Security of Messages Exchanged between Servers and Relay Agents
RFC 8213

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/rfc8213.
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) [RFC2131]
 and the Bootstrap Protocol [RFC1542] have no guidance for how to
 secure messages exchanged between servers and relay agents. The
 Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315]
 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 [RFC7258], 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.
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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 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.
 This document uses terminology from [RFC1542], [RFC2131], and
 [RFC3315].
3. Security of Messages Exchanged between Servers and Relay Agents
 For DHCPv6 [RFC3315], 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 [RFC3315] when this
 specification is followed.
 For DHCPv4 [RFC2131], 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) [RFC3046], vendor-specific
 information (for example, the options defined in [CableLabs-DHCP]),
 and Access-Network-Identifier option(s) [RFC7839], are protected from
 pervasive monitoring and other attacks.
 Relay agents and servers MUST be able to exchange messages using the
 IPsec mechanisms described in [RFC4301] 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.
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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 [RFC7321],
 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
 [RFC7839].
 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)
 [RFC7296] 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 [RFC4107].
 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.
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RFC 8213 DHCP Relay/Server Security August 2017
 Note: As using IPsec with multicast has additional complexities (see
 [RFC5374]), 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 [RFC7844] 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 [RFC4552], 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 [RFC6221], 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.
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RFC 8213 DHCP Relay/Server Security August 2017
6. References
6.1. Normative References
 [RFC1542] Wimer, W., "Clarifications and Extensions for the
 Bootstrap Protocol", RFC 1542, DOI 10.17487/RFC1542,
 October 1993, <http://www.rfc-editor.org/info/rfc1542>.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119,
 DOI 10.17487/RFC2119, March 1997,
 <http://www.rfc-editor.org/info/rfc2119>.
 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
 RFC 2131, DOI 10.17487/RFC2131, March 1997,
 <http://www.rfc-editor.org/info/rfc2131>.
 [RFC3315] 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/RFC3315, July
 2003, <http://www.rfc-editor.org/info/rfc3315>.
 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
 December 2005, <http://www.rfc-editor.org/info/rfc4301>.
 [RFC7321] 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/RFC7321, August 2014,
 <http://www.rfc-editor.org/info/rfc7321>.
 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
 May 2017, <http://www.rfc-editor.org/info/rfc8174>.
6.2. Informative References
 [CableLabs-DHCP]
 CableLabs, "CableLabs' DHCP Options Registry",
 <https://apps.cablelabs.com/specification/CL-SP-CANN-DHCP-
 Reg>.
 [RFC3046] Patrick, M., "DHCP Relay Agent Information Option",
 RFC 3046, DOI 10.17487/RFC3046, January 2001,
 <http://www.rfc-editor.org/info/rfc3046>.
Volz & Pal Standards Track [Page 6]
RFC 8213 DHCP Relay/Server Security August 2017
 [RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
 Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC4107,
 June 2005, <http://www.rfc-editor.org/info/rfc4107>.
 [RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
 for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
 <http://www.rfc-editor.org/info/rfc4552>.
 [RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast
 Extensions to the Security Architecture for the Internet
 Protocol", RFC 5374, DOI 10.17487/RFC5374, November 2008,
 <http://www.rfc-editor.org/info/rfc5374>.
 [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,
 <http://www.rfc-editor.org/info/rfc6221>.
 [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
 Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
 2014, <http://www.rfc-editor.org/info/rfc7258>.
 [RFC7296] 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/RFC7296, October
 2014, <http://www.rfc-editor.org/info/rfc7296>.
 [RFC7839] Bhandari, S., Gundavelli, S., Grayson, M., Volz, B., and
 J. Korhonen, "Access-Network-Identifier Option in DHCP",
 RFC 7839, DOI 10.17487/RFC7839, June 2016,
 <http://www.rfc-editor.org/info/rfc7839>.
 [RFC7844] Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
 Profiles for DHCP Clients", RFC 7844,
 DOI 10.17487/RFC7844, May 2016,
 <http://www.rfc-editor.org/info/rfc7844>.
 [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.
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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 [RFC3315] 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
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