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Versions: 00 01 02 03

Network Working Group                                           R. Hibbs
Internet-Draft                                  Richard Barr Hibbs, P.E.
Expires: December 12, 2003                                      C. Smith
                                                        Sun Microsystems
                                                                 B. Volz
                                                                M. Zohar
                                                           June 13, 2003

 Dynamic Host Configuration Protocol for IPv4 (DHCPv4) Threat Analysis

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at http://

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   This Internet-Draft will expire on December 12, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   DHCPv4 (RFC 2131) is a stable, widely used protocol for configuration
   of host systems in a TCP/IPv4 network.  It did not provide for
   authentication of clients and servers, nor did it provide for data
   confidentiality.  This is reflected in the original "Security
   Considerations" section of RFC 2131, which identifies a few threats
   and leaves development of any defenses against those threats to
   future work.  Beginning in about 1995 DHCP security began to attract

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   attention from the Internet community, eventually resulting in the
   publication of RFC 3118 in 2001.  Although RFC 3118 was a mandatory
   prerequisite for the DHCPv4 Reconfigure Extension, RFC 3203, it has
   had no known usage by any commercial or private implementation since
   its adoption.  The DHC Working Group has adopted a work item for 2003
   to review and modify or replace RFC 3118 to afford a workable, easily
   deployed security mechanism for DHCPv4.  This memo provides a
   comprehensive threat analysis of the Dynamic Host Configuration
   Protocol for use both as RFC 2131 advances from Draft Standard to
   Full Standard and to support our chartered work improving the
   acceptance and deployment of RFC 3118.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1   Issues for Consideration . . . . . . . . . . . . . . . . . .  3
   1.2   Assumptions  . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.3   Scope of this Memo . . . . . . . . . . . . . . . . . . . . .  3
   1.4   Use of Key Words . . . . . . . . . . . . . . . . . . . . . .  4
   2.    General threats to DHCPv4  . . . . . . . . . . . . . . . . .  5
   2.1   Denial-of-Service Attacks  . . . . . . . . . . . . . . . . .  5
   2.1.1 Refusal to Configure Clients . . . . . . . . . . . . . . . .  5
   2.1.2 Client Masquerading  . . . . . . . . . . . . . . . . . . . .  5
   2.1.3 Flooding . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.2   Client Misconfiguration  . . . . . . . . . . . . . . . . . .  5
   2.3   Theft of Service . . . . . . . . . . . . . . . . . . . . . .  6
   2.4   Packet Insertion, Deletion, or Modification  . . . . . . . .  6
   3.    Weaknesses of RFC 3118 . . . . . . . . . . . . . . . . . . .  7
   3.1   Key Exposure . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.2   Key Distribution . . . . . . . . . . . . . . . . . . . . . .  7
   3.3   Replay attacks . . . . . . . . . . . . . . . . . . . . . . .  7
   3.4   Protocol Agreement Difficulties  . . . . . . . . . . . . . .  8
   4.    DHCPv4 Security Requirements . . . . . . . . . . . . . . . .  9
   4.1   Environments . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.2   Capabilities . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.3   Musings on the Key Distribution Problem  . . . . . . . . . . 10
   4.4   Data Confidentiality . . . . . . . . . . . . . . . . . . . . 10
   4.4.1 "Public" Data in DHCP Packets  . . . . . . . . . . . . . . . 11
   4.4.2 Protecting Other Data in DHCP Options  . . . . . . . . . . . 11
   5.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 12
   6.    Security Considerations  . . . . . . . . . . . . . . . . . . 13
   7.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
         Normative References . . . . . . . . . . . . . . . . . . . . 15
         Informative References . . . . . . . . . . . . . . . . . . . 16
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
         Intellectual Property and Copyright Statements . . . . . . . 18

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1. Introduction

   DHCPv4 as defined in RFC 1541 [1] and RFC 2131 [2] does not provide
   any form of communication security, confidentiality, data integrity,
   or peer entity authentication.

   A design team was formed at IETF-55 in Atlanta in November 2002 to
   look at DHCPv4 and RFC 3118 [3] to document security requirements for
   DHCPv4.  RFC 3118 defines the current security mechanisms for DHCPv4.

   Unfortunately, RFC 3118 has neither been implemented nor deployed to
   date.  There is widespread feeling that its current restriction to
   manual keying of clients limits its deployment.  The DHC Working
   Group seeks to rectify this situation by defining security mechanisms
   for DHCPv4 that have better deployment properties.

1.1 Issues for Consideration

   Specific issues to be considered include:

   o  Improved key management and scalability.

   o  Security for messages passed between relay agents and servers.

   o  The increased usage of DHCPv4 on insecure (e.g., wireless) and
      public LANs.

   o  The need for clients to be able to authenticate servers, without
      simultaneously requiring client authentication by the server.

1.2 Assumptions

   This document assumes that the reader is familiar with both the base
   DHCPv4 protocol as defined in RFC 2131 and the DHCPv4 authentication
   extension as defined in RFC 3118, and does not attempt to provide a
   tutorial on either.

1.3 Scope of this Memo

   This document confines its analysis to DHCPv4, as defined in RFC 2131
   and RFC 2132 [4] and DHCP Authentication, as defined in RFC 3118.

   Excluded from our analysis are

   o  The DHCP Reconfigure Extension (FORCERENEW), RFC 3203 [10]:   the
      authors believe it is appropriate to put the onus to provide the
      analysis on those who are interested in moving it forward.

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   o  DHCP Failover Protocol, as defined in [11]:  the server-to-server
      protocol it uses differs significantly from DHCP.

   o  DHCP-DNS Interaction, as defined in [12]:   securing communication
      between DHCP servers and DNS servers is a DNS update security
      issue and therefore out of scope for the DHC working group.

   o  DHCPv6, as defined in [13]:   while we believe that authentication
      techniques developed for DHCPv4 would generally be applicable to
      DHCPv6, there are fundamental differences between the two
      protocols and RFC 3118 specifies DHCPv4-style message and options
      formats, so we have chosen to concentrate on DHCPv4.

   o  DHCP Lease Query, as defined in [14]:  because of lack of maturity

1.4 Use of Key Words

   The key words "MUST," "MUST NOT," "REQUIRED," "SHALL," "SHALL NOT,"
   document are to be interpreted as described in RFC 2119. [5]

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2. General threats to DHCPv4

   These are the classes of threats to the base DHCPv4 protocol.  Not
   all of these are DHCP-specific, nor can all the concerns listed be
   addressed by DHCP authentication.

2.1 Denial-of-Service Attacks

2.1.1 Refusal to Configure Clients

   This includes rogue servers that don't give addresses or give bad
   addresses in addition to simply ignoring client requests.

   A rogue server could respond with either partial information (i.e.,
   missing the IP address, yet containing other information) or a
   non-routable (or otherwise bad) IP address.  Another possibility is a
   rogue server that responds to DHCPDISCOVER messages (with DHCPOFFER
   messages) but fails to respond to DHCPREQUEST messages.  This might
   cause a client to repeatedly fail to be configured.  Clients should
   take steps to ensure that they subsequently ignore such servers.

2.1.2 Client Masquerading

   This includes clients that send out bogus requests or masquerade as
   legal clients to use up addresses, or just consume server/network
   resources.  The term "legal clients," in this case, refers to client
   hardware ("chaddr") address or client identifier (client-ID)
   restrictions (lists) configured into the server through some
   mechanism not described in RFC 2131.  Some sites may use such a
   mechanism to restrict the clients that are allowed addresses.  A
   rogue client listens to DHCPv4 traffic and captures a few chaddrs or
   client-IDs and starts using them.

2.1.3 Flooding

   A rogue client can flood the network with (near-) continuous DHCPv4
   request messages, depleting the IP addresses and consuming bandwidth.
   As most servers are implemented as a stateless query-response model,
   network traffic is effectively doubled with the message pair.  DHCPv4
   is particularly susceptible to such attacks because initial packet
   exchanges are broadcast.  We mention this attack only for
   completeness, as there is little or nothing that a DHCP server can do
   to prevent such an attack and we will not discuss it further.

2.2 Client Misconfiguration

   This includes rogue servers that give out bad configuration
   information, or relay agents or elements that alter packets between

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   client and server.  The result is misconfiguration (such as fake
   gateways or DNS servers) of clients and potentially worse attacks by
   directing traffic through a bogus router or web spoofing or other
   traffic interception or redirection.  This category is usually part
   of another attack, be it theft of service, business espionage, or
   business interruption including denial of service.

2.3 Theft of Service

   By "theft of service" we mean the taking of an unused address for
   network access or the use of an assigned address not belonging to the
   client, in contrast with "client masquerading" (Section 2.1.2) which
   refers specifically to the use of a legitimate client's chaddr or
   client identifier.

   Instantiation of an unauthorized client for purposes of using network
   resources or services is only partially preventable using
   client-server authentication techniques.  Additional host and
   application security is required to prevent theft of service, and
   such layer 5 and higher functions are declared out of scope for this

2.4 Packet Insertion, Deletion, or Modification

   If a client (or server or relay agent) is known to crash or shut down
   when invalid packets of some type are sent, this could be simply
   another type of denial of service attack.  Likewise, simply deleting
   certain packet types would eventually result in client lease
   expiration, a denial of service attack.  A rogue relay agent or other
   host would typically use packet insertion and deletion to interrupt
   service.  In a different vein, the modification of packets in the
   DHCP exchange may be used to facilitate many different types of
   attacks on either client or server.

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3. Weaknesses of RFC 3118

   An authentication mechanism for DHCPv4 protocol messages was
   developed in RFC 3118, proposing two basic authentication mechanisms
   and the means for extending the repertoire of methods as needed.
   Because the configuration token method (protocol 0) relies on
   exchanging clear-text authentication tokens between as yet
   unconfigured DHCPv4 clients and DHCPv4 servers, it does not scale
   well beyond relatively small networks.  It is also vulnerable to
   message interception.  Delayed authentication (protocol 1) focuses on
   solving the intradomain authentication problem where the out-of-band
   exchange of a shared secret is feasible.  Methods that are more
   computationally intensive are particularly susceptible to Denial of
   Service attacks through flooding.

3.1 Key Exposure

   The configuration token protocol, protocol 0, utilizes clear-text
   authentication tokens (i.e., passwords), providing only weak entity
   authentication and no message authentication.  This protocol is
   vulnerable to interception and provides only the most rudimentary
   protection against inadvertently instantiated DHCP servers.  It also
   leaks the key before knowing whether the server supports protocol 0.

3.2 Key Distribution

   Both protocols 0 and 1 suffer from the lack of a means to easily,
   quickly, and reliably distribute authentication tokens used in the
   protocols.  In many environments, some existing key distribution
   mechanism is presumed to be trusted and reliable, with strong
   administrative procedures and a security-conscious user population in
   place, leaving only the selection and specification of an appropriate
   cryptographic algorithm as a concern of the protocol designer.

   Relying on out-of-band methods to distribute and manage tens or
   hundreds of thousands of tokens is a significant barrier to
   widespread implementation of either protocol 0 or 1.

   Key distribution presents a significant system management challenge
   that is in marked contrast with DHCP itself, a protocol that has been
   successfully deployed in environments that make few demands or
   assumptions.  If we are to hope for similarly successful deployment
   of authentication for DHCP, a means for mitigating (if not
   eliminating) these difficulties must be offered.

3.3 Replay attacks

   Since the configuration token protocol, protocol 0, passes a

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   clear-text authentication token, any host on the same subnet would be
   able to capture it.  Delayed authentication, protocol 1, is not
   susceptible to replay attacks since it contains a nonce value
   generated by the source and a message authentication code (MAC) which
   provides both message and entity authentication.

3.4 Protocol Agreement Difficulties

   An a priori agreement is presumed to have taken place between client
   and server on the authentication protocol to use.  No mechanism is
   provided to allow for the discovery of supported protocols, nor is
   there a facility for negotiation.  The only way to express
   non-support of a protocol is by failing to respond.

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4. DHCPv4 Security Requirements

   DHCPv4 was developed in an era when computers were primarily used in
   business and university environments.  Security was either not a
   concern or was addressed by controlling physical access stemming from
   the belief that intrusion into critical systems was most likely to
   come from an external source.  Now, with wireless access points and
   ubiquitous networking, physical access control mechanisms are no
   longer sufficient, and security and privacy issues are a major

4.1 Environments

   The following environments can be expected for DHCPv4

   o  Network size from a few hosts to hundreds of thousands of hosts.

   o  Network topology from a single subnet to Class-A networks.

   o  Network location from a single room to international dispersion.

   o  Wired, broadcast wireless, and directional wireless media.

4.2 Capabilities

   The following are essential elements of DHCPv4 Security:

   o  Clients MUST be able to authenticate servers (to prevent
      misconfigured clients and assure that the correct servers are
      being contacted).

   o  Servers MUST be able to authenticate clients (use of hardware
      addresses and client-IDs provides no real security but is all that
      is easily possible today).  Better mechanisms are needed for
      servers to identify clients to which they will offer service (to
      prevent IP address pool depletion, for example).

   o  Administrators MUST be able to choose between four authentication

      *  No authentication required.

      *  Mutual authentication required.

      *  Client authenticates server.

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      *  Server authenticates client.

   o  Integrity of DHCP packet exchanges MUST be assured.

4.3 Musings on the Key Distribution Problem

   The authors believe that only by addressing scalability issues with
   key distribution can RFC 3118 achieve wide deployment.  While it is
   not our intention to describe solutions in this document, we admit
   that we find the model used today by browsers and secure web servers
   attractive:   certificates are distributed with the client
   implementation (web browser); users have the ability to extend the
   certificates that they will accept, install their own certificates
   (should client identification be required), and choose which
   certificate to present to servers requesting a client's identity.

   Analogously, DHCPv4 servers could make use of certificates just as
   web servers do, while DHCPv4 clients could be distributed with
   appropriate certificate authority certificates (trust anchors).
   Self-signed certificates are, of course, a possibility, should an
   organization wish full control over its domain of trust.

   Should this path be pursued, we believe that certificate revocation
   will be the major problem to confront, just as it is in the browser/
   web server environment today.  Revocation of client certificates
   (which we believe would occur, on the whole, much more frequently
   than revocation of server certificates), would require only ordinary
   care in certificate validation by the DHCP server.

   Revocation of server certificates is more complex because of the
   difficulty updating client configuration, as well as the inability of
   clients to rely on certificate revocation lists while in the process
   of performing IP address and configuration management.

4.4 Data Confidentiality

   Data Confidentiality was not provided for in the original DHCP
   protocol as defined in RFC 2131 or any of the subsequent RFCs.
   Historically, DHCP was mainly used to assign IP addresses and return
   configuration options such as the local gateway and DNS information.

   Over time the DHCP protocol has evolved, deployments are extending
   beyond physically secure intranets to public networks in hotspots,
   caf‰s, airports, and at home over broadband. and we are seeing an
   accompanying proliferation of new configuration options.

   DHCP has, in fact, become so successful that it is now used to

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   transport a great deal of configuration data that could be obtained
   in a variety of other ways.  It is certainly possible that a client
   or server might wish to reveal some of these data only to a
   properly-authenticated peer.

4.4.1 "Public" Data in DHCP Packets

   We assume that any information that may be gleaned directly from the
   network using, for example, Ethernet promiscuous mode is not
   confidential.  It could be argued that over time more and more
   communication will be encrypted, so that less information could be
   gleaned from the network traffic.  Taking this into consideration,
   the IP packet payload might be encrypted, but not the IP header
   itself since it is required for packet routing.  As a result none of
   the IP header fields are confidential.  IP addresses included in the
   header are therefore not confidential.

   Although the IP packet payload (which would include the UDP or TCP
   header) might normally be encrypted, some protocols have made
   explicit decisions not to encrypt their exchanges, declaring their
   data public.  DNS is such a protocol [15].  Thus we may also treat
   DNS domain and server information as public.

   Commonly-used routing protocols such as BGP (RFC 1771) [6], RIP (RFC
   1721) [7], and router discovery (RFC 1256) [8] also normally send
   advertisements in the clear and we therefore extend our treatment of
   public DHCP data to routing information.

4.4.2 Protecting Other Data in DHCP Options

   Some DHCP options (e.g., relay agent options, RFC 3046 [9]) or option
   families (site or vendor options) admit no analysis because the data
   carried by them may be of unknown sensitivity.  Analysis of their
   confidentiality requirements must be done by their users.

   Should some data require confidentiality, it may be possible to
   exploit the "public" data above to allow a two-step configuration
   process in which sufficient client configuration is first obtained by
   the normal DHCPDISCOVER/OFFER/REQUEST/ACK exchange, and private data
   subsequently transmitted over a secure communications channel (such
   as IPsec) using DHCPINFORM.

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5. IANA Considerations

   None known.

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6. Security Considerations

   This entire memo presents a threat analysis of the DHCPv4 protocol.

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7. Acknowledgements

   This document is the result of work undertaken the by DHCP working
   group, beginning at 55th IETF meeting in Atlanta.  The authors would
   also like to acknowledge contributions from others not directly
   involved in writing this memo, including John Beatty and Vipul Gupta
   of Sun Microsystems, and Ralph Droms of cisco Systems.

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Normative References

   [1]  Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
        October 1993.

   [2]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        March 1997.

   [3]  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
        RFC 3118, June 2001.

   [4]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
        Extensions", RFC 2132, March 1997.

   [5]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [6]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
        RFC 1771, March 1995.

   [7]  Malkin, G., "RIP Version 2 Protocol Analysis", RFC 1721,
        November 1994.

   [8]  Deering, S., "ICMP Router Discovery Messages", RFC 1256,
        September 1991.

   [9]  Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
        January 2001.

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Informative References

   [10]  T'Joens, Y., Hublet, C. and P. De Schrijver, "DHCP reconfigure
         extension", RFC 3203, December 2001.

   [11]  Droms, R. and K. Kinnear, "DHCP Failover Protocol",
         draft-ietf-dhc-failover-12 (work in progress), March 2003.

   [12]  Rekhter, Y. and M. Stapp, "The DHCP Client FQDN Option",
         draft-ietf-dhc-fqdn-option-05 (work in progress), November

   [13]  Droms, R., "Dynamic Host Configuration Protocol for IPv6
         (DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress),
         November 2002.

   [14]  Woundy, R. and K. Kinnear, "DHCP Lease Query",
         draft-ietf-dhc-leasequery-04 (work in progress), November 2002.

   [15]  Atkins, D. and R. Austein, "Threat Analysis Of The Domain Name
         System", draft-ietf-dnsext-dns-threats-02 (work in progress),
         November 2002.

   [16]  Haller, N. and R. Atkinson, "On Internet Authentication", RFC
         1704, October 1994.

   [17]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on
         Security Considerations", draft-iab-sec-cons-03 (work in
         progress), February 2003.

   [18]  Schnizlein, J., Polk, J. and M. Linsner, "DHC Location Object
         within GEOPRIV", draft-ietf-geopriv-dhcp-lo-option-00 (work in
         progress), January 2003.

   [19]  Morris, R., "A Weakness in the 4.2 BSD UNIX TCP/IP Software",
         CSTR 117, 1985.

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Authors' Addresses

   Richard Barr Hibbs
   Richard Barr Hibbs, P.E.
   952 Sanchez Street
   San Francisco, California  94114-3362

   Phone: +1 415 648 3920
   Fax:   +1 415 648 9017
   EMail: rbhibbs@pacbell.net

   Carl Smith
   Sun Microsystems, Inc.
   901 San Antonio Road
   Palo Alto, California  94303-4900

   EMail: cs@eng.sun.com

   Bernie Volz
   959 Concord Street
   Framingham, Massachusetts  01701-4682

   Phone: +1 508 875 3162
   EMail: Bernie.Volz@ericsson.com

   Mimi Zohar
   IBM T. J. Watson Research Center
   19 Skyline Drive
   Hawthorne, New York  10532-2134

   Phone: 1 914 784 7606
   EMail: zohar@us.ibm.com

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   This document and the information contained herein is provided on an


   Funding for the RFC Editor function is currently provided by the
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