wdiff draft-ietf-dhc-subnet-option-02.txt draft-ietf-dhc-subnet-option-03.txt

Network Working Group G. Waters
INTERNET-DRAFT Nortel Networks
June 1999

The Subnet Selection Option for DHCP

<draft-ietf-dhc-subnet-option-02.txt>

<draft-ietf-dhc-subnet-option-03.txt>
Thursday, June 24, 1999, 10:43 AM

Status of this Memo

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provisions of Section 10 of RFC2026.

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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...........................................3 Option Definition................................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

To select the subnet that a DHCP server should use
when selecting on which to allocate an address. This option takes precedence over other
methods that address, the DHCP server may use to determine
determines the subnet on from which
to select the request originated, and then
selects an address. Two existing methods of determining address on the originating subnet or on a subnet that is on
the same network segment as the originating subnet. The subnet from
which to select an address are: the request originates can be determined by:

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

o To use Using the subnet address of the local interface on which the
packet was DHCP
server received by the packet.

This memo defines a new DHCP server.

Methods other than option, the two described above may exist.

The subnet selection option,
which allows the DHCP client to specify the subnet on which to
allocate an address. This option is useful for, but not limited to, takes precedence over the
class of devices methods
that have a packet-handling plane (e.g.: switching,
routing functionality) 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 control plane device (e.g.:
a RAS device) that is allocating addresses on behalf of its clients.
In this case the device management would be allocating addresses through DHCP and control functionality). The control plane
then managing those addresses among its clients.

In this scenario, the device is network connected and
there is to a private "internal"
network on which the DHCP server connected to that network. The packet-handling
plane may or may not would be network connected, however, in either case
there located. The device is no network also
connected DHCP server available to this plane. The
control plane 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 network IP connected to the packet-handling plane, external networks,
although inside the device there is connectivity between the two planes may communicate using some method (e.g.: an internal data bus).

There
and external networks (e.g.: though the backplane).

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Recall that the device is a requirement to allocate allocating addresses for devices connected to its clients on the
external networks to which the packet-handling plane is connected.

Since and that there is no network IP connectivity between the
internal network and the external networks. The DHCP server requests cannot
originate from the external networks since packets cannot be routed
between the external network and the
packet-handling plane, internal network. Thus, the control plane DHCP
requests must allocate addresses using originate from the DHCP on behalf of internal network. The problem with
originating the packet-handling plane. Since DHCP requests from the control
plane internal network is requesting the address, that the
DHCP server would normally

Waters Expires: Jun 1999 + 6 months [Page 2] will allocate addresses on the address 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 which the control plane is
connected, which internal subnet, but
would not be the desired result.

If include the subnet selection option specified by this memo is included in containing the
DHCPDISCOVER/DHCPREQUEST message then the server should allocate an address on of
the external subnet or network segment that is specified by this
option. The option would specify an address on one of which it requires the packet-
handling plane's subnets. address. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are subnet
selection option instructs the DHCP server to be interpreted allocate the address on
the requested subnet as described in [RFC2119]. 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 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 the DHCP server is allocating an address and this option
is present, present then the DHCP server MUST use allocate the address on either:

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

o A a subnet on the same network segment as the subnet specified in the
option;

on which to allocate an address.
subnet selection option.

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 an identical copy of the
option to any client that sends it, regardless of whether or not the
client requests it the option in a parameter request list. Clients using
this option must 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 should continue to operate unchanged.

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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.

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 IP address on which the DHCP client
will accept DHCP packets (e.g.: the address of the subnet connected to
the internal network).

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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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. Under the current DHCP security model there is
no methods available to circumvent this type of attack.

6. References

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

[RFC2131] Droms, R., 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

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.

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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
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.

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