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.
Copyright (C) The Internet Society (2000). All Rights Reserved.
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
1.1. Motivational Example........................................2
2. Subnet Selection Option Definition............................3
3. Intellectual Property.........................................4
4. IANA Considerations...........................................4
6. Security Considerations.......................................5
8. Editor's Addresses............................................6
9. Full Copyright Statement......................................7
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
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
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
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
4. IANA Considerations
IANA has assigned a value of 118 for the DHCP option code described
in this document.
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
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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R. "Dynamic Host Configuration Protocol", RFC 2131,
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
8. Editor's Address
310-875 Carling Avenue,
Ottawa, Ontario K1S 5P1
Phone: +1 613-765-0249
9. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
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