draft-ietf-dhc-relay-agent-ipsec-01.txt   draft-ietf-dhc-relay-agent-ipsec-02.txt 
DHC R. Droms DHC R. Droms
Internet-Draft Cisco Systems, Inc. Internet-Draft Cisco Systems, Inc.
Expires: May 22, 2004 November 22, 2003 Expires: November 27, 2005 May 26, 2005
Authentication of DHCP Relay Agent Options Using IPsec Authentication of DHCP Relay Agent Options Using IPsec
<draft-ietf-dhc-relay-agent-ipsec-01.txt> draft-ietf-dhc-relay-agent-ipsec-02.txt
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Copyright Notice Copyright Notice
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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).
1. Requirements Terminology 1. DHCP Terminology
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 [1].
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 2. 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
them to servers as unicast messages. In network environments like them to servers as unicast messages. In network environments like
DOCSIS data-over-cable and xDSL, for example, it has proven useful DOCSIS data-over-cable and xDSL, for example, it has proven useful
for the relay agent to add information to the DHCP message before for the relay agent to add information to the DHCP message before
forwarding it, using the relay agent information option, RFC 3046 forwarding it, using the relay agent information option, RFC 3046
[2]. The kind of information that a relay agent adds is often used in [1]. The kind of information that a relay agent adds is often used
the server's decision making about the addresses and configuration in 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 explicitly excludes message, the DHCP Authentication specification explicitly excludes
relay agent data from that authentication. relay agent data from that authentication.
skipping to change at page 2, line 33 skipping to change at page 2, line 31
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 explicitly 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. Deployment of Relay Agents in a DHCP Service 3. Deployment of Relay Agents in a DHCP Service
DHCP relay agents forward messages between DHCP clients and DHCP DHCP relay agents forward messages between DHCP clients and DHCP
servers, so that the DHCP service can be provided without requiring a servers, so that the DHCP service can be provided without requiring a
DHCP service on each network segment. Usually, there is a DHCP relay DHCP service on each network segment. Usually, there is a DHCP relay
agent on the same network segment as the client, and the relay agent agent on the same network segment as the client, and the relay agent
forwards messages directly between the client and DHCP server, as forwards messages directly between the client and DHCP server, as
illustrated in Figure 1. illustrated in Figure 1.
.______ ______
_____ / \ _____ / \
+------+ / \ +-------+ / \ +------+ +------+ / \ +-------+ / \ +------+
| DHCP |--|Network|--| Relay |--| internet |--| DHCP | | DHCP |--|Network|--| Relay |--| internet |--| DHCP |
|client| |Segment| |Agent A| \ / |server| |client| |Segment| |Agent A| \ / |server|
+------+ \_____/ +-------+ \______/ +------+ +------+ \_____/ +-------+ \______/ +------+
. .
Deployment of a DHCP relay agent to forward messages between a DHCP Deployment of a DHCP relay agent to forward messages between a DHCP
client and a DHCP server client and a DHCP server
Figure 1 Figure 1
skipping to change at page 4, line 5 skipping to change at page 4, line 5
to forward DHCP messages to relay agent B. Relay agent B is to forward DHCP messages to relay agent B. Relay agent B is
configured to forward DHCP messages to relay agent C, which is, in configured to forward DHCP messages to relay agent C, which is, in
turn, configured to forward DHCP messages to the DHCP server turn, configured to forward DHCP messages to the DHCP server
In the case where multiple relay agents are deployed between the DHCP In the case where multiple relay agents are deployed between the DHCP
client and server, the responses from the server to the client are client and server, the responses from the server to the client are
sent directly to the relay agent closest to the DHCP client. In 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 Figure 2, the DHCP server will send its responses to the DHCP client
directly to relay agent A. directly to relay agent A.
.______ ______
_____ / \ _____ / \
+------+ / \ +-------+ / \ +------+ / \ +-------+ / \
| DHCP |--|Network|--| Relay |--| internet | | DHCP |--|Network|--| Relay |--| internet |
|client| |Segment| |Agent A| \ / |client| |Segment| |Agent A| \ /
+------+ \_____/ +-------+ \______/ +------+ \_____/ +-------+ \______/
| |
. | |
+-------+ +-------+
| Relay | | Relay |
|Agent B| |Agent B|
+-------+ +-------+
| |
. | ______
.______
/ \ / \
/ \ / \
| internet | | internet |
\ / \ /
\______/ \______/
| |
. |
+-------+ +-------+
| Relay | | Relay |
|Agent C| |Agent C|
+-------+ +-------+
| |
| ______
.______
/ \ / \
/ \ / \
| internet | | internet |
\ / \ /
\______/ \______/
| |
. |
+-------+ +-------+
| Relay | | Relay |
|Agent D| |Agent D|
+-------+ +-------+
| |
| ______
.______
/ \ / \
/ \ +------+ / \ +------+
| internet |--| DHCP | | internet |--| DHCP |
\ / |server| \ / |server|
\______/ +------+ \______/ +------+
Deployment of multiple relay agents between a DHCP client and server Deployment of multiple relay agents between a DHCP client and server
Figure 2 Figure 2
5. Relay Agent Message Threat Model 4. Relay Agent Message Threat Model
DHCP messages are forwarded by DHCP relay agents between DHCP clients DHCP messages are forwarded by DHCP relay agents between DHCP clients
and DHCP servers. The messages exchanged between relay agents and and DHCP servers. The messages exchanged between relay agents and
servers, in addition to carrying the contents of the messages between servers, in addition to carrying the contents of the messages between
the clients and server, may carry additional information in relay the clients and server, may carry additional information in relay
agent information options. The information in the relay agent agent information options. The information in the relay agent
information options may be used by the relay agent, for example to information options may be used by the relay agent, for example to
track the physical interface to which a DHCP client is attached, and 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 by the server, for example to affect the selection of an IP address
and other configuration information to be assigned to the client. and other configuration information to be assigned to the client.
skipping to change at page 5, line 35 skipping to change at page 5, line 32
The attacks available to a malicious attacker through the relay The attacks available to a malicious attacker through the relay
information option include inserting new relay information options, information option include inserting new relay information options,
modifying the contents of existing relay information options or modifying the contents of existing relay information options or
deleting relay information options. There is no attack available deleting relay information options. There is no attack available
through examining the contents of relay information options so there through examining the contents of relay information options so there
is no requirement for privacy of the contents of relay information is no requirement for privacy of the contents of relay information
options. options.
A malicious attacker might mount the following denial of service A malicious attacker might mount the following denial of service
attacks against a DHCP client: attacks against a DHCP client:
o Change the contents of the Agent Circuit ID sub-option or the 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 Agent Remote ID sub-option [1], causing the relay agent to be
unable to return DHCP messages from the server to the client unable to return DHCP messages from the server to the client
o Change the contents of the DOCSIS Device Class sub-option [9], o Change the contents of the DOCSIS Device Class sub-option [9],
causing the DHCP server to provide incorrect configuration causing the DHCP server to provide incorrect configuration
parameters to a DOCSIS device parameters to a DOCSIS device
o Change the contents of the Link Selection sub-option [10], causing o Change the contents of the Link Selection sub-option [10], causing
the DHCP server to assign an IP address from an incorrect subnet the DHCP server to assign an IP address from an incorrect subnet
to the DHCP client to the DHCP client
In some networks, hosts are assigned to different VLANs that provide In some networks, hosts are assigned to different VLANs that provide
different types of access to the network depending on the identity of 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 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 assigned to an internal company VLAN or an isolated VLAN that
provides only external Internet access depending on the identity of provides only external Internet access depending on the identity of
the host. A malicious attacker might mount the following attacks the host. A malicious attacker might mount the following attacks
skipping to change at page 6, line 8 skipping to change at page 5, line 49
the DHCP server to assign an IP address from an incorrect subnet the DHCP server to assign an IP address from an incorrect subnet
to the DHCP client to the DHCP client
In some networks, hosts are assigned to different VLANs that provide In some networks, hosts are assigned to different VLANs that provide
different types of access to the network depending on the identity of 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 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 assigned to an internal company VLAN or an isolated VLAN that
provides only external Internet access depending on the identity of provides only external Internet access depending on the identity of
the host. A malicious attacker might mount the following attacks the host. A malicious attacker might mount the following attacks
designed to gain unauthorized network access: designed to gain unauthorized network access:
o Change the contents of the Link Selection sub-option to cause the o Change the contents of the Link Selection sub-option to cause the
DHCP client to be assigned an IP address from an inappropriate DHCP client to be assigned an IP address from an inappropriate
VLAN VLAN
o Change the contents of the RADIUS Attributes sub-option [11] to o Change the contents of the RADIUS Attributes sub-option [11] to
cause the DHCP client to be authorized to access inappropriate cause the DHCP client to be authorized to access inappropriate
network resources network resources
o Replay an earlier DHCP message that contained a valid RADIUS o Replay an earlier DHCP message that contained a valid RADIUS
Attriubtes sub-option to cause the DHCP client to be authorized to Attributes sub-option to cause the DHCP client to be authorized to
access inappropriate network resources access inappropriate network resources
6. Use of IPsec to secure DHCP messages 5. Use of IPsec to secure DHCP messages
Relay agents and servers can use IPsec mechanisms [3] to exchange Relay agents and servers can use IPsec mechanisms [2] to exchange
messages securely as described in this section. If there is a single messages securely as described in this section. If there is a single
relay agent between the DHCP client, there MUST be an IPsec trust relay agent between the DHCP client, there is an IPsec trust
relationship established between the relay agent and the DHCP server. relationship established between the relay agent and the DHCP server.
In Figure 1, relay agent A and the DHCP server must have an IPsec In Figure 1, relay agent A and the DHCP server must have an IPsec
session through which DHCP messages are exchanged. session through which DHCP messages are exchanged.
If a client message is relayed through multiple relay agents, there If a client message is relayed through multiple relay agents, there
must be independent, pairwise IPsec sessions among the relay agents. are independent, pairwise IPsec sessions among the relay agents. In
In a deployment with multiple relay agents, the relay agents are a deployment with multiple relay agents, the relay agents are assumed
assumed to belong to a single administrative domain or otherwise have to belong to a single administrative domain or otherwise have the
the ability to establish IPsec sessions. For example, in Figure 2, ability to establish IPsec sessions. For example, in Figure 2, there
there must be an IPsec session between pairs of relay agents A and B, must be an IPsec session between pairs of relay agents A and B, B and
B and C, and C and D. There must also be be a IPsec session between C, and C and D. There must also be be a IPsec session between relay
relay agent D and the DHCP server. In addition, there must be an agent D and the DHCP server. In addition, there must be an IPsec
IPsec session between the DHCP server and relay agent A, for messages session between the DHCP server and relay agent A, for messages that
that will be returned from the server directly to relay agent A. 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 addresses Selectors: Relay agents are manually configured with the 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 to be forwarded. Each relay agent and server that will
be using IPsec for securing DHCP messages must also be be using IPsec for securing DHCP messages must also be
configured with a list of the relay agents to which configured with a list of the relay agents to which
messages will be returned. The selectors for the relay messages will be returned. The selectors for the relay
agents and servers will be the pairs of addresses agents and servers will be the pairs of addresses
defining relay agents and servers that exchange DHCP 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 [3].
Mode: Relay agents and servers use transport mode and ESP [4].
The information in DHCP messages is not generally The information in DHCP messages is not generally
considered confidential, so encryption need not be used considered confidential, so encryption need not be used
(i.e., NULL 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. an organization, public key schemes are not necessary.
Because the relay agents and servers must be manually Because the relay agents and servers must be manually
configured, manually configured key management may configured, manually configured key management may
suffice, but does not provide defense against replayed suffice, but does not provide defense against replayed
messages. Accordingly, IKE with preshared secrets SHOULD messages. Accordingly, if replay protection is required,
be supported. IKE with public keys MAY be supported. IKE [4] with preshared secrets must be used. IKE with
public keys may be used.
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 should only be accepted from DHCP peers as identified in
the local 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 available for servers and for relay agents in more
featureful devices used in enterprise and core ISP featureful devices used in enterprise and core ISP
networks. IPsec is less likely to be available for relay networks. IPsec is less likely to be available for relay
agents in low end devices primarily used in the home or agents in low end devices primarily used in the home or
small office markets. small office markets.
7. IANA Considerations 6. 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.
8. Security Considerations 7. Security Considerations
The threat model for messages exchanged between DHCP relay agents and The threat model for messages exchanged between DHCP relay agents and
DHCP servers is described in Section 5. In Section 6, this DHCP servers is described in Section 4. In Section 5, this
specification describes a mechanism that can be used to provide 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 use of IPsec for securing relay agent options in DHCP messages The use of IPsec for securing relay agent options in DHCP messages
requires: requires:
o the existence of an IPsec implementation available to the relay o the existence of an IPsec implementation available to the relay
agents and DHCP servers agents and DHCP servers
o that the DHCP relay agents and servers be under appropriate o that the DHCP relay agents and servers be under appropriate
administrative control so that IPsec sessions can be established administrative control so that IPsec sessions can be established
among the relay agents and servers among the relay agents and servers
o manual configuration of the participants, including manual o manual configuration of the participants, including manual
distribution of key distribution of key
9. Acknowledgments The dhc WG has developed two documents describing authentication of
DHCP relay agent options to accommodate the requirements of different
deployment scenarios: this document and "The Authentication Suboption
for the DHCP Relay Agent Option" [12]. In deployments where IPsec is
readily available and pairwise keys can be managed efficiently, the
use of IPsec as described in this document may be appropriate. If
IPsec is not available or there are multiple relay agents for which
multiple keys must be managed, the protocol described in "The
Authentication Suboption for the DHCP Relay Agent Option" may be
appropriate. As is the case whenever two alternatives are available,
local network administration can choose whichever is more
appropriate. Because the relay agents and the DHCP server are all in
the same administrative domain, the appropriate mechanism can be
configured on all interoperating DHCP server elements.
8. 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 9. References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 9.1 Normative references
Levels", BCP 14, RFC 2119, March 1997.
[2] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, [1] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001. January 2001.
[3] Kent, S. and R. Atkinson, "Security Architecture for the [2] 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 [3] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998. (ESP)", RFC 2406, November 1998.
Informative References [4] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
9.2 Informative References
[5] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, [5] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
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 [9] Jones, D. and R. Woundy, "The DOCSIS (Data-Over-Cable Service
Interface Specifications) Device Class DHCP (Dynamic Host Interface Specifications) Device Class DHCP (Dynamic Host
Configuration Protocol) Relay Agent Information Sub-option", Configuration Protocol) Relay Agent Information Sub-option",
RFC 3256, April 2002. RFC 3256, April 2002.
[10] Kinnear, K., Stapp, M., Johnson, R. and J. Kumarasamy, "Link [10] Kinnear, K., Stapp, M., Johnson, R., and J. Kumarasamy, "Link
Selection sub-option for the Relay Agent Information Option for Selection sub-option for the Relay Agent Information Option for
DHCPv4", RFC 3527, April 2003. DHCPv4", RFC 3527, April 2003.
[11] Droms, R. and J. Schnizlein, "RADIUS Attributes Sub-option for [11] Droms, R. and J. Schnizlein, "RADIUS Attributes Sub-option for
the DHCP Relay Agent Information Option", the DHCP Relay Agent Information Option",
draft-ietf-dhc-agentopt-radius-02 (work in progress), November draft-ietf-dhc-agentopt-radius-08 (work in progress),
2002. September 2004.
[12] Stapp, M. and T. Lemon, "The Authentication Suboption for the
DHCP Relay Agent Option", draft-ietf-dhc-auth-suboption-05
(work in progress), August 2004.
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
EMail: rdroms@cisco.com Email: rdroms@cisco.com
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Acknowledgment Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
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