The IPv4 Subnet Selection Option for DHCP
draft-ietf-dhc-subnet-option-07

Network Working Group                                        G. Waters 
 INTERNET-DRAFT                                         Nortel Networks 
                                                         September 2000 
                                                                        
                                      
                                                                        
                                       

                 The IPv4 Subnet Selection Option for DHCP 
                                       

                   <draft-ietf-dhc-subnet-option-07.txt> 
                    Monday, September 11, 2000, 10:07 AM 

 Status of this Memo 

   This document is an Internet-Draft and is in full conformance with all 
   provisions of Section 10 of RFC2026. 

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 Copyright Notice 

   Copyright (C) The Internet Society (2000).  All Rights Reserved. 

 Abstract 

   This memo defines a new DHCP option for selecting the subnet on which 
   to allocate an address. This option would override a DHCP server's 
   normal methods of selecting the subnet on which to allocate an address 
   for a client. 

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   Table of Contents 

   1. Introduction......................................................2 
      1.1. Motivational Example.........................................2 
   2. Subnet Selection Option Definition................................3 
   3. Intellectual Property.............................................4 
   4. IANA Considerations...............................................5 
   5. Acknowledgements..................................................5 
   6. Security Considerations...........................................5 
   7. References........................................................5 
   8. Editor's Addresses................................................6 
   9. Full Copyright Statement..........................................6 
    

 1. Introduction 

  
   The Dynamic Host Configuration Protocol (DHCP) [RFC2131] provides a 
   framework for passing configuration information to hosts on a TCP/IP 
   network. RFC 2132 [RFC2132] specifies DHCP option configuration 
   information that may be carried in DHCP packets to/from the DHCP 
   server and the DHCP client. This document specifies a new DHCP option. 

   To select the subnet on which to allocate an address, the DHCP server 
   determines the subnet from which the request originated, and then 
   selects an address on the originating subnet or on a subnet that is on 
   the same network segment as the originating subnet. The subnet from 
   which the request originates can be determined by: 

   o Using the subnet address of the giaddr field in the DHCP packet 
     header, or if the giaddr field is zero; 

   o Using the subnet address of the local interface on which the DHCP 
     server received the packet. 

   This memo defines a new DHCP option, the subnet selection option, 
   which allows the DHCP client to specify the subnet on which to 
   allocate an address. This option takes precedence over the methods 
   that the DHCP server uses to determine the subnet on which to select 
   an address. 

   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 [RFC2119]. 

 1.1. Motivational Example 

   An example of where this option could be useful is in a device (e.g.: 
   a RAS device) that is allocating addresses on behalf of its clients.  
   In this case the device would be allocating addresses through DHCP and 
   then managing those addresses among its clients. 

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   In this scenario, the device is connected to a private "internal" 
   network on which the DHCP server would be located. The device is also 
   connected to one or more service providing "external" networks (i.e.: 
   the networks that the device's clients are connected to). Furthermore, 
   the internal network is not IP connected to the external networks, 
   although inside the device there is connectivity between the internal 
   and external networks (e.g.: though the backplane).  

   Recall that the device is allocating addresses for its clients on the 
   external networks and that there is no IP connectivity between the 
   internal network and the external networks. The DHCP requests cannot 
   originate from the external networks since packets cannot be routed 
   between the external network and the internal network. Thus, the DHCP 
   requests must originate from the internal network. The problem with 
   originating the DHCP requests from the internal network is that the 
   DHCP server will allocate addresses on the internal network's subnet, 
   when what is required are addresses on the external subnets. The 
   subnet selection option provides a solution to this problem. 

   The device would send its DHCP request on the internal subnet, but 
   would include the subnet selection option containing the address of 
   the external subnet on which it requires the address. The subnet 
   selection option instructs the DHCP server to allocate the address on 
   the requested subnet as opposed to the normal operation of allocating 
   the address on the subnet from which the DHCP request originated. 

 2. Subnet Selection Option Definition 

   The subnet selection option is a DHCP option. The option contains a 
   single IPv4 address that is the address of a subnet. The value for the 
   subnet address is determined by taking any IPv4 address on the subnet 
   and ANDing that address with the subnet mask (i.e.: the network and 
   subnet bits are left alone and the remaining (address) bits are set to 
   zero). When the DHCP server is configured to respond to this option, 
   is allocating an address, and this option is present then the DHCP 
   server MUST allocate the address on either: 

   o the subnet specified in the subnet selection option, or; 

   o a subnet on the same network segment as the subnet specified in the 
     subnet selection option. 

   The format of the option is: 

        Code   Len        IPv4 Address 
       +-----+-----+-----+-----+-----+-----+ 
       | TBD |  4  | A1  | A2  | A3  | A4  | 
       +-----+-----+-----+-----+-----+-----+ 
    

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   Servers configured to support this option MUST return an identical 
   copy of the option to any client that sends it, regardless of whether 
   or not the client requests the option in a parameter request list. 
   Clients using this option MUST discard DHCPOFFER or DHCPACK packets 
   that do not contain this option. 

   This option does not require changes to operations or features of the 
   DHCP server other than to select the subnet on which to allocate an 
   address. For example, the handling of DHCPDISCOVER for an unknown 
   subnet should continue to operate unchanged. 

   When this option is present and the server is configured to support 
   this option, the server MUST NOT offer an address that is not on the 
   requested subnet or network segment. Servers that do not understand 
   this option will allocate an address using their normal algorithms and 
   will not return this option in the DHCPOFFER or DHCPACK. In this case 
   the client will discard the DHCPOFFER or DHCPACK. Servers that 
   understand this option but are administratively configured to ignore 
   the option MUST ignore the option, use their normal algorithms to 
   allocate an address, and MUST NOT return this option in the DHCPOFFER 
   or DHCPACK. In this case the client will discard the DHCPOFFER or 
   DHCPACK. 

   During an address renew, the DHCP server may send a DHCPACK directly 
   to the allocated address, however packets from the DHCP server may not 
   be routable to the address. Thus, in all packets that the DHCP client 
   sends that contain the subnet selection option, the giaddr field in 
   the BOOTP header MUST be set to an IPv4 address on which the DHCP 
   client will accept DHCP packets (e.g.: the address on the subnet 
   connected to the internal network). 

   The IPv4 address to which a DHCP server sends a reply to MUST be the 
   same as it would chose when this option is not present. 

 3. Intellectual Property 

   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 implementers or users of this specification 
   can be obtained from the IETF Secretariat. 

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   The IETF invites any interested party to bring to its attention any 
   copyrights, patents or patent applications, or other proprietary 
   rights which may cover technology that may be required to practice 
   this standard.  Please address the information to the IETF Executive 
   Director. 

 4. IANA Considerations 

   IANA has assigned a value of TBD for the DHCP option code described in 
   this document. 

 5. Acknowledgements 

   This document is the result of work undertaken the by DHCP working 
   group. Thanks to Ted Lemon, Tim Aston and Ralph Droms for their 
   helpful comments in this work. 

 6. Security Considerations 

   DHCP currently provides no authentication or security mechanisms. 
   Potential exposures to attack are discussed is section 7 of the 
   protocol specification [RFC2131]. 

   The subnet selection option allows for the DHCP client to specify the 
   subnet on which to allocate an address. This would allow a client to 
   perform a more complete address-pool exhaustion attack since the 
   client would no longer be restricted to attacking address-pools on 
   just its local subnet.  

   Servers that implement the subnet selection option MUST by default 
   disable use of the feature; it must specifically be enabled through 
   configuration. Moreover, a server SHOULD provide the ability to 
   selectively enable use of the feature under restricted conditions, 
   e.g., by enabling use of the option only from explicitly configured 
   client-ids, enabling its use only by clients on a particular subnet, 
   or restricting the subnets (as indicated in the subnet selection 
   option) from which addresses may be requested. 

 7. References 

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 
        Requirement Levels", RFC 2119, BCP 14, March 1997. 

   [RFC2131] Droms, R. "Dynamic Host Configuration Protocol", RFC 2131, 
        March 1997. 

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

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 8. Editor's Addresses 

   Glenn Waters 
   Nortel Networks 
   310-875 Carling Avenue, 
   Ottawa, Ontario K1S 5P1 
   Canada 

   Phone:  +1 613-795-0249 
   Email:  gww@nortelnetworks.com 

 9. Full Copyright Statement 

   Copyright (C) The Internet Society (2000).  All Rights Reserved. 

   This document and translations of it may be copied and furnished to 
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   the copyright notice or references to the Internet Society or other 
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   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 
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   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT 
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   WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 
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