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RFC 3046 - DHCP Relay Agent Information Option

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Network Working Group                                         M. Patrick
Request for Comments: 3046                                  Motorola BCS
Category: Standards Track                                   January 2001

                  DHCP Relay Agent Information Option

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 Internet Society (2001).  All Rights Reserved.


   Newer high-speed public Internet access technologies call for a
   high-speed modem to have a local area network (LAN) attachment to one
   or more customer premise hosts.  It is advantageous to use the
   Dynamic Host Configuration Protocol (DHCP) as defined in RFC 2131 to
   assign customer premise host IP addresses in this environment.
   However, a number of security and scaling problems arise with such
   "public" DHCP use.  This document describes a new DHCP option to
   address these issues.  This option extends the set of DHCP options as
   defined in RFC 2132.

   The new option is called the Relay Agent Information option and is
   inserted by the DHCP relay agent when forwarding client-originated
   DHCP packets to a DHCP server.  Servers recognizing the Relay Agent
   Information option may use the information to implement IP address or
   other parameter assignment policies.  The DHCP Server echoes the
   option back verbatim to the relay agent in server-to-client replies,
   and the relay agent strips the option before forwarding the reply to
   the client.

   The "Relay Agent Information" option is organized as a single DHCP
   option that contains one or more "sub-options" that convey
   information known by the relay agent.  The initial sub-options are
   defined for a relay agent that is co-located in a public circuit
   access unit.  These include a "circuit ID" for the incoming circuit,
   and a "remote ID" which provides a trusted identifier for the remote
   high-speed modem.

Table of Contents

   1   Introduction...........................................  2
   1.1 High-Speed Circuit Switched Data Networks..............  2
   1.2 DHCP Relay Agent in the Circuit Access Equipment.......  4
   2.0 Relay Agent Information Option.........................  5
   2.1 Agent Operation........................................  6
   2.1.1 Reforwarded DHCP requests............................  7
   2.2 Server Operation.......................................  7
   3.0 Relay Agent Information Suboptions.....................  8
   3.1 Agent Circuit ID.......................................  8
   3.2 Agent Remote ID........................................  9
   4.0 Issues Resolved........................................  9
   5.0 Security Considerations................................ 10
   6.0 IANA Considerations.................................... 11
   7.0 Intellectual Property Notice........................... 12
   8.0 References............................................. 12
   9.0 Glossary............................................... 13
   10.0 Author's Address...................................... 13
   11.0 Full Copyright Statement ............................. 14

1   Introduction

1.1 High-Speed Circuit Switched Data Networks

   Public Access to the Internet is usually via a circuit switched data
   network.  Today, this is primarily implemented with dial-up modems
   connecting to a Remote Access Server.  But higher speed circuit
   access networks also include ISDN, ATM, Frame Relay, and Cable Data
   Networks.  All of these networks can be characterized as a "star"
   topology where multiple users connect to a "circuit access unit" via
   switched or permanent circuits.

   With dial-up modems, only a single host PC attempts to connect to the
   central point.  The PPP protocol is widely used to assign IP
   addresses to be used by the single host PC.

   The newer high-speed circuit technologies, however, frequently
   provide a LAN interface (especially Ethernet) to one or more host
   PCs.  It is desirable to support centralized assignment of the IP
   addresses of host computers connecting on such circuits via DHCP.
   The DHCP server can be, but usually is not, co-implemented with the
   centralized circuit concentration access device.  The DHCP server is
   often connected as a separate server on the "Central LAN" to which
   the central access device (or devices) attach.

   A common physical model for high-speed Internet circuit access is
   shown in Figure 1, below.

                   +---------------+                          |
     Central       |   Circuit     |-- ckt 1--- Modem1-- Host-|- Host A
     LAN     |     |   Access      |                     Lan  |- Host B
             |     |   Unit 1      |                          |- Host C
             |-----|               |--                        |
             |     |(relay agent)  |...
+---------+  |     +---------------+
|  DHCP   |--|
| Server  |  |
+---------+  |
             |     +---------------+
+---------+  |     |   Circuit     |-- ckt 1--- Modem2-- Host--- Host D
| Other   |  |     |   Access      |                     Lan
| Servers |--|-----|   Unit 2      |
|  (Web,  |  |     |               |-- ckt 2--- Modem3-- Host--- Host E
|   DNS)  |  |     |(relay agent)  |...                  Lan
|         |        +---------------+

         Figure 1:  DHCP High Speed Circuit Access Model

   Note that in this model, the "modem" connects to a LAN at the user
   site, rather than to a single host.  Multiple hosts are implemented
   at this site.  Although it is certainly possible to implement a full
   IP router at the user site, this requires a relatively expensive
   piece of equipment (compared to typical modem costs).  Furthermore, a
   router requires an IP address not only for every host, but for the
   router itself.  Finally, a user-side router requires a dedicated
   Logical IP Subnet (LIS) for each user.  While this model is
   appropriate for relatively small corporate networking environments,
   it is not appropriate for large, public accessed networks.  In this
   scenario, it is advantageous to implement an IP networking model that
   does not allocate an IP address for the modem (or other networking
   equipment device at the user site), and especially not an entire LIS
   for the user side LAN.

   Note that using this method to obtain IP addresses means that IP
   addresses can only be obtained while communication to the central
   site is available.  Some host lan installations may use a local DHCP
   server or other methods to obtain IP addresses for in-house use.

1.2 DHCP Relay Agent in the Circuit Access Unit

   It is desirable to use DHCP to assign the IP addresses for public
   high-speed circuit access.  A number of circuit access units (e.g.,
   RAS's, cable modem termination systems, ADSL access units, etc)
   connect to a LAN (or local internet) to which is attached a DHCP

   For scaling and security reasons, it is advantageous to implement a
   "router hop" at the circuit access unit, much like high-capacity
   RAS's do today.  The circuit access equipment acts as both a router
   to the circuits and as the DHCP relay agent.

   The advantages of co-locating the DHCP relay agent with the circuit
   access equipment are:

   DHCP broadcast replies can be routed to only the proper circuit,
   avoiding, say, the replication of the DCHP reply broadcast onto
   thousands of access circuits;

   The same mechanism used to identify the remote connection of the
   circuit (e.g., a user ID requested by a Remote Access Server acting
   as the circuit access equipment) may be used as a host identifier by
   DHCP, and used for parameter assignment.  This includes centralized
   assignment of IP addresses to hosts.  This provides a secure remote
   ID from a trusted source -- the relay agent.

   A number of issues arise when forwarding DHCP requests from hosts
   connecting publicly accessed high-speed circuits with LAN connections
   at the host.  Many of these are security issues arising from DHCP
   client requests from untrusted sources.  How does the relay agent
   know to which circuit to forward replies?  How does the system
   prevent  DHCP IP exhaustion attacks?  This is when an attacker
   requests all available IP addresses from a DHCP server by sending
   requests with fabricated client MAC addresses.  How can an IP address
   or LIS be permanently assigned to a particular user or modem?  How
   does one prevent "spoofing" of client identifier fields used to
   assign IP addresses?  How does one prevent denial of service by
   "spoofing" other client's MAC addresses?

   All of these issues may be addressed by having the circuit access
   equipment, which is a trusted component, add information to DHCP
   client requests that it forwards to the DHCP server.

2.0 Relay Agent Information Option

   This document defines a new DHCP Option called the Relay Agent
   Information Option.  It is a "container" option for specific agent-
   supplied sub-options.  The format of the Relay Agent Information
   option is:

          Code   Len     Agent Information Field
         |  82  |   N  |  i1  |  i2  |  i3  |  i4  |      |  iN  |

   The length N gives the total number of octets in the Agent
   Information Field.  The Agent Information field consists of a
   sequence of SubOpt/Length/Value tuples for each sub-option, encoded
   in the following manner:

          SubOpt  Len     Sub-option Value
         |  1   |   N  |  s1  |  s2  |  s3  |  s4  |      |  sN  |
          SubOpt  Len     Sub-option Value
         |  2   |   N  |  i1  |  i2  |  i3  |  i4  |      |  iN  |

   No "pad" sub-option is defined, and the Information field shall NOT
   be terminated with a 255 sub-option.  The length N of the DHCP Agent
   Information Option shall include all bytes of the sub-option
   code/length/value tuples.  Since at least one sub-option must be
   defined, the minimum Relay Agent Information length is two (2).  The
   length N of the sub-options shall be the number of octets in only
   that sub-option's value field.  A sub-option length may be zero.  The
   sub-options need not appear in sub-option code order.

   The initial assignment of DHCP Relay Agent Sub-options is as follows:

                 DHCP Agent              Sub-Option Description
                 Sub-option Code
                 ---------------         ----------------------
                     1                   Agent Circuit ID Sub-option
                     2                   Agent Remote ID Sub-option

2.1 Agent Operation

   Overall adding of the DHCP relay agent option SHOULD be configurable,
   and SHOULD be disabled by default.  Relay agents SHOULD have separate
   configurables for each sub-option to control whether it is added to
   client-to-server packets.

   A DHCP relay agent adding a Relay Agent Information field SHALL add
   it as the last option (but before 'End Option' 255, if present) in
   the DHCP options field of any recognized BOOTP or DHCP packet
   forwarded from a client to a server.

   Relay agents receiving a DHCP packet from an untrusted circuit with
   giaddr set to zero (indicating that they are the first-hop router)
   but with a Relay Agent Information option already present in the
   packet SHALL discard the packet and increment an error count.  A
   trusted circuit may contain a trusted downstream (closer to client)
   network element (bridge) between the relay agent and the client that
   MAY add a relay agent option but not set the giaddr field.  In this
   case, the relay agent does NOT add a "second" relay agent option, but
   forwards the DHCP packet per normal DHCP relay agent operations,
   setting the giaddr field as it deems appropriate.

   The mechanisms for distinguishing between "trusted" and "untrusted"
   circuits are specific to the type of circuit termination equipment,
   and may involve local administration.  For example, a Cable Modem
   Termination System may consider upstream packets from most cable
   modems as "untrusted", but an ATM switch terminating VCs switched
   through a DSLAM may consider such VCs as "trusted" and accept a relay
   agent option added by the DSLAM.

   Relay agents MAY have a configurable for the maximum size of the DHCP
   packet to be created after appending the Agent Information option.
   Packets which, after appending the Relay Agent Information option,
   would exceed this configured maximum size shall be forwarded WITHOUT
   adding the Agent Information option.  An error counter SHOULD be
   incremented in this case.  In the absence of this configurable, the
   agent SHALL NOT increase a forwarded DHCP packet size to exceed the
   MTU of the interface on which it is forwarded.

   The Relay Agent Information option echoed by a server MUST be removed
   by either the relay agent or the trusted downstream network element
   which added it when forwarding a server-to-client response back to
   the client.

   The agent SHALL NOT add an "Option Overload" option to the packet or
   use the "file" or "sname" fields for adding Relay Agent Information
   option.  It SHALL NOT parse or remove Relay Agent Information options
   that may appear in the sname or file fields of a server-to-client
   packet forwarded through the agent.

   The operation of relay agents for specific sub-options is specified
   with that sub-option.

   Relay agents are NOT required to monitor or modify client-originated
   DHCP packets addressed to a server unicast address.  This  includes
   the DHCP-REQUEST sent when entering the RENEWING state.

   Relay agents MUST NOT modify DHCP packets that use the IPSEC
   Authentication Header or IPSEC Encapsulating Security Payload [6].

2.1.1 Reforwarded DHCP requests

   A DHCP relay agent may receive a client DHCP packet forwarded from a
   BOOTP/DHCP relay agent closer to the client.  Such a packet will have
   giaddr as non-zero, and may or may not already have a DHCP Relay
   Agent option in it.

   Relay agents configured to add a Relay Agent option which receive a
   client DHCP packet with a nonzero giaddr SHALL discard the packet if
   the giaddr spoofs a giaddr address implemented by the local agent

   Otherwise, the relay agent SHALL forward any received DHCP packet
   with a valid non-zero giaddr WITHOUT adding any relay agent options.
   Per RFC 2131, it shall also NOT modify the giaddr value.

2.2 Server Operation

   DHCP servers unaware of the Relay Agent Information option will
   ignore the option upon receive and will not echo it back on
   responses.  This is the specified server behavior for unknown

   DHCP servers claiming to support the Relay Agent Information option
   SHALL echo the entire contents of the Relay Agent Information option
   in all replies.  Servers SHOULD copy the Relay Agent Information
   option as the last DHCP option in the response.  Servers SHALL NOT
   place the echoed Relay Agent Information option in the overloaded
   sname or file fields.  If a server is unable to copy a full Relay
   Agent Information field into a response, it SHALL send the response
   without the Relay Information Field, and SHOULD increment an error
   counter for the situation.

   The operation of DHCP servers for specific sub-options is specified
   with that sub-option.

   Note that DHCP relay agents are not required to monitor unicast DHCP
   messages sent directly between the client and server (i.e., those
   that aren't sent via a relay agent).  However, some relay agents MAY
   chose to do such monitoring and add relay agent options.
   Consequently, servers SHOULD be prepared to handle relay agent
   options in unicast messages, but MUST NOT expect them to always be

3.0 Relay Agent Information Sub-options

3.1 Agent Circuit ID Sub-option

   This sub-option MAY be added by DHCP relay agents which terminate
   switched or permanent circuits.  It encodes an agent-local identifier
   of the circuit from which a DHCP client-to-server packet was
   received.  It is intended for use by agents in relaying DHCP
   responses back to the proper circuit.  Possible uses of this field

       - Router interface number
       - Switching Hub port number
       - Remote Access Server port number
       - Frame Relay DLCI
       - ATM virtual circuit number
       - Cable Data virtual circuit number

   Servers MAY use the Circuit ID for IP and other parameter assignment
   policies.  The Circuit ID SHOULD be considered an opaque value, with
   policies based on exact string match only; that is, the Circuit ID
   SHOULD NOT be internally parsed by the server.

   The DHCP server SHOULD report the Agent Circuit ID value of current
   leases in statistical reports (including its MIB) and in logs.  Since
   the Circuit ID is local only to a particular relay agent, a circuit
   ID should be qualified with the giaddr value that identifies the
   relay agent.

          SubOpt   Len     Circuit ID
         |  1   |   n  |  c1  |  c2  |  c3  |  c4  |  c5  |  c6  | ...

3.2 Agent Remote ID Sub-option

   This sub-option MAY be added by DHCP relay agents which terminate
   switched or permanent circuits and have mechanisms to identify the
   remote host end of the circuit.  The Remote ID field may be used to
   encode, for instance:

       -- a "caller ID" telephone number for dial-up connection
       -- a "user name" prompted for by a Remote Access Server
       -- a remote caller ATM address
       -- a "modem ID" of a cable data modem
       -- the remote IP address of a point-to-point link
       -- a remote X.25 address for X.25 connections

   The remote ID MUST be globally unique.

   DHCP servers MAY use this option to select parameters specific to
   particular users, hosts, or subscriber modems.  The option SHOULD be
   considered an opaque value, with policies based on exact string match
   only; that is, the option SHOULD NOT be internally parsed by the

   The relay agent MAY use this field in addition to or instead of the
   Agent Circuit ID field to select the circuit on which to forward the
   DHCP reply (e.g., Offer, Ack, or Nak).  DHCP servers SHOULD report
   this value in any reports or MIBs associated with a particular

          SubOpt   Len     Agent Remote ID
         |  2   |   n  |  r1  |  r2  |  r3  |  r4  |  r5  |  r6  | ...

4.0 Issues Resolved

   The DHCP relay agent option resolves several issues in an environment
   in which untrusted hosts access the internet via a circuit based
   public network.  This resolution assumes that all DHCP protocol
   traffic by the public hosts traverse the DHCP relay agent and that
   the IP network between the DHCP relay agent and the DHCP server is

   Broadcast Forwarding

      The circuit access equipment forwards the normally broadcasted
      DHCP response only on the circuit indicated in the Agent Circuit

   DHCP Address Exhaustion

      In general, the DHCP server may be extended to maintain a database
      with the "triplet" of

            (client IP address,  client MAC address,  client remote ID)

      The DHCP server SHOULD implement policies that restrict the number
      of IP addresses to be assigned to a single remote ID.

   Static Assignment

      The DHCP server may use the remote ID to select the IP address to
      be assigned.  It may permit static assignment of IP addresses to
      particular remote IDs, and disallow an address request from an
      unauthorized remote ID.

   IP Spoofing

      The circuit access device may associate the IP address assigned by
      a DHCP server in a forwarded DHCP Ack packet with the circuit to
      which it was forwarded.  The circuit access device MAY prevent
      forwarding of IP packets with source IP addresses -other than-
      those it has associated with the receiving circuit.  This prevents
      simple IP spoofing attacks on the Central LAN, and IP spoofing of
      other hosts.

   Client Identifier Spoofing

      By using the agent-supplied Agent Remote ID option, the untrusted
      and as-yet unstandardized client identifier field need not be used
      by the DHCP server.

   MAC Address Spoofing

      By associating a MAC address with an Agent Remote ID, the DHCP
      server can prevent offering an IP address to an attacker spoofing
      the same MAC address on a different remote ID.

5.0 Security Considerations

   DHCP as currently defined provides no authentication or security
   mechanisms.  Potential exposures to attack are discussed in section 7
   of the DHCP protocol specification in RFC 2131 [1].

   This document introduces mechanisms to address several security
   attacks on the operation of IP address assignment, including IP
   spoofing, Client ID spoofing, MAC address spoofing, and DHCP server

   address exhaustion.  It relies on an implied trusted relationship
   between the DHCP Relay Agent and the DHCP server, with an assumed
   untrusted DHCP client.  It introduces a new identifer, the "Remote
   ID", that is also assumed to be trusted.  The Remote ID is provided
   by the access network or modem and not by client premise equipment.
   Cryptographic or other techniques to authenticate the remote ID are
   certainly possible and encouraged, but are beyond the scope of this

   This option is targeted towards environments in which the network
   infrastructure -- the relay agent, the DHCP server, and the entire
   network in which those two devices reside -- is trusted and secure.
   As used in this document, the word "trusted" implies that
   unauthorized DHCP traffic cannot enter the trusted network except
   through secured and trusted relay agents and that all devices
   internal to the network are secure and trusted.  Potential deployers
   of this option should give careful consideration to the potential
   security vulnerabilities that are present in this model before
   deploying this option in actual networks.

   Note that any future mechanisms for authenticating DHCP client to
   server communications must take care to omit the DHCP Relay Agent
   option from server authentication calculations.  This was the
   principal reason for organizing the DHCP Relay Agent Option as a
   single option with sub-options, and for requiring the relay agent to
   remove the option before forwarding to the client.

   While it is beyond the scope of this document to specify the general
   forwarding algorithm of public data circuit access units, note that
   automatic reforwarding of IP or ARP broadcast packets back downstream
   exposes serious IP security risks.  For example, if an upstream
   broadcast DHCP-DISCOVER or DHCP-REQUEST were re-broadcast back
   downstream, any public host may easily spoof the desired DHCP server.

6.0 IANA Considerations

   IANA is required to maintain a new number space of "DHCP Relay Agent
   Sub-options", located in the BOOTP-DHCP Parameters Registry.  The
   initial sub-options are described in section 2.0 of this document.

   IANA assigns future DHCP Relay Agent Sub-options with a "IETF
   Consensus" policy as described in RFC 2434 [3].  Future proposed
   sub-options are to be referenced symbolically in the Internet-Drafts
   that describe them, and shall be assigned numeric codes by IANA when
   approved for publication as an RFC.

7.0 Intellectual Property Notices

   This section contains two notices as required by [5] for standards
   track documents.

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   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

8.0 References

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

   [2]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
        Extension", RFC 2132, March 1997.

   [3]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

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

   [5]  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
        9, RFC 2026, October 1996.

   [6]  Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

9.0 Glossary

   DSLAM   Digital Subscriber Link Access Multiplexer
   IANA    Internet Assigned Numbers Authority
   LIS     Logical IP Subnet
   MAC     Message Authentication Code
   RAS     Remote Access Server

10.0 Author's Address

   Michael Patrick
   Motorola Broadband Communications Sector
   20 Cabot Blvd., MS M4-30
   Mansfield, MA 02048

   Phone: (508) 261-5707
   EMail: michael.patrick@motorola.com

11.0  Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   or assist in its implementation may be prepared, copied, published
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   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
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   the copyright notice or references to the Internet Society or other
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   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an


   Funding for the RFC Editor function is currently provided by the
   Internet Society.


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