Network Working Group R. Johnson
Internet-Draft J. Kumarasamy
Expires: May 19, 2008 K. Kinnear
M. Stapp
Cisco
November 16, 2007
Virtual Subnet Selection Option
draft-ietf-dhc-vpn-option-07.txt
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Copyright (C) The IETF Trust (2007).
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Abstract
This memo defines existing usage for the Virtual Subnet Selection
(VSS) information option. It is intended for use primarily by DHCP
proxy clients in situations where VSS information needs to be passed
to the DHCP server for proper address allocation to take place.
The option number currently in use is 221. This memo documents the
current usage of the option in agreement with [8], which declares
that any pre-existing usages of option numbers in the range 128 - 223
should be documented and the working group will try to officially
assign those numbers to those options.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. VSS Information Definition . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Introduction
There is a growing use of Virtual Private Network (VPN)
configurations. The growth comes from many areas; individual client
systems needing to appear to be on the home corporate network even
when traveling, ISPs providing extranet connectivity for customer
companies, etc. In some of these cases there is a need for the DHCP
server to know the VPN (hereafter called a "Virtual Subnet Selector"
or "VSS") from which an address, and other resources, should be
allocated.
If the allocation is being done through a DHCP relay, then a relay
sub-option could be included. In some cases, however an IP address
is being sought by a DHCP proxy on behalf of a client (which may be
assigned the address via a different protocol). In this case, there
is a need to include VSS information relating to the client as a DHCP
option.
A good example might be a dial-in aggregation device where PPP [10]
addresses are acquired via DHCP and then given to the remote customer
system via IPCP [9]. In a network where such a device is used to
aggregate PPP dial-in from multiple companies, each company may be
assigned a unique VSS.
This memo defines a new DHCP [4] option, the VSS Information option,
which allows the DHCP client to specify the VSS Information needed in
order to allocate an address. If the receiving DHCP server
understands the VSS Information option, this information may be used
in conjunction with other information in determining the subnet on
which to select an address as well as other information such as DNS
server, default router, etc.
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2. Conventions
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 [3].
This document also uses the following terms:
DHCP Client
DHCP Client or "Client" is an Internet host using DHCP to obtain
configuration parameters such as a network address.
DHCP Server
A DHCP Server or "Server" is an Internet host that returns
configuration parameters to DHCP Clients.
DHCP relay agent
A DHCP relay agent is a third-party agent that transfers BOOTP and
DHCP messages between clients and servers residing on different
subnets, per [1] and [2].
downstream
Downstream is the direction from the access concentrator towards
the subscriber.
upstream
Upstream is the direction from the subscriber towards the access
concentrator.
VSS information
Information about a VPN necessary to allocate an address to a DHCP
client on that VPN and necessary to forward a DHCP reply packet to
a DHCP client on that VPN.
VPN
Virtual private network. A network which appears to the client to
be a private network.
VPN Identifier
The VPN-ID is defined by [6] to be a sequence of 7 octets.
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3. VSS Information Definition
The VSS Information option is a DHCP option [5]. The option contains
generalized VSS information in one of two formats: NVT ASCII VPN
identifier, or RFC2685 VPN-ID [6].
The format of the option is:
Code Len Type VSS Information octets
+-----+-----+------+-----+-----+-----+---
| 221 | n | t | v1 | v2 | v3 | ...
+-----+-----+------+-----+-----+-----+---
Type: 0 NVT ASCII VPN identifier
1 RFC2685 VPN-ID
2-255 Not Allowed
Figure 1
The option minimum length (n) is 2.
There are two types of identifiers which can be placed in the VSS
Information Option. The first type of identifier which can be placed
in the VSS Information Option is an NVT ASCII string. It MUST NOT be
terminated with a zero byte.
The second type of identifier which can be placed in the VSS
Information Option is an RFC2685 VPN-ID [6], which is typically 7
octets (3 of VPN OUI followed by 4 of VPN index) in length (though it
can be any length as far as the VSS Information Option is concerned).
If the type field is set to zero (0), it indicates that all following
bytes of the option contain a NVT ASCII string. This string MUST NOT
be terminated with a zero byte.
If the type field is set to one (1), it indicates that all following
bytes should be interpreted in agreement with RFC2685 as a VPN
Identifier, typically 7 octets.
All other values of the type field are invalid as of this memo and
VSS options containing any other value than zero (0) or one (1)
SHOULD be ignored.
Since this option is placed in the packet in order to change the VPN
on which an IP address is allocated for a particular DHCP client, one
presumes that an allocation on that VPN is necessary for correct
operation. If this presumption is correct, then a client which
places this option in a packet and doesn't receive it in the
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returning packet should drop the packet since the IP address that was
allocated will not be in the correct VPN. If an IP address that is
not on the requested VPN is not required, then the client is free to
accept the IP address that is not on the VPN that the was requested.
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.
This option provides the DHCP server additional information upon
which to make a determination of address to be assigned. The DHCP
server, if it is configured to support this option, should use this
information in addition to other options included in the DHCPDISCOVER
packet in order to assign an IP address for DHCP client.
In the event that a Virtual Subnet Selection option and a Virtual
Subnet Selection sub-option [12] are both received in a particular
DHCP client packet, the information from the Virtual Subnet Selection
sub-option MUST be used in preference to the information in the
Virtual Subnet Selection option. This reasoning behind this approach
is that the relay-agent is almost certainly more trusted than the
DHCP client, and therefore information in the relay-agent-information
option that conflicts with information in the packet generated by the
DHCP client is more likely to be correct.
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 should consider
discarding the DHCPOFFER or DHCPACK, as mentioned above. 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 such that the client will know that the
allocated address is not in the VPN requested and will consider this
information in deciding whether or not to accept the DHCPOFFER. In
other words, this option MUST NOT appear in a DHCPOFFER or DHCPACK
from a server unless it was used by the server in making or updating
the address allocation requested.
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4. Security Considerations
Message authentication in DHCP for intradomain use where the out-of-
band exchange of a shared secret is feasible is defined in [11].
Potential exposures to attack are discussed in section 7 of the DHCP
protocol specification in [4].
The VSS Information option could be used by a client in order to
obtain an IP address from a VPN other than the one where it should.
Another possible defense would be for the DHCP relay to insert a
Relay option containing a VSS Information Relay Sub-option, which
would override the DHCP VSS Information option.
This option 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 VSS Information 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 VSSs from which addresses may be requested.
Implementations should consider using the DHCP Authentication option
[11] in order to provide a higher level of security if it is deemed
necessary in their environment.
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5. IANA Considerations
IANA is requested to assign DHCP option number 221 for this option,
in accordance with [8].
While the type byte of the Virtual Subnet Selection option defines a
number space that could be managed by IANA, expansion of this number
space is not anticipated and so creation of a registry of these
numbers is not required by this document. In the event that
additional values for the type byte are defined in subsequent
documents, IANA should at that time create a registry for these type
bytes. New values for the type byte may only be defined by IETF
Consensus, as described in [7]. Basically, this means that they are
defined by RFCs approved by the IESG.
Moreover, any changes or additions to the type byte codes MUST be
made concurrently in the type byte codes of the VSS Information
Option. The type bytes and data formats of the VSS Information
Option and VSS Information Relay Sub-option MUST always be identical.
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6. Acknowledgements
This document is the result of work done within Cisco Systems.
Thanks to Kim Kinnear, Mark Stapp, and Jay Kumarasamy for their work
on this option definition and the other related work for which this
is necessary.
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7. References
7.1. Normative References
[1] Croft, B. and J. Gilmore, "Bootstrap Protocol (BOOTP)",
RFC 951, September 1985.
[2] Wimer, W., "Clarifications and Extensions for the Bootstrap
Protocol", RFC 1542, October 1993.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[5] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[6] Fox, B. and B. Gleeson, "Virtual Private Networks Identifier",
RFC 2685, September 1999.
[7] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[8] Volz, B., "Reclassifying Dynamic Host Configuration Protocol
version 4 (DHCPv4) Options", RFC 3942, November 2004.
7.2. Informative References
[9] McGregor, G., "The PPP Internet Protocol Control Protocol
(IPCP)", RFC 1332, May 1992.
[10] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[11] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[12] Kinnear, K., "Virtual Subnet Selection Sub-Option for the Relay
Agent Information Option for DHCPv4",
draft-ietf-dhc-agent-vpn-id-05 (work in progress),
November 2007.
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Authors' Addresses
Richard A. Johnson
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
US
Phone: +1 408 526 4000
Email: raj@cisco.com
Jay Kumarasamy
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
US
Phone: +1 408 526 4000
Email: jayk@cisco.com
Kim Kinnear
Cisco Systems
250 Apollo Drive
Chelmsford, MA 01824
US
Phone: +1 978 244 8000
Email: kkinnar@cisco.com
Mark Stapp
Cisco Systems
250 Apollo Drive
Chelmsford, MA 01824
US
Phone: +1 978 244 8000
Email: mjs@cisco.com
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