DHCP Server Identifier Override Suboption
draft-ietf-dhc-server-override-05

Network Working Group                                         R. Johnson
Internet-Draft                                             J. Jumarasamy
Expires: May 19, 2008                                         K. Kinnear
                                                                M. Stapp
                                                     Cisco Systems, Inc.
                                                       November 16, 2007

               DHCP Server Identifier Override Suboption
                 draft-ietf-dhc-server-override-05.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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Abstract

   This memo defines a new suboption of the DHCP relay information
   option which allows the DHCP relay to specify a new value for the
   Server Identifier option, which is inserted by the DHCP Server.  This
   allows the DHCP relay to act as the actual DHCP server such that
   RENEW DHCPREQUESTs will come to the relay instead of going to the
   server directly.  This gives the relay the opportunity to include the
   Relay Agent option with appropriate suboptions even on DHCP RENEW
   messages.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Server Identifier Override Suboption Definition  . . . . . . .  5
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   6.  Intellectual Property Rights and Copyright . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12

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1.  Introduction

   There are many situations where the DHCP relay is involved and can
   insert a relay agent option [3] with appropriate suboptions easily
   into DHCP DISCOVER messages.  Once the lease has been granted,
   however, future DHCP RENEWAL messages are sent directly to the DHCP
   Server as specified in the Server Identifier option.  This means that
   the relay may not see the DHCP RENEWAL messages (depending upon
   network topology) and thus can not provide the same relay agent
   option information in the RENEWAL messages.

   This new DHCP relay agent suboption, Server Identifier override,
   allows the relay to tell the DHCP server what value to place into the
   Server Identifier option [5].  Using this, the relay agent can force
   RENEWAL messages to come to it instead of the server.  The relay may
   then insert the relay agent option with appropriate suboptions and
   relay the DHCPREQUEST to the actual server.  In this fashion the DHCP
   server will be provided with the same relay agent information upon
   renewals (such as Circuit-ID, Remote-ID, Device Class, etc.) as was
   provided in the initial DISCOVER message.  In effect, this makes a
   RENEWAL into a REBINDING.

   This new suboption could also be used by the DHCP relay in order to
   allow the relay to appear as the actual DHCP server to the client.
   This has the advantage that the relay can more easily keep up-to-date
   information about leases granted, etc.

   In short, this new suboption allows the DHCPv4 relay to function in
   the same fashion as the DHCPv6 relay [7] currently does.

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2.  Conventions

   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 [1].

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3.  Server Identifier Override Suboption Definition

   The format of the suboption is:

   Code   Len    Overriding Server Identifier address
   +-----+-----+-----+-----+-----+-----+
   | TBD |  n  | a1  | a2  | a3  | a4  |
   +-----+-----+-----+-----+-----+-----+

                                 Figure 1

   The option length (n) is 4.  The octets "a1" through "a4" specify the
   value which MUST be inserted into the Server Identifier option by the
   DHCP Server upon reply.

   DHCP Servers which implement this Relay Suboption MUST use this
   value, if present, as the value to insert into the Server Identifier
   option whenever responding to a DHCP Client.

   If a DHCP Server does not understand/implement this Relay Suboption,
   it will ignore the Suboption, and thus will insert it's own
   appropriate interface address as the Server Identifier address.  In
   this case, the DHCP Relay will not receive RENEW DHCPREQUEST packets
   from the client.  When configuring a DHCP Relay to use this
   Suboption, the administrator of the Relay should take into account
   whether or not the DHCP Server to which the packet will be relayed
   will correctly understand this Suboption.

   When servicing a DHCPREQUEST packet the DHCP Server would normally
   look at the Server Identifier option for verification that the
   address specified there is one of the addresses associated with the
   DHCP Server, silently ignoring the DHCPREQUEST if it does not match a
   configured DHCP Server interface address.  If the DHCPREQUEST packet
   contains a Server Identifier Override Suboption, however, comparison
   should be made between this suboption and the Server Identifier
   option.  If both of the Server Identifier Override Suboption and the
   Server Identifier Option specify the same address, then the Server
   should accept the DHCPREQUEST packet for processing, regardless of
   whether or not the Server Identifier Option matchs a DHCP Server
   interface.

   The DHCP Relay should fill in the giaddr field when relaying the
   packet just as it normally would do.

   In a situation where the DHCP Relay is configured to forward packets
   to more than one server, the DHCP Relay should forward all DHCP
   packets to all servers.  This applies to DHCP RENEW packets as well.

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   The intent is that the DHCP Relay should not need to maintain state
   information about the DHCP lease.

   DHCP Relays using this suboption SHOULD also implement and use the
   DHCPv4 Relay Agent Flags Suboption [4] in order to specify whether
   the DHCP Relay received the original packet as a broadcast or
   unicast.  The DHCP Server receiving a packet containing the Server
   Identifier Override Suboption may use this additional information in
   processing the packet.

   Note that if the DHCP Relay becomes inaccessible by the DHCP Client
   or loses network access to the DHCP Server, further DHCP RENEW
   packets from the DHCP Client may not be properly processed and the
   DHCP Client's lease may time out.

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4.  Security Considerations

   Message authentication in DHCP for intradomain use where the out-of-
   band exchange of a shared secret is feasible is defined in [6].
   Potential exposures to attack are discussed in section 7 of the DHCP
   protocol specification in [2].

   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 [8] SHOULD be deployed as well.

   If a rogue DHCP relay were inserted between the client and the
   server, it could redirect clients to it using this suboption.  This
   would allow such a system to later deny RENEW DHCPREQUEST and thus
   force clients to discontinue use of their allocated address.  It
   could also allow the rogue relay to change, insert, or delete DHCP
   options in DHCPACK messages and extend leases beyond what the server
   has allowed.  This interception, however, would need to be done
   during the initial DISCOVER and OFFER phase, since the suboption
   value SHOULD be ignored by the server during RENEWAL state.  DHCP
   Authentication [6] and/or DHCP Relay Agent option authentication [8]
   would address this case.  (Note that, as is always the case, lack of
   DHCP Authentication would allow a rogue DHCP relay to change the
   Server-ID option in the DHCPOFFER and DHCPACK packets without
   detection.  This threat is not new to the Server-ID-Override
   suboption.)

   This draft does not add any new vulnerabilities that were not already
   present, except in the case where DHCP authentication is already in
   place and DHCP clients require its use.  It is suggested that DHCP
   Authentication and DHCP Relay Agent Option Authentication SHOULD be
   deployed when this option is used, or protection should be provided
   against the insertion of rogue DHCP relays and server.

   This relay sub-option is not intended, by itself, to provide any
   additional security benefits.

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5.  IANA Considerations

   IANA is requested to assign a suboption number for the Server
   Identifier Override Suboption from the DHCP Relay Agent Information
   Option [3] suboption number space.

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6.  Intellectual Property Rights and Copyright

   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
   rights.

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7.  References

7.1.  Normative References

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

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

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

   [4]  Kinnear, K., Normoyle, M., and M. Stapp, "The Dynamic Host
        Configuration Protocol Version 4 (DHCPv4) Relay Agent Flags
        Suboption", RFC 5010, September 2007.

7.2.  Informative References

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

   [6]  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
        RFC 3118, June 2001.

   [7]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
        Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
        RFC 3315, July 2003.

   [8]  Stapp, M. and T. Lemon, "The Authentication Suboption for the
        Dynamic Host Configuration Protocol (DHCP) Relay Agent Option",
        RFC 4030, March 2005.

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Authors' Addresses

   Richard A. Johnson
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA  95134
   US

   Phone: +1 408 526 4000
   Email: raj@cisco.com

   Jay Kumarasamy
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA  95134
   US

   Phone: +1 408 526 4000
   Email: jayk@cisco.com

   Kim Kinnear
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA  95134
   US

   Phone: +1 408 526 4000
   Email: kkinnear@cisco.com

   Mark Stapp
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA  95134
   US

   Phone: +1 408 526 4000
   Email: mjs@cisco.com

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