The DHCPv4 Relay Agent Identifier Sub-Option

The information below is for an old version of the document that is already published as an RFC
Document Type RFC Internet-Draft (dhc WG)
Authors Bharat Joshi  , D.T.V. Rao  , Mark Stapp 
Last updated 2015-10-14 (latest revision 2013-02-18)
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DHC                                                             B. Joshi
Internet-Draft                                        D. Ramakrishna Rao
Intended status: Standards Track                            Infosys Ltd.
Expires: August 23, 2013                                        M. Stapp
                                                     Cisco Systems, Inc.
                                                       February 19, 2013

              The DHCPv4 Relay Agent Identifier Suboption


   This document defines a new Relay Agent Identifier suboption for the
   Dynamic Host Configuration Protocol's (DHCP) Relay Agent Information
   option.  The suboption 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
   suboption allows a DHCP relay agent to include the identifier in the
   DHCP messages it sends.

Status of this Memo

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   This Internet-Draft will expire on August 23, 2013.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Example Use-Cases . . . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  Bulk Leasequery . . . . . . . . . . . . . . . . . . . . . . 3
     3.2.  Industrial Ethernet . . . . . . . . . . . . . . . . . . . . 3
   4.  Suboption 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  . . . . . . . . . . . . . . . . . . 7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   The Dynamic Host Configuration Protocol for IPv4 (DHCPv4) [RFC2131]
   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 [RFC3046].  Servers that
   recognize the relay agent information option echo it back in their

   This specification introduces a Relay Agent Identifier (Relay-Id)
   suboption for the Relay Agent Information option.  The Relay-Id
   suboption carries a sequence of octets that is intended to uniquely
   identify the relay agent within the administrative domain.  In this
   document, an administrative domain consist of all DHCP servers and
   relay agents that communicate with each other.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   DHCPv4 terminology is defined in [RFC2131], and the DHCPv4 Relay
   Agent Information Option in [RFC3046].

3.  Example Use-Cases

3.1.  Bulk Leasequery

   There has been quite a bit of recent interest in extending the DHCP
   Leasequery protocol [RFC4388] to accommodate some additional
   situations.  There is a recent document
   [I-D.ietf-dhc-dhcpv4-bulk-leasequery] proposing 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 -

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   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.

4.  Suboption Format

   Format of the Relay Agent Identifier suboption:

       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)                       .
      .                                                               .



      length            the number of octets in the suboption
                        (excluding the suboption 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

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   identifier used by a relay device SHOULD be committed to stable
   storage, unless the relay device can regenerate the value upon

   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, 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 has last seen with a lease that is being RENEWed.
   Other deployments may prefer to use the Server Identifier Override
   suboption [RFC5107] 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

   Administrators should 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 re-use an
   existing identifier or set of identifiers that are already guaranteed
   to be unique (e.g., UUID [RFC4122]).

   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

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   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, this 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.

   In some cases it may be that the DHCP server has configurable event
   notification, and that a duplicate relay-id would cause some event
   that could trigger a notification, and that would never happen in any
   other case.  In this scenario, administrators should take advantage
   of this feature.  This is not a perfect solution, because it will not
   work until such an event occurs.

   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.

   This is not an exhaustive list of strategies.  We suggest an
   additional strategy in the security considerations section;
   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, if none of these
   will work.

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 [RFC3046] and
   [RFC4030].  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 suboption for DHCP relay agent information option
   [RFC4030] SHOULD be deployed as well.  It also helps in avoiding
   duplication of relay identifiers by malicious entities.  However,
   implementation of authentication suboption for DHCP relay agent
   information option [RFC4030] is not a must to support relay-id

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6.2.  Factory Floor Scenario

   One possible use case for the relay-id suboption 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

   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.

   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
   the relay-id, that is physically attached to the installation
   location of the relay agent device, and that 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

   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

   We request that IANA assign a new suboption code from the registry of
   DHCP Agent Sub-Option Codes maintained in

      Relay Agent Identifier Suboption [TBA]

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8.  Acknowledgments

   Thanks to Bernie Volz, David W. Hankins, Pavan Kurapati and Ted Lemon
   for providing valuable suggestions.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RFC3046]  Patrick, M., "DHCP Relay Agent Information Option",
              RFC 3046, January 2001.

   [RFC4030]  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

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              July 2005.

   [RFC4388]  Woundy, R. and K. Kinnear, "Dynamic Host Configuration
              Protocol (DHCP) Leasequery", RFC 4388, February 2006.

   [RFC5107]  Johnson, R., Kumarasamy, J., Kinnear, K., and M. Stapp,
              "DHCP Server Identifier Override Suboption", RFC 5107,
              February 2008.

              Kinnear, K., Stapp, M., Joshi, B., and N. Russell, "Bulk
              DHCPv4 Lease Query",
              draft-ietf-dhc-dhcpv4-bulk-leasequery-07 (work in
              progress), October 2012.

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Authors' Addresses

   Bharat Joshi
   Infosys Ltd.
   44 Electronics City, Hosur Road
   Bangalore  560 100


   D.T.V Ramakrishna Rao
   Infosys Ltd.
   44 Electronics City, Hosur Road
   Bangalore  560 100


   Mark Stapp
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719

   Phone: +1 978 936 0000

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