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IETF RFC 8213
Last modified on Friday, August 4th, 2017
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Internet Engineering Task Force (IETF) B. Volz
Request for Comments: 8213 Y. Pal
Category: Standards Track Cisco Systems
ISSN: 2070-1721 August 2017
Security of Messages Exchanged between Servers and Relay Agents
 Abstract
The Dynamic Host Configuration Protocol for IPv4 (DHCPv4) has no
guidance for how to secure messages exchanged between servers and
relay agents. The Dynamic Host Configuration Protocol for IPv6
(DHCPv6) states that IPsec should be used to secure messages
exchanged between servers and relay agents but does not require
encryption. With recent concerns about pervasive monitoring and
other attacks, it is appropriate to require securing relay-to-relay
and relay-to-server communication for DHCPv6 and relay-to-server
communication for DHCPv4.
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
http://www.rfc-editor.org/info/RFC 8213.
Copyright Notice
Copyright (c) 2017 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.
Volz & Pal Standards Track PAGE 1 
RFC 8213 DHCP Relay/Server Security August 2017
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................2
2. Requirements Language and Terminology ...........................3
3. Security of Messages Exchanged between Servers and Relay
Agents ..........................................................3
4. Security Considerations .........................................5
5. IANA Considerations .............................................5
6. References ......................................................6
6.1. Normative References .......................................6
6.2. Informative References .....................................6
Acknowledgments ....................................................8
Authors' Addresses .................................................8
1. Introduction
The Dynamic Host Configuration Protocol for IPv4 (DHCPv4) [RFC 2131]
and the Bootstrap Protocol [RFC 1542] have no guidance for how to
secure messages exchanged between servers and relay agents. The
Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC 3315]
states that IPsec should be used to secure messages exchanged between
servers and relay agents but does not recommend encryption. With
recent concerns about pervasive monitoring [RFC 7258], it is
appropriate to require use of IPsec with encryption for relay-to-
server communication for DHCPv4 and require use of IPsec with
encryption for relay-to-relay and relay-to-server communication for
DHCPv6.
This document specifies the optional requirements for relay agent and
server implementations to support IPsec authentication and encryption
and recommends that operators enable this IPsec support.
Volz & Pal Standards Track PAGE 2
RFC 8213 DHCP Relay/Server Security August 2017
2. Requirements Language and Terminology
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 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
This document uses terminology from [RFC 1542], [RFC 2131], and
[RFC 3315].
3. Security of Messages Exchanged between Servers and Relay Agents
For DHCPv6 [RFC 3315], this specification REQUIRES relay and server
implementations to support IPsec encryption of relay-to-relay and
relay-to-server communication as documented below. The remainder of
this section replaces the text in Section 21.1 of [RFC 3315] when this
specification is followed.
For DHCPv4 [RFC 2131], this specification REQUIRES relay and server
implementations to support IPsec encryption of relay-to-server
communication as documented below.
This specification RECOMMENDS that operators enable IPsec for this
communication.
By using IPsec with encryption for this communication, potentially
sensitive client message and relay included information, such as the
DHCPv4 Relay Agent Information option (82) [RFC 3046], vendor-specific
information (for example, the options defined in [CableLabs-DHCP]),
and Access-Network-Identifier option(s) [RFC 7839], are protected from
pervasive monitoring and other attacks.
Relay agents and servers MUST be able to exchange messages using the
IPsec mechanisms described in [RFC 4301] with the conditions below.
If a client message is relayed through multiple relay agents (relay
chain), each of the relay agents MUST have established independent,
pairwise trust relationships. That is, if messages from client C
will be relayed by relay agent A to relay agent B and then to the
server, relay agents A and B MUST be configured to use IPsec for the
messages they exchange, and relay agent B and the server MUST be
configured to use IPsec for the messages they exchange.
Volz & Pal Standards Track PAGE 3
RFC 8213 DHCP Relay/Server Security August 2017
Relay agents and servers use IPsec with the following conditions:
Selectors Relay agents are manually configured with the
addresses of the relay agent or server to which DHCP
messages are to be forwarded. Each relay agent and
server that will be using IPsec for securing DHCP
messages MUST also be configured with a list of the
relay agents to which messages will be returned.
The selectors for the relay agents and servers will
be the pairs of addresses defining relay agents and
servers and the direction of DHCP message exchange
on DHCPv4 UDP port 67 or DHCPv6 UDP port 547.
Mode Relay agents and servers MUST use IPsec in transport
mode and use Encapsulating Security Payload (ESP).
Encryption and authentication algorithms
This document REQUIRES combined mode algorithms for
ESP authenticated encryption, ESP encryption
algorithms, and ESP authentication algorithms as per
Sections 2.1, 2.2, and 2.3 of [RFC 7321],
respectively. Encryption is required as relay
agents may forward unencrypted client messages as
well as include additional sensitive information,
such as vendor-specific information (for example,
the options defined in [CableLabs-DHCP]) and the
Access-Network-Identifier Option defined in
[RFC 7839].
Key management Because both relay agents and servers tend to be
managed by a single organizational entity, public
key schemes MAY be optional. Manually configured
key management MAY suffice but does not provide
defense against replayed messages. Accordingly,
Internet Key Exchange Protocol Version 2 (IKEv2)
[RFC 7296] with pre-shared secrets SHOULD be
supported. IKEv2 with public keys MAY be supported.
Additional information on manual vs. automated key
management and when one should be used over the
other can be found in [RFC 4107].
Security policy DHCP messages between relay agents and servers MUST
only be accepted from DHCP peers as identified in
the local configuration.
Authentication Shared keys, indexed to the source IP address of the
received DHCP message, are adequate in this
application.
Volz & Pal Standards Track PAGE 4
RFC 8213 DHCP Relay/Server Security August 2017
Note: As using IPsec with multicast has additional complexities (see
[RFC 5374]), relay agents SHOULD be configured to forward DHCP
messages to unicast addresses.
4. Security Considerations
The security model specified in this document is hop by hop. For
DHCPv6, there could be multiple relay agents between a client and
server, and each of these hops needs to be secured. For DHCPv4,
there is no support for multiple relays.
As this document only mandates securing messages exchanged between
relay agents and servers, the message exchanges between clients and
the first-hop relay agent or server are not secured. Clients may
follow the recommendations in [RFC 7844] to minimize what information
they expose or make use of secure DHCPv6 [SEC-DHCPv6] to secure
communication between the client and server.
As mentioned in Section 14 of [RFC 4552], the following are known
limitations of the usage of manual keys:
o As the sequence numbers cannot be negotiated, replay protection
cannot be provided. This leaves DHCP insecure against all the
attacks that can be performed by replaying DHCP packets.
o Manual keys are usually long lived (changing them often is a
tedious task). This gives an attacker enough time to discover the
keys.
It should be noted that if the requirements in this document are
followed, while the DHCP traffic on the wire between relays and
servers is encrypted, the unencrypted data may still be available
through other attacks on the DHCP servers, relays, and related
systems. Securing these systems and the data in databases and logs
also needs to be considered on both the systems themselves and when
transferred over a network (i.e., to network attached storage for
backups or to operational support systems).
Use of IPsec as described herein is also applicable to Lightweight
DHCPv6 Relay Agents [RFC 6221], as they have a link-local address that
can be used to secure communication with their next-hop relay(s).
5. IANA Considerations
This document makes no request of IANA.
Volz & Pal Standards Track PAGE 5
RFC 8213 DHCP Relay/Server Security August 2017
6. References
6.1. Normative References
[RFC 1542] Wimer, W., "Clarifications and Extensions for the
Bootstrap Protocol", RFC 1542, DOI 10.17487/RFC 1542,
October 1993, <http://www.rfc-editor.org/info/RFC 1542>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC 2131, March 1997,
<http://www.rfc-editor.org/info/RFC 2131>.
[RFC 3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC 3315, July
2003, <http://www.rfc-editor.org/info/RFC 3315>.
[RFC 4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC 4301,
December 2005, <http://www.rfc-editor.org/info/RFC 4301>.
[RFC 7321] McGrew, D. and P. Hoffman, "Cryptographic Algorithm
Implementation Requirements and Usage Guidance for
Encapsulating Security Payload (ESP) and Authentication
Header (AH)", RFC 7321, DOI 10.17487/RFC 7321, August 2014,
<http://www.rfc-editor.org/info/RFC 7321>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <http://www.rfc-editor.org/info/RFC 8174>.
6.2. Informative References
[CableLabs-DHCP]
CableLabs, "CableLabs' DHCP Options Registry",
<https://apps.cablelabs.com/specification/CL-SP-CANN-DHCP-
Reg>.
[RFC 3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, DOI 10.17487/RFC 3046, January 2001,
<http://www.rfc-editor.org/info/RFC 3046>.
Volz & Pal Standards Track PAGE 6
RFC 8213 DHCP Relay/Server Security August 2017
[RFC 4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC 4107,
June 2005, <http://www.rfc-editor.org/info/RFC 4107>.
[RFC 4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, DOI 10.17487/RFC 4552, June 2006,
<http://www.rfc-editor.org/info/RFC 4552>.
[RFC 5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast
Extensions to the Security Architecture for the Internet
Protocol", RFC 5374, DOI 10.17487/RFC 5374, November 2008,
<http://www.rfc-editor.org/info/RFC 5374>.
[RFC 6221] Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A.
Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221,
DOI 10.17487/RFC 6221, May 2011,
<http://www.rfc-editor.org/info/RFC 6221>.
[RFC 7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC 7258, May
2014, <http://www.rfc-editor.org/info/RFC 7258>.
[RFC 7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC 7296, October
2014, <http://www.rfc-editor.org/info/RFC 7296>.
[RFC 7839] Bhandari, S., Gundavelli, S., Grayson, M., Volz, B., and
J. Korhonen, "Access-Network-Identifier Option in DHCP",
RFC 7839, DOI 10.17487/RFC 7839, June 2016,
<http://www.rfc-editor.org/info/RFC 7839>.
[RFC 7844] Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
Profiles for DHCP Clients", RFC 7844,
DOI 10.17487/RFC 7844, May 2016,
<http://www.rfc-editor.org/info/RFC 7844>.
[SEC-DHCPv6]
Li, L., Jiang, S., Cui, Y., Jinmei, T., Lemon, T., and D.
Zhang, "Secure DHCPv6", Work in Progress,
draft-ietf-dhc-sedhcpv6-21, February 2017.
Volz & Pal Standards Track PAGE 7
RFC 8213 DHCP Relay/Server Security August 2017
Acknowledgments
The motivation for this document was several IESG DISCUSSes on recent
DHCP relay agent options.
Thanks to Kim Kinnear, Jinmei Tatuya, Francis Dupont, and Tomek
Mrugalski for reviewing and helping to improve the document. Thanks
to the authors of [RFC 3315] for the original Section 21.1 text.
Authors' Addresses
Bernie Volz
Cisco Systems, Inc.
1414 Massachusetts Ave
Boxborough, MA 01719
United States of America
Email: volz@cisco.com
Yogendra Pal
Cisco Systems
Cessna Business Park
Varthur Hobli, Outer Ring Road
Bangalore, Karnataka 560103
India
Email: yogpal@cisco.com
Volz & Pal Standards Track PAGE 8
RFC TOTAL SIZE: 17657 bytes
PUBLICATION DATE: Friday, August 4th, 2017
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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