Network Working Group Kim Kinnear
INTERNET DRAFT Mark Stapp
Richard Johnson
Jay Kumarasamy
Cisco Systems
July 2001
Expires January 2002
VPN Identifier sub-option
for the Relay Agent Information Option
<draft-ietf-dhc-agent-vpn-id-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 (2001). All Rights Reserved.
Abstract
In some environments, a relay agent resides in a network element
which also has access to one or more VPNs. If one DHCP server wishes
to offer service to DHCP clients on those different VPNs the DHCP
server needs to know the VPN on which each client resides. The vpn-
id sub-option of the relay-agent-information option is used by the
relay agent to tell the DHCP server the VPN for every DHCP request it
passes on to the DHCP server, and is also used to properly forward
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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 this information in the relay-agent-
information option in the client's DHCP requests. This document
defines a sub-option for the relay-agent-information option which
contains the vpn-id, and which allows the relay agent to communicate
the VPN association 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 vpn-id 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 vpn-id sub-option to the relay agent in the
relay-agent-information option back to the relay agent.
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
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In this architecture, the relay agent knows the VPN for each of the
DHCP clients, and inserts that information in the vpn-id sub-option
in every DHCP request it forwards onto 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 vpn-id
sub-option as well.
When the relay agent receives a DHCP reply packet from the server
with a vpn-id sub-option, it will forward the packet onto the proper
VPN based on the value of the vpn-id 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.
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 "VPN"
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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. VPN identifier sub-option definition
The vpn-id sub-option MAY be used by any DHCP relay agent which
desires to specify the VPN from which a DHCP client request was sent.
The vpn-id sub-option contains a generalized VPN identifier.
The format of the option is:
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 vpn-id
sub-option. The first type of identifier which can be placed in the
vpn-id 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 vpn-id 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 vpn-id
sub-option is concerned).
A relay agent which recieves a DHCP request from a DHCP client on a
VPN SHOULD include a vpn-id 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 vpn-id sub-option SHOULD be sufficient for
the relay agent to properly route any DHCP reply packet returned from
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Internet Draft VPN-ID sub-option July 2001
the DHCP server to the DHCP client for which it is destined. Servers
supporting this sub-option MUST return an identical copy of the sub-
option in the relay-agent-info option to any relay-agent that sends
it.
In the event that a vpn-id option and a vpn-id sub-option are both
received in a particular DHCP client packet, the information from the
vpn-id sub-option MUST be used in preference to the information in
the vpn-id option.
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.
In some cases, a DHCP server may use the vpn-id sub-option to inform
a relay agent that a particular DHCP client is associated with a par-
ticular VPN. It does this by sending the vpn-id 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 back to the DHCP client.
4. Security
DHCP currently provides no authentication or security mechanisms.
Potential exposures to attack are discussed is section 7 of the pro-
tocol specification [RFC2131]. The vpn-id sub-option could allow a
program masquerading as a relay agent to obtain addresses on other
VPNs than the one on which it resides, possibly aiding in an
address-pool exhaustion attack on that VPN.
This attack can be partially prevented by the relay agent not for-
warding 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 different 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. Certainly many leases could be offered, which would
result in a form of address-pool exhaustion.
Under the current DHCP security model there are no methods available
to completely circumvent this type of attack.
5. IANA Considerations
This document defines a number space for the type byte of the vpn-id
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sub-option. Certain allowable values for this byte are defined in
this specification. New values may only be defined by IETF Con-
sensus, 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-id option. The
type bytes and data formats of the vpn-id option and vpn-id sub-
option MUST always be identical.
6. Acknowledgments
None (yet).
7. 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.
8. Author's information
Kim Kinnear
Mark Stapp
Cisco Systems
250 Apollo Drive
Chelmsford, MA 01824
Phone: (978) 244-8000
EMail: kkinnear@cisco.com
mjs@cisco.com
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Internet Draft VPN-ID sub-option July 2001
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
9. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to oth-
ers, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and dis-
tributed, in whole or in part, without restriction of any kind, provided
that the above copyright notice and this paragraph are included on all
such copies and derivative works. However, this document itself may not
be modified in any way, such as by removing the copyright notice or
references to the Internet Society or other Internet organizations,
except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet Stan-
dards process must be followed, or as required to translate it into
languages other than English.
The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an "AS
IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK
FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT
LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT
INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FIT-
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