Internet Engineering Task Force (IETF) C. Porfiri
Request for Comments: 9928 Ericsson
Category: Standards Track S. Krishnan
ISSN: 2070-1721 Cisco
J. Arkko
M. Kühlewind
Ericsson
February 2026
DHCPv4 over DHCPv6 with Relay Agent Support
Abstract
This document describes a mechanism for networks with legacy
IPv4-only clients to use services provided by DHCPv4 over DHCPv6 in a
Relay Agent. RFC 7341 specifies the use of DHCPv4 over DHCPv6 in the
client only. This document specifies an approach based on RFC 7341
that allows a Relay Agent to implement the DHCP 4o6 DHCPv4 over DHCPv6
encapsulation and decapsulation of DHCPv4 messages in DHCPv6 messages
on behalf of a DHCPv4 client.
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 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9928.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
1.1. Applicability Scope
2. Conventions and Definitions
3. DHCPv4-over-DHCPv6 Relay Agent (4o6RA)
3.1. Intermediate Relays
3.2. 4o6RA and Topology Discovery
4. Deployment Considerations
5. Security Considerations
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Appendix A. Example Use Case: Topology Discovery for IPv4-Only
Radio Unit in 3GPP RAN with Switched Fronthaul
Acknowledgments
Authors' Addresses
1. Introduction
[RFC7341] describes a transport mechanism for carrying DHCPv4
[RFC2131] messages using DHCPv6 [RFC9915] for dynamic provisioning of
IPv4 addresses and other DHCPv4-specific configuration parameters
across IPv6-only networks. The deployment of [RFC7341] requires
support in DHCP clients and at the DHCPv6 server. However, if a
client is embedded in a host that only supports IPv4 and cannot
easily be replaced or updated (which could be due to any number of
technical or business reasons), this approach does not work.
Similarly, the specifications for DHCPv6 Relay Agents such as
Lightweight DHCPv6 Relay Agent (LDRA) specification defined in [RFC9915],
which also refers to [RFC6221] or for the Lightweight DHCPv6 Relay Agent
(L3RA) [RFC9915] do
(LDRA) behavior, does not foresee the possibility provide any mechanism to handle legacy
DHCPv4, other than implementing DHCP 4o6 in except by requiring the client. client to implement the DHCPv4 over
DHCPv6 encapsulation and decapsulation.
This document specifies a solution based on [RFC7341] that can be
implemented in intermediate nodes such as switches or routers,
without putting any requirements on clients. No new protocols or
extensions are needed; instead, this document specifies a new use
case for [RFC7341] that allows a Relay Agent to perform the DHCP 4o6 DHCPv4
over DHCPv6 encapsulation and decapsulation instead of the client.
1.1. Applicability Scope
The mechanisms described in this document apply to the configuration
phase of hosts that need to receive an IPv4 address when a DHCP
server for IPv4 [RFC2131] is not reachable directly from the host.
Furthermore, the host is unable to implement a DHCP client conformant
to [RFC7341], as it is connected to an IPv4-only network. However,
there is a DHCPv6 server that can provide IPv4 addresses by means of
the mechanisms specified in [RFC7341].
2. Conventions and Definitions
The following terms and abbreviations are used in this document:
DHCP:
If not otherwise specified, DHCP refers
Refers to DHCPv4 and/or DHCPv6.
DHCPv4:
DHCP as defined in [RFC2131].
DHCPv4 over DHCPv6 (DHCP 4o6):
The architecture, the procedures, and the protocols specified in
the DHCPv4-over-DHCPv6 document [RFC7341]. if not otherwise specified.
DHCP Relay Agent:
This is
Refers to a common concept in all of the following protocols,
although the details differ between them: the Bootstrap Protocol
(BOOTP) [RFC0951] [RFC1542], DHCPv4 [RFC2131] [RFC2132], and
DHCPv6 [RFC9915].
DHCPv4:
Refers to DHCP as defined in [RFC2131].
DHCPv4 over DHCPv6 (DHCP 4o6):
Refers to the architecture, the procedures, and the protocols
specified in the DHCPv4-over-DHCPv6 document [RFC7341].
DHCPv4-over-DHCPv6 Relay Agent (4o6RA):
Refers to a Relay Agent that implements the DHCP 4o6 transport as
specified in this document.
Layer 3 Relay Agent (L3RA):
Refers to a DHCP Relay Agent as specified in [RFC9915] that is not
a LDRA.
Lightweight DHCPv6 Relay Agent (LDRA):
This is
Refers to an extension of the original DHCPv6 Relay Agent
specification, to allow Layer 2 (L2) only devices to perform a
Relay Agent function [RFC6221].
DHCPv4-over-DHCPv6 Relay Agent (4o6RA):
Refers to a Relay Agent that implements the 4o6 transport as
specified in this document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. DHCPv4-over-DHCPv6 Relay Agent (4o6RA)
This document assumes a network where IPv4-only hosts are connected
to a network that supports IPv6 and limited IPv4 services.
To address such a network setup, this document extends DHCPv6 Relay
Agents with DHCPv4 over DHCPv6, as shown in Figure 1.
.-----------. .-----------.
| | | |
+--------+-+ L2 +-+-----------+-+ IPv6 +-+--------+
| DHCPv4 | Network | DHCPv6 | Network | DHCP 4o6 |
| Client +---------+ Relay Agent +---------+ Server |
| | | with 4o6RA | | |
+--------+-+ +-+-----------+-+ +-+--------+
| | | |
'-----------' '-----------'
Figure 1: Architecture Example with Legacy DHCP Client
This document specifies the encapsulation and decapsulation specified
in [RFC7341] to be performed in the Relay Agent without requiring any
changes on the DHCPv4 client. In this case, it is up to the Relay
Agent to provide the full DHCP 4o6 support, and the legacy DHCPv4
client is not aware that it is being served via a DHCP 4o6 service.
As the 4o6RA acts as a DHCP 4o6 client, all prerequisites and
configurations that apply to the DHCP client in Section 5 of
[RFC7341] are also applied to the 4o6RA.
As the 4o6RA takes the role of the client in respect to [RFC7341], it
is responsible for determining a suitable interface where it acts as
a DHCPv6 client, and it is responsible for locating a suitable DHCPv6
server or Relay Agent and obtaining the necessary IPv6 configuration.
As specified in [RFC7341], the 4o6RA, acting as DHCP 4o6 client,
therefore has to request the DHCP 4o6 Server Address option from the
server by sending the Option Request option as described in [RFC9915]
before it can use the DHCP 4o6 transport.
To maintain interoperability with existing DHCPv6 relays and servers,
the message format is unchanged from [RFC9915]. The 4o6RA implements
the same message types as a DHCPv6 Relay Agent (see Section 6 of
[RFC7341]).
However, in this specification, the 4o6RA, instead of the client,
creates the DHCPV4-QUERY message and encapsulates the DHCP request
message received from the legacy DHCPv4 client.
When the DHCPV4-RESPONSE message is received by the DHCP 4o6 Relay
Agent, it looks for the DHCPv4 message option within this message.
If this option is not found or the DHCPv4-RESPONSE message is not well-
formed,
well-formed, it MUST be discarded. If the DHCPv4 message option is
present and correct, the 4o6RA MUST extract the DHCPv4 message and
forward the encapsulated DHCPv4-RESPONSE to the requesting DHCPv4
client, given that the encapsulated DHCPv4-RESPONSE is correct and
can be actually forwarded.
Layer 2 (L2) Relay Agents receiving DHCPV4-QUERY or DHCPV4-RESPONSE
messages MUST handle them as specified in Section 6 of [RFC6221].
In any given environment, DHCPv6 servers to which DHCPV4-QUERY
requests are routed are expected to be compliant with DHCP 4o6
according to [RFC7341]. No additional requirements on DHCPv6 servers
are set by this specification.
3.1. Intermediate Relays
Intermediate relays shall behave according to Section 10 of
[RFC7341].
3.2. 4o6RA and Topology Discovery
In some networks, the configuration of a host may depend on the
topology. However, when a new host attaches to a network, it may be
unaware of the topology and, consequently, how it has to be
configured.
DHCPv4 [RFC2131] and DHCPv6 [RFC9915] specifications describe how
addresses can typically be allocated to clients based on network
topology information provided by a DHCP relay.
Address/prefix allocation decisions are integral to the allocation of
addresses and prefixes in DHCP, as described in detail in [RFC7969].
This specification aims to guarantee that the 4o6RA does not break
any legacy capability when used for topology discovery.
Topology discovery as described in [RFC7969] differs between IPv4 and
IPv6 as follows:
* IPv4: When using DHCP on IPv4, only the first Relay Agent SHOULD
set the giaddr field (Section 3.1 of [RFC7969]). Thus, in a
network that has more than one Relay Agent, only part of the
topology is transported via DHCPv4.
* IPv6: When using DHCPv6, all Relay Agents SHOULD send link-address
and Interface-ID options that provide information about the
complete path between the DHCPv6 client and the DHCPv6 server to
the DHCPv6 server.
In Layer 2 networks, Lightweight DHCPv6 Relay Agents (LDRAs)
[RFC6221] can be used.
When provided, the topology information is available at the DHCPv6
server in the form of a sequence of the link-address field and
Interface-ID option.
Then, topology information for the given IP address can be obtained
from the DHCPv6 server and used for configuration or other purposes.
[RFC7341] enables the client to use DHCPv6 for topology discovery
even within a DHCPv4 context, as the DHCPv6 Relay Agent knows the
interface where the encapsulated DHCP request is received. However,
as shown in Figure 2, the introduction of DHCP 4o6 at the edge of the
IPv6 network hides the Layer 2 network from the DHCPv6 RA. As such,
moving DHCP 4o6 to an intermediate node rather than performing it at
the client breaks the topology propagation, as 4o6RA-only solutions
do not provide any interface information in the encapsulated message.
.-----------------. .-------------------------.
| L2 Network | | IPv6 Network |
+--------+-+ +---------+ +-+---+---+ +--------+ +-+--------+
| DHCPv4 | | L2 | | 4o6 | | DHCPv6 | | DHCP 4o6 |
| Client +--+ Switch +--+ Relay +----+ Relay +-------+ Server |
| | | | | Agent | | Agent | | |
+--------+-+ +---------+ +-+---+---+ +--------+ +-+--------+
| | | |
'-----------------' '-------------------------'
Figure 2: Broken Topology Information
In order to provide full topology information, it is RECOMMENDED that
any implementation of 4o6RA be combined with an LDRA implementation
[RFC6221] in a back-to-back structure and that the LDRA
implementation includes a mechanism to obtain interface information
that can be used to provide the Interface-ID option to outgoing
DHCPV4-QUERY messages, as specified in Section 5.3.2 of [RFC6221].
The internal mechanisms to exchange interface information, their
format, and whether the interface information contains an indication
that a 4o6RA is involved, are out of the scope for this document.
The resulting architecture is shown in Figure 3 where the Relay Agent
is implementing 4o6RA and LDRA and has an internal interface to
propagate topology information from 4o6RA to LDRA.
.-----------------. .------------------------.
| L2 Network or | | IPv6 Network |
| IPv6-Only Link | | |
+--------+-+ +---------+ +-+---+--+---------+ +------+---+
| DHCPv4 | | L2 | | 4o6 | LDRA | | DHCP 4o6 |
| Client +--+ Switch +--+ Relay + RFC 6221+------+ Server |
| | | | | Agent | | | |
+--------+-+ +---------+ +-+---+--+---------+ +------+---+
| | | |
'-----------------' '------------------------'
Figure 3: Topology Information Preserved with LDRA
In a simple case, where the same node hosts the 4o6RA and the DHCP
4o6 server, it might be enough to only use 4o6RA, as shown in
Figure 4. 4, where CPE stands for "Customer Premises Equipment".
.-----------.
| L2 Network |
+--------+-+ +-+------+----------+
| DHCP | | 4o6 | DHCP 4o6 |
| Client +---------+ Relay + Server |
| on CPE | | Agent | |
+--------+-+ +-+------+----------+
| |
'-----------'
Figure 4: Topology Information Preserved by 4o6 Relay Agent in
DHCP Server
4. Deployment Considerations
As clients are unaware of the presence of 4o6RA, the network
deployment needs to ensure that all DHCPv4 broadcast and unicast
messages to and from clients are steered via a 4o6RA. This can be
achieved by placing the 4o6RA in a central position that can
intercept all traffic from the clients or by using Network Address
Translation (NAT) with the 4o6RA address for unicast messages.
5. Security Considerations
This document specifies the applicability of DHCP 4o6 in a scenario
where legacy IPv4 clients are connected to 4o6 DHCP 4o6 Relay Agents that
perform the encapsulation and decapsulation. This document does not
change anything else in the DHCP 4o6 specification; specification [RFC7341];
therefore, the security considerations of [RFC7341] that document still apply.
Specifically, since the legacy IPv4 client is not aware of the
encapsulation and decapsulation, 4o6RA has to provide the protections
that are specified in the security considerations in Section 12 of
[RFC7341].
The mechanisms defined here differ from [RFC7341] as they allow the
DHCP client to send and receive DHCPv4 messages, whereas in
[RFC7341], the client only sends DHCPv6 messages. This makes it
possible that in improperly configured networks where the client is
located on the same Layer 2 scope of a DHCPv4 server, DHCPv4 messages
could reach a DHCPv4 server without using the 4o6RA. While this can
cause erroneous state in both clients and servers and potentially
even lead to misconfigurations that impact reachability, this is seen
as a deployment error rather than a security concern. Further, even
though this mechanism may be used for attacks from within the
network, this is not a new concern introduced by this specification.
More generally, legacy IPv4 clients are not aware of this mechanism;
however, even when DHCP 4o6 is used, the client does not have any
control about the information provided by the Relay Agent. As such,
this change does not raise any additional security concerns.
6. IANA Considerations
This document has no IANA actions.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6221] Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A.
Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221,
DOI 10.17487/RFC6221, May 2011,
<https://www.rfc-editor.org/info/rfc6221>.
[RFC7341] Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport",
RFC 7341, DOI 10.17487/RFC7341, August 2014,
<https://www.rfc-editor.org/info/rfc7341>.
[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>.
[RFC9915] Mrugalski, T., Volz, B., Richardson, M., Jiang, S., and T.
Winters, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", STD 102, RFC 9915, DOI 10.17487/RFC9915,
January 2026, <https://www.rfc-editor.org/info/rfc9915>.
7.2. Informative References
[RFC0951] Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951,
DOI 10.17487/RFC0951, September 1985,
<https://www.rfc-editor.org/info/rfc951>.
[RFC1542] Wimer, W., "Clarifications and Extensions for the
Bootstrap Protocol", RFC 1542, DOI 10.17487/RFC1542,
October 1993, <https://www.rfc-editor.org/info/rfc1542>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<https://www.rfc-editor.org/info/rfc2132>.
[RFC7969] Lemon, T. and T. Mrugalski, "Customizing DHCP
Configuration on the Basis of Network Topology", RFC 7969,
DOI 10.17487/RFC7969, October 2016,
<https://www.rfc-editor.org/info/rfc7969>.
Appendix A. Example Use Case: Topology Discovery for IPv4-Only Radio
Unit in 3GPP RAN with Switched Fronthaul
In 3GPP mobile network architecture, the User Equipment (UE) is
connected via a Radio Access Network (RAN). RAN is built up with
Baseband Units (BBs) (BBUs) and Radio Units (RUs). Radio A radio Fronthaul Network (FH)
network connects RUs and BBs. BBUs. Each RU and BB BBU is an IP host, and
they may support IPv4 only, IPv6 only, or both, depending on the
vendor and the model. Each RU is unique as it is tied to a set of
antennas, and each antenna is serving a specific Cell and Sector.
Each RU is configured by the BB depending on the Cell and Sectors it
serves. However, that dependency is only specified by the cabling
between RUs and antennas. BBs can be cabled to RUs directly or via a
Layer 2 switched network.
+--------+
| RU2 +-----+
| | |
+--------+ |
|
+--------+ |
| RU3 | |
| +--+ | +-----------+
+--------+ | +--| |
+-----| Baseband |
| |
+--------+ +-----| Unit |
| RU4 +--+ +--| |
| | | +-----------+
+--------+ |
|
+--------+ |
| RU2 +-----+
| |
+--------+
Figure 5: 3GPP RAN Where RUs Are Cabled Directly to BB
In Figure 5, the BB is directly cabled to a set of RUs, and the BB
can recognize the relationship between RUs and Cell/Sectors based on
the cabling between the RUs and antennas.
When BBs and RUs are connected via a Layer 2 switched network, the
added level of complexity requires the BBs to have a deeper knowledge
of the topology in order to properly configure the RUs, involving
knowledge of all the cabling in the switched network.
Examples for switched networks are shown in Section 3 of [RFC7969]
and demonstrate the different levels of complexity. An example of a
FH is depicted in Figure 6.
+--------+
| RU1 | P1 +-+------+ | |
| +--------| | L2RA | | +----+------+ |
+--------+ | +------+ | | | L3RA | |
| L2 | +--| +------+ |
+--------+ P2 | Switch | | | | |
| RU2 +--------| #1 +-----| | Router +----|
| | +--------+ | +-----------+ | +---------+
+--------+ | | | |
| +--| DHCP |
+--------+ | | | Server |
| RU3 | P1 +-+------+ | | | #1 |
| +--------| | L2RA | | +-----------+ | +---------+
+--------+ | +------+ | | | |
| L2 | +--| Baseband | |
+--------+ P2 | Switch | | | Unit | |
| RU4 +--------| #2 +-----| | +----|
| | +--------+ | +-----------+ |
+--------+ | |
Figure 6: 3GPP RAN with Layer 2 Switched Fronthaul Example
If IPv6 is used and all RUs are capable of DHCPv6 in Figure 6, DHCP
topology knowledge can be used for solving the RU configuration
problem. Such solution would use the topology discovery mechanisms
described in Section 3.2 of [RFC7969].
If RUs are capable of IPv4 only but implement a DHCP 4o6 client
according to [RFC7341], the same topology discovery mechanisms are
applicable.
If RUs are capable of IPv4 only and cannot implement a DHCP 4o6
client according to [RFC7341], the topology discovery mechanisms
described in Section 3.2 of [RFC7969] can be used by introducing
4o6RA in the switches as described in this document.
Acknowledgments
The authors would like to acknowledge interesting discussions in this
problem space with Sarah Gannon, Ines Ramadza, and Siddharth Sharma,
as well as reviews and comments provided by Éric Vyncke, Mohamed
Boucadair, David Lamparter, Michael Richardson, Alan DeKok, Dale
Worley, Paul Wouters, Deb Cooley, Erik Kline, Ketan Talaulikar, Mike
Bishop, and Roman Danyliw.
Authors' Addresses
Claudio Porfiri
Ericsson
Email: claudio.porfiri@ericsson.com
Suresh Krishnan
Cisco
Email: suresh.krishnan@gmail.com
Jari Arkko
Ericsson
Email: jari.arkko@ericsson.com
Mirja Kühlewind
Ericsson
Email: mirja.kuehlewind@ericsson.com