Network Working Group R. Johnson
Internet-Draft J. Jumarasamy
Expires: April 24, 2006 K. Kinnear
M. Stapp
Cisco Systems, Inc.
October 21, 2005
DHCP Server Identifier Override Suboption
draft-ietf-dhc-server-override-03.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 which allows the DHCP relay to specify a new value for the
Server Identifier option, which is inserted by the DHCP Server. This
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
server directly. This gives the relay the opportunity to include the
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Relay Agent option with appropriate suboptions even on DHCP RENEW
messages.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Server Identifier Override Suboption Definition . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . 7
6. Intellectual Property Rights and Copyright . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . 10
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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 Identifier 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 Identifier override,
allows the relay to tell the DHCP server what value to place into the
Server Identifier 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 DHCPREQUEST 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. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
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3. Server Identifier Override Suboption Definition
The format of the suboption is:
Code Len Overriding Server Identifier address
+-----+-----+-----+-----+-----+-----+
| TBD | n | a1 | a2 | a3 | a4 |
+-----+-----+-----+-----+-----+-----+
Figure 1
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
DHCP Server upon reply.
DHCP Servers which implement this Relay Suboption MUST use this
value, if present, as the value to insert into the Server Identifier
option whenever responding to a DHCP Client.
If a DHCP Server does not understand/implement this Relay Suboption,
it will ignore the Suboption, and thus will insert it's own
appropriate interface address as the Server Identifier address. In
this case, the DHCP Relay will not receive RENEW DHCPREQUEST packets
from the client. When configuring a DHCP Relay to use this
Suboption, the administrator of the Relay should take into account
whether or not the DHCP Server to which the packet will be relayed
will correctly understand this Suboption.
When servicing a DHCPREQUEST packet the DHCP Server would normally
look at the Server Identifier option for verification that the
address specified there is one of the addresses associated with the
DHCP Server, silently ignoring the DHCPREQUEST if it does not match a
configured DHCP Server interface address. If the DHCPREQUEST packet
contains a Server Identifier Override Suboption, however, comparison
should be made between this suboption and the Server Identifier
option. If both of the Server Identifier Override Suboption and the
Server Identifier Option specify the same address, then the Server
should accept the DHCPREQUEST packet for processing, regardless of
whether or not the Server Identifier Option matchs a DHCP Server
interface.
The DHCP Relay should fill in the giaddr field when relaying the
packet just as it normally would do.
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4. 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 DHCPREQUEST 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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5. IANA Considerations
IANA is requested to assign a suboption number for the Server
Identifier Override Suboption from the DHCP Relay Agent Information
Option [3] suboption number space. None.
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6. 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.
7. 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 3046,
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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