Network Working Group R. Hibbs
Internet-Draft Richard Barr Hibbs, P.E.
Expires: December 12, 2003 C. Smith
Sun Microsystems
B. Volz
Ericsson
M. Zohar
IBM
June 13, 2003
Dynamic Host Configuration Protocol for IPv4 (DHCPv4) Threat Analysis
draft-ietf-dhc-v4-threat-analysis-00
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on December 12, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). 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
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,"
"SHOULD," "SHOULD NOT," "RECOMMENDED," "MAY," and "OPTIONAL" in this
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
analysis.
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
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.
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
paradigms:
* 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
2002.
[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
USA
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
USA
EMail: cs@eng.sun.com
Bernie Volz
Ericsson
959 Concord Street
Framingham, Massachusetts 01701-4682
USA
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
USA
Phone: 1 914 784 7606
EMail: zohar@us.ibm.com
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