Network Working Group R. Hibbs
Internet-Draft Richard Barr Hibbs, P.E.
Expires: October 29, 2004 C. Smith
C & C Catering
B. Volz
Cisco Systems, Inc.
M. Zohar
IBM T. J. Watson Research Center
April 30, 2004
Dynamic Host Configuration Protocol for IPv4 (DHCPv4) Threat Analysis
draft-ietf-dhc-v4-threat-analysis-02
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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This Internet-Draft will expire on October 29, 2004.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
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
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and leaves development of any defenses against those threats to
future work. Beginning in about 1995 DHCP security began to attract
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Issues for Consideration . . . . . . . . . . . . . . . . . 4
1.2 Assumptions . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Scope of this Memo . . . . . . . . . . . . . . . . . . . . 4
1.4 Use of Key Words . . . . . . . . . . . . . . . . . . . . . 5
2. General Threats to DHCPv4 . . . . . . . . . . . . . . . . . . 5
2.1 Denial-of-Service Attacks . . . . . . . . . . . . . . . . 5
2.1.1 Refusal to Configure Clients . . . . . . . . . . . . . 5
2.1.2 Impersonating Clients . . . . . . . . . . . . . . . . 5
2.1.3 Flooding . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Client Misconfiguration . . . . . . . . . . . . . . . . . 6
2.3 Theft of Network Service . . . . . . . . . . . . . . . . . 6
2.4 Packet Insertion, Deletion, or Modification . . . . . . . 7
3. Weaknesses of RFC 3118 . . . . . . . . . . . . . . . . . . . . 7
3.1 Key Exposure . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Key Distribution . . . . . . . . . . . . . . . . . . . . . 7
3.3 Replay attacks . . . . . . . . . . . . . . . . . . . . . . 8
3.4 Protocol Agreement Difficulties . . . . . . . . . . . . . 8
4. DHCPv4 Security Requirements . . . . . . . . . . . . . . . . . 8
4.1 Environments . . . . . . . . . . . . . . . . . . . . . . . 8
4.2 Capabilities . . . . . . . . . . . . . . . . . . . . . . . 8
4.3 Musings on the Key Distribution Problem . . . . . . . . . 9
4.4 Data Confidentiality . . . . . . . . . . . . . . . . . . . 10
4.4.1 "Public" Data in DHCP Packets . . . . . . . . . . . . 10
4.4.2 Protecting Data in DHCP Options . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1 01 Draft . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.2 02 Draft . . . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 12
9.2 Informative References . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 15
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1. Introduction
DHCPv4 as defined in RFC 1541 [RFC1541] and RFC 2131 [RFC2131] 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 [RFC3118] 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 [RFC2132] and DHCP Authentication, as defined in RFC
3118.
Excluded from our analysis are:
o Securing messages between relay agents and servers: work is
already underway on this, see [auth-subopt] and [relay-ipsec].
o DHCP Reconfigure Extension (FORCERENEW), RFC 3203 [RFC3203]: the
authors believe it is appropriate to put the onus to provide the
analysis on those who are interested in moving that work forward.
o DHCP Failover Protocol, as defined in [failover]: the
server-to-server protocol used differs significantly from DHCP.
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o DHCP-DNS Interaction, as defined in [fqdn]: 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 RFC 3315 [RFC3315]: 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 [leasequery]: because of lack of
maturity.
1.4 Use of Key Words
The key words "MUST," "MUST NOT," "REQUIRED," "SHALL," "SHALL NOT,"
"SHOULD," "SHOULD NOT," "RECOMMENDED," "MAY," and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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
A rogue DHCP server can refuse to configure clients by responding
with either partial information (i.e., missing the IP address, yet
containing other information) or a non-routable (or otherwise bad) IP
address. Or, the server may respond to DHCPDISCOVER messages (with
DHCPOFFER messages) but then ignore the subsequent client DHCPREQUEST
messages. This may cause a client to repeatedly fail to be
configured, though clients could take steps to ensure that they
subsequently ignore such servers for a period of time.
2.1.2 Impersonating Clients
A rogue client can impersonate a client or many clients, by using
another client's client identifer (client identifier option) and/or
hardware address (chaddr) or by generating these identifiers. This
may be done to:
o Obtain addresses when hardware address or client identifier
restrictions (lists) are configured into the site's 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 identifiers and starts using them.
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o Simulate many clients to consume all available addresses. The
rogue client may either hold on to these addresses (until the
leases expire) or decline the addresses (by sending a DHCPDECLINE)
in the hopes that the server will remove the declined address from
use for a longer period of time.
o Create havoc on the subnet by injecting fake messages on behalf of
other clients that prematurely release (DHCPRELEASE) or decline
(DHCPDECLINE) their addresses. A rogue client listens to DHCPv4
traffic and gleans client identity and address information and
uses this information to inject fake messages.
2.1.3 Flooding
A rogue client can flood the network with (near-) continuous DHCPv4
request messages thereby consuming processing resources and network
bandwidth.
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 the
client could just as well send messages of other protocols; and we
will not discuss it further.
2.2 Client Misconfiguration
Rogue servers may give out bad configuration information (such as
fake gateways or DNS servers), or relay agents or other network
elements may alter packets between a client and server, to cause the
client to be misconfigured, or to enable future man-in-the-middle
attacks. 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 Network Service
By "theft of network 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. We mention this attack only
for completeness, as there is little or nothing that a DHCP server,
itself, can do to prevent such an attack. 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
analysis.
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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 (DHCPREQUEST to renew or rebind a lease) 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. For example, a DHCPACK could be modified to a DHCPNAK,
thereby denying the client a lease.
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. The
configuration token method (protocol 0) relies on exchanging
clear-text authentication tokens between as yet unconfigured DHCPv4
clients and DHCPv4 servers. 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.
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 such out-of-band methods to distribute and manage tens or
hundreds of thousands of tokens is a significant barrier to the
widespread implementation of either protocol 0 or 1.
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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
clear-text authentication token, the token would be visible to any
host on the same subnet. 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.
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
concern.
4.1 Environments
The following environments can be expected for DHCPv4
implementations:
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.
o Movement between different media and networks.
4.2 Capabilities
The following are essential elements of DHCPv4 security:
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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
paradigms:
* No authentication required.
* Mutual authentication required.
* Client authenticates server.
* Server authenticates client.
o Integrity of DHCP packet exchanges MUST be assured.
Not all capabilities may be needed or desired in all situations.
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: trusted root 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 the 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 a 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 configurations, as well as the inability
of clients to rely on certificate revocation lists while in the
process of performing IP address and configuration management.
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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,
cafes, 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
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 switched, encrypted, or secured at the physical
layer, so that less information could easily be gleaned from the
network traffic.
Taking encryption 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. Similarly, the hardware addresses are also 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 [dns-threats]. Thus we may also
treat DNS domain and server information as public.
Commonly-used routing protocols such as BGP (RFC 1771) [RFC1771], RIP
(RFC 1721) [RFC1721], and router discovery (RFC 1256) [RFC1256] also
normally send advertisements in the clear and we therefore extend our
definition of public DHCP data to include routing information
options.
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4.4.2 Protecting Data in DHCP Options
Some DHCP options (e.g., relay agent options, RFC 3046 [RFC3046]) 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.
Other DHCPv4 options contain opaque data, not subject to
interpretation by a DHCPv4 server. With no RFC-based definition of
the data content of these options, we can offer no opinion of the
sensitivity of the data, and leave any confidentiality treatment to
other work.
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.
5. IANA Considerations
None known.
6. Security Considerations
This entire memo presents a threat analysis of the DHCPv4 protocol.
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. And Bernard
Aboba and Mark Stapp for their careful reviews and suggestions.
8. Change Log
This section summaries the changes made to this Internet-Draft as it
has evolved.
8.1 01 Draft
No significant changes were made:
o Updated author information.
o Converted to using xml2rfc to generate the draft.
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o Removed unused references.
o Added the Change Log section.
8.2 02 Draft
The following changes were made:
o Updated author information.
o Added text to 1.3 to exclude security for messages passed between
relay agents and servers as there are two Internet-Drafts on this
subject.
o Reworded several sections, particularily in section 2.
o Revised and renamed section 2.1.2; now includes more attacks.
o Revised section 2.1.3.
o Minor revisions to section 3, 3.2, and 3.2.
o Added more to section 4.4.2.
o Other minor insertions, deletions, and modifications based on
comments from Bernard Aboba and Mark Stapp and to otherwise
improve the document.
9. References
9.1 Normative References
[RFC1256] Deering, S., "ICMP Router Discovery Messages", RFC 1256,
September 1991.
[RFC1541] Droms, R., "Dynamic Host Configuration Protocol", RFC
1541, October 1993.
[RFC1721] Malkin, G., "RIP Version 2 Protocol Analysis", RFC 1721,
November 1994.
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
3046, January 2001.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
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Messages", RFC 3118, June 2001.
9.2 Informative References
[RFC3203] T'Joens, Y., Hublet, C. and P. De Schrijver, "DHCP
reconfigure extension", RFC 3203, December 2001.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and
M. Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[auth-subopt]
Stapp, M. and T. Lemon, "The Authentication Suboption for
the DHCP Relay Agent Option",
draft-ietf-dhc-auth-suboption-03 (work in progress),
February 2004.
[dns-threats]
Atkins, D. and R. Austein, "Threat Analysis Of The Domain
Name System", draft-ietf-dnsext-dns-threats-06 (work in
progress), February 2004.
[failover]
Droms, R. and K. Kinnear, "DHCP Failover Protocol",
draft-ietf-dhc-failover-12 (work in progress), December
2003.
[fqdn] Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option",
draft-ietf-dhc-fqdn-option-06 (work in progress), October
2003.
[leasequery]
Woundy, R., "DHCP Lease Query",
draft-ietf-dhc-leasequery-07 (work in progress), March
2004.
[relay-ipsec]
Droms, R., "Authentication of DHCP Relay Agent Options
Using IPsec", draft-ietf-dhc-relay-agent-ipsec-01 (work in
progress), November 2003.
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Authors' Addresses
Richard Barr Hibbs
Richard Barr Hibbs, P.E.
952 Sanchez Street
San Francisco, California 94114-3362
USA
Phone: +1 415 648 3920
Fax: +1 415 648 9017
EMail: rbhibbs@pacbell.net
Carl Smith
C & C Catering
1121 Holly St
Alameda, California 94502
USA
EMail: islandia@alumni.ucsd.edu
Bernard Volz
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
Phone: +1 978 936 0382
EMail: volz@cisco.com
Mimi Zohar
IBM T. J. Watson Research Center
19 Skyline Drive
Hawthorne, New York 10532-2134
USA
Phone: +1 914 784 7606
EMail: zohar@us.ibm.com
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