draft-ietf-dhc-server-override-05.txt   rfc5107.txt 
Network Working Group R. Johnson Network Working Group R. Johnson
Internet-Draft J. Jumarasamy Request for Comments: 5107 J. Jumarasamy
Expires: May 19, 2008 K. Kinnear Category: Standards Track K. Kinnear
M. Stapp M. Stapp
Cisco Systems, Inc. Cisco Systems, Inc.
November 16, 2007 February 2008
DHCP Server Identifier Override Suboption DHCP Server Identifier Override Suboption
draft-ietf-dhc-server-override-05.txt
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months Status of This Memo
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on May 19, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2007). 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.
Abstract Abstract
This memo defines a new suboption of the DHCP relay information This memo defines a new suboption of the DHCP relay information
option which allows the DHCP relay to specify a new value for the option that allows the DHCP relay to specify a new value for the
Server Identifier option, which is inserted by the DHCP Server. This Server Identifier option, which is inserted by the DHCP Server. This
allows the DHCP relay to act as the actual DHCP server such that allows the DHCP relay to act as the actual DHCP server such that
RENEW DHCPREQUESTs will come to the relay instead of going to the RENEW DHCPREQUESTs will come to the relay instead of going to the
server directly. This gives the relay the opportunity to include the server directly. This gives the relay the opportunity to include the
Relay Agent option with appropriate suboptions even on DHCP RENEW Relay Agent option with appropriate suboptions even on DHCP RENEW
messages. messages.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Server Identifier Override Suboption Definition . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 4. Server Identifier Override Suboption Definition . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6. Intellectual Property Rights and Copyright . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. Intellectual Property Rights and Copyright . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 8.2. Informative References . . . . . . . . . . . . . . . . . . 5
Intellectual Property and Copyright Statements . . . . . . . . . . 12
1. Introduction 1. Introduction
There are many situations where the DHCP relay is involved and can There are many situations where a DHCP relay agent is involved, and
insert a relay agent option [3] with appropriate suboptions easily it can easily insert a Relay Agent Information option [3] with
into DHCP DISCOVER messages. Once the lease has been granted, appropriate suboptions into DHCPDISCOVER messages. Once the lease
however, future DHCP RENEWAL messages are sent directly to the DHCP has been granted, however, future DHCPREQUEST messages sent by a
Server as specified in the Server Identifier option. This means that client in RENEWING state are sent directly to the DHCP server, as
the relay may not see the DHCP RENEWAL messages (depending upon specified in the Server Identifier option. In this case, the relay
network topology) and thus can not provide the same relay agent may not see these DHCPREQUEST messages (depending upon network
option information in the RENEWAL messages. topology) and thus cannot insert the Relay Agent Information option
in the DHCPREQUEST messages.
This new DHCP relay agent suboption, Server Identifier override, This DHCP relay agent suboption, Server Identifier Override, allows
allows the relay to tell the DHCP server what value to place into the the relay agent to tell the DHCP server what value to place into the
Server Identifier option [5]. Using this, the relay agent can force Server Identifier option [5]. Using this, the relay agent can force
RENEWAL messages to come to it instead of the server. The relay may a host in RENEWING state to send DHCPREQUEST messages to the relay
then insert the relay agent option with appropriate suboptions and agent instead of directly to the server. The relay agent then has
relay the DHCPREQUEST to the actual server. In this fashion the DHCP the opportunity to insert the Relay Agent Information option with
server will be provided with the same relay agent information upon appropriate suboptions and relay the DHCPREQUEST to the actual
renewals (such as Circuit-ID, Remote-ID, Device Class, etc.) as was server. In this fashion, the DHCP server will be provided with the
provided in the initial DISCOVER message. In effect, this makes a same relay agent information upon renewals (such as Circuit-ID,
RENEWAL into a REBINDING. Remote-ID, Device Class, etc.) as was provided in the initial
DHCPDISCOVER message.
This new suboption could also be used by the DHCP relay in order to
allow the relay to appear as the actual DHCP server to the client.
This has the advantage that the relay can more easily keep up-to-date
information about leases granted, etc.
In short, this new suboption allows the DHCPv4 relay to function in In short, this new suboption allows the DHCPv4 relay to function in
the same fashion as the DHCPv6 relay [7] currently does. the same fashion as the DHCPv6 relay [7] currently does.
2. Conventions 2. Conventions
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 [1]. document are to be interpreted as described in [1].
3. Server Identifier Override Suboption Definition 3. Terminology
This document uses DHCP terminology as defined in section 1.5 of RFC
2131 [2], with the exception of the term "DHCP relay agent" replacing
"BOOTP relay agent".
Other terms used in this document:
o RENEW DHCPREQUEST - a DHCPREQUEST message sent by a client in
RENEWING state
4. Server Identifier Override Suboption Definition
The format of the suboption is: The format of the suboption is:
Code Len Overriding Server Identifier address Code Len Overriding Server Identifier Address
+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+
| TBD | n | a1 | a2 | a3 | a4 | | 11 | n | a1 | a2 | a3 | a4 |
+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+
Figure 1 Figure 1
The option length (n) is 4. The octets "a1" through "a4" specify the The option length (n) is 4. The octets "a1" through "a4" specify the
value which MUST be inserted into the Server Identifier option by the value that MUST be inserted into the Server Identifier option by the
DHCP Server upon reply. DHCP Server upon reply.
DHCP Servers which implement this Relay Suboption MUST use this DHCP servers that implement this Relay Agent Information suboption
value, if present, as the value to insert into the Server Identifier MUST use this value, if present in a DHCP message received from a
option whenever responding to a DHCP Client. client, as the value to insert into the Server Identifier option in
the corresponding response. The DHCP server must also record the
address in the suboption for use in subsequent messages to the DHCP
client until the next DHCP message is received from the DHCP relay
agent.
If a DHCP Server does not understand/implement this Relay Suboption, If a DHCP server does not understand/implement this Relay Information
it will ignore the Suboption, and thus will insert it's own suboption, it will ignore the suboption, and thus it will insert its
appropriate interface address as the Server Identifier address. In own appropriate interface address in the Server Identifier option.
this case, the DHCP Relay will not receive RENEW DHCPREQUEST packets In this case, the DHCP Relay will not receive RENEW DHCPREQUEST
from the client. When configuring a DHCP Relay to use this messages from the client. When configuring a DHCP relay agent to use
Suboption, the administrator of the Relay should take into account this suboption, the administrator of the relay agent should take into
whether or not the DHCP Server to which the packet will be relayed account whether or not the DHCP server to which the message will be
will correctly understand this Suboption. relayed will correctly understand this suboption.
When servicing a DHCPREQUEST packet the DHCP Server would normally When servicing a DHCPREQUEST message, the DHCP server would normally
look at the Server Identifier option for verification that the look at the Server Identifier option for verification that the
address specified there is one of the addresses associated with the address specified there is one of the addresses associated with the
DHCP Server, silently ignoring the DHCPREQUEST if it does not match a DHCP server, silently ignoring the DHCPREQUEST if it does not match a
configured DHCP Server interface address. If the DHCPREQUEST packet configured DHCP server interface address. If the DHCPREQUEST message
contains a Server Identifier Override Suboption, however, comparison contains a Server Identifier Override suboption, however, comparison
should be made between this suboption and the Server Identifier should be made between the address in this suboption and the Server
option. If both of the Server Identifier Override Suboption and the Identifier option. If both the Server Identifier Override suboption
Server Identifier Option specify the same address, then the Server and the Server Identifier option specify the same address, then the
should accept the DHCPREQUEST packet for processing, regardless of server should accept the DHCPREQUEST message for processing,
whether or not the Server Identifier Option matchs a DHCP Server regardless of whether or not the Server Identifier option matches a
interface. DHCP server interface.
The DHCP Relay should fill in the giaddr field when relaying the
packet just as it normally would do.
In a situation where the DHCP Relay is configured to forward packets The DHCP relay agent should fill in the giaddr field when relaying
to more than one server, the DHCP Relay should forward all DHCP the message, just as it normally would do.
packets to all servers. This applies to DHCP RENEW packets as well.
The intent is that the DHCP Relay should not need to maintain state In a situation where the DHCP relay agent is configured to forward
information about the DHCP lease. messages to more than one server, the DHCP relay agent SHOULD forward
all DHCP messages to all servers. This applies to RENEW DHCPREQUEST
messages as well. The intent is that the DHCP relay agent should not
need to maintain state information about the DHCP lease.
DHCP Relays using this suboption SHOULD also implement and use the DHCP relay agents implementing this suboption SHOULD also implement
DHCPv4 Relay Agent Flags Suboption [4] in order to specify whether and use the DHCPv4 Relay Agent Flags Suboption [4] in order to
the DHCP Relay received the original packet as a broadcast or specify whether the DHCP relay agent received the original message as
unicast. The DHCP Server receiving a packet containing the Server a broadcast or unicast. The DHCP server receiving a message
Identifier Override Suboption may use this additional information in containing the Server Identifier Override Suboption may use this
processing the packet. additional information in processing the message.
Note that if the DHCP Relay becomes inaccessible by the DHCP Client Note that if the DHCP relay agent becomes inaccessible by the DHCP
or loses network access to the DHCP Server, further DHCP RENEW client or loses network access to the DHCP server, further RENEW
packets from the DHCP Client may not be properly processed and the DHCPREQUEST messages from the DHCP client may not be properly
DHCP Client's lease may time out. processed and the DHCP client's lease may time out.
4. Security Considerations 5. Security Considerations
Message authentication in DHCP for intradomain use where the out-of- Message authentication in DHCP for intradomain use where the out-of-
band exchange of a shared secret is feasible is defined in [6]. band exchange of a shared secret is feasible is defined in [6].
Potential exposures to attack are discussed in section 7 of the DHCP Potential exposures to attack are discussed in Section 7 of the DHCP
protocol specification in [2]. protocol specification in [2].
The DHCP Relay Agent option depends on a trusted relationship between The DHCP Relay Agent Information option depends on a trusted
the DHCP relay agent and the server, as described in section 5 of RFC relationship between the DHCP relay agent and the DHCP server, as
3046. While the introduction of fraudulent relay-agent options can described in Section 5 of RFC 3046. While the introduction of
be prevented by a perimeter defense that blocks these options unless fraudulent DHCP relay agent information options can be prevented by a
the relay agent is trusted, a deeper defense using the authentication perimeter defense that blocks these options unless the DHCP relay
option for relay agent options [8] SHOULD be deployed as well. agent is trusted, a deeper defense using the authentication suboption
for DHCP relay agent information option [8] SHOULD be deployed as
If a rogue DHCP relay were inserted between the client and the well.
server, it could redirect clients to it using this suboption. This
would allow such a system to later deny RENEW DHCPREQUEST and thus
force clients to discontinue use of their allocated address. It
could also allow the rogue relay to change, insert, or delete DHCP
options in DHCPACK messages and extend leases beyond what the server
has allowed. This interception, however, would need to be done
during the initial DISCOVER and OFFER phase, since the suboption
value SHOULD be ignored by the server during RENEWAL state. DHCP
Authentication [6] and/or DHCP Relay Agent option authentication [8]
would address this case. (Note that, as is always the case, lack of
DHCP Authentication would allow a rogue DHCP relay to change the
Server-ID option in the DHCPOFFER and DHCPACK packets without
detection. This threat is not new to the Server-ID-Override
suboption.)
This draft does not add any new vulnerabilities that were not already If a rogue DHCP relay agent were inserted between the DHCP client and
present, except in the case where DHCP authentication is already in the DHCP server, it could redirect clients to itself using this
place and DHCP clients require its use. It is suggested that DHCP suboption. This would allow such a system to later deny RENEW
Authentication and DHCP Relay Agent Option Authentication SHOULD be DHCPREQUESTs and thus force clients to discontinue use of their
deployed when this option is used, or protection should be provided allocated addresses. It could also allow the rogue relay to change,
against the insertion of rogue DHCP relays and server. insert, or delete DHCP options in DHCPACK messages and extend leases
beyond what the server has allowed. DHCP authentication [6] and/or
DHCP Relay Agent Information option authentication [8] would address
this case. (Note that, as is always the case, lack of DHCP
authentication would allow a rogue DHCP relay agent to change the
Server Identifier Override option in the DHCPOFFER and DHCPACK
messages without detection. This threat is not new to the Server
Identifier Override suboption.)
This document does not add any new vulnerabilities that were not
already present, except in the case where DHCP authentication is
already in place, and DHCP clients require its use. It is suggested
that DHCP Authentication and DHCP Relay Agent Option Authentication
SHOULD be deployed when this option is used, or protection should be
provided against the insertion of rogue DHCP relay agents between the
client and server.
This relay sub-option is not intended, by itself, to provide any This relay suboption is not intended, by itself, to provide any
additional security benefits. additional security benefits.
5. IANA Considerations 6. IANA Considerations
IANA is requested to assign a suboption number for the Server IANA has assigned a suboption number (11) for the Server Identifier
Identifier Override Suboption from the DHCP Relay Agent Information Override Suboption from the DHCP Relay Agent Information Option [3]
Option [3] suboption number space. suboption number space.
6. Intellectual Property Rights and Copyright 7. Intellectual Property Rights and Copyright
The IETF has been notified of intellectual property rights claimed in The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this regard to some or all of the specification contained in this
document. For more information consult the online list of claimed document. For more information consult the online list of claimed
rights. rights.
7. References 8. References
7.1. Normative References 8.1. 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", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, [2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997. March 1997.
[3] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, [3] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001. January 2001.
[4] Kinnear, K., Normoyle, M., and M. Stapp, "The Dynamic Host [4] Kinnear, K., Normoyle, M., and M. Stapp, "The Dynamic Host
Configuration Protocol Version 4 (DHCPv4) Relay Agent Flags Configuration Protocol Version 4 (DHCPv4) Relay Agent Flags
Suboption", RFC 5010, September 2007. Suboption", RFC 5010, September 2007.
7.2. Informative References 8.2. Informative References
[5] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor [5] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997. Extensions", RFC 2132, March 1997.
[6] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", [6] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001. RFC 3118, June 2001.
[7] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. [7] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 3315, July 2003. RFC 3315, July 2003.
skipping to change at page 11, line 14 skipping to change at page 6, line 22
Authors' Addresses Authors' Addresses
Richard A. Johnson Richard A. Johnson
Cisco Systems, Inc. Cisco Systems, Inc.
170 W. Tasman Dr. 170 W. Tasman Dr.
San Jose, CA 95134 San Jose, CA 95134
US US
Phone: +1 408 526 4000 Phone: +1 408 526 4000
Email: raj@cisco.com EMail: raj@cisco.com
Jay Kumarasamy Jay Kumarasamy
Cisco Systems, Inc. Cisco Systems, Inc.
170 W. Tasman Dr. 170 W. Tasman Dr.
San Jose, CA 95134 San Jose, CA 95134
US US
Phone: +1 408 526 4000 Phone: +1 408 526 4000
Email: jayk@cisco.com EMail: jayk@cisco.com
Kim Kinnear Kim Kinnear
Cisco Systems, Inc. Cisco Systems, Inc.
170 W. Tasman Dr. 170 W. Tasman Dr.
San Jose, CA 95134 San Jose, CA 95134
US US
Phone: +1 408 526 4000 Phone: +1 408 526 4000
Email: kkinnear@cisco.com EMail: kkinnear@cisco.com
Mark Stapp Mark Stapp
Cisco Systems, Inc. Cisco Systems, Inc.
170 W. Tasman Dr. 170 W. Tasman Dr.
San Jose, CA 95134 San Jose, CA 95134
US US
Phone: +1 408 526 4000 Phone: +1 408 526 4000
Email: mjs@cisco.com EMail: mjs@cisco.com
Full Copyright Statement Full Copyright Statement
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors contained in BCP 78, and except as set forth therein, the authors
retain all their rights. retain all their rights.
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
skipping to change at page 12, line 44 skipping to change at line 316
attempt made to obtain a general license or permission for the use of attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr. http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at this standard. Please address the information to the IETF at
ietf-ipr@ietf.org. ietf-ipr@ietf.org.
Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
 End of changes. 39 change blocks. 
152 lines changed or deleted 140 lines changed or added

This html diff was produced by rfcdiff 1.34. The latest version is available from http://tools.ietf.org/tools/rfcdiff/