Network Working Group G. Waters
INTERNET-DRAFT Nortel Networks
June 1999
The Subnet Selection Option for DHCP
<draft-ietf-dhc-subnet-option-02.txt>
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 (1999). 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
2. Subnet Selection Option...........................................3
3. Intellectual Property.............................................4
4. Acknowledgements..................................................4
5. Security Considerations...........................................4
6. References........................................................5
7. Editor's Addresses................................................5
8. Full Copyright Statement..........................................5
1. Introduction
This memo was produced by the DHCP Working Group and defines a new
DHCP option that specifies the subnet that a DHCP server should use
when selecting an address. This option takes precedence over other
methods that the DHCP server may use to determine the subnet on which
to select an address. Two existing methods of determining the subnet
on which to select an address are:
o To use the subnet address of the giaddr field in the DHCP packet,
or if the giaddr field is zero;
o To use the subnet address of the local interface on which the
packet was received by the DHCP server.
Methods other than the two described above may exist.
The subnet selection option is useful for, but not limited to, the
class of devices that have a packet-handling plane (e.g.: switching,
routing functionality) and a control plane (e.g.: device management
and control functionality). The control plane is network connected and
there is a DHCP server connected to that network. The packet-handling
plane may or may not be network connected, however, in either case
there is no network connected DHCP server available to this plane. The
control plane is not network connected to the packet-handling plane,
although the two planes may communicate using some method (e.g.: an
internal data bus).
There is a requirement to allocate addresses for devices connected to
the networks to which the packet-handling plane is connected.
Since there is no network connectivity between the DHCP server and the
packet-handling plane, the control plane must allocate addresses using
the DHCP on behalf of the packet-handling plane. Since the control
plane is requesting the address, the DHCP server would normally
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allocate the address on the subnet on which the control plane is
connected, which would not be the desired result.
If the option specified by this memo is included in the
DHCPDISCOVER/DHCPREQUEST message then the server should allocate an
address on the subnet or network segment that is specified by this
option. The option would specify an address on one of the packet-
handling plane's subnets.
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].
2. Subnet Selection Option
The subnet selection option is a DHCP option. The option contains a
single IP address that is the address of a subnet. The value for the
subnet address is determined by taking any IP 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 this option is present, the DHCP server MUST use either:
o The subnet specified in the option, or;
o A subnet on the same network segment as the subnet specified in the
option;
on which to allocate an address.
The format of the option is:
Code Len IP Address
+-----+-----+-----+-----+-----+-----+
| TBD | 4 | A1 | A2 | A3 | A4 |
+-----+-----+-----+-----+-----+-----+
In order to ensure backwards compatibility of clients that do support
this option when communicating with DHCP servers that do not support
this option, the DHCP client SHOULD check that an allocated address is
on the requested subnet or network segment. The client SHOULD NOT
respond to a DHCPOFFER or DHCPACK of an address that is not on the
requested subnet or network segment.
Servers supporting this option MUST return the option to any client
that sends it, regardless of whether or not the client requests it in
a parameter request list. Clients using this option must discard
DHCPOFFER or DHCPACK packets that do not contain this option.
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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 may continue to operate unchanged.
When this option is present and the server supports this option, the
server MUST NOT offer an address that is not on the requested subnet
or network segment.
The IP 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.
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. 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.
5. Security Considerations
DHCP currently provides no authentication or security mechanisms.
Potential exposures to attack are discussed is section 7 of the
protocol specification [RFC2131].
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6. 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.
7. Editor's Addresses
Glenn Waters
Nortel Networks
310-875 Carling Avenue,
Ottawa, Ontario K1S 5P1
Canada
Phone: +1 613-798-4925
Email: gww@nortelnetworks.com
8. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
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