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IETF RFC 6925
Last modified on Monday, April 29th, 2013
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Internet Engineering Task Force (IETF) B. Joshi
Request for Comments: 6925 R. Desetti
Category: Standards Track Infosys Ltd.
ISSN: 2070-1721 M. Stapp
Cisco Systems, Inc.
April 2013
The DHCPv4 Relay Agent Identifier Sub-Option
 Abstract
This document defines a new Relay Agent Identifier sub-option for the
Dynamic Host Configuration Protocol (DHCP) Relay Agent Information
option. The sub-option carries a value that uniquely identifies the
relay agent device within the administrative domain. The value is
normally administratively configured in the relay agent. The sub-
option allows a DHCP relay agent to include the identifier in the
DHCP messages it sends.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6925.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Joshi, et al. Standards Track PAGE 1 
RFC 6925 The Relay Agent ID Sub-Option April 2013
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................2
3. Example Use Cases ...............................................3
3.1. Bulk Leasequery ............................................3
3.2. Industrial Ethernet ........................................3
4. Sub-Option Format ...............................................4
5. Identifier Stability ............................................4
5.1. Identifier Uniqueness ......................................5
6. Security Considerations .........................................6
6.1. Forged Relay ID Attacks ....................................6
6.2. Factory-Floor Scenario .....................................6
7. IANA Considerations .............................................7
8. Acknowledgments .................................................7
9. References ......................................................7
9.1. Normative References .......................................7
9.2. Informative References .....................................8
1. Introduction
The Dynamic Host Configuration Protocol for IPv4 (DHCPv4) [RFC 2131]
provides IP addresses and configuration information for IPv4 clients.
It includes a relay agent capability, in which network elements
receive broadcast messages from clients and forward them to DHCP
servers as unicast messages. In many network environments, relay
agents add information to the DHCP messages before forwarding them,
using the Relay Agent Information option [RFC 3046]. Servers that
recognize the Relay Agent Information option echo it back in their
replies.
This specification introduces a Relay Agent Identifier (Relay-ID)
sub-option for the Relay Agent Information option. The Relay-ID sub-
option carries a sequence of octets that is intended to uniquely
identify the relay agent within the administrative domain. In this
document, an administrative domain consists of all DHCP servers and
relay agents that communicate with each other.
2. 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].
DHCPv4 terminology is defined in [RFC 2131], and the DHCPv4 Relay
Agent Information option is defined in [RFC 3046].
Joshi, et al. Standards Track PAGE 2
RFC 6925 The Relay Agent ID Sub-Option April 2013
3. Example Use Cases
3.1. Bulk Leasequery
There has been quite a bit of recent interest in extending the DHCP
Leasequery protocol [RFC 4388] to accommodate some additional
situations. [RFC 6926] proposes a variety of enhancements to the
existing Leasequery protocol. The document describes a use case
where a relay agent queries DHCP servers using the relay identifier
to retrieve all the leases allocated through the relay agent.
3.2. Industrial Ethernet
DHCP typically identifies clients based on information in their DHCP
messages, such as the Client-Identifier option or the value of the
chaddr field. In some networks, however, the location of a client --
its point of attachment to the network -- is a more useful
identifier. In factory-floor networks (commonly called 'industrial'
networks), for example, the role a device plays is often fixed and
based on its location. Using manual address configuration is
possible (and is common), but it would be beneficial if DHCP
configuration could be applied to these networks.
One way to provide connection-based identifiers for industrial
networks is to have the network elements acting as DHCP relay agents
supply information that a DHCP server could use as a client
identifier. A straightforward way to form identifier information is
to combine something that is unique within the scope of the network
element, such as a port/slot value, with something that uniquely
identifies that network element, such as a Relay Agent Identifier.
Joshi, et al. Standards Track PAGE 3
RFC 6925 The Relay Agent ID Sub-Option April 2013
4. Sub-Option Format
Format of the Relay Agent Identifier sub-option:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|SUBOPT_RELAY_ID| length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
. .
. identifier (variable) .
. .
+---------------------------------------------------------------+
Where:
SUBOPT_RELAY_ID 12
length the number of octets in the sub-option
(excluding the sub-option ID and length fields);
the minimum length is one.
identifier the identifying data
5. Identifier Stability
If the relay identifier is to be meaningful, it has to be stable. A
relay agent SHOULD use a single identifier value consistently. The
identifier used by a relay device SHOULD be committed to stable
storage, unless the relay device can regenerate the value upon
reboot.
If the Relay-ID configured in a relay agent is not unique within its
administrative domain, resource allocation problems may occur as the
DHCP server attempts to allocate the same resource to devices behind
two different relay agents. Therefore, a Relay-ID configured in a
relay agent MUST be unique within its administrative domain. To aid
in ensuring uniqueness of Relay-IDs, relay agents SHOULD make their
relay identifiers visible to their administrators via their user
interface, through a log entry, through a MIB field, or through some
other mechanism.
Implementors of relay agents should note that the identifier needs to
be present in all DHCP message types where its value is being used by
the DHCP server. The relay agent may not be able to add the Relay
Agent Information option to all messages, such as RENEW messages sent
as IP unicasts. In some deployments, that might mean that the server
has to be willing to continue to associate the relay identifier it
Joshi, et al. Standards Track PAGE 4
RFC 6925 The Relay Agent ID Sub-Option April 2013
has last seen with a lease that is being RENEWed. Other deployments
may prefer to use the Server Identifier Override sub-option [RFC 5107]
to permit the relay device to insert the Relay Agent Information
option into all relayed messages.
Handling situations where a relay agent device is replaced is another
aspect of stability. One of the use cases for the relay identifier
is to permit a server to associate clients' lease bindings with the
relay device connected to the clients. If the relay device is
replaced because it has failed or been upgraded, it may be desirable
for the new device to continue to provide the same relay identifier
as the old device. Therefore, if a relay agent supports Relay-ID,
the Relay-ID should be administratively configurable.
5.1. Identifier Uniqueness
It is strongly recommended that administrators take special care to
ensure that Relay-IDs configured in their relay agents are not
duplicated. There are a number of strategies that may be used to
achieve this.
Administrators may use a strategy to configure unique Relay-IDs. One
such strategy is that a Relay-ID on a relay agent may reuse an
existing identifier or set of identifiers that are already guaranteed
to be unique (e.g., Universally Unique Identifier (UUID) [RFC 4122]).
For administrators who are already using a provisioning system to
manage their networking infrastructure, it may work to enumerate
relay agents on the basis of roles and then, as a second step, assign
those roles to specific relay agents or groups of relay agents. In
such a scenario, when a replacement relay agent is first seen by the
DHCP server, it could trigger a configuration event on the
provisioning system, and the new relay agent could be assigned to the
role of the relay agent it is replacing.
It may be that the DHCP server has configurable event notification
and that a duplicate Relay-ID would trigger this notification.
Administrators can take advantage of this feature to work out whether
the duplication is real and unintended or whether the original relay
agent is being replaced.
A network management/provisioning system may also be able to collect
a full list of all relay agents on the network. It may then notice
that more than one device reports the same Relay-ID. In such a case,
the provisioning system could notify the administrator of the fault,
which could then be corrected.
Joshi, et al. Standards Track PAGE 5
RFC 6925 The Relay Agent ID Sub-Option April 2013
This is not an exhaustive list of strategies. We suggest an
additional strategy in the Security Considerations section. If none
of these strategies will work, administrators are also encouraged to
consider the specifics of their own network configuration to see if
there is some way to detect duplicate Relay-IDs other than the ones
listed here.
6. Security Considerations
6.1. Forged Relay ID Attacks
Security issues with the Relay Agent Information option and its use
by servers in address assignment are discussed in [RFC 3046] and
[RFC 4030]. 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 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 sub-option for the DHCP Relay Agent Information option
[RFC 4030] SHOULD be deployed as well. It also helps in avoiding
duplication of relay identifiers by malicious entities. However,
implementation of the authentication sub-option for the DHCP Relay
Agent Information option [RFC 4030] is not a must to support the
Relay-ID sub-option.
6.2. Factory-Floor Scenario
One possible use case for the Relay-ID sub-option is the automated
configuration of machines on a factory floor. In this situation,
various sections of the factory floor might be on their own network
links with a relay agent interposed between those links and the DHCP
server. The Relay-ID of each relay agent might cause special
configurations to be downloaded to those devices to control their
behavior.
If a relay agent was deployed on the factory floor in such a
situation, with an incorrect Relay-ID, there is the potential that
devices could be misconfigured in a way that could produce incorrect
results, cause physical damage, or even create hazardous conditions
for workers.
In deployment scenarios like this one, administrators must use some
dependable technique to ensure that such misconfigurations do not
occur. It is beyond the scope of this document to provide a complete
list of such techniques.
Joshi, et al. Standards Track PAGE 6
RFC 6925 The Relay Agent ID Sub-Option April 2013
However, as an example, a relay agent device intended for use in such
a scenario could require the use of a hardware token that contains a
Relay-ID that is physically attached to the installation location of
the relay agent device and can be connected to and disconnected from
the relay agent device without the use of special tools. Such a
relay agent device should not be operable when this hardware token is
not connected to it: either it should fail because it presents an
unknown identifier to the DHCP server, or it should simply refuse to
relay DHCP packets until the token is connected to it.
A relay agent device that does not provide a clear mitigation
strategy for a scenario where misconfiguration could have damaging or
hazardous consequences should not be deployed in such a scenario.
7. IANA Considerations
IANA has assigned a new sub-option code from the "DHCP Relay Agent
Sub-Option Codes" registry maintained at
http://www.iana.org/assignments/bootp-dhcp-parameters.
Relay Agent Identifier Sub-Option 12
8. Acknowledgments
Thanks to Bernie Volz, David W. Hankins, Pavan Kurapati, and Ted
Lemon for providing valuable suggestions.
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 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
3046, January 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.
Joshi, et al. Standards Track PAGE 7
RFC 6925 The Relay Agent ID Sub-Option April 2013
9.2. Informative References
[RFC 4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122, July
2005.
[RFC 4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration
Protocol (DHCP) Leasequery", RFC 4388, February 2006.
[RFC 5107] Johnson, R., Kumarasamy, J., Kinnear, K., and M. Stapp,
"DHCP Server Identifier Override Suboption", RFC 5107,
February 2008.
[RFC 6926] Kinnear, K., Stapp, M., Desetti, R., Joshi, B., Russell,
N., Kurapati, P., and B. Volz, "DHCPv4 Bulk Leasequery",
RFC 6926, April 2013.
Authors' Addresses
Bharat Joshi
Infosys Ltd.
44 Electronics City, Hosur Road
Bangalore 560 100
India
EMail: bharat_joshi@infosys.com
URI: http://www.infosys.com/
D.T.V Ramakrishna Rao
Infosys Ltd.
44 Electronics City, Hosur Road
Bangalore 560 100
India
EMail: ramakrishnadtv@infosys.com
URI: http://www.infosys.com/
Mark Stapp
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
Phone: +1 978 936 0000
EMail: mjs@cisco.com
Joshi, et al. Standards Track PAGE 8
RFC TOTAL SIZE: 17664 bytes
PUBLICATION DATE: Monday, April 29th, 2013
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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