draft-ietf-dhc-relay-agent-ipsec-00.txt   draft-ietf-dhc-relay-agent-ipsec-01.txt 
DHC R. Droms DHC R. Droms
Internet-Draft Cisco Systems, Inc. Internet-Draft Cisco Systems, Inc.
Expires: February 27, 2004 August 29, 2003 Expires: May 22, 2004 November 22, 2003
Authentication of DHCP Relay Agent Options Using IPsec Authentication of DHCP Relay Agent Options Using IPsec
<draft-ietf-dhc-relay-agent-ipsec-00.txt> <draft-ietf-dhc-relay-agent-ipsec-01.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract Abstract
The DHCP Relay Agent Information Option (RFC 3046) conveys The DHCP Relay Agent Information Option (RFC 3046) conveys
information between a DHCP relay agent and a DHCP server. This information between a DHCP relay agent and a DHCP server. This
specification defines a mechanism for securing the messages specification defines a mechanism for securing the messages
exchanged between a relay agent and a server using IPsec (RFC 2401). exchanged between a relay agent and a server using IPsec (RFC 2401).
Table of Contents
1. Requirements Terminology . . . . . . . . . . . . . . . . . . . . 3
2. DHCP Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Use of IPsec to secure DHCP messages . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. IPsec Considerations . . . . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
Normative references . . . . . . . . . . . . . . . . . . . . . . 5
Informative References . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . . 7
1. Requirements Terminology 1. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1]. document are to be interpreted as described in RFC 2119 [1].
2. DHCP Terminology 2. DHCP Terminology
This document uses the terms "DHCP server" (or "server") and "DHCP This document uses the terms "DHCP server" (or "server") and "DHCP
client" (or "client") as defined in RFC 2131. The term "DHCP relay client" (or "client") as defined in RFC 2131. The term "DHCP relay
agent" refers to a "BOOTP relay agent" as defined in RFC 2131. agent" refers to a "BOOTP relay agent" as defined in RFC 2131.
3. Introduction 3. Introduction
DHCP (RFC 2131 [5]) provides IP addresses and configuration DHCP (RFC 2131 [5]) provides IP addresses and configuration
information for DHCP clients. It includes a relay agent capability information for DHCP clients. It includes a relay agent capability
(RFC 951 [6], RFC 1542 [7]), in which processes within the network (RFC 951 [6], RFC 1542 [7]), in which processes within the network
infrastructure receive broadcast messages from clients and forward infrastructure receive broadcast messages from clients and forward
skipping to change at page 3, line 37 skipping to change at page 2, line 28
[2]. The kind of information that a relay agent adds is often used in [2]. The kind of information that a relay agent adds is often used in
the server's decision making about the addresses and configuration the server's decision making about the addresses and configuration
parameters that the client should receive. The way that the relay parameters that the client should receive. The way that the relay
agent data is used in server decision-making tends to make that data agent data is used in server decision-making tends to make that data
very important, and highlights the importance of the trust very important, and highlights the importance of the trust
relationship between the relay agent and the server. relationship between the relay agent and the server.
The existing DHCP Authentication specification (RFC 3118) [8] only The existing DHCP Authentication specification (RFC 3118) [8] only
secures communication between the DHCP client and server. Because secures communication between the DHCP client and server. Because
relay agent information is added after the client has signed its relay agent information is added after the client has signed its
message, the DHCP Authentication specification explictly excludes message, the DHCP Authentication specification explicitly excludes
relay agent data from that authentication. relay agent data from that authentication.
The goals of this specification is to define a method that a relay The goals of this specification is to define a method that a relay
agent can use to: agent can use to:
1. protect the integrity of the data that the relay adds 1. protect the integrity of the data that the relay adds
2. provide replay protection for that data 2. provide replay protection for that data
3. leverage the existing IPsec mechanism 3. leverage the existing IPsec mechanism
4. Use of IPsec to secure DHCP messages 4. Deployment of Relay Agents in a DHCP Service
Relay agents and servers that exchange messages securely can use DHCP relay agents forward messages between DHCP clients and DHCP
IPsec mechanisms [3] as described in this section. If a client servers, so that the DHCP service can be provided without requiring a
message is relayed through multiple relay agents, each of the relay DHCP service on each network segment. Usually, there is a DHCP relay
agents must have established independent, pairwise trust agent on the same network segment as the client, and the relay agent
relationships. That is, if messages from client C will be relayed by forwards messages directly between the client and DHCP server, as
relay agent A to relay agent B and then to the server, relay agents A illustrated in Figure 1.
and B must be configured to use IPSec for the messages they exchange,
and relay agent B and the server must be configured to use IPSec for .______
the messages they exchange. _____ / \
+------+ / \ +-------+ / \ +------+
| DHCP |--|Network|--| Relay |--| internet |--| DHCP |
|client| |Segment| |Agent A| \ / |server|
+------+ \_____/ +-------+ \______/ +------+
.
Deployment of a DHCP relay agent to forward messages between a DHCP
client and a DHCP server
Figure 1
In some deployments, there may be more than one relay agent between
the DHCP client and server. In Figure 2, relay agent A is configured
to forward DHCP messages to relay agent B. Relay agent B is
configured to forward DHCP messages to relay agent C, which is, in
turn, configured to forward DHCP messages to the DHCP server
In the case where multiple relay agents are deployed between the DHCP
client and server, the responses from the server to the client are
sent directly to the relay agent closest to the DHCP client. In
Figure 2, the DHCP server will send its responses to the DHCP client
directly to relay agent A.
.______
_____ / \
+------+ / \ +-------+ / \
| DHCP |--|Network|--| Relay |--| internet |
|client| |Segment| |Agent A| \ /
+------+ \_____/ +-------+ \______/
|
. |
+-------+
| Relay |
|Agent B|
+-------+
|
. |
.______
/ \
/ \
| internet |
\ /
\______/
|
. |
+-------+
| Relay |
|Agent C|
+-------+
|
|
.______
/ \
/ \
| internet |
\ /
\______/
|
. |
+-------+
| Relay |
|Agent D|
+-------+
|
|
.______
/ \
/ \ +------+
| internet |--| DHCP |
\ / |server|
\______/ +------+
Deployment of multiple relay agents between a DHCP client and server
Figure 2
5. Relay Agent Message Threat Model
DHCP messages are forwarded by DHCP relay agents between DHCP clients
and DHCP servers. The messages exchanged between relay agents and
servers, in addition to carrying the contents of the messages between
the clients and server, may carry additional information in relay
agent information options. The information in the relay agent
information options may be used by the relay agent, for example to
track the physical interface to which a DHCP client is attached, and
by the server, for example to affect the selection of an IP address
and other configuration information to be assigned to the client.
Because the information carried in the relay agent information option
may affect the behavior of relay agents and servers, operation of a
DHCP service may be disrupted through malicious attacks on DHCP
messages carrying relay agent information options.
The attacks available to a malicious attacker through the relay
information option include inserting new relay information options,
modifying the contents of existing relay information options or
deleting relay information options. There is no attack available
through examining the contents of relay information options so there
is no requirement for privacy of the contents of relay information
options.
A malicious attacker might mount the following denial of service
attacks against a DHCP client:
o Change the contents of the Agent Circuit ID sub-option or the
Agent Remote ID sub-option [2], causing the relay agent to be
unable to return DHCP messages from the server to the client
o Change the contents of the DOCSIS Device Class sub-option [9],
causing the DHCP server to provide incorrect configuration
parameters to a DOCSIS device
o Change the contents of the Link Selection sub-option [10], causing
the DHCP server to assign an IP address from an incorrect subnet
to the DHCP client
In some networks, hosts are assigned to different VLANs that provide
different types of access to the network depending on the identity of
the host or the user of that host. For example, a host might be
assigned to an internal company VLAN or an isolated VLAN that
provides only external Internet access depending on the identity of
the host. A malicious attacker might mount the following attacks
designed to gain unauthorized network access:
o Change the contents of the Link Selection sub-option to cause the
DHCP client to be assigned an IP address from an inappropriate
VLAN
o Change the contents of the RADIUS Attributes sub-option [11] to
cause the DHCP client to be authorized to access inappropriate
network resources
o Replay an earlier DHCP message that contained a valid RADIUS
Attriubtes sub-option to cause the DHCP client to be authorized to
access inappropriate network resources
6. Use of IPsec to secure DHCP messages
Relay agents and servers can use IPsec mechanisms [3] to exchange
messages securely as described in this section. If there is a single
relay agent between the DHCP client, there MUST be an IPsec trust
relationship established between the relay agent and the DHCP server.
In Figure 1, relay agent A and the DHCP server must have an IPsec
session through which DHCP messages are exchanged.
If a client message is relayed through multiple relay agents, there
must be independent, pairwise IPsec sessions among the relay agents.
In a deployment with multiple relay agents, the relay agents are
assumed to belong to a single administrative domain or otherwise have
the ability to establish IPsec sessions. For example, in Figure 2,
there must be an IPsec session between pairs of relay agents A and B,
B and C, and C and D. There must also be be a IPsec session between
relay agent D and the DHCP server. In addition, there must be an
IPsec session between the DHCP server and relay agent A, for messages
that will be returned from the server directly to relay agent A.
Relay agents and servers that support secure relay agent to server or Relay agents and servers that support secure relay agent to server or
relay agent to relay agent communication use IPsec under the relay agent to relay agent communication use IPsec under the
following conditions: following conditions:
Selectors: Relay agents are manually configured with the Selectors: Relay agents are manually configured with the addresses
addresses of the relay agent or server to which DHCP messages are of the relay agent or server to which DHCP messages are
to be forwarded. Each relay agent and server that will be using to be forwarded. Each relay agent and server that will
IPsec for securing DHCP messages must also be configured with a be using IPsec for securing DHCP messages must also be
list of the relay agents to which messages will be returned. The configured with a list of the relay agents to which
selectors for the relay agents and servers will be the pairs of messages will be returned. The selectors for the relay
addresses defining relay agents and servers that exchange DHCP agents and servers will be the pairs of addresses
defining relay agents and servers that exchange DHCP
messages on the DHCP UDP ports 67 and 68. messages on the DHCP UDP ports 67 and 68.
Mode: Relay agents and servers use transport mode and ESP [4]. The Mode: Relay agents and servers use transport mode and ESP [4].
information in DHCP messages is not generally considered The information in DHCP messages is not generally
confidential, so encryption need not be used (i.e., NULL considered confidential, so encryption need not be used
encryption can be used). (i.e., NULL encryption can be used).
Key management: Because the relay agents and servers are used within Key management: Because the relay agents and servers are used within
an organization, public key schemes are not necessary. Because an organization, public key schemes are not necessary.
the relay agents and servers must be manually configured, manually Because the relay agents and servers must be manually
configured key management may suffice, but does not provide configured, manually configured key management may
defense against replayed messages. Accordingly, IKE with suffice, but does not provide defense against replayed
preshared secrets SHOULD be supported. IKE with public keys MAY be messages. Accordingly, IKE with preshared secrets SHOULD
supported. be supported. IKE with public keys MAY be supported.
Security policy: DHCP messages between relay agents and servers Security policy: DHCP messages between relay agents and servers
should only be accepted from DHCP peers as identified in the local should only be accepted from DHCP peers as identified in
configuration. the local configuration.
Authentication: Shared keys, indexed to the source IP address of the Authentication: Shared keys, indexed to the source IP address of the
received DHCP message, are adequate in this application. received DHCP message, are adequate in this application.
Availability: Appropriate IPsec implementations are likely to be Availability: Appropriate IPsec implementations are likely to be
available for servers and for relay agents in more featureful available for servers and for relay agents in more
devices used in enterprise and core ISP networks. IPsec is less featureful devices used in enterprise and core ISP
likely to be available for relay agents in low end devices networks. IPsec is less likely to be available for relay
primarily used in the home or small office markets. agents in low end devices primarily used in the home or
small office markets.
5. IANA Considerations 7. IANA Considerations
There are no IANA considerations for the authentication mechanisms There are no IANA considerations for the authentication mechanisms
described in this document. described in this document.
6. Security Considerations 8. Security Considerations
This specification describes a mechanism that can be used to provide The threat model for messages exchanged between DHCP relay agents and
DHCP servers is described in Section 5. In Section 6, this
specification describes a mechanism that can be used to provide
authentication and message integrity protection to the messages authentication and message integrity protection to the messages
between DHCP relay agents and DHCP servers. between DHCP relay agents and DHCP servers.
The authentication sub-option protocol requires configuration of The use of IPsec for securing relay agent options in DHCP messages
relay agents and servers with shared secret keys. requires:
7. IPsec Considerations o the existence of an IPsec implementation available to the relay
agents and DHCP servers
The use of IPsec for securing relay agent options in DHCP messages o that the DHCP relay agents and servers be under appropriate
requires the existence of an IPsec implementation available to the administrative control so that IPsec sessions can be established
relay agents and DHCP servers. It also requires manual configuration among the relay agents and servers
of the participants, including manual distribution of keys.
8. Acknowledgments o manual configuration of the participants, including manual
distribution of key
9. Acknowledgments
The need for this specification was made clear by comments made by The need for this specification was made clear by comments made by
Thomas Narten and John Schnizlein at IETF 53. Thomas Narten and John Schnizlein at IETF 53.
Normative references Normative references
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[2] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, [2] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001. January 2001.
[3] Kent, S. and R. Atkinson, "Security Architecture for the [3] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998. Internet Protocol", RFC 2401, November 1998.
[4] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload [4] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998. (ESP)", RFC 2406, November 1998.
skipping to change at page 6, line 10 skipping to change at page 8, line 47
March 1997. March 1997.
[6] Croft, B. and J. Gilmore, "Bootstrap Protocol", RFC 951, [6] Croft, B. and J. Gilmore, "Bootstrap Protocol", RFC 951,
September 1985. September 1985.
[7] Wimer, W., "Clarifications and Extensions for the Bootstrap [7] Wimer, W., "Clarifications and Extensions for the Bootstrap
Protocol", RFC 1542, October 1993. Protocol", RFC 1542, October 1993.
[8] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", [8] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001. RFC 3118, June 2001.
[9] Jones, D. and R. Woundy, "The DOCSIS (Data-Over-Cable Service
Interface Specifications) Device Class DHCP (Dynamic Host
Configuration Protocol) Relay Agent Information Sub-option",
RFC 3256, April 2002.
[10] Kinnear, K., Stapp, M., Johnson, R. and J. Kumarasamy, "Link
Selection sub-option for the Relay Agent Information Option for
DHCPv4", RFC 3527, April 2003.
[11] Droms, R. and J. Schnizlein, "RADIUS Attributes Sub-option for
the DHCP Relay Agent Information Option",
draft-ietf-dhc-agentopt-radius-02 (work in progress), November
2002.
Author's Address Author's Address
Ralph Droms Ralph Droms
Cisco Systems, Inc. Cisco Systems, Inc.
1414 Massachusetts Ave. 1414 Massachusetts Ave.
Boxborough, MA 01719 Boxborough, MA 01719
USA USA
Phone: +1 978.936.1674 Phone: +1 978.936.1674
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