wdiff draft-ietf-dhc-v4-threat-analysis-02.txt draft-ietf-dhc-v4-threat-analysis-03.txt

Network Working Group R. Hibbs
Internet-Draft
INTERNET-DRAFT Richard Barr Hibbs, P.E.
Expires: October 29, 2004
Category: Informational C. Smith
Expires: December 15, 2006 C & C Catering
B. Volz
Cisco Systems, Inc.
M. Zohar
IBM T. J. Watson Research Center
April 30, 2004
June 13, 2006

Dynamic Host Configuration Protocol for IPv4 (DHCPv4)
Threat Analysis
draft-ietf-dhc-v4-threat-analysis-02

Status of this Memo

<draft-ietf-dhc-v4-threat-analysis-03.txt>
Saved: Tuesday, June 13, 2006, 12:56:25

Intellectual Property Rights

By submitting this Internet-Draft, I certify each author represents that any
applicable patent or other IPR claims of which I am he or she is aware
have been or will be disclosed, and any of which I become he or she becomes
aware will be disclosed, in accordance with
RFC 3668. Section 6 of BCP 79.

Status of this Memo

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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 In about 1995 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 Ipv4 (DHCPv4) 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 . . . . . . . . . . . . . . . . . . . . . . . . . 4

1 Introduction................................................4
1.1 Issues for Consideration . . . . . . . . . . . . . . . . . 4 Consideration...............................4
1.2 Assumptions . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Exclusions.............................................4
2 Use of Key Words............................................5
3 Applicability...............................................5
3.1 Assumptions............................................5
3.2 Scope of this Memo . . . . . . . . . . . . . . . . . . . . Memo.....................................5
4
1.4 Use of Key Words . . . . . . . . . . . . . . . . . . . . . 5
2. General Threats threats to DHCPv4 . . . . . . . . . . . . . . . . . . 5
2.1 DHCPv4...................................5
4.1 Denial-of-Service Attacks . . . . . . . . . . . . . . . . 5
2.1.1 Attacks..............................5
4.1.1 Refusal to Configure Clients . . . . . . . . . . . . . 5
2.1.2 Clients.....,...............5
4.1.2 Impersonating Clients . . . . . . . . . . . . . . . . 5
2.1.3 Flooding . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Clients.............,..............5
4.1.3 Flooding...........................,.............6
4.2 Client Misconfiguration . . . . . . . . . . . . . . . . . 6
2.3 Misconfiguration................................6
4.3 Theft of Network Service . . . . . . . . . . . . . . . . . 6
2.4 Service...............................6
4.4 Packet Insertion, Deletion, or Modification . . . . . . . 7
3. Modification............7
5 Weaknesses of RFC 3118 . . . . . . . . . . . . . . . . . . . . 7
3.1 3118......................................7
5.1 Key Exposure . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Exposure...........................................7
5.2 Key Distribution . . . . . . . . . . . . . . . . . . . . . 7
3.3 Distribution.......................................7
5.3 Replay attacks . . . . . . . . . . . . . . . . . . . . . . 8
3.4 attacks.........................................8
5.4 Protocol Agreement Difficulties . . . . . . . . . . . . . 8
4. Difficulties........................8
5.5 DHCPv4 Relay Agents Excluded...........................8
5.6 Unanticipated Infrastructure Changes...................8
6 DHCPv4 Security Requirements . . . . . . . . . . . . . . . . . 8
4.1 Environments . . . . . . . . . . . . . . . . . . . . . . . 8
4.2 Capabilities . . . . . . . . . . . . . . . . . . . . . . . 8
4.3 Requirements................................9
6.1 Environments...........................................9
6.2 Capabilities..........................................10
6.3 Musings on the Key Distribution Problem . . . . . . . . . 9
4.4 Problem...............10
6.4 Data Confidentiality . . . . . . . . . . . . . . . . . . . 10
4.4.1 Confidentiality..................................11
6.4.1 "Public" Data in DHCP Packets . . . . . . . . . . . . 10
4.4.2 Packets...................12
6.4.2 Protecting Data in DHCP Options . . . . . . . . . . . 11
5. Options.................12
6.5 Host versus User Authentication.......................12
6.5.1 Why do we make this distinction?................13
6.5.2 Is one mechanism sufficient?....................13
7 IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Considerations........................................14
8 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 Considerations....................................14
9 Acknowledgements...........................................14
10 References................................................14
10.1 Normative References . . . . . . . . . . . . . . . . . . . . 12
9.2 References.................................14
10.2 Informative References . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 15

1. References...............................15
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 [RFC3118] 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

O Improved key management and scalability.
o

O Security for messages passed between relay agents and servers.
o

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

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

O Does use of the base
DHCPv4 protocol as defined in RFC 2131 and Relay Agent Information Option imply 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 need
for authenticated messages between DHCP Authentication, as defined in RFC
3118. servers and relay
agents?

1.2 Exclusions

Excluded from our analysis are:
o

O Securing messages between relay agents and servers: work is
already underway on this, see [auth-subopt] [RFC4030] and [relay-ipsec].
o

O DHCP Reconfigure Extension (FORCERENEW), RFC 3203 (FORCERENEW) [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
[Editor's note: despite repeated calls on the DHC Working Group
mailing list to identify even a single implementation of
FORCERENEW, we are unable to put forward an example of its use.]

O DHCP Failover Protocol, as defined in [failover]: the
server-to-server server-
to-server protocol used differs significantly from DHCP.

o DHCP, and
there has been no recent work on the [failover] draft.

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

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

O DHCP Lease Query, as defined in [leasequery]: [RFC4388]: because of lack of
maturity.

1.4

2 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].

3 Applicability

3.1 Assumptions

This document assumes that the reader is familiar with both the base
DHCPv4 protocol as defined in [RFC2131] and the DHCPv4
authentication extension as defined in [RFC3118], and does not
attempt to provide a tutorial on either.

3.2 Scope of this Memo

This document confines its analysis to DHCPv4, as defined in
[RFC2131] and [RFC2132] and DHCP Authentication, as defined in
[RFC3118].

4 General Threats 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

4.1 Denial-of-Service Attacks

2.1.1

4.1.1 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) information), or a non-routable (or otherwise bad)
IP
address. Or, 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 some time.

2.1.2

4.1.2 Impersonating Clients

A rogue client can impersonate a client or many clients, by using
another client's client identifer identifier (client identifier option) and/or
hardware address (chaddr) or by generating these identifiers. This
may be done to:
o

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.

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 period.

O Create havoc on the subnet by injecting fake messages on behalf
of other clients that clients, prematurely release releasing (DHCPRELEASE) or decline
declining (DHCPDECLINE) their addresses. A rogue client listens
to DHCPv4 traffic and gleans gleams client identity and address
information and uses this information to inject fake messages.

2.1.3

4.1.3 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 protocols, so we
will not discuss it further.

2.2

4.2 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 potentially worse cause future man-in-the-middle 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

4.3 Theft of Network Service

By "theft of network service" 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, 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.

2.4

4.4 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.

5 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

5.1 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

5.2 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.

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

5.3 Replay attacks

Since the configuration token protocol, protocol 0, passes a
clear-text 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

5.4 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 non-
support of a protocol is by failing to respond.

5.5 DHCPv4 Relay Agents Excluded

[RFC3118] is defined exclusively for client-server communication.
The role of relay agents has expanded somewhat from their earliest
definition to include a DHCP option carrying relay agent information
via sub-options [RFC3046]. An authentication sub-option for the
relay agent information option has been defined by [RFC4030], though
it only defines a single protocol, symmetrical shared-secret keys,
to protect the contents of the messages between DHCP relay agents
and servers. Work-in-progress to protect the interaction between
relay agents and servers using IPSEC [relay-ipsec] seems to have
halted, with no recent work.

5.6 Unanticipated Infrastructure Changes

Rapid commit, defined by [RFC4039], specifies how a two-message
exchange between client and server can dramatically decrease the
elapsed time for address assignment, a feature becoming significant
as more and more highly mobile devices desire an abbreviated address
assignment phase for short duration communications.

While a two-message exchange by itself does not affect the overall
security of the communications, it has two side effects. First, the
delayed authentication protocol simply cannot be used as the
DHCPOFFER message required to return a nonce value to the client is
not present. Second, as noted by the authors of [RFC4039], without
authentication it is considerably more likely to consume addresses,
increasing the risk of one type of denial of service attack.

CableLabs client configuration [RFC3495] adds another two-tier
option to the DHCPv4 options list, defining an initial set of sub-
options for passing configuration information specific to CableLabs
VoIP and possible future services. Although several of the sub-
options contain potentially sensitive information regarding Kerberos
tickets to be used by cable modems and media terminal adapters. The
option specification does not require used of DHCP authentication,
although it would seem to be necessary. The authors of this memo do
not wish to go so far as to recommend that DHCP authentication be a
requirement for any or all DHCP options, but the CableLabs client
configuration option will likely not be the last option that could
benefit from a robust, workable authentication implementation.

Passive duplicate address detection [p-dad] is a relatively new
proposal to replace the use of ICMPECHO and ARP messages by DHCP
clients and servers to improve the duplicate address detection
process by passively listening to message traffic and developing a
table of matches between chaddr, DHCP client identifier, and IP
address that would be periodically transmitted to interested DHCP
servers. The presence of an entry for a particular IP address would
signify that it is known to be in use, so a DHCP server could
exclude the address from its pool of addresses available for
assignment.

The address usage collector (AUC) defined by [p-dad] does not use
the DHCP client-server protocol, nor does it function by actively
handling DHCP messages, so its implementation would not affect
authenticated DHCP messages. However, DHC Working Group discussion
of the [p-dad] draft raised the point that the AUC must capture the
client identifiers used in DHCP message exchanges. If DHCP
authentication were enabled, an AUC of necessity would be required
to have the same authentication configuration data (protocol,
algorithm, and key) as the clients and servers, certainly a
consideration for scalability and risk assessment.

6 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

6.1 Environments

The following environments can be expected for DHCPv4
implementations:
o

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

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

O Network location location, from a single room to international dispersion.
o

O Wired, broadcast wireless, and directional wireless media.
o

O Movement between different media and networks.

4.2

6.2 Capabilities

The following are essential elements of DHCPv4 security:

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

2. 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 whom they will offer service
(to prevent IP address pool depletion, for example).
o

3. Administrators MUST be able to choose between four
authentication paradigms:
*

a. No authentication required.
*

b. Mutual authentication required.
*

c. Client authenticates server.
*

d. Server authenticates client.
o

4. Integrity of DHCP packet exchanges MUST be assured.

Not all capabilities may be needed or desired in all situations.

4.3

6.3 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 several models used today by browsers and secure web
servers
attractive: trusted as well as token-based user authentication schemes such as
the RSA SecureID token to be 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. Security tokens that combine a
secure seed value with the current time of day using a cryptographic
algorithm to produce effectively a random one-time pad are
relatively inexpensive and widely available.

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, 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
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), 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.

4.4

Using a security token device today is mostly to identify a person
requesting access to a private resource. Key fobs, USB dongles, or
wallet cards are in the possession of a user, and in conjunction
with a user name, password, and possibly other information confirms
two of the three classic dimensions of provable identity (something
you know--user name and password, something you have--the security
token, and something you are--biometrics typically satisfy this
dimension.)

We envision a security token becoming part of the host system's
hardware complement in the near future, such that the token then
becomes not a user identity validator, but a host system validator.
It is common today to have a system service tag or serial number
that is machine-readable. Some hardware configurations include
processors with readable serial numbers as well. What we lack is a
secure means to generate a cryptographically random key that cannot
be easily defeated by software or component swapping.

Either of these approaches offers a simple way to avoid the classic
key distribution problem, though neither is totally without cost.
Somehow, somewhere, a token's identifiers (seed and algorithm) must
be recorded by the DHCP and other interested servers, or the
certificate infrastructure must be in place. We see no way to
eliminate administrative issues associated with security, but we can
see an end to passwords written on a sticky note.

An interesting Internet-Draft by Alper Yegin et al. [eap-auth]
proposed the use of EAP for DHCP authentication using the delayed
method. Their draft required modifications to several components of
an AAA solution, but illustrated how delayed authentication could be
"bootstrapped" using tools at our disposal.

That idea is also suggested by [RFC4014] Ralph Droms and John
Schnizlein, who define a DHCP option code for RADIUS information
provided by an NAS, similar to the mechanism in [eap-auth]. These
two documents taken together may provide a third solution to the key
distribution problem.

6.4 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 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 will wish to reveal some of these data only to a
properly-authenticated properly
authenticated peer.

4.4.1

6.4.1 "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 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 Thus, we may
also treat DNS domain and server information as public.

Commonly-used

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 treatment of public DHCP data
to include routing information
options.

4.4.2 information.

6.4.2 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 Users must be done by do 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
own analysis of the data, and leave any confidentiality treatment to
other work. confidentiality.

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.

6.5 Host versus User Authentication

[RFC3118] is concerned specifically with DHCP clients and servers
authenticating themselves to each other if required by an
administrative domain. This is not the same thing as authenticating
users for establishing their Identity, access rights, permissions,
or other matters relating to what they can view or do once connected
to the network.

6.5.1 Why do we make this distinction?

Host authentication provides only assurance that the hosts
connecting to a network are recognized. This may be for several
reasons, including:

O Requirement to restrict network access from "foreign" hosts to
ensure consistent technical support or meet other regulatory
requirements such as double-insulation or non-sparking.

O Requirement to restrict network access from "unsecured" (for
instance, non-TEMPEST compliant) hosts in a high security
network.

O Requirement to restrict network access from unknown hosts whose
identity has not been recorded by existing administrative
procedures, saving troubleshooting and administrative effort.

User authentication focuses on the individual users of a host system,
seeking to uniquely identify someone to establish their rights to
view, print, add, alter, delete, edit, modify, copy, or manipulate
any data maintained by an organization, run software programs, or
affect changes in the environment. This may be for such reasons as:

O Requirement to be compliant with regulations such as HIPAA, SOX,
and GLBA designed to safeguard and protect confidentiality and
various reporting requirements.

O Requirement to maintain reasonable controls over access to
certain critical systems such as utility power grids, water and
sewage treatment plants, the network infrastructure itself, and
life safety systems of all sorts.

O Requirement to maintain administrative controls over certain
sensitive information such as trade secrets and personnel data

6.5.2 Is one mechanism sufficient?

Full discussion of this question is beyond the scope of this memo,
but the simple answer is, "probably not." This has a significant
implication for the claims of certain techniques such as
"sandboxing" of unverified users, restricting their network access
to a user registration web site until their identity has been
established. Simply put, limiting network access is not DHCP
authentication, although it does represent a very workable approach
to user authentication in many cases.

Recall that a user can be identified by "something they know,"
"something they have," and "something they are." While the same
could be said to be true of host systems, the authors point out that
while we do not pretend to understand all of the ways that future
developers might imbue hosts with the tools to independently create
attacks on our infrastructure, we will assert that users will be the
greatest risk for some time still.

DHCP Authentication addresses only host authentication from the
belief that other, existing mechanisms such as IPSEC, SSL, and VPN
can protect the content of communications, with Identity Management
and other technologies can protect applications and data.

A properly authenticated host could still launch a denial of service
attack, corrupt sensitive data, or wreak other havoc, which
underscores our point that host and user authentication are
different.

A well-secured network needs both.

7 IANA Considerations

None known.

8 Security Considerations

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

9 Acknowledgements

This document is the result of work undertaken the by DHCP working
group, beginning at the 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 Systems,
Bernard Aboba of Microsoft, and Mark Stapp of Cisco Systems for
their careful reviews and helpful 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.

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.

10 References

9.1

10.1 Normative References

[RFC1256] Deering, S., "ICMP Router Discovery Messages", Messages," RFC 1256,
September 1991.

[RFC1541] Droms, R., "Dynamic Host Configuration Protocol", Protocol," RFC 1541,
October 1993.

[RFC1721] Malkin, G., "RIP Version 2 Protocol Analysis", Analysis," RFC 1721,
November 1994.

[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-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", 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", Option," RFC
3046, January 2001.

[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
Messages",
Messages," RFC 3118, June 2001.

9.2

[RFC4014] Droms, R. and J. Schnizlein, " Remote Authentication Dial-
In User Service (RADIUS) Attributes Suboption for the Dynamic
Host Configuration Protocol (DHCP) Relay Agent Information
Option," RFC 4014, February 2005.

10.2 Informative References

[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3," RFC 2026, BCP 9, October 1996.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3203] T'Joens, Y., Hublet, C. Hublet and P. De Schrijver, "DHCP
reconfigure extension",
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)",
(DHCPv6)," RFC 3315, July 2003.

[auth-subopt]

[RFC3495] Beser, B. and P. Duffy, "Dynamic Host Configuration
Protocol (DHCP) Option for CableLabs Client Configuration," RFC
3495, March 2003.

[RFC3594] Duffy, P., PacketCable Security Ticket Control Sub-Option
for the DHCP CableLabs Client Configuration (CCC) Option"," RFC
3594, September 2003.

[RFC3978] Bradner, S., "IETF Rights in Contributions," RFC 3978, BCP
78, March 2005.

[RFC3979] Bradner, S., "Intellectual Property Rights in IETF
Technology," RFC 3979, BCP 79, March 2005.

[RFC4030] Stapp, M. and T. Lemon, "The Authentication Suboption for
the DHCP Relay Agent Option",
draft-ietf-dhc-auth-suboption-03 (work in progress), Option," draft-ietf-dhc-auth-suboption-03),
RFC 4030, March 2005.

[RFC4039] Park, S., P. Kim and B. Volz, "Rapid Commit Option for the
Dynamic Host Configuration Protocol version 4 (DHCPv4)," RFC
4039, March 2005.

[RFC4388] Woundy, R., and Kinnear, K., "Dynamic Host Configuration
Protocol (DHCP) Leasequery," February 2004. 2006.

[dns-threats] Atkins, D. and R. Austein, "Threat Analysis Of The
Domain Name System", draft-ietf-dnsext-dns-threats-06 System," draft-ietf-dnsext-dns-threats-07 (work in
progress), February April 2004.

[draft2223bis] Reynolds, J. and R. Braden, "Instructions to Request
for Comments (RFC) Authors," draft-rfc-editor-rfc2223bis-08.txt
(work in progress), August 2004.

[eap-auth] Yegin, A., H. Tschofenig and D. Forsberg, "Bootstrapping
RFC3118 Delayed DHCP Authentication Using EAP-based Network
Access Authentication," draft-yegin-eap-boot-rfc3118-02, (work
in progress), March 2006.

[failover] Droms, R. and K. Kinnear, "DHCP Failover Protocol", Protocol,"
draft-ietf-dhc-failover-12 (work in progress), December 2003.
[Editor's note: at the time of publication of this memo, the
Failover Internet-Draft has expired.]

[fqdn] Stapp, M. M., Volz, B., 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
Option," draft-ietf-dhc-fqdn-option-13 (work in progress), March
2004.
2006.

[p-dad] Forte, A., S. Shin and H. Schulzrinne, "Passive Duplicate
Address Detection for Dynamic Host Configuration Protocol
(DHCP)," draft-forte-dhc-passive-dad-02 (work-in-progress), June
2006.

[relay-ipsec] Droms, R., "Authentication of DHCP Relay Agent Options
Using IPsec", draft-ietf-dhc-relay-agent-ipsec-01 Ipsec," draft-ietf-dhc-relay-agent-ipsec-02 (work in
progress), November 2003. May 2005. [Editor's note: at the time of
publication of this memo, the Relay IPSEC Internet-Draft has
expired.]
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:
E-Mail: rbhibbs@pacbell.net

Carl Smith
C & C Catering
1121 Holly St Street
Alameda, California 94502
USA

EMail:

E-Mail: islandia@alumni.ucsd.edu

Bernard Volz
Cisco Systems, Inc.
1414 Massachusetts Ave. Avenue
Boxborough, MA Massachusetts 01719
USA

Phone: +1 978 936 0382
EMail:
E-Mail: 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:
E-Mail: zohar@us.ibm.com

Full Copyright Statement

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property Statement

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights.
Information on the IETF's procedures with respect to rights in IETF Documents RFC
documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.

Disclaimer of Validity ietf-
ipr@ietf.org.

Acknowledgement

Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).

APPENDIX: NOTES

This document and appendix will be removed in its entirety before the memo goes
to Working Group Last Call.

ISSUES LIST

This section summarizes issues raised in this memo that require
resolution by the DHC Working Group.

1. Because of the specific exception for inclusion of the Relay
Agent Information Option [RFC3046] in cases where it does not
fit in the main payload portion of a DHCP response packet,
should the use of DHCP Authentication be mandated in place of
the Relay Agent Authentication suboption?

2. Does the CableLabs Client Configuration option [RFC3495] require
the use of DHCP Authentication to protect sensitive information contained herein are provided
about Kerberos domains and keys?

3. Should the DHC Working Group promote the development of a new
authentication protocol based on the use of certificates?

4. Should the DHC Working Group solicit an
"AS IS" basis update from the authors
of the EAP-based authentication protocol [eap-auth] and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

5. Should the DHC Working Group promote the development of a new
authentication protocol based on hardware security tokens?

CHANGE LOG

This document is subject section summarizes the changes made to this memo as it has
evolved.

"-01" Draft

No significant changes were made from initial ("-0") version:

O Updated author information.

O Removed unused references.

O Added the rights, licenses Change Log section.

"-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 restrictions contained servers, as there are two Internet-
Drafts on this subject.

O Reworded several sections in BCP 78, section 2.

O Revised and
except as set forth therein, 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 Other minor insertions, deletions, and modifications based on
comments from Bernard Aboba and Mark Stapp and to otherwise
improve the authors retain document.

"-03" Draft

The draft was updated, correcting minor spelling, grammatical, and
typographical errors, and modified in the following ways:

O Removed Section 8, "Change Log," to APPENDIX and added an issues
list section.

O Replaced all their rights.

Acknowledgment

Funding Internet-Draft boilerplate with the most current
versions.

O Renumbered document sections.

O Updated author information.

O Updated references for I-Ds advanced to RFCs.

O Added normative and informative references.

O Added discussion of Relay Agents (section 3.5.)

O Added section 3.6, discussing the RFC Editor function side effects of infrastruture
changes from the Rapid Commit and CableLabs configuration
options, and the newly proposed Address Usage Collector (AUC)
for passive duplicate address detection.

O Expanded section 4.3, "Musings on the Key Distribution Problem"
to include description of hardware-based key generation tokens.

O Added section 4.5, "Host versus User Authentication," to help
clarify the problem [RFC3118] is currently provided by intended to address.

O Added discussion of the
Internet Society. RADIUS-based authentication proposal
described in [RFC4014].

O Added discussion of the EAP-based authentication protocol
proposed by A. Yegin et al.

O Inserted Intellectual Property Rights statement on first page.

O Performed general spelling, grammar, and typography update of
entire memo text.

O Reviewed all drafts used as references, updated as necessary.