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IETF RFC 5010
The Dynamic Host Configuration Protocol Version 4 (DHCPv4) Relay Agent Flags Suboption
Last modified on Thursday, September 6th, 2007
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Network Working Group K. Kinnear
Request for Comments: 5010 M. Normoyle
Category: Standards Track M. Stapp
Cisco Systems, Inc.
September 2007
The Dynamic Host Configuration Protocol Version 4 (DHCPv4)
Relay Agent Flags 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.
Abstract
This memo defines a new suboption of the Dynamic Host Configuration
Protocol (DHCP) relay agent information option that allows the DHCP
relay to specify flags for the forwarded packet. One flag is defined
to indicate whether the DHCP relay received the packet via a unicast
or broadcast packet. This information may be used by the DHCP server
to better serve clients based on whether their request was originally
broadcast or unicast.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Terminology . . . . . . . . . . . . . . . . . . . 2
3. The Flags Suboption . . . . . . . . . . . . . . . . . . . . . . 3
4. DHCP Relay Agent Behavior . . . . . . . . . . . . . . . . . . . 3
5. DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . . 5
Kinnear, et al. Standards Track PAGE 1 
RFC 5010 Relay Agent Flags Suboption September 2007
1. Introduction
Any time a client's DHCP packet is broadcast, a local DHCP relay will
process its request and forward it on to the DHCP server. When the
DHCP relay performs this function, it can be configured to use the
DHCP relay agent information option to forward additional information
to the DHCP server, which the server may then use to alter its
processing algorithms. Once the lease has been granted, however,
future DHCP DHCPREQUEST/RENEWAL messages are unicast directly to the
DHCP Server [RFC 2131] [RFC 2132] [RFC 3046].
In general, DHCP servers may also make subtle (and sometimes not so
subtle) changes in their processing algorithms depending on whether
or not the DHCP server received the message as a unicast packet from
the DHCP client directly, a broadcast packet from the DHCP client on
a locally connected network, or a unicast packet from a DHCP Relay
Agent, which has forwarded on a packet broadcast from a DHCP client
connected to a network local to the DHCP Relay Agent.
In some situations, DHCP Clients may unicast their DHCPREQUEST/RENEW
packets to the DHCP Relay Agent, which will forward the packet on to
the DHCP server. In these cases, the DHCP server cannot tell whether
the packet was broadcast or unicast by the DHCP client, and so it may
be unable to process the DHCP client packets in the manner that it
would if it knew whether the original DHCP packet was broadcast or
unicast. For example, a server might be willing to NAK a client in
the REBINDING state based on a determination that the client's
address does not match its location in the network, but might not be
willing to do so if the client is in the RENEWING state.
The purpose of the suboption described in this document is to allow
the DHCP relay to specify flags for the forwarded packet. These
flags can be used to describe DHCP client attributes that are useful
to the DHCP server, but can only be detected by the local DHCP relay.
The DHCP server can use the information provided by the DHCP relay to
improve its processing algorithms.
One flag is defined to indicate whether the DHCP relay received the
packet via a unicast or broadcast packet. This allows the DHCP
server to know if a packet forwarded on by a DHCP Relay Agent was
broadcast or unicast to the DHCP Relay Agent.
2. Requirements Terminology
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].
Kinnear, et al. Standards Track PAGE 2
RFC 5010 Relay Agent Flags Suboption September 2007
3. The Flags Suboption
The Flags suboption provides an extensible suboption definition for
several possible flags. The first flag defined is the unicast flag.
The format of the suboption is:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Length | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code The suboption code (10).
Length The suboption length, 1 octet.
Flags The Relay Agent flags for this forwarded packet.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|U| MBZ |
+-+-+-+-+-+-+-+-+
U: UNICAST flag
unicast = 1
broadcast = 0
MBZ: MUST BE ZERO (reserved for future use)
4. DHCP Relay Agent Behavior
A DHCP relay agent that claims to conform to this specification MUST
include this suboption in every Relay Agent Information Option
[RFC 3046] it adds to a forwarded DHCP request. In this way, the DHCP
server can distinguish a request forwarded from a DHCP relay agent
that does not support the relay-agent-flags suboption from a request
forwarded by a DHCP relay agent that supports the relay-agent-flags
suboption, and which received the request that is being forwarded in
a broadcast packet.
To put this another way, A DHCP relay agent that supports the relay-
agent-flags suboption MUST always include it in every relay-agent-
information-option that it inserts into packets that it forwards on
to the DHCP server, whether the packet it is forwarding was received
as a broadcast or as a unicast. This is because the DHCP server will
Kinnear, et al. Standards Track PAGE 3
RFC 5010 Relay Agent Flags Suboption September 2007
be dealing with DHCP relay agents that support the relay-agent-flags
suboption as well as DHCP relay agents that do not support the relay-
agent-flags suboption.
5. DHCP Server Behavior
This option provides additional information to the DHCP server. The
DHCP server MAY use this information to make processing decisions
regarding the DHCP Client's packet that it is processing. For
instance, knowledge of the broadcast or unicast reception of a packet
by a DHCP relay agent could be used when making the processing
decisions required to implement Load Balancing [RFC 3074]. A load-
balancing server may be willing to respond to a REBINDING client, but
the server cannot determine the client's state without this
additional indication.
The option length is one octet. If the DHCP server receives a relay-
agent-flags suboption that is longer than one octet, it MUST evaluate
the first octet.
Note to implementors: In specifying the behavior of new flags bits in
the future, careful attention must be paid to compatibility with
earlier implementations. If additional flags values are defined in
the future, it will not always be possible to distinguish between
messages from relay agents who understand the new value and set its
value to 'zero', and relay agents who are simply setting a series of
unassigned bits to 'zero'. It would be a mistake to specify
significant behavior changes based on 'zero' values of flags
specified in the future.
6. Security Considerations
Message authentication in DHCP for intradomain use, where the out-of-
band exchange of a shared secret is feasible, is defined in
[RFC 3118]. Potential exposures to attack are discussed in Section 7
of the DHCP protocol specification in [RFC 2131].
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 [RFC 4030] SHOULD be
deployed as well.
Kinnear, et al. Standards Track PAGE 4
RFC 5010 Relay Agent Flags Suboption September 2007
7. IANA Considerations
IANA has assigned a suboption number (10) for the Flags Suboption
from the DHCP Relay Agent Information Option [RFC 3046] suboption
number space.
8. Acknowledgements
Thanks to David Hankins for realizing the problems created by the
server-id-override option document and for helping us understand the
value of finally solving this problem in a way that has general
applicability.
9. References
9.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC 2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC 3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, January 2001.
[RFC 3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
Messages", RFC 3118, June 2001.
[RFC 4030] Stapp, M. and T. Lemon, "The Authentication Suboption for
the Dynamic Host Configuration Protocol (DHCP) Relay Agent
Option", RFC 4030, March 2005.
9.2. Informative References
[RFC 3074] Volz, B., Gonczi, S., Lemon, T., and R. Stevens, "DHC Load
Balancing Algorithm", RFC 3074, February 2001.
Kinnear, et al. Standards Track PAGE 5
RFC 5010 Relay Agent Flags Suboption September 2007
Authors' Addresses
Kim Kinnear
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
US
Phone: +1 978 936 0000
EMail: kkinnear@cisco.com
Marie Normoyle
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
US
Phone: +1 978 936 0000
EMail: mnormoyle@cisco.com
Mark Stapp
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
US
Phone: +1 978 936 0000
EMail: mjs@cisco.com
Kinnear, et al. Standards Track PAGE 6
RFC 5010 Relay Agent Flags Suboption September 2007
Full Copyright Statement
Copyright © The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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Kinnear, et al. Standards Track PAGE 7
The Dynamic Host Configuration Protocol Version 4 (DHCPv4) Relay Agent Flags Suboption
RFC TOTAL SIZE: 13834 bytes
PUBLICATION DATE: Thursday, September 6th, 2007
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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