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RFC 5010 - The Dynamic Host Configuration Protocol Version 4 (DH

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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.


   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

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 [RFC2131] [RFC2132] [RFC3046].

   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",
   document are to be interpreted as described in [RFC2119].

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
   [RFC3046] 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

   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 [RFC3074].  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
   [RFC3118].  Potential exposures to attack are discussed in Section 7
   of the DHCP protocol specification in [RFC2131].

   The DHCP Relay Agent option depends on a trusted relationship between
   the DHCP relay agent and the server, as described in Section 5 of
   [RFC3046].  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 [RFC4030] SHOULD be
   deployed as well.

7.  IANA Considerations

   IANA has assigned a suboption number (10) for the Flags Suboption
   from the DHCP Relay Agent Information Option [RFC3046] 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

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.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.

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

   [RFC3118]  Droms, R. and W. Arbaugh, "Authentication for DHCP
              Messages", RFC 3118, June 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

   [RFC3074]  Volz, B., Gonczi, S., Lemon, T., and R. Stevens, "DHC Load
              Balancing Algorithm", RFC 3074, February 2001.

Authors' Addresses

   Kim Kinnear
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719

   Phone: +1 978 936 0000
   EMail: kkinnear@cisco.com

   Marie Normoyle
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719

   Phone: +1 978 936 0000
   EMail: mnormoyle@cisco.com

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

   Phone: +1 978 936 0000
   EMail: mjs@cisco.com

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