rfc3046.txt

<VC++网络游戏建摸与实现>源代码


RFC 3046          DHCP Relay Agent Information Option       January 2001


   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
   there.

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
   include:

       - 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  | ...
         +------+------+------+------+------+------+------+------+--







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RFC 3046          DHCP Relay Agent Information Option       January 2001


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
   server.

   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
   client.

          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
   uncompromised.

   Broadcast Forwarding

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




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RFC 3046          DHCP Relay Agent Information Option       January 2001


   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



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RFC 3046          DHCP Relay Agent Information Option       January 2001


   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
   document.

   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.






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RFC 3046          DHCP Relay Agent Information Option       January 2001


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
   rights.

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.








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RFC 3046          DHCP Relay Agent Information Option       January 2001


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: [phone]
   EMail: [e-mail]


































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RFC 3046          DHCP Relay Agent Information Option       January 2001


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
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   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
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS 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.

Acknowledgement

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



















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