Network Working Group R. Johnson
Request for Comments: 5107 J. Jumarasamy
Category: Standards Track K. Kinnear
M. Stapp
Cisco Systems, Inc.
January 2008
DHCP Server Identifier Override Suboption
Status of This Memo
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.
Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This memo defines a new suboption of the DHCP relay information
option that 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 . . . . . . . . . . . . . . . . . . . . . . . . . 2 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Server Identifier Override Suboption Definition . . . . . . . . 2
4. 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
5. 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
6. 6
7. Intellectual Property Rights and Copyright . . . . . . . . . . 5
7. 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. 6
8.1. Normative References . . . . . . . . . . . . . . . . . . . 5
7.2. 6
8.2. Informative References . . . . . . . . . . . . . . . . . . 5 6
1. Introduction
There are many situations where the a DHCP relay agent is involved, and
it can easily insert a relay agent Relay Agent Information option [3] with
appropriate suboptions into DHCP DISCOVER DHCPDISCOVER messages. Once the lease
has been granted, however, future DHCP RENEWAL DHCPREQUEST messages sent by a
client in RENEWING state are sent directly to the DHCP Server, server, as
specified in the Server Identifier option. This
means that In this case, the relay
may not see the DHCP RENEWAL these DHCPREQUEST messages (depending upon network
topology) and thus cannot provide insert the same relay agent Relay Agent Information option information
in the RENEWAL DHCPREQUEST messages.
This new DHCP relay agent suboption, Server Identifier override, Override, allows
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
RENEWAL messages
a host in RENEWING state to come send DHCPREQUEST messages to it the relay
agent instead of directly to the server. The relay may agent then has
the opportunity to insert the relay agent Relay Agent Information 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
DHCPDISCOVER 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.
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].
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:
Code Len Overriding Server Identifier Address
+-----+-----+-----+-----+-----+-----+
| 11 | n | a1 | a2 | a3 | a4 |
+-----+-----+-----+-----+-----+-----+
Figure 1
The option length (n) is 4. The octets "a1" through "a4" specify the
value that MUST be inserted into the Server Identifier option by the
DHCP Server upon reply.
DHCP Servers servers that implement this Relay Suboption Agent Information suboption
MUST use this value, if present, present in a DHCP message received from a
client, as the value to insert into the Server Identifier option
whenever responding in
the corresponding response. The DHCP server must also record the
address in the suboption for use in subsequent messages to a the DHCP
client until the next DHCP message is received from the DHCP Client. relay
agent.
If a DHCP Server server does not understand/implement this Relay Suboption, Information
suboption, it will ignore the Suboption, suboption, and thus it will insert it's its
own appropriate interface address as in the Server Identifier address. option.
In this case, the DHCP Relay will not receive RENEW DHCPREQUEST packets
messages from the client. When configuring a DHCP Relay relay agent to use
this
Suboption, suboption, the administrator of the Relay relay agent should take into
account whether or not the DHCP Server server to which the packet message will be
relayed will correctly understand this Suboption. suboption.
When servicing a DHCPREQUEST packet, message, the DHCP Server 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, server, silently ignoring the DHCPREQUEST if it does not match a
configured DHCP Server server interface address. If the DHCPREQUEST packet message
contains a Server Identifier Override Suboption, suboption, however, comparison
should be made between the address in this suboption and the Server
Identifier option. If both the Server Identifier Override Suboption suboption
and the Server Identifier Option option specify the same address, then the Server
server should accept the DHCPREQUEST packet message for processing,
regardless of whether or not the Server Identifier Option option matches a
DHCP Server server interface.
The DHCP Relay relay agent should fill in the giaddr field when relaying
the
packet, message, just as it normally would do.
In a situation where the DHCP Relay relay agent is configured to forward packets
messages to more than one server, the DHCP Relay relay agent SHOULD forward
all DHCP
packets messages to all servers. This applies to DHCP RENEW packets DHCPREQUEST
messages as well. The intent is that the DHCP Relay relay agent should not
need to maintain state information about the DHCP lease.
DHCP Relays using relay agents implementing this suboption SHOULD also implement
and use the DHCPv4 Relay Agent Flags Suboption [4] in order to
specify whether the DHCP Relay relay agent received the original packet message as
a broadcast or unicast. The DHCP Server server receiving a packet message
containing the Server Identifier Override Suboption may use this
additional information in processing the packet. message.
Note that if the DHCP Relay relay agent becomes inaccessible by the DHCP Client
client or loses network access to the DHCP Server, server, further DHCP RENEW
packets
DHCPREQUEST messages from the DHCP Client client may not be properly
processed and the DHCP Client's client's lease may time out.
4.
5. 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 Information option depends on a trusted
relationship between the DHCP relay agent and the DHCP server, as
described in Section 5 of RFC 3046. While the introduction of
fraudulent relay-agent DHCP relay agent information options can be prevented by a
perimeter defense that blocks these options unless the DHCP relay
agent is trusted, a deeper defense using the authentication
option suboption
for DHCP relay agent options information option [8] SHOULD be deployed as
well.
If a rogue DHCP relay agent were inserted between the DHCP client and
the DHCP server, it could redirect clients to it itself using this
suboption. This would allow such a system to later deny RENEW DHCPREQUEST
DHCPREQUESTs and thus force clients to discontinue use of their
allocated address. addresses. 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 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
authentication would allow a rogue DHCP relay agent to change the
Server-ID
Server Identifier Override option in the DHCPOFFER and DHCPACK packets
messages without detection. This threat is not new to the Server-ID-Override 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 relays relay agents between the
client and server.
This relay sub-option suboption is not intended, by itself, to provide any
additional security benefits.
5.
6. IANA Considerations
IANA has assigned a suboption number (11) for the Server Identifier
Override Suboption from the DHCP Relay Agent Information Option [3]
suboption number space.
6.
7. 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.
7.
8. References
7.1.
8.1. 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.
8.2. 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.
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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