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Internet Engineering Task Force (IETF)
Request for Comments: 8213
Category: Standards Track
ISSN: 2070-1721
B. Volz
Y. Pal
Cisco Systems
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 © 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

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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.

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.

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.

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:

  • 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.
  • 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.

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>.
   
   [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.

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