Request for Comments: DRAFT Richard Johnson
Kim Kinnear
Mark Stapp
Jay Kumarasamy
Cisco Systems, Inc.
July 2001
Expires January 2001
DHCP VPN Information option
<draft-ietf-dhc-vpn-option-00.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 (2000). All Rights Reserved.
Johnson, R. Expires January 2002 [Page 1]
DRAFT DHCP VPN Information option July 2001
Abstract
This memo defines a new DHCP option for passing VPN information
between the DHCP client and the DHCP server. It is intended for use
primarily by DHCP proxy clients in situations where VPN information
needs to be passed to the DHCP server for proper address allocation
to take place.
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
client to communicate to the DHCP server the VPN from which an
address, and other resources, should be allocated. Currently there
is no way to pass this information.
This option would most likely not be used by an actual DHCP end-user
client such as a workstation or laptop. It is primarily intended to
be used by a DHCP proxy client which would be using DHCP in order to
allocate an IP address on behalf of some other protocol or client.
This memo defines a new DHCP option, the VPN Information option,
which allows the DHCP client to specify the VPN Information needed in
order to allocate an address. If the receiving DHCP server
understands the VPN 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.
VPN Information Option Definition
The VPN Information option is a DHCP option. The option contains
generalized VPN information in one of two formats: NVT ASCII VPN
identifier, or RFC2685 VPN-ID.
The format of the option is:
Code Len Type VPN Information octets
+-----+-----+------+-----+-----+-----+---
| TBD | n | t | v1 | v2 | v3 | ...
+-----+-----+------+-----+-----+-----+---
Type: 0 NVT ASCII VPN identifier
1 RFC2685 VPN-ID
2-255 Not Allowed
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The option minimum length (n) is 2.
There are two types of identifiers which can be placed in the VPN
Information Suboption. The first type of identifier which can be
placed in the VPN Information Suboption 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 VPN
Information Suboption is an RFC2685 VPN-ID [RFC 2685], which is
typically 14 hex digits in length (though it can be any length as far
as the VPN Information Suboption 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 14 hex digits.
All other values of the type field are invalid as of this memo and
VPN options containing any other value than zero (0) or one (1)
SHOULD be ignored.
Any VPN information contained in a DHCP Relay suboption SHOULD
override the information contained in this VPN Information option.
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 provides the DHCP server additional information upon
which to make a determination of address to be assigned. The DHCP
server, if it is configure 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 VPN Informmation Option and a VPN Information
Relay Suboption are both received in a particular DHCP client packet,
the information from the VPN Information Suboption MUST be used in
preference to the information in the VPN Information Option.
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
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DRAFT DHCP VPN Information option July 2001
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. In other words, this
option MUST not appear in a DHCPOFFER from a server unless it was
used by the server in making the address allocation requested.
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 VPN 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.
DHCP relays MAY choose to remove the option before passing on
DHCPDISCOVER packets. Another possible defense would be for the DHCP
relay to insert a Relay option containing a VPN Information
Suboption, which would override the DHCP VPN 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 VPN 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 VPNs from which addresses may be requested.
IANA Considerations
IANA has assigned a value of TBD for the DHCP option code described
in this document. No assignment of values for the type field need be
made at this time. New values may only be defined by IETF Consensus,
as described in [RFC 2434]. 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 VPN Information
Option. The type bytes and data formats of the VPN Information
Option and VPN Information Suboption MUST always be identical.
Johnson, R. Expires January 2002 [Page 4]
DRAFT DHCP VPN Information option July 2001
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.
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.
[RFC2685] Fox, B. and Gleeson, B., "Virtual Private Networks
Identifier", RFC 2685, September 1999
Author Information:
Richard Johnson
Jay Kumarasamy
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
Phone: (408) 526-4000
EMail: jayk@cisco.com
raj@cisco.com
Kim Kinnear
Mark Stapp
Cisco Systems
250 Apollo Drive
Chelmsford, MA 01824
Phone: (978) 244-8000
EMail: kkinnear@cisco.com
mjs@cisco.com
Johnson, R. Expires January 2002 [Page 5]
DRAFT DHCP VPN Information option July 2001
Johnson, R. Expires January 2002 [Page 6]
