DHCPv4-over-DHCPv6 (DHCP 4o6) Transport

Internet Engineering Task Force (IETF)                            Q. Sun
Request for Comments: 7341                                        Y. Cui
Category: Standards Track                            Tsinghua University
ISSN: 2070-1721                                             M. Siodelski
                                                                     ISC
                                                             S. Krishnan
                                                                Ericsson
                                                               I. Farrer
                                                     Deutsche Telekom AG
                                                             August 2014


                DHCPv4-over-DHCPv6 (DHCP 4o6) Transport

Abstract

   IPv4 connectivity is still needed as networks migrate towards IPv6.
   Users require IPv4 configuration even if the uplink to their service
   provider supports IPv6 only.  This document describes a mechanism for
   obtaining IPv4 configuration information dynamically in IPv6 networks
   by carrying DHCPv4 messages over DHCPv6 transport.  Two new DHCPv6
   messages and two new DHCPv6 options are defined for this purpose.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7341.

Copyright Notice

   Copyright (c) 2014 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
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   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 ....................................................3
   2. Requirements Language ...........................................3
   3. Terminology .....................................................3
   4. Applicability ...................................................4
   5. Architecture Overview ...........................................4
   6. New DHCPv6 Messages .............................................6
      6.1. Message Types ..............................................6
      6.2. Message Formats ............................................6
      6.3. DHCPv4-query Message Flags .................................7
      6.4. DHCPv4-response Message Flags ..............................7
   7. New DHCPv6 Options ..............................................7
      7.1. DHCPv4 Message Option Format ...............................7
      7.2. DHCP 4o6 Server Address Option Format ......................8
   8. Use of the DHCPv4-query Unicast Flag ............................9
   9. DHCP 4o6 Client Behavior .......................................10
   10. Relay Agent Behavior ..........................................12
   11. DHCP 4o6 Server Behavior ......................................12
   12. Security Considerations .......................................13
   13. IANA Considerations ...........................................14
   14. Contributors List .............................................14
   15. References ....................................................14
      15.1. Normative References .....................................14
      15.2. Informative References ...................................15

1.  Introduction

   As the migration towards IPv6 continues, IPv6-only networks will
   become more prevalent.  In such networks, IPv4 connectivity will
   continue to be provided as a service over IPv6-only networks.  In
   addition to provisioning IPv4 addresses for clients of this service,
   other IPv4 configuration parameters may also be needed (e.g.,
   addresses of IPv4-only services).

   This document describes a transport mechanism to carry DHCPv4
   messages using the DHCPv6 protocol for the dynamic provisioning of
   IPv4 addresses and other DHCPv4 specific configuration parameters
   across IPv6-only networks.  It leverages the existing DHCPv4
   infrastructure, e.g., failover, DNS updates, DHCP Leasequery, etc.

   When IPv6 multicast is used to transport DHCP 4o6 messages, another
   benefit is that the operator can gain information about the
   underlying IPv6 network to which the DHCP 4o6 client is connected
   from the DHCPv6 relay agents through which the request has passed.

2.  Requirements Language

   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].

3.  Terminology

   This document makes use of the following terms:

   CPE:
      Customer Premises Equipment (also known as Customer Provided
      Equipment), which provides access for devices connected to a Local
      Area Network (LAN), typically at the customer's site/home, to the
      Internet Service Provider's (ISP's) network.

   DHCP 4o6 client (or client):
      A DHCP client supporting both the DHCPv6 protocol [RFC3315] as
      well as the DHCPv4 over DHCPv6 protocol described in this
      document.  Such a client is capable of requesting IPv6
      configuration using DHCPv6 and IPv4 configuration using DHCPv4
      over DHCPv6.

   DHCP 4o6 server (or server):
      A DHCP server that is capable of processing DHCPv4 packets
      encapsulated in the DHCPv4 Message option (defined below).

   DHCPv4 over DHCPv6:
      A protocol (described in this document) used to carry DHCPv4
      messages in the payload of DHCPv6 messages.

4.  Applicability

   The mechanism described in this document is not universally
   applicable.  This is intended as a special-purpose mechanism that
   will be implemented on nodes that must obtain IPv4 configuration
   information using DHCPv4 in specific environments where native DHCPv4
   is not available.  Such nodes are expected to follow the advice in
   Section 9; nodes that do not require this functionality are expected
   not to implement it, or not to enable it by default.  This mechanism
   may be enabled using an administrative control, or it may be enabled
   automatically in accordance with the needs of some dual-stack
   transition mechanism such as [LW4OVER6].  Such mechanisms are beyond
   the scope of this document.

5.  Architecture Overview

   The architecture described here addresses a typical use case, where a
   DHCP client's uplink supports IPv6 only and the Service Provider's
   network supports IPv6 and limited IPv4 services.  In this scenario,
   the client can only use the IPv6 network to access IPv4 services, so
   IPv4 services must be configured using IPv6 as the underlying network
   protocol.

   Although the purpose of this document is to address the problem of
   communication between the DHCPv4 client and the DHCPv4 server, the
   mechanism that it describes does not restrict the transported
   messages types to DHCPv4 only.  As the DHCPv4 message is a special
   type of BOOTP message, BOOTP messages [RFC951] MAY also be
   transported using the same mechanism.

   DHCP clients may be running on CPE devices, end hosts, or any other
   device that supports the DHCP-client function.  This document uses
   the CPE as an example for describing the mechanism.  This does not
   preclude any end host, or other device requiring IPv4 configuration,
   from implementing DHCPv4 over DHCPv6 in the future.

   This mechanism works by carrying DHCPv4 messages encapsulated within
   the newly defined DHCPv6 messages.  The DHCPv6-relay encapsulation is
   used solely to deliver DHCPv4 packets to a DHCPv4-capable server, and
   does not allocate any IPv6 addresses nor does it provide
   IPv6-configuration information to the client.  Figure 1, below,
   illustrates one possible deployment architecture of this mechanism.

   The DHCP 4o6 client implements a new DHCPv6 message called
   DHCPv4-query, which carries a DHCPv4 message encapsulated in the new
   DHCPv4 Message option.  The DHCPv6 message can be transmitted either
   via DHCPv6 Relay Agents or directly to the DHCP 4o6 server.

   The server replies with a new DHCPv6 message called DHCPv4-response,
   which carries the DHCPv4 message from the server, encapsulated in the
   DHCPv4 Message option.

                 _____________             _____________
                /             \           /             \
                |             |           |             |
       +--------+-+  IPv6   +-+-----------+-+  IPv6   +-+--------+
       | DHCP 4o6 | Network |    DHCPv6     | Network | DHCP 4o6 |
       |  Client  +---------+  Relay Agent  +---------+  Server  |
       | (on CPE) |         |               |         |          |
       +--------+-+         +-+-----------+-+         +-+--------+
                |             |           |             |
                \_____________/           \_____________/


                      Figure 1: Architecture Overview

   Before the client can use DHCPv4 over DHCPv6, it MUST obtain the
   necessary IPv6 configuration.  The client requests the DHCP 4o6
   Server Address option from the server by sending the option code in
   an Option Request option as described in [RFC3315].  If the server
   responds with the DHCP 4o6 Server Address option, it is an indication
   to the client to attempt using DHCPv4 over DHCPv6 to obtain IPv4
   configuration.  Otherwise, the client MUST NOT use DHCPv4 over DHCPv6
   to request IPv4 configuration.

   The client obtains the address(es) of the DHCP 4o6 server(s) from the
   DHCP 4o6 Server Address option and uses it (them) to communicate with
   the DHCP 4o6 servers as described in Section 9.  If the DHCP 4o6
   Server Address option contains no addresses (is empty), the client
   uses the well-known All_DHCP_Relay_Agents_and_Servers multicast
   address to communicate with the DHCP 4o6 server(s).

   Before applying for an IPv4 address via a DHCPv4-query message, the
   client must identify a suitable network interface for the address.
   Once the request is acknowledged by the server, the client can
   configure the address and other relevant parameters on this
   interface.  The mechanism for determining a suitable interface is out
   of the scope of the document.

6.  New DHCPv6 Messages

   Two new DHCPv6 messages carry DHCPv4 messages between the client and
   the server using the DHCPv6 protocol: DHCPv4-query and
   DHCPv4-response.  This section describes the structures of these
   messages.

6.1.  Message Types

   DHCPV4-QUERY (20):  The DHCP 4o6 client sends a DHCPv4-query message
      to a DHCP 4o6 server.  The DHCPv4 Message option carried by this
      message contains a DHCPv4 message that the DHCP 4o6 client uses to
      request IPv4 configuration parameters from the server.

   DHCPV4-RESPONSE (21):  A DHCP 4o6 server sends a DHCPv4-response
      message to a DHCP 4o6 client.  It contains a DHCPv4 Message option
      carrying a DHCPv4 message in response to a DHCPv4 message received
      by the server in the DHCPv4 Message option of the DHCPv4-query
      message.

6.2.  Message Formats

   Both DHCPv6 messages defined in this document share the following
   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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    msg-type   |                     flags                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                            options                            .
     .                           (variable)                          .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 2: The Format of DHCPv4-query and DHCPv4-response Messages

   msg-type:  Identifies the message type.  It can be either
      DHCPV4-QUERY (20) or DHCPV4-RESPONSE (21) corresponding to the
      contained DHCPv4-query or DHCPv4-response, respectively.

   flags:  Specifies flags providing additional information required by
      the server to process the DHCPv4 message encapsulated in the
      DHCPv4-query message, or required by the client to process a
      DHCPv4 message encapsulated in the DHCPv4-response message.

   options:  Options carried by the message.  The DHCPv4 Message Option
      (described in Section 7.1) MUST be carried by the message.  Only
      DHCPv6 options for IPv4 configuration may be included in this
      field.  It MUST NOT contain DHCPv6 options related solely to IPv6,
      or IPv6-only service configuration.

6.3.  DHCPv4-query Message Flags

   The "flags" field of the DHCPv4-query is used to carry additional
   information that may be used by the server to process the
   encapsulated DHCPv4 message.  Currently, only one bit of this field
   is used.  Remaining bits are reserved for the future use.  The
   "flags" field has the following format:

          0                   1                   2
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |U|                    MBZ                      |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 3: DHCPv4-query Flags Format

   U:   Unicast flag.  If set to 1, it indicates that the DHCPv4 message
        encapsulated within the DHCPv4-query message would be sent to a
        unicast address if it were sent using IPv4.  If this flag is set
        to 0, it indicates that the DHCPv4 message would be sent to the
        broadcast address if it were sent using IPv4.  The usage of the
        flag is described in detail in Section 8.

   MBZ: Bits MUST be set to zero when sending and MUST be ignored when
        receiving.

6.4.  DHCPv4-response Message Flags

   This document introduces no flags to be carried in the "flags" field
   of the DHCPv4-response message.  They are all reserved for future
   use.  The DHCP 4o6 server MUST set all bits of this field to 0 and
   the DHCP 4o6 client MUST ignore the content in this field.

7.  New DHCPv6 Options

7.1.  DHCPv4 Message Option Format

   The DHCPv4 Message option carries a DHCPv4 message that is sent by
   the client or the server.  Such messages exclude any IP or UDP
   headers.

   The format of the DHCPv4 Message option is:

      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-code          |           option-len          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                        DHCPv4-message                         .
     .                                                               .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 4: DHCPv4 Message Option Format

   option-code:  OPTION_DHCPV4_MSG (87).

   option-len:  Length of the DHCPv4 message.

   DHCPv4-message:  The DHCPv4 message sent by the client or the server.
      In a DHCPv4-query message, it contains a DHCPv4 message sent by a
      client.  In a DHCPv4-response message, it contains a DHCPv4
      message sent by a server in response to a client.

7.2.  DHCP 4o6 Server Address Option Format

   The DHCP 4o6 Server Address option is sent by a server to a client
   requesting IPv6 configuration using DHCPv6 [RFC3315].  It carries a
   list of DHCP 4o6 servers' IPv6 addresses that the client should
   contact to obtain IPv4 configuration.  This list may include
   multicast and unicast addresses.  The client sends its requests to
   all unique addresses carried in this option.

   This option may also carry no IPv6 addresses, which instructs the
   client to use the All_DHCP_Relay_Agents_and_Servers multicast address
   as the destination address.

   The presence of this option in the server's response indicates to the
   client that it should use DHCPv4 over DHCPv6 to obtain IPv4
   configuration.  If the option is absent, the client MUST NOT enable
   DHCPv4-over-DHCPv6 functionality.

   The format of the DHCP 4o6 Server Address option is:

      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-code         |           option-len          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                        IPv6 Address(es)                       .
     .                                                               .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 5: DHCP 4o6 Servers Address Option Format

   option-code:  OPTION_DHCP4_O_DHCP6_SERVER (88).

   option-len:  Length of the IPv6 address(es) carried by the option,
      i.e., multiple of 16 octets.  Minimal length of this option is 0.

   IPv6 Address:  Zero or more IPv6 addresses of the DHCP 4o6 server(s).

8.  Use of the DHCPv4-query Unicast Flag

   A DHCPv4 client conforming to [RFC2131] may send its DHCPREQUEST
   message to either a broadcast or unicast address depending on its
   state.  For example, a client in the RENEWING state uses a unicast
   address to contact the DHCPv4 server to renew its lease.  A client in
   the REBINDING state uses a broadcast address.

   In DHCPv4 over DHCPv6, IPv6 is used to deliver DHCPv4 messages to the
   DHCP 4o6 server.  There is no relation between the outer IPv6 address
   and the inner DHCPv4 message.  As a result, the server is unable to
   determine whether the received DHCPv4 messages should have been sent
   using broadcast or unicast in IPv4 by checking the IPv6 address.

   In order to allow the server to determine the client's state, the
   Unicast flag is carried in the DHCPv4-query message.  The client MUST
   set this flag to 1 when the DHCPv4 message would have been sent to
   the unicast address if using DHCPv4 over IPv4.  This flag MUST be set
   to 0 if the DHCPv4 client would have sent the message to the
   broadcast address in IPv4.  The choice whether a given message should
   be sent to a broadcast or unicast address is made based on the
   [RFC2131] and its extensions.

   Note: The Unicast flag reflects how the DHCPv4 packet would have been
   sent; not how the DHCPv6 packet itself is sent.

9.  DHCP 4o6 Client Behavior

   The client MUST obtain necessary IPv6 configuration from a DHCPv6
   server before using DHCPv4 over DHCPv6.  The client requests the DHCP
   4o6 Server Address option using the Option Request option (ORO) in
   every Solicit, Request, Renew, Rebind, and Information-request
   message.  If the DHCPv6 server includes the DHCP 4o6 Server Address
   option in its response, it is an indication that the client can use
   DHCPv4 over DHCPv6 to obtain the IPv4 configuration (by sending
   DHCPv4 messages encapsulated in DHCPv4-query messages).

   The client MUST NOT use DHCPv4 over DHCPv6 to request IPv4
   configuration if the DHCPv6 server does not include the DHCP 4o6
   Server Address option.  If the IPv6 configuration that contained the
   DHCP 4o6 Server Address option subsequently expires, or if the
   renewed IPv6 configuration does not contain the DHCP 4o6 Server
   Address option, the client MUST stop using DHCPv4 over DHCPv6 to
   request or renew IPv4 configuration.  However, the client continues
   to request DHCP 4o6 Server Address option in the messages sent to the
   DHCPv6 server as long as it desires to use DHCPv4 over DHCPv6.

   It is possible in a multihomed configuration for there to be more
   than one DHCPv6 configuration containing a DHCP 4o6 Server Address
   Option active at the same time.  In this case, the configurations are
   treated as being independent, so that when any such configuration is
   active, a DHCPv4-over-DHCPv6 function may be enabled for that
   configuration.

   An implementation may also treat such configurations as being
   exclusive, such that only one is kept active at a time.  In this
   case, the client keeps the same configuration active continuously as
   long as it is valid.  If that configuration becomes invalid but one
   or more other configurations remain valid, the client activates one
   of the remaining valid configurations.

   Which strategy to follow is dependent on the implementation: keeping
   multiple configurations active at the same time may provide useful
   redundancy in some applications but may be needlessly complex in
   other cases.

   If the client receives the DHCP 4o6 Server Address option and DHCPv4
   [RFC2131] is used on the interface over which the DHCPv6 option was
   received, the client MUST stop using the IPv4 configuration received
   using DHCPv4 on this interface.  The client MAY send a DHCPRELEASE to
   the DHCPv4 server to relinquish an existing lease as described in
   Section 4.4.6 of [RFC2131].  The client MUST NOT use DHCPv4 on this
   interface as long as it receives DHCP 4o6 Server Address option in
   the messages received from the DHCPv6 server.

   If the client receives a DHCP 4o6 Server Address option that contains
   no IP addresses, i.e., the option is empty, the client MUST send its
   requests to the All_DHCP_Relay_Agents_and_Servers multicast address.
   If there is a list of IP addresses in the option, the client SHOULD
   send requests to each unique address carried by the option.

   If the client obtained stateless IPv6 configuration by sending an
   Information-request message to the server, the client MUST follow the
   rules in [RFC4242] to periodically refresh the DHCPv4-over-DHCPv6
   configuration (i.e., list of DHCP 4o6 servers) as well as other
   configuration data.  The client that obtained stateful IPv6
   configuration will refresh the status of DHCPv4-over-DHCPv6 function
   when extending a lifetime of acquired IPv6 address (Renew and Rebind
   messages).

   The client MUST employ an IPv6 address of an appropriate scope from
   which to source the DHCPv4-query message.  When the client sends a
   DHCPv4-query message to the multicast address, it MUST use a link-
   local address as the source address as described in [RFC3315].  When
   the client sends a DHCPv4-query message using unicast, the source
   address MUST be an address of appropriate scope, acquired in advance.

   The client generates a DHCPv4 message and stores it verbatim in the
   DHCPv4 Message option carried by the DHCPv4-query message.  The
   client MUST put exactly one DHCPv4 Message option into a single
   DHCPv4-query message.  The client MUST NOT request the DHCP 4o6
   Server Address option in the DHCPv4-query message.

   The client MUST follow the rules defined in Section 8 when setting
   the Unicast flag based on the DHCPv4 destination.

   On receiving a DHCPv4-response message, the client MUST look for the
   DHCPv4 Message option within this message.  If this option is not
   found, the DHCPv4-response message is discarded.  If the DHCPv4
   Message option is present, the client extracts the DHCPv4 message it
   contains and processes it as described in Section 4.4 of [RFC2131].

   When dealing with IPv4 configuration, the client MUST follow the
   normal DHCPv4 retransmission requirements and strategy as specified
   in Section 4.1 of [RFC2131].  There are no explicit transmission
   parameters associated with a DHCPv4-query message, as this is
   governed by the DHCPv4 "state machine" [RFC2131].

   The client MUST implement [RFC4361] to ensure that the device
   correctly identifies itself.  It MUST send a 'client identifier'
   option when using DHCPv4 over DHCPv6.

10.  Relay Agent Behavior

   When a DHCPv6 relay agent receives a DHCPv4-query message, it may not
   recognize this message.  The unknown message MUST be forwarded as
   described in [RFC7283].

   A DHCPv6 relay agent that can recognize DHCP 4o6 messages MAY allow
   the configuration of a separate set of destination addresses for such
   messages in addition to the destination addresses used for relaying
   the other DHCPv6 messages.  To implement this function, the relay
   checks the received DHCPv6 message type and forwards according to the
   following logic:

   1.  If the message type is DHCPV4-QUERY, the packet is relayed to the
       configured DHCP 4o6 Server's address(es) in the form of a normal
       DHCPv6 packet (i.e., DHCPv6/UDP/IPv6).

   2.  For any other DHCPv6 message type, forward according to section
       20 of [RFC3315].

   The above logic only allows for separate relay destinations
   configured on the relay agent closest to the client (single relay
   hop).  Multiple relaying hops are not considered in the case of
   separate relay destinations.

11.  DHCP 4o6 Server Behavior

   When the server receives a DHCPv4-query message from a client, it
   searches for the DHCPv4 Message option.  The server discards a packet
   without this option.  In addition, the server MAY notify an
   administrator about the receipt of this malformed packet.  The
   mechanism for this notification is out of scope for this document.

   If the server finds a valid DHCPv4 Message option, it extracts the
   original DHCPv4 message.  Since the DHCPv4 message is encapsulated in
   the DHCPv6 message, it lacks the information that is typically used
   by the DHCPv4 server, implementing [RFC2131], to make address-
   allocation decisions, e.g., giaddr for relayed messages and IPv4
   address of the interface that the server is using to communicate with
   a directly connected client.  Therefore, the DHCP 4o6 server
   allocates addresses according to the policies on local address
   assignment determined by the server administrator.  For example, if
   the DHCPv4-query message has been sent via a relay, the server MAY
   use the link-address field of the Relay-forward message as a lookup
   for the IPv4 subnet from which to assign a DHCPv4 address.  If the
   DHCPv4-query message has been sent from a directly connected client,
   the server MAY use the IPv6 source address of the message to
   determine the appropriate IPv4 subnet to use for DHCPv4 address
   assignment.

   Alternatively, the server may act as a DHCPv4 relay agent and forward
   the DHCPv4 packet to a "normal" DHCPv4 server.  The details of such a
   solution have not been considered by the working group; describing
   that solution is out of scope of this document and is left as future
   work should the need for it arise.

   The server SHOULD use the "flags" field of the DHCPv4-query message
   to create a response (server to client DHCPv4 message).  The use of
   this field is described in detail in Section 8.

   When an appropriate DHCPv4 response is created, the server places it
   in the payload of a DHCPv4 Message option, which it puts into the
   DHCPv4-response message.

   If the DHCPv4-query message was received directly by the server, the
   DHCPv4-response message MUST be unicast from the interface on which
   the original message was received.

   If the DHCPv4-query message was received in a Relay-forward message,
   the server creates a Relay-reply message with the DHCPv4-response
   message in the payload of a Relay Message option, and responds as
   described in Section 20.3 of [RFC3315].

12.  Security Considerations

   In this specification, DHCPv4 messages are encapsulated in the newly
   defined option and messages.  This is similar to the handling of the
   current relay agent messages.  In order to bypass firewalls or
   network authentication gateways, a malicious attacker may leverage
   this feature to convey other messages using DHCPv6, i.e., use DHCPv6
   as a form of encapsulation.  However, the potential risk from this is
   no more severe than that with the current DHCPv4 and DHCPv6 practice.

   It is possible for a rogue server to reply with a DHCP 4o6 Server
   Address option containing duplicated IPv6 addresses, which could
   cause an amplification attack.  To avoid this, the client MUST check
   if there are duplicate IPv6 addresses in a DHCP 4o6 Server Address
   option when receiving one.  The client MUST ignore any but the first
   instance of each address.

   When considering whether to enable DHCPv4-over-DHCPv6, one important
   consideration is that when it is enabled, this gives the DHCPv6
   server the ability to shut off DHCPv4 traffic, and, consequently,
   IPv4 traffic, on the interface that is configured to do DHCPv4-over-
   DHCPv6.  For this reason, DHCPv4-over-DHCPv6 should only be enabled
   in situations where there is a clear trust relationship that
   eliminates this concern.  For instance, a CPE device can safely
   enable this on its WAN interface, because it is reasonable to assume
   that an ISP will not accidentally configure DHCPv4 over DHCPv6
   service on that link, and that it will be impractical for an attacker
   to set up a rogue DHCPv6 server in the ISP's network.

13.  IANA Considerations

   IANA has allocated two DHCPv6 option codes for use by
   OPTION_DHCPV4_MSG (87) and OPTION_DHCP4_O_DHCP6_SERVER (88) from the
   "Option Codes" table.  Also, IANA has allocated two DHCPv6 message
   type codes for the DHCPV4-QUERY (20) and DHCPV4-RESPONSE (21) from
   the "Message Types" table of the "Dynamic Host Configuration Protocol
   for IPv6 (DHCPv6)" registry.  Both tables can be found at
   <http://www.iana.org/assignments/dhcpv6-parameters/>.

14.  Contributors List

   Many thanks to Ted Lemon, Bernie Volz, Tomek Mrugalski, Cong Liu, and
   Yuchi Chen for their great contributions to the specification.

15.  References

15.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, 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.

   [RFC4242]  Venaas, S., Chown, T., and B. Volz, "Information Refresh
              Time Option for Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 4242, November 2005.

   [RFC4361]  Lemon, T. and B. Sommerfeld, "Node-specific Client
              Identifiers for Dynamic Host Configuration Protocol
              Version Four (DHCPv4)", RFC 4361, February 2006.

   [RFC7283]  Cui, Y., Sun, Q., and T. Lemon, "Handling Unknown DHCPv6
              Messages", RFC 7283, July 2014.

15.2.  Informative References

   [LW4OVER6]
              Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and I.
              Farrer, "Lightweight 4over6: An Extension to the DS-Lite
              Architecture", Work in Progress, June 2014.

   [RFC951]   Croft, B. and J. Gilmore, "Bootstrap Protocol", RFC 951,
              September 1985.

Authors' Addresses

   Qi Sun
   Tsinghua University
   Beijing  100084
   P.R. China

   Phone: +86-10-6278-5822
   EMail: sunqi@csnet1.cs.tsinghua.edu.cn


   Yong Cui
   Tsinghua University
   Beijing  100084
   P.R. China

   Phone: +86-10-6260-3059
   EMail: yong@csnet1.cs.tsinghua.edu.cn


   Marcin Siodelski
   Internet Systems Consortium
   950 Charter Street
   Redwood City, CA  94063
   USA

   EMail: msiodelski@gmail.com


   Suresh Krishnan
   Ericsson
   8400 Blvd. Decarie
   Town of Mount Royal, Quebec
   Canada

   EMail: suresh.krishnan@ericsson.com


   Ian Farrer
   Deutsche Telekom AG
   CTO-ATI, Landgrabenweg 151
   Bonn, NRW  53227
   Germany

   EMail: ian.farrer@telekom.de