INTERNET-DRAFT K. Luehrs
Dynamic Host Configuration Working Group CableLabs
Expires December 2003 R. Woundy
Comcast Cable
J. Bevilacqua
YAS Corporation
N. Davoust
YAS Corporation
June 2003
KDC Server Address Sub-option
<draft-ietf-dhc-suboptions-kdc-serveraddress-04.txt>
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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) DHCP option code for conveying the network
addresses of Key Distribution Center (KDC) servers.
1. Introduction
A CCC DHCP Option code providing the 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 PS 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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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 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
+------+-----+-----+-----+-----+-----+-----+-----+--
| TBD | 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
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
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Internet Draft KDC Server Address Sub-option June 2003
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 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 certified) 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 insure 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.
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.
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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 is requested to assign and
register a sub-option code of the CCC option to the KDC Server
Address sub-option.
5. Normative References
[1] Beser, B. and P. Duffy, "DHCP Option for CableLabs Client
Configuration", RFC 3495, March 2003.
[2] "CableHome 1.1 Specification SP-CH1.1-I01-030418", CableLabs,
April 2003, http://www.cablelabs.com/projects/cablehome/
specifications/.
[3] "CableHome 1.0 Specification SP-CH1.0-I04-030411", CableLabs,
April 2003, 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
6. Authors' Addresses
Kevin Luehrs
CableLabs
400 Centennial Parkway
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
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Internet Draft KDC Server Address Sub-option June 2003
Nancy Davoust
YAS Corporation
300 Brickstone Square
Andover, MA 01810
Phone: (978) 749-9999
EMail: nancy@yas.com
7. Full Copyright Statement
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Acknowledgement
Funding for the RFC Editor function is currently provided by
the Internet Society.
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