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rfc:rfc5107

Network Working Group R. Johnson Request for Comments: 5107 J. Jumarasamy Category: Standards Track K. Kinnear

                                                              M. Stapp
                                                   Cisco Systems, Inc.
                                                         February 2008
             DHCP Server Identifier Override 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.

Abstract

 This memo defines a new suboption of the DHCP relay information
 option that 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  . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 2
 3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2
 4.  Server Identifier Override Suboption Definition . . . . . . . . 3
 5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
 6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
 7.  Intellectual Property Rights and Copyright  . . . . . . . . . . 5
 8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
   8.2.  Informative References  . . . . . . . . . . . . . . . . . . 5

Johnson, et al. Standards Track [Page 1] RFC 5107 Server ID Override Suboption February 2008

1. Introduction

 There are many situations where a DHCP relay agent is involved, and
 it can easily insert a Relay Agent Information option [3] with
 appropriate suboptions into DHCPDISCOVER messages.  Once the lease
 has been granted, however, future DHCPREQUEST messages sent by a
 client in RENEWING state are sent directly to the DHCP server, as
 specified in the Server Identifier option.  In this case, the relay
 may not see these DHCPREQUEST messages (depending upon network
 topology) and thus cannot insert the Relay Agent Information option
 in the DHCPREQUEST messages.
 This DHCP relay agent suboption, Server Identifier Override, allows
 the relay agent to tell the DHCP server what value to place into the
 Server Identifier option [5].  Using this, the relay agent can force
 a host in RENEWING state to send DHCPREQUEST messages to the relay
 agent instead of directly to the server.  The relay agent then has
 the opportunity to insert the Relay Agent Information 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
 DHCPDISCOVER message.
 In short, this new suboption allows the DHCPv4 relay to function in
 the same fashion as the DHCPv6 relay [7] currently does.

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].

3. Terminology

 This document uses DHCP terminology as defined in section 1.5 of RFC
 2131 [2], with the exception of the term "DHCP relay agent" replacing
 "BOOTP relay agent".
 Other terms used in this document:
 o  RENEW DHCPREQUEST - a DHCPREQUEST message sent by a client in
    RENEWING state

Johnson, et al. Standards Track [Page 2] RFC 5107 Server ID Override Suboption February 2008

4. Server Identifier Override Suboption Definition

 The format of the suboption is:
 Code   Len    Overriding Server Identifier Address
 +-----+-----+-----+-----+-----+-----+
 | 11  |  n  | a1  | a2  | a3  | a4  |
 +-----+-----+-----+-----+-----+-----+
                               Figure 1
 The option length (n) is 4.  The octets "a1" through "a4" specify the
 value that MUST be inserted into the Server Identifier option by the
 DHCP Server upon reply.
 DHCP servers that implement this Relay Agent Information suboption
 MUST use this value, if present in a DHCP message received from a
 client, as the value to insert into the Server Identifier option in
 the corresponding response.  The DHCP server must also record the
 address in the suboption for use in subsequent messages to the DHCP
 client until the next DHCP message is received from the DHCP relay
 agent.
 If a DHCP server does not understand/implement this Relay Information
 suboption, it will ignore the suboption, and thus it will insert its
 own appropriate interface address in the Server Identifier option.
 In this case, the DHCP Relay will not receive RENEW DHCPREQUEST
 messages from the client.  When configuring a DHCP relay agent to use
 this suboption, the administrator of the relay agent should take into
 account whether or not the DHCP server to which the message will be
 relayed will correctly understand this suboption.
 When servicing a DHCPREQUEST message, 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 message
 contains a Server Identifier Override suboption, however, comparison
 should be made between the address in this suboption and the Server
 Identifier option.  If both the Server Identifier Override suboption
 and the Server Identifier option specify the same address, then the
 server should accept the DHCPREQUEST message for processing,
 regardless of whether or not the Server Identifier option matches a
 DHCP server interface.
 The DHCP relay agent should fill in the giaddr field when relaying
 the message, just as it normally would do.

Johnson, et al. Standards Track [Page 3] RFC 5107 Server ID Override Suboption February 2008

 In a situation where the DHCP relay agent is configured to forward
 messages to more than one server, the DHCP relay agent SHOULD forward
 all DHCP messages to all servers.  This applies to RENEW DHCPREQUEST
 messages as well.  The intent is that the DHCP relay agent should not
 need to maintain state information about the DHCP lease.
 DHCP relay agents implementing this suboption SHOULD also implement
 and use the DHCPv4 Relay Agent Flags Suboption [4] in order to
 specify whether the DHCP relay agent received the original message as
 a broadcast or unicast.  The DHCP server receiving a message
 containing the Server Identifier Override Suboption may use this
 additional information in processing the message.
 Note that if the DHCP relay agent becomes inaccessible by the DHCP
 client or loses network access to the DHCP server, further RENEW
 DHCPREQUEST messages from the DHCP client may not be properly
 processed and the DHCP client's lease may time out.

5. 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 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 [8] SHOULD be deployed as
 well.
 If a rogue DHCP relay agent were inserted between the DHCP client and
 the DHCP server, it could redirect clients to itself using this
 suboption.  This would allow such a system to later deny RENEW
 DHCPREQUESTs and thus force clients to discontinue use of their
 allocated addresses.  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.  DHCP authentication [6] and/or
 DHCP Relay Agent Information option authentication [8] would address
 this case.  (Note that, as is always the case, lack of DHCP
 authentication would allow a rogue DHCP relay agent to change the
 Server Identifier Override option in the DHCPOFFER and DHCPACK
 messages without detection.  This threat is not new to the Server
 Identifier Override suboption.)

Johnson, et al. Standards Track [Page 4] RFC 5107 Server ID Override Suboption February 2008

 This document 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 relay agents between the
 client and server.
 This relay suboption is not intended, by itself, to provide any
 additional security benefits.

6. IANA Considerations

 IANA has assigned a suboption number (11) for the Server Identifier
 Override Suboption from the DHCP Relay Agent Information Option [3]
 suboption number space.

7. 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.

8. References

8.1. 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.

8.2. 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.

Johnson, et al. Standards Track [Page 5] RFC 5107 Server ID Override Suboption February 2008

 [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.

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

Johnson, et al. Standards Track [Page 6] RFC 5107 Server ID Override Suboption February 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights 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; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at
 ietf-ipr@ietf.org.

Johnson, et al. Standards Track [Page 7]

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