Internet Engineering Task Force (IETF) Q. Sun

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.

Sun, et al. Standards Track [Page 1]

RFC 7341 DHCPv4 over DHCPv6 August 2014

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
(http://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
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

Sun, et al. Standards Track [Page 2]

RFC 7341 DHCPv4 over DHCPv6 August 2014

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).

Sun, et al. Standards Track [Page 3]

RFC 7341 DHCPv4 over DHCPv6 August 2014

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.

Sun, et al. Standards Track [Page 4]

RFC 7341 DHCPv4 over DHCPv6 August 2014

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.

Sun, et al. Standards Track [Page 5]

RFC 7341 DHCPv4 over DHCPv6 August 2014

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.

Sun, et al. Standards Track [Page 6]

RFC 7341 DHCPv4 over DHCPv6 August 2014

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.

Sun, et al. Standards Track [Page 7]

RFC 7341 DHCPv4 over DHCPv6 August 2014

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.

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RFC 7341 DHCPv4 over DHCPv6 August 2014

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.

Sun, et al. Standards Track [Page 9]

RFC 7341 DHCPv4 over DHCPv6 August 2014

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.

Sun, et al. Standards Track [Page 10]

RFC 7341 DHCPv4 over DHCPv6 August 2014

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.

Sun, et al. Standards Track [Page 11]

RFC 7341 DHCPv4 over DHCPv6 August 2014

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,

Sun, et al. Standards Track [Page 12]

RFC 7341 DHCPv4 over DHCPv6 August 2014

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-

Sun, et al. Standards Track [Page 13]

RFC 7341 DHCPv4 over DHCPv6 August 2014

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.

Sun, et al. Standards Track [Page 14]

RFC 7341 DHCPv4 over DHCPv6 August 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.

Sun, et al. Standards Track [Page 15]

RFC 7341 DHCPv4 over DHCPv6 August 2014

Authors' Addresses

Qi Sun
Tsinghua University
Beijing 100084
P.R. China

Phone: +86-10-6278-5822
EMail: [email protected]

Yong Cui
Tsinghua University
Beijing 100084
P.R. China

Phone: +86-10-6260-3059
EMail: [email protected]

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

EMail: [email protected]

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

EMail: [email protected]

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

EMail: [email protected]

Sun, et al. Standards Track [Page 16]