Network Working Group R. Johnson
Internet-Draft J. Jumarasamy
Expires: December 31, 2005 K. Kinnear
M. Stapp
Cisco Systems, Inc.
June 29, 2005
DHCP Server-ID Override Suboption
draft-ietf-dhc-server-override-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo defines a new suboption of the DHCP relay information
option [6] which allows the DHCP relay to specify a new value for the
Server-ID option, which is inserted by the DHCP Server. In some
cases it is convenient for the DHCP relay to act as the actual DHCP
server such that DHCP RENEWAL requests will come to the relay instead
of going to the server directly. This gives the relay the
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opportunity to include the Relay Agent option with appropriate
suboptions even on RENEWAL messages.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Server-ID Override Suboption Definition . . . . . . . . . . . 4
3. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Intellectual Property Rights and Copyright . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . 9
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1. Introduction
There are many situations where the DHCP relay is involved and can
insert a relay agent option with appropriate suboptions easily into
DHCP DISCOVER messages. Once the lease has been granted, however,
future DHCP RENEWAL messages are sent directly to the DHCP Server as
specified in the Server-ID option. This means that the relay may not
see the DHCP RENEWAL messages (depending upon network topology) and
thus can not provide the same relay agent option information in the
RENEWAL messages.
This new DHCP relay agent suboption, Server-ID override, allows the
relay to tell the DHCP server what value to place into the Server-ID
option. Using this, the relay agent can force RENEWAL messages to
come to it instead of the server. The relay may then insert the
relay agent option with appropriate suboptions and relay the request
to the actual server. In this fashion the DHCP server will be
provided with the same relay agent information upon renewals (such as
Circuit-ID, Remote-ID, Device Class, etc.) as was provided in the
initial DISCOVER message. In effect, this makes a RENEWAL into a
REBINDING.
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
the same fashion as the DHCPv6 relay currently does.
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2. Server-ID Override Suboption Definition
The format of the suboption is:
Code Len Overriding Server-ID address
+-----+-----+-----+-----+-----+-----+
| TBD | n | a1 | a2 | a3 | a4 |
+-----+-----+-----+-----+-----+-----+
Figure 1
The option length (n) is 4. The octets "a1" through "a4" specify the
value which SHOULD be inserted into the Server-ID option by the DHCP
Server upon reply.
DHCP Servers SHOULD use this value, if present, as the value to
insert into the Server-ID option whenever responding to a DHCP
Client.
When servicing a DHCP REQUEST packet the DHCP Server would normally
look at the Server-ID option for verification that the address
specified there is one of the addresses associated with the DHCP
Server, silently ignoring the REQUEST if it does not match a
configured DHCP Server interface address. If the REQUEST packet
contains a Server-ID Override Suboption, however, comparison should
be made between this suboption and the Server-ID option. If both of
the Server-ID Override Suboption and the Server-ID Option specify the
same address, then the Server should accept the REQUEST packet for
processing, regardless of whether or not the Server-ID Option matchs
a DHCP Server interface.
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3. Security Considerations
Message authentication in DHCP for intradomain use where the out-of-
band exchange of a shared secret is feasible is defined in [3].
Potential exposures to attack are discussed in section 7 of the DHCP
protocol specification in [2].
The DHCP Relay Agent option depends on a trusted relationship between
the DHCP relay agent and the server, as described in section 5 of RFC
3046. While the introduction of fraudulent relay-agent options can
be prevented by a perimeter defense that blocks these options unless
the relay agent is trusted, a deeper defense using the authentication
option for relay agent options [4] SHOULD be deployed as well.
If a rogue DHCP relay were inserted between the client and the
server, it could redirect clients to it using this suboption. This
would allow such a system to later deny renew requests and thus force
clients to discontinue use of their allocated address. 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. Either DHCP Authentication [3]
or DHCP Relay Agent option authentication [4] would address this
case.
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4. IANA Considerations
None.
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5. Intellectual Property Rights and Copyright
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights."
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
6. References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[3] Droms, R., "Authentication for DHCP Messages", RFC 3118,
June 2001.
[4] Stapp, M., "The Authentication Suboption for the DHCP Relay
Agent Option", RFC 4030, March 2005.
[5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
[6] Patrick, M., "DHCP Relay Agent Information Option", RFC 3096,
November 2004.
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Authors' Addresses
Richard A. Johnson
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
US
Phone: +1 408 526 4000
Email: raj@cisco.com
Jay Kumarasamy
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
US
Phone: +1 408 526 4000
Email: jayk@cisco.com
Kim Kinnear
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
US
Phone: +1 408 526 4000
Email: kkinnear@cisco.com
Mark Stapp
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
US
Phone: +1 408 526 4000
Email: mjs@cisco.com
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This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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