC2563] defines an Auto-Configure DHCPv4 option to disable IPv4
   link-local address configuration for IPv4 clients.  Clients can
   support both, neither or just one of IPv6-Only Preferred and Auto-
   Configure options.  If a client sends both IPv6-Only Preferred and
   Auto-Configure options the network administrator can prevent the host
   from configuring an IPv4 link-local address on an IPv6-mostly
   network.  To achieve this the server needs to send DHCPOFFER which
   contains a 'yiaddr' of 0x00000000, and the Auto-Configure flag saying
   "DoNotAutoConfigure".

   However special care should be taken in a situation when a server
   supports both options and receives just IPv6-Only Preferred option
   from a client.  Section 2.3 of [RFC2563] states that if no address is
   chosen for the host (which would be the case for IPv6-only capable
   clients on IPv6-mostly network) then: "If the DHCPDISCOVER does not
   contain the Auto-Configure option, it is not answered."  Such
   behavior would be undesirable for clients supporting the IPv6-Only
   Preferred option without supporting the Auto-Configure option as they
   would not receive any response from the server and would keep asking,
   instead of disabling DHCPv4 for V6ONLY_WAIT seconds.  Therefore the
   following update is made to Section 2.3 of [RFC2563]"

   OLD TEXT:

   ---

   However, if no address is chosen for the host, a few additional steps
   MUST be taken.

   If the DHCPDISCOVER does not contain the Auto-Configure option, it is
   not answered.

   ---



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   NEW TEXT:

   ---

   However, if no address is chosen for the host, a few additional steps
   MUST be taken.

   If the DHCPDISCOVER does not contain the Auto-Configure option and
   the IPv6-Only Preferred option is not present, it is not answered.
   If the DHCPDISCOVER does not contain the Auto-Configure option but
   contains the IPv6-Only Preferred option, the processing rules for the
   IPv6-Only Preferred option apply.

   ---

3.4.  Constants and Configuration Variables

   V6ONLY_WAIT     The time for which the client SHOULD stop the DHCPv4
                   configuration process. The value MUST NOT be less
                   than MIN_V6ONLY_WAIT seconds. Default: 1800 seconds
   MIN_V6ONLY_WAIT The lower boundary for V6ONLY_WAIT. Value: 300
                   seconds

4.  IPv6-Only Transition Technologies Considerations

   Until IPv6 adoption in the Internet reaches 100%, communication
   between an IPv6-only host and IPv4-only destination requires some
   form of transition mechanism deployed in the network.  At the time of
   writing, the only such mechanism that is widely supported by end
   hosts is NAT64 [RFC6146] (either with or without 464XLAT).  Therefore
   the IPv6-only Preferred option is only sent by hosts capable of
   operating on NAT64 networks.  In a typical deployment scenario, a
   network administrator would not configure the DHCPv4 server to return
   the IPv6-only Preferred option unless the network provides NAT64
   service.

   Hypothetically, it is possible for multiple transition technologies
   to coexist.  In such scenario some form of negotiation would be
   required between a client and a server to ensure that the transition
   technology supported by the client is the one the network provides.
   However it seems unlikely that any new transition technology would
   arise and be widely adopted in any foreseeable future.  Therefore
   adding support for non-existing technologies seems to be suboptimal
   and the proposed mechanism implies that NAT64 is used to facilitate
   connectivity between IPv6 and IPv4.  In the unlikely event that a new
   transition mechanism becomes widely deployed, the applicability of
   the IPv6-Only-Preferred option to that mechanism will depend on the
   nature of the new mechanism.  If the new mechanism is designed in



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   such a way that it's fully transparent for hosts that support NAT64
   and the IPv6-Only-Preferred option, then the option can continue to
   be used with the new mechanism.  If the new mechanism is not
   compatible with NAT64, and implementation on the host side is
   required to support it, then a new DHCPv4 option needs to be defined.

   It should be also noted that declaring a host (technically, a host
   interface) IPv6-only capable is a policy decision.  For example,

   o  An operating system vendor may make such decision and configure
      their DHCPv4 clients to send the IPv6-Only Preferred option by
      default if the OS has 464XLAT CLAT [RFC6877] enabled.

   o  An enterprise network administrator may provision the corporate
      hosts as IPv6-only capable if all applications users are supposed
      to run have been tested in an IPv6-only environment (or if 464XLAT
      CLAT is enabled on the devices).

   o  IoT devices may be shipped in IPv6-only capable mode if they are
      designed to connect to IPv6-enabled cloud destination only.

5.  IANA Considerations

   The IANA is requested to assign a new DHCPv4 Option code for the
   IPv6-Only Preferred option from the BOOTP Vendor Extensions and
   DHCPv4 Options registry, located at https://www.iana.org/assignments/
   bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml#options .  If
   possible, please assign option code 108.

   +-----------+-----------+--------+----------+-----------------------+
   | Tag       | Name      | Data   | Meaning  | Reference             |
   |           |           | Length |          |                       |
   +-----------+-----------+--------+----------+-----------------------+
   | TBD       | IPv6-only | 4      | Number   | draft-ietf-dhc-v6only |
   | (proposed | Preferred |        | of       |                       |
   | value:    | option    |        | seconds  |                       |
   | 108)      |           |        | to       |                       |
   |           |           |        | disable  |                       |
   |           |           |        | DHCPv4   |                       |
   |           |           |        | for      |                       |
   +-----------+-----------+--------+----------+-----------------------+

                                  Table 1








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6.  Security Considerations

   An attacker might send a spoofed DHCPOFFER containing IPv6-only
   Preferred option with the value field set to a large number, such as
   0xffffffff, effectively disabling DHCPv4 on clients supporting the
   option.  If the network is IPv4-only such clients would lose
   connectivity, while on a dual-stack network without NAT64 service
   only connectivity to IPv4-only destinations would be affected.  The
   recovery would require triggering a network attachment event.
   However it should be noted that if the network does not provide
   protection from a rogue DHCPv4 server the similar attack vector can
   be executed by offering an invalid address and setting the Lease Time
   option value field to 0xffffffff.  The latter attack would affect all
   hosts, not just hosts that support the IPv6-only Preferred option.
   Therefore the security measures against rogue DHCPv4 servers would be
   sufficient to prevent the attacks specific to IPv6-only Preferred
   option.  Additionally such attacks can only be executed if the victim
   prefers the rogue DHCPOFFER over the legitimate ones.  Therefore for
   the attack to be successful the attacker needs to know the selection
   criteria used by the client and to be able to make its rogue offer
   more preferable.

   It should be noted that disabling IPv4 on a host upon receiving the
   IPv6-only Preferred option from the DHCPv4 server protects the host
   from IPv4-related attacks and therefore could be considered a
   security feature as it reduces the attack surface.

7.  Acknowledgements

   Thanks to the following people (in alphabetical order) for their
   review and feedback: Mohamed Boucadair, Martin Duke, Russ Housley,
   Sheng Jiang, Benjamin Kaduk, Murray Kucherawy, Ted Lemon, Roy
   Marples, Bjorn Mork, Alvaro Retana, Peng Shuping, Pascal Thubert,
   Bernie Volz, Eric Vyncke, Robert Wilton.  Authors would like to thank
   Bob Hinden and Brian Carpenter for the initial idea of signaling
   IPv6-only capability to hosts.  Special thanks to Erik Kline, Mark
   Townsley and Maciej Zenczykowski for the discussion which led to the
   idea of signalling IPv6-only capability over DHCPv4.

8.  References

8.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.




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   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC2563]  Troll, R., "DHCP Option to Disable Stateless Auto-
              Configuration in IPv4 Clients", RFC 2563,
              DOI 10.17487/RFC2563, May 1999,
              <https://www.rfc-editor.org/info/rfc2563>.

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              DOI 10.17487/RFC3927, May 2005,
              <https://www.rfc-editor.org/info/rfc3927>.

   [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,
              <https://www.rfc-editor.org/info/rfc4039>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

   [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,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC4957]  Krishnan, S., Ed., Montavont, N., Njedjou, E., Veerepalli,
              S., and A. Yegin, Ed., "Link-Layer Event Notifications for
              Detecting Network Attachments", RFC 4957,
              DOI 10.17487/RFC4957, August 2007,
              <https://www.rfc-editor.org/info/rfc4957>.

   [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, <https://www.rfc-editor.org/info/rfc6146>.

   [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,
              <https://www.rfc-editor.org/info/rfc6147>.





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   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, April 2013,
              <https://www.rfc-editor.org/info/rfc6877>.

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












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