RFC3011 - The IPv4 Subnet Selection Option for DHCP

Network Working Group G. Waters

Request for Comments: 3011 Nortel Networks

Category: Standards Track November 2000

The IPv4 Subnet Selection Option for DHCP

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 Dynamic Host Configuration Protocol (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.

Table of Contents

1. IntrodUCtion..................................................1

1.1. Motivational Example........................................2

2. Subnet Selection Option Definition............................3

3. Intellectual Property.........................................4

4. IANA Considerations...........................................4

5. Acknowledgements..............................................5

6. Security Considerations.......................................5

7. References....................................................5

8. Editor's Addresses............................................6

9. Full Copyright Statement......................................7

1. Introduction

The Dynamic Host Configuration Protocol (DHCP) [RFC2131] provides a

framework for passing configuration information to hosts on a TCP/IP

network. RFC2132 [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.

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

+-----+-----+-----+-----+-----+-----+

118 4 A1 A2 A3 A4

+-----+-----+-----+-----+-----+-----+

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.

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 118 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.

W. Mark Townsley and Pratik Gupta originally published a subnet

selection option Internet Draft in July 1997. The work in this

document was not based on the original work but it does achieve the

same goals.

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", BCP 14, RFC2119, March 1997.

[RFC2131] Droms, R. "Dynamic Host Configuration Protocol", RFC2131,

March 1997.

[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor

Extensions", RFC2132, March 1997.

8. Editor's Address

Glenn Waters

Nortel Networks

310-875 Carling Avenue,

Ottawa, Ontario K1S 5P1

Canada

Phone: +1 613-765-0249

EMail: gww@nortelnetworks.com

9. Full Copyright Statement

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Acknowledgement

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