draft-ietf-dhc-suboptions-kdc-serveraddress-04.txt   rfc3634.txt 
INTERNET-DRAFT K. Luehrs Network Working Group K. Luehrs
Dynamic Host Configuration Working Group CableLabs Request for Comments: 3634 CableLabs
Expires December 2003 R. Woundy Category: Standards Track R. Woundy
Comcast Cable Comcast Cable
J. Bevilacqua J. Bevilacqua
YAS Corporation
N. Davoust N. Davoust
YAS Corporation YAS Corporation
June 2003 December 2003
KDC Server Address Sub-option Key Distribution Center (KDC) Server Address Sub-option for
<draft-ietf-dhc-suboptions-kdc-serveraddress-04.txt> the Dynamic Host Configuration Protocol (DHCP)
CableLabs Client Configuration (CCC) Option
Status of this Memo Status of this Memo
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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
This document defines a new sub-option for the CableLabs Client This document defines a new sub-option for the CableLabs Client
Configuration (CCC) DHCP option code for conveying the network Configuration (CCC) Dynamic Host Configuration Protocol (DHCP) option
addresses of Key Distribution Center (KDC) servers. code for conveying the network addresses of Key Distribution Center
(KDC) servers.
1. Introduction 1. Introduction
A CCC DHCP Option code providing the KDC server address will be A CableLabs Client Configuration (CCC) Dynamic Host Configuration
needed for CableHome-compliant residential gateways configured to Protocol (DHCP) Option code providing the Key Distribution Center
use Kerberos for authentication as the first step in establishing (KDC) server address will be needed for CableHome-compliant
a secure SNMPv3 link between the PS and the SNMP entity in the residential gateways configured to use Kerberos for authentication as
cable operator's data network. 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 The CCC DHCP option code will be used to address specific needs of
CableLabs client devices during their configuration processes. This CableLabs client devices during their configuration processes. This
document proposes a sub-option for the CCC DHCP option. document proposes a sub-option for the CCC DHCP option.
Luehrs, Woundy, Bevilacqua,& Davoust Expires December 2003 [Page 1] Internet Draft KDC Server Address Sub-option June 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.
Configuration of a class of CableLabs client devices described in The class of devices assumed in [2] and [3] is unlike the class of
[2] and [3] will require a DHCP sub-option to provide the client devices considered in [1], which perform a DNS lookup of the Kerberos
with the network address of a KDC server in the cable operator's Realm name to find the KDC server network address.
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 This document proposes a sub-option of the CCC DHCP option code for
code for use with CableLabs client devices. The proposed sub-option use with CableLabs client devices. The proposed sub-option encodes
encodes an identifier for the network address of each of one or more an identifier for the network address of each of one or more Key
Key Distribution Center servers with which the CableLabs client Distribution Center servers with which the CableLabs client device
device exchanges security information. exchanges security information.
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY" The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY" in
in this document are to be interpreted as described in RFC 2119 [4]. this document are to be interpreted as described in BCP 14, RFC 2119
[4].
2. Key Distribution Center IP Address Sub-option 2. Key Distribution Center IP Address Sub-option
CableHome specifications will specify the Key Distribution CableHome specifications will specify the Key Distribution Center
Center network address encoding as a sub-option of the CCC DHCP network address encoding as a sub-option of the CCC DHCP Option code.
Option code. This field will be used to inform the client device of This field will be used to inform the client device of the network
the network address of one or more Key Distribution Center servers. address of one or more Key Distribution Center servers.
The encoding of the KDC Server Address sub-option will adhere to the 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 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 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 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 priority. The format of the KDC Server Address sub-option of the CCC
option code is as shown below: option code is as shown below:
SubOpt Len Address 1 Address 2 SubOpt Len Address 1 Address 2
+------+-----+-----+-----+-----+-----+-----+-----+-- +------+-----+-----+-----+-----+-----+-----+-----+--
| TBD | n | a1 | a2 | a3 | a4 | a1 | a2 | ... | 10 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
+------+-----+-----+-----+-----+-----+-----+-----+-- +------+-----+-----+-----+-----+-----+-----+-----+--
3. Security Considerations 3. Security Considerations
This document relies upon the DHCP protocol [5] for authentication This document relies upon the DHCP protocol [5] for authentication
and security, i.e., it does not provide security in excess of what and security, i.e., it does not provide security in excess of what
DHCP is (or will be) providing. Potential exposures to attack in DHCP is (or will be) providing. Potential exposures to attack in the
the DHCP protocol are discussed in section 7 of the DHCP protocol DHCP protocol are discussed in section 7 of the DHCP protocol
specification [5] and in Authentication for DHCP Messages [6]. specification [5] and in Authentication for DHCP Messages [6].
The CCC option can be used to misdirect network traffic by providing The CCC option can be used to misdirect network traffic by providing
incorrect DHCP server addresses, incorrect provisioning server incorrect DHCP server addresses, incorrect provisioning server
addresses, and incorrect Kerberos realm names to a CableLabs client addresses, and incorrect Kerberos realm names to a CableLabs client
device. This misdirection can lead to several threat scenarios. A device. This misdirection can lead to several threat scenarios. A
Denial of Service (DoS) attack can result from address information Denial of Service (DoS) attack can result from address information
being simply invalid. A man-in-the-middle attack can be mounted by being simply invalid. A man-in-the-middle attack can be mounted by
providing addresses to a potential snooper. A malicious service providing addresses to a potential snooper. A malicious service
provider can steal customers from the customer selected service provider can steal customers from the customer selected service
provider, by altering the Kerberos realm designation. provider, by altering the Kerberos realm designation.
These threats are mitigated by several factors. These threats are mitigated by several factors.
Within the cable delivery architecture required by CableLabs' Within the cable delivery architecture required by CableLabs'
PacketCable, DOCSIS, and CableHome specifications, the DHCP client PacketCable, DOCSIS, and CableHome specifications, the DHCP client is
is connected to a network through a cable modem and the CMTS. The connected to a network through a cable modem and the Cable Modem
CMTS is explicitly configured with a set of DHCP servers to which Termination System (CMTS). The CMTS is explicitly configured with a
DHCP requests are forwarded. Further, a correctly configured CMTS set of DHCP servers to which DHCP requests are forwarded. Further, a
will only allow downstream traffic from specific IP addresses/ correctly configured CMTS will only allow downstream traffic from
ranges. specific IP addresses/ ranges.
Assuming that server addresses were successfully spoofed to the Assuming that server addresses were successfully spoofed to the point
point that a malicious client device was able to contact a KDC, the that a malicious client device was able to contact a KDC, the client
client device must still present valid certificates to the KDC device must still present valid certificates to the KDC before being
before being service enabled. Given the computational overhead of service enabled. Given the computational overhead of the certificate
the certificate validation process, this situation could present a validation process, this situation could present a DoS opportunity.
DoS opportunity.
It is possible for a malicious (although certified) service It is possible for a malicious (although certificate enabled) service
provider to redirect a customer from the customer's selected service provider to redirect a customer from the customer's selected service
provider. It is assumed that all service providers permitted onto provider. It is assumed that all service providers permitted onto an
an access providers network are trusted entities that will cooperate access providers network are trusted entities that will cooperate to
to insure peaceful coexistence. If a service provider is found to ensure peaceful coexistence. If a service provider is found to be
be redirecting customers, this should be handled as an redirecting customers, this should be handled as an administrative
administrative matter between the access provider and the service matter between the access provider and the service provider.
provider.
Another safeguard that can be taken by service providers to limit Another safeguard that can be taken by service providers to limit
their exposure to their KDC server(s) is to configure their network their exposure to their KDC server(s) is to configure their network
so that the KDC(s) reside on a separate subnetwork. so that the KDC(s) reside on a separate subnetwork.
Service providers can further protect their KDC server(s) by placing Service providers can further protect their KDC server(s) by placing
a firewall in front of the KDC(s)only allowing connections needed a firewall in front of the KDC(s)only allowing connections needed
for its current provisioning processes. The IP temporary addresses for its current provisioning processes. The IP temporary addresses
given the client devices from the DHCP server could be sent directly 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 to the firewall from the DHCP server to open a hole for Kerberos
messages only for those particular IP addresses for a short period messages only for those particular IP addresses for a short period of
of time. If this was used it would be recommended that service time. If this was used it would be recommended that service
providers authenticate their DHCP server to the KDC as well. This providers authenticate their DHCP server to the KDC as well. This
could be done via password authentication rather than digital could be done via password authentication rather than digital
certificate due to the co-location of the DHCP server to the KDC. certificate due to the co-location of the DHCP server to the KDC.
Finally, Kerberos requires mutual client-server authentication. Finally, Kerberos requires mutual client-server authentication.
Therefore, the client device must authenticate itself with its Therefore, the client device must authenticate itself with its
digital certificate and the KDC is required to authenticate it to digital certificate and the KDC is required to authenticate it to the
the client device. If a hacker tries to redirect the client device client device. If a hacker tries to redirect the client device by
by replacing the service provider-configured KDC Server Address replacing the service provider-configured KDC Server Address sub-
sub-option with another IP address, it is not likely to be a valid option with another IP address, it is not likely to be a valid
service provider's KDC server and authentication will fail. service provider's KDC server and authentication will fail.
4. IANA Considerations 4. IANA Considerations
The KDC Server Address sub-option described in this document is The KDC Server Address sub-option described in this document is
intended to be a sub-option of the CableLabs Client Configuration intended to be a sub-option of the CableLabs Client Configuration
(CCC) option described in [1]. IANA is requested to assign and (CCC) option described in [1]. IANA has assigned and registered
register a sub-option code of the CCC option to the KDC Server sub-option code 10 of the CCC option to the KDC Server Address sub-
Address sub-option. option.
5. Normative References 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.
6. Normative References
[1] Beser, B. and P. Duffy, "DHCP Option for CableLabs Client [1] Beser, B. and P. Duffy, "DHCP Option for CableLabs Client
Configuration", RFC 3495, March 2003. Configuration", RFC 3495, March 2003.
[2] "CableHome 1.1 Specification SP-CH1.1-I01-030418", CableLabs, [2] "CableHome 1.1 Specification", CableLabs,
April 2003, http://www.cablelabs.com/projects/cablehome/ http://www.cablelabs.com/projects/cablehome/specifications/.
specifications/.
[3] "CableHome 1.0 Specification SP-CH1.0-I04-030411", CableLabs, [3] "CableHome 1.0 Specification", CableLabs,
April 2003, http://www.cablelabs.com/projects/cablehome/ http://www.cablelabs.com/projects/cablehome/specifications/.
specifications/.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement [4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[5] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March [5] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
1997. March 1997.
[6] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC [6] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
3118, June 2001 RFC 3118, June 2001
6. Authors' Addresses 7. Authors' Addresses
Kevin Luehrs Kevin Luehrs
CableLabs CableLabs
400 Centennial Parkway 858 Coal Creek Circle
Louisville, CO 80027 Louisville, CO 80027
Phone: (303) 661-9100 Phone: (303) 661-9100
EMail: k.luehrs@cablelabs.com EMail: k.luehrs@cablelabs.com
Richard Woundy Richard Woundy
Comcast Cable Comcast Cable
27 Industrial Drive 27 Industrial Drive
Chelmsford, MA 01824 Chelmsford, MA 01824
Phone: (978) 244-4010 Phone: (978) 244-4010
EMail: richard_woundy@cable.comcast.com EMail: richard_woundy@cable.comcast.com
John Bevilacqua John Bevilacqua
YAS Corporation YAS Corporation
300 Brickstone Square 300 Brickstone Square
Andover, MA 01810 Andover, MA 01810
Phone: (978) 749-9999 Phone: (978) 749-9999
EMail: john@yas.com EMail: john@yas.com
Nancy Davoust Nancy Davoust
YAS Corporation YAS Corporation
300 Brickstone Square 300 Brickstone Square
Andover, MA 01810 Andover, MA 01810
Phone: (978) 749-9999 Phone: (978) 749-9999
EMail: nancy@yas.com EMail: nancy@yas.com
7. Full Copyright Statement 8. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
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and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
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are included on all such copies and derivative works. However, this included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
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