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Versions: (draft-johnson-dhc-server-override)
00 01 02 03 04 05 RFC 5107
Network Working Group R. Johnson
Internet-Draft J. Jumarasamy
Expires: May 19, 2008 K. Kinnear
M. Stapp
Cisco Systems, Inc.
November 16, 2007
DHCP Server Identifier Override Suboption
draft-ietf-dhc-server-override-05.txt
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007).
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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
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 . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Intellectual Property Rights and Copyright . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Introduction
There are many situations where the DHCP relay is involved and can
insert a relay agent option [3] 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 [5]. 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 [7] 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 [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.
In a situation where the DHCP Relay is configured to forward packets
to more than one server, the DHCP Relay should forward all DHCP
packets to all servers. This applies to DHCP RENEW packets as well.
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The intent is that the DHCP Relay should not need to maintain state
information about the DHCP lease.
DHCP Relays using this suboption SHOULD also implement and use the
DHCPv4 Relay Agent Flags Suboption [4] in order to specify whether
the DHCP Relay received the original packet as a broadcast or
unicast. The DHCP Server receiving a packet containing the Server
Identifier Override Suboption may use this additional information in
processing the packet.
Note that if the DHCP Relay becomes inaccessible by the DHCP Client
or loses network access to the DHCP Server, further DHCP RENEW
packets from the DHCP Client may not be properly processed and the
DHCP Client's lease may time out.
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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 [6].
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 [8] 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. 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
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 relays and server.
This relay sub-option is not intended, by itself, to provide any
additional security benefits.
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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.
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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.
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7. References
7.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[3] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001.
[4] Kinnear, K., Normoyle, M., and M. Stapp, "The Dynamic Host
Configuration Protocol Version 4 (DHCPv4) Relay Agent Flags
Suboption", RFC 5010, September 2007.
7.2. Informative References
[5] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[6] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[7] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 3315, July 2003.
[8] Stapp, M. and T. Lemon, "The Authentication Suboption for the
Dynamic Host Configuration Protocol (DHCP) Relay Agent Option",
RFC 4030, March 2005.
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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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