Network Working Group K. Luehrs
Request for Comments: 3634 CableLabs
Category: Standards Track R. Woundy
Comcast Cable
J. Bevilacqua
N. Davoust
YAS Corporation
December 2003
Key Distribution Center (KDC) Server Address Sub-option for
the Dynamic Host Configuration Protocol (DHCP)
CableLabs Client Configuration (CCC) Option
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines a new sub-option for the CableLabs Client
Configuration (CCC) Dynamic Host Configuration Protocol (DHCP) option
code for conveying the network addresses of Key Distribution Center
(KDC) servers.
1. Introduction
A CableLabs Client Configuration (CCC) Dynamic Host Configuration
Protocol (DHCP) Option code providing the Key Distribution Center
(KDC) server address will be needed for CableHome-compliant
residential gateways configured to use Kerberos for authentication as
the first step in establishing a secure SNMPv3 link between the
Portal Service (PS) logical element [2,3] in residential gateways,
and the SNMP entity in the cable operator's data network.
The CCC DHCP option code will be used to address specific needs of
CableLabs client devices during their configuration processes. This
document proposes a sub-option for the CCC DHCP option.
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RFC 3634 KDC Server Address Sub-option December 2003
Configuration of a class of CableLabs client devices described in [2]
and [3] will require a DHCP sub-option to provide the client with the
network address of a KDC server in the cable operator's data network.
The class of devices assumed in [2] and [3] is unlike the class of
devices considered in [1], which perform a DNS lookup of the Kerberos
Realm name to find the KDC server network address.
This document proposes a sub-option of the CCC DHCP option code for
use with CableLabs client devices. The proposed sub-option encodes
an identifier for the network address of each of one or more Key
Distribution Center servers with which the CableLabs client device
exchanges security information.
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY" in
this document are to be interpreted as described in BCP 14, RFC 2119
[4].
2. Key Distribution Center IP Address Sub-option
CableHome specifications will specify the Key Distribution Center
network address encoding as a sub-option of the CCC DHCP Option code.
This field will be used to inform the client device of the network
address of one or more Key Distribution Center servers.
The encoding of the KDC Server Address sub-option will adhere to the
format of an IPv4 address. The minimum length for this option is 4
octets, and the length MUST always be a multiple of 4. If multiple
KDC Servers are listed, they MUST be listed in decreasing order of
priority. The format of the KDC Server Address sub-option of the CCC
option code is as shown below:
SubOpt Len Address 1 Address 2
+------+-----+-----+-----+-----+-----+-----+-----+--
| 10 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
+------+-----+-----+-----+-----+-----+-----+-----+--
3. Security Considerations
This document relies upon the DHCP protocol [5] for authentication
and security, i.e., it does not provide security in excess of what
DHCP is (or will be) providing. Potential exposures to attack in the
DHCP protocol are discussed in section 7 of the DHCP protocol
specification [5] and in Authentication for DHCP Messages [6].
The CCC option can be used to misdirect network traffic by providing
incorrect DHCP server addresses, incorrect provisioning server
addresses, and incorrect Kerberos realm names to a CableLabs client
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RFC 3634 KDC Server Address Sub-option December 2003
device. This misdirection can lead to several threat scenarios. A
Denial of Service (DoS) attack can result from address information
being simply invalid. A man-in-the-middle attack can be mounted by
providing addresses to a potential snooper. A malicious service
provider can steal customers from the customer selected service
provider, by altering the Kerberos realm designation.
These threats are mitigated by several factors.
Within the cable delivery architecture required by CableLabs'
PacketCable, DOCSIS, and CableHome specifications, the DHCP client is
connected to a network through a cable modem and the Cable Modem
Termination System (CMTS). The CMTS is explicitly configured with a
set of DHCP servers to which DHCP requests are forwarded. Further, a
correctly configured CMTS will only allow downstream traffic from
specific IP addresses/ ranges.
Assuming that server addresses were successfully spoofed to the point
that a malicious client device was able to contact a KDC, the client
device must still present valid certificates to the KDC before being
service enabled. Given the computational overhead of the certificate
validation process, this situation could present a DoS opportunity.
It is possible for a malicious (although certificate enabled) service
provider to redirect a customer from the customer's selected service
provider. It is assumed that all service providers permitted onto an
access providers network are trusted entities that will cooperate to
ensure peaceful coexistence. If a service provider is found to be
redirecting customers, this should be handled as an administrative
matter between the access provider and the service provider.
Another safeguard that can be taken by service providers to limit
their exposure to their KDC server(s) is to configure their network
so that the KDC(s) reside on a separate subnetwork.
Service providers can further protect their KDC server(s) by placing
a firewall in front of the KDC(s) only allowing connections needed
for its current provisioning processes. The IP temporary addresses
given the client devices from the DHCP server could be sent directly
to the firewall from the DHCP server to open a hole for Kerberos
messages only for those particular IP addresses for a short period of
time. If this was used it would be recommended that service
providers authenticate their DHCP server to the KDC as well. This
could be done via password authentication rather than digital
certificate due to the co-location of the DHCP server to the KDC.
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RFC 3634 KDC Server Address Sub-option December 2003
Finally, Kerberos requires mutual client-server authentication.
Therefore, the client device must authenticate itself with its
digital certificate and the KDC is required to authenticate it to the
client device. If a hacker tries to redirect the client device by
replacing the service provider-configured KDC Server Address sub-
option with another IP address, it is not likely to be a valid
service provider's KDC server and authentication will fail.
4. IANA Considerations
The KDC Server Address sub-option described in this document is
intended to be a sub-option of the CableLabs Client Configuration
(CCC) option described in [1]. IANA has assigned and registered
sub-option code 10 of the CCC option to the KDC Server Address sub-
option.
5. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property 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; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication 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 implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
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RFC 3634 KDC Server Address Sub-option December 2003
6. Normative References
[1] Beser, B. and P. Duffy, "DHCP Option for CableLabs Client
Configuration", RFC 3495, March 2003.
[2] "CableHome 1.1 Specification", CableLabs,
http://www.cablelabs.com/projects/cablehome/specifications/.
[3] "CableHome 1.0 Specification", CableLabs,
http://www.cablelabs.com/projects/cablehome/specifications/.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[6] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001
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RFC 3634 KDC Server Address Sub-option December 2003
7. Authors' Addresses
Kevin Luehrs
CableLabs
858 Coal Creek Circle
Louisville, CO 80027
Phone: (303) 661-9100
EMail: k.luehrs@cablelabs.com
Richard Woundy
Comcast Cable
27 Industrial Drive
Chelmsford, MA 01824
Phone: (978) 244-4010
EMail: richard_woundy@cable.comcast.com
John Bevilacqua
YAS Corporation
300 Brickstone Square
Andover, MA 01810
Phone: (978) 749-9999
EMail: john@yas.com
Nancy Davoust
YAS Corporation
300 Brickstone Square
Andover, MA 01810
Phone: (978) 749-9999
EMail: nancy@yas.com
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RFC 3634 KDC Server Address Sub-option December 2003
8. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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