Network Working Group Kim Kinnear
INTERNET DRAFT Mark Stapp
Richard Johnson
Jay Kumarasamy
Cisco Systems
February 2005
Expires July 2005
Virtual Subnet Selection Sub-Option
for the Relay Agent Information Option
<draft-ietf-dhc-agent-vpn-id-03.txt>
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract
In some environments, a relay agent resides in a network element
which also has access to one or more virtual private networks (VPNs).
If one DHCP server wishes to offer service to DHCP clients on those
different VPNs the DHCP server needs to know information about the
VPN on which each client resides. The virtual-subnet-selection sub-
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option of the relay-agent-information option is used by the relay
agent to tell the DHCP server important information about the VPN
(called the Virtual Subnet Selection information, or VSS) for every
DHCP request it passes on to the DHCP server, and is also used to
properly forward any DHCP reply that the DHCP server sends back to
the relay agent.
1. Introduction
There exist situations where there are multiple VPNs serviced by one
or more network elements which also contain relay agents. These
VPNs contain DHCP clients, and there is a desire to allow one DHCP
server to supply the full range of DHCP services to these DHCP
clients.
The network element which contains the relay agent typically is also
the network element which knows about the VPN association of the DHCP
client and could include information about the VPN in the relay-
agent-information option in the client's DHCP requests. This informa-
tion about the VPN is called the Virtual Subnet Selection informa-
tion, or VSS information. This document defines a sub-option for the
relay-agent-information option which contains this VSS information,
and which allows the relay agent to communicate the VSS information
to the DHCP server.
When the DHCP server sends its response to the relay agent for for-
warding back to the DHCP client, the relay agent will also need to
use the virtual-subnet-selection sub-option to determine to which VPN
to send the DHCP response.
This sub-option can also be used by the DHCP server to inform a relay
agent that a particular DHCP client is associated with a particular
VPN by sending the virtual-subnet-selection sub-option in the relay-
agent-information option back to the relay agent.
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Consider the following architecture:
+--------+ +---------------+
| DHCP | IP x| Relay Agent | IP z
| Server |-.......-| and +---+-------+-------+
+--------+ | VPN manager | | | |
+---+-----------+ | | |
|IP y +-----+ +--+--+ +--+--+
+-+-----+ |Host1| |Host2| |Host3|
| | +-----+ +-----+ +-----+
| |
+-----+ +--+--+ VPN 2
|Host1| |Host2|
+-----+ +-----+
VPN 1
In this architecture, the relay agent knows the VPN for each of the
DHCP clients, and inserts the VSS information about the VPN in the
virtual-subnet-selection sub-option in every DHCP request it forwards
on to the DHCP server.
When the DHCP server copies over the relay-agent-information option
from the request to the reply packet, it will copy over the virtual-
subnet-selection sub-option as well.
When the relay agent receives a DHCP reply packet from the server
with a virtual-subnet-selection sub-option, it will forward the
packet onto the proper VPN based on the VSS information in the
virtual-subnet-selection sub-option.
2.
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 RFC 2119 [RFC 2119].
This document uses the following terms:
o "DHCP client"
A DHCP client is an Internet host using DHCP to obtain confi-
guration parameters such as a network address.
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o "DHCP relay agent"
A DHCP relay agent is a third-party agent that transfers BOOTP
and DHCP messages between clients and servers residing on dif-
ferent subnets, per [RFC 951] and [RFC 1542].
o "DHCP server"
A DHCP server is an Internet host that returns configuration
parameters to DHCP clients.
o "downstream"
Downstream is the direction from the access concentrator towards
the subscriber.
o "upstream"
Upstream is the direction from the subscriber towards the access
concentrator.
o "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.
o "VPN"
Virtual private network. A network which appears to the client
to be a private network.
o "VPN Identifier"
The VPN-ID is defined by [RFC2685] to be a sequence of 14 hex
digits.
3. Virtual Subnet Selection Sub-Option Definition
The virtual-subnet-selection sub-option MAY be used by any DHCP relay
agent which desires to specify the VSS information about a VPN from
which a DHCP client request was sent.
The virtual-subnet-selection sub-option contains a generalized way to
specify the VSS information about a VPN.
The format of the option is:
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SubOpt Len Type VPN identifier
+------+------+------+------+------+------+---
| TBD | n | t | id1 | id2 | id3 | ...
+------+------+------+------+------+------+---
Type: 0 NVT ASCII VPN identifier
1 RFC2685 VPN-ID
2-255 Not Allowed
There are two types of identifiers which can be placed in the
virtual-subnet-selection sub-option. The first type of identifier
which can be placed in the virtual-subnet-selection sub-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 virtual-
subnet-selection sub-option 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 virtual-subnet-selection sub-option is concerned).
All other values of the type field are invalid as of this memo and
VSS sub-options containing any other value than zero (0) or one (1)
SHOULD be ignored.
A relay agent which recieves a DHCP request from a DHCP client on a
VPN SHOULD include a virtual-subnet-selection sub-option in the
relay-agent-information option that it inserts in the DHCP packet
prior to forwarding it on to the DHCP server.
The value placed in the virtual-subnet-selection sub-option SHOULD be
sufficient for the relay agent to properly route any DHCP reply
packet returned from the DHCP server to the DHCP client for which it
is destined. Servers supporting this sub-option MUST return an
instance of this sub-option in the relay-agent-info option to any
relay-agent that sends it. Servers SHOULD return the an exact copy
of the sub-option unless they desire to change the VPN on which a
client was configured, which would typically be a very unusual thing
to do.
In the event that a virtual-subnet-selection option and a virtual-
subnet-selection sub-option 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.
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Relay agents which include this sub-option when forwarding DHCP
client requests MUST discard DHCPOFFER or DHCPACK packets that do not
contain this sub-option in their associated relay-agent-info options.
This does not imply any memory of the particular packets forwarded
with this sub-option included. Rather, the expectation is that the
relay agent will use whatever algorithm that it used on the DHCPDIS-
COVER and DHCPREQUEST packets to decide to include this sub-option on
the DHCPOFFER and DHCPACK packets to decide if they MUST have this
sub-option included in their relay-agent-info options.
In some cases, a DHCP server may use the virtual-subnet-selection
sub-option to inform a relay agent that a particular DHCP client is
associated with a particular VPN. It does this by sending the
virtual-subnet-selection sub-option with the appropriate information
to the relay agent in the relay-agent-information option. If the
relay agent is unable to honor the DHCP server's requirement to place
the DHCP client into that VPN it MUST drop the packet and not send it
to the DHCP client.
This sub-option SHOULD NOT be used without also making use of some
form of authentication for relay-agent-information option.
4. Security
Message authentication in DHCP for intradomain use where the out-of-
band exchange of a shared secret is feasible is defined in [RFC
3118]. Potential exposures to attack are discussed in section 7 of
the DHCP protocol specification in [RFC 2131].
The virtual-subnet-selection sub-option could be used by a client in
order to obtain an IP address from a VPN other than the one on which
it resides. This attack can be partially prevented by the relay
agent not forwarding any DHCP packet which already contains a relay-
agent-information option.
Any program which unicasts a DHCP packet to the DHCP server with a
relay-agent-information option in it with a vpn-id for a different
VPN would cause the DHCP server to allocate an address from that dif-
ferent VPN, but since the DHCP server cannot (in general) communicate
directly back to the program that sent in the malicious DHCP packet,
the entire cycle of creating a lease will not be completed. Cer-
tainly many leases could be offered, which would result in a tem-
poraty form of address-pool exhaustion.
Servers that implement the virtual-subnet-selection sub-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
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conditions, e.g., by enabling use of the option only from explicitly
configured client-ids, enabling its use only by clients on a particu-
lar subnet, or restricting the VPNs from which addresses may be
requested.
5. IANA Considerations
IANA has assigned the value of TBD for the VPN Identifier sub-option
from the DHCP Relay Agent Sub-options space [RFC 3046] for the VPN
Identifier sub-option defined in Section 3.
This document defines a number space for the type byte of the
virtual-subnet-selection sub-option. Certain allowable values for
this byte are defined in this specification (see Section 3). 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 virtual-subnet-
selection option. The type bytes and data formats of the virtual-
subnet-selection option and virtual-subnet-selection sub-option MUST
always be identical.
6. Acknowledgments
None.
7. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC 2132] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor
Extensions", Internet RFC 2132, March 1997.
[RFC 2685] Fox, B., Gleeson, B., "Virtual Private Networks Identif-
ier", Internet RFC 2685, September 1999.
[RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
3046, January 2001.
[RFC 3118] Droms, R. "Authentication for DHCP Messages", RFC 3118,
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June 2001.
8. Author's information
Kim Kinnear
Mark Stapp
Cisco Systems
1414 Massachusetts Ave.
Boxborough, Massachusetts 01719
Phone: (978) 936-0000
EMail: kkinnear@cisco.com
mjs@cisco.com
Jay Kumarasamy
Richard Johnson
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
Phone: (408) 526-4000
EMail: jayk@cisco.com
raj@cisco.com
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Copyright (C) The Internet Society (2005). This document is subject to
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