Internet DRAFT - draft-ietf-dhc-vpn-option
draft-ietf-dhc-vpn-option
DHC Working Group Kim Kinnear
Internet Draft Richard Johnson
Updates: 3046 Mark Stapp
Intended Status: Standards Track Cisco Systems
Expires: July 26, 2012 Jay Kumarasamy
January 26, 2012
Virtual Subnet Selection Options for DHCPv4 and DHCPv6
<draft-ietf-dhc-vpn-option-15.txt>
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Abstract
This memo defines a Virtual Subnet Selection (VSS) option for each of
DHCPv4 and DHCPv6, and a VSS sub-option carried in the DHCPv4 relay-
agent-information option. These are intended for use by DHCP
clients, relay agents, and proxy clients in situations where VSS
information needs to be passed to the DHCP server for proper address
or prefix allocation to take place.
For the DHCPv4 option and relay-agent-information sub-option, this
memo documents existing usage as per RFC 3942 [RFC3942]. This memo
updates RFC 3046 [RFC3046] regarding details relating to copying of
sub-options (see Section 8).
Table of Contents
1. Introduction................................................. 3
2. Terminology.................................................. 4
3. Virtual Subnet Selection Option and Sub-Options Definitions.. 5
3.1. DHCPv4 Virtual Subnet Selection Option..................... 5
3.2. DHCPv4 Virtual Subnet Selection Sub-Option................. 6
3.3. DHCPv4 Virtual Subnet Selection Control Sub-Option......... 6
3.4. DHCPv6 Virtual Subnet Selection Option..................... 7
3.5. Virtual Subnet Selection Type and Information.............. 7
4. Overview of Virtual Subnet Selection Usage................... 8
4.1. VPN assignment by the DHCP relay agent..................... 9
4.2. VPN assignment by the DHCP server.......................... 12
4.3. Required Support........................................... 14
4.4. Alternative VPN assignment approaches...................... 14
5. Relay Agent Behavior......................................... 14
5.1. VPN assignment by the DHCP server.......................... 16
5.2. DHCP Leasequery............................................ 17
6. Client Behavior.............................................. 17
7. Server Behavior.............................................. 18
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7.1. Returning the DHCPv4 or DHCPv6 Option...................... 19
7.2. Returning the DHCPv4 Sub-Option............................ 20
7.3. Making sense of conflicting VSS information................ 21
8. Updates to RFC 3046.......................................... 21
9. Security..................................................... 22
10. IANA Considerations......................................... 23
11. Acknowledgments............................................. 23
12. References.................................................. 24
12.1. Normative References...................................... 24
12.2. Informative References.................................... 24
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.
This memo defines a Virtual Subnet Selection (VSS) option for each of
DHCPv4 and DHCPv6, and a VSS sub-option carried in the DHCPv4 relay-
agent-information option. These are intended for use by DHCP
clients, relay agents, and proxy clients in situations where VSS
information needs to be passed to the DHCP server for proper address
or prefix allocation to take place. If the receiving DHCP server
understands the VSS option or sub-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.
If the allocation is being done through a DHCPv4 relay, then the
relay-agent-information sub-option defined here should be included.
In some cases, however, an IP address is being sought by a DHCPv4
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 DHCPv4 option.
If the allocation is being done through a DHCPv6 relay, then the
DHCPv6 VSS option defined in this document should be included in the
Relay-forward and Relay-reply message going between the DHCPv6 relay
and server. In some cases, addresses or prefixes are being sought by
a DHCPv6 proxy on behalf of a client. In this case, there is a need
for the client itself to supply the VSS information using the DHCPv6
VSS option in the messages that it sends to the DHCPv6 server.
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In the remaining text of this document, when a DHCPv6 address is
indicated the same information applies to DHCPv6 Prefix Delegation
[RFC3633] as well.
In the remaining text of this document, when the term VSS sub-option
is used, it refers to the VSS sub-option carried in the DHCPv4
relay-agent-information option.
2. Terminology
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 [RFC2119].
This document uses the following terms:
o "DHCP client"
A DHCP client is a host using DHCP to obtain configuration
parameters such as a network address.
o "DHCP proxy"
A DHCP proxy is a DHCP client which acquires IP addresses not
for its own use, but rather on behalf of another entity. There
are a variety of ways that a DHCP proxy can supply the addresses
it acquires to other entities that need them.
o "DHCP relay agent"
A DHCP relay agent is an agent that transfers BOOTP and DHCP
messages between clients and servers residing on different
subnets, per [RFC951], [RFC1542], and [RFC3315].
o "DHCP server"
A DHCP server is a host that returns configuration parameters to
DHCP clients.
o "DHCPv4 option"
An option used to implement a capability defined by the DHCPv4
RFCs [RFC2131][RFC2132]. These options have one-octet code and
size fields.
o "DHCPv4 sub-option"
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As used in this document, a DHCPv4 sub-option refers to a sub-
option of the relay-agent-information option [RFC3046]. These
sub-options have one-octet code and size fields.
o "DHCPv6 option"
An option used to implement a capability defined by the DHCPv6
RFC [RFC3315]. These options have two-octet code and size
fields.
o "Global VPN"
Indicates that the address being described belongs to the set of
addresses not part of any VPN. In other words, the normal
address space operated on by DHCP. This includes private
addresses, for example the 10.x.x.x addresses as well as the
other private subnets that are not routed on the open internet.
o "NVT ASCII Identifier"
A Network Virtual Terminal (NVT) identifier is an identifier
containing only characters from the ASCII repetoire and using
the Network Virtual Terminal encoding (see Appendix B in
[RFC5198]).
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 7 octets.
3. Virtual Subnet Selection Option and Sub-Options Definitions
The Virtual Subnet Selection options and sub-options contain a
generalized way to specify the VSS information about a VPN. There
are two options and two sub-options defined in this section. The
actual VSS information is identical both options and one of the two
sub-options.
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3.1. DHCPv4 Virtual Subnet Selection Option
The format of the option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Length | Type | VSS Info ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code The option code (221).
Length The option length, minimum 1 octets.
Type and VSS Information -- see Section 3.5
3.2. DHCPv4 Virtual Subnet Selection Sub-Option
This is a sub-option of the relay-agent-information option [RFC3046].
The format of the sub-option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Length | Type | VSS Info. ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code The sub-option code (151).
Length The sub-option length, minimum 1 octets.
Type and VSS Information -- see Section 3.5.
3.3. DHCPv4 Virtual Subnet Selection Control Sub-Option
This is a sub-option of the relay-agent-information option [RFC3046].
The format of the sub-option is:
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code The sub-option code (TBD).
Length The sub-option length, 0.
This sub-option only appears in the DHCPv4 relay-agent-information
option. In a DHCP request, it indicates that a DHCPv4 VSS sub-option
is also present in the relay-agent-information option. In a DHCP
reply, if it appears in the relay-agent-information option, it
indicates that the DHCP server did not understand any DHCPv4 VSS
sub-option that also appears in the relay-agent-information option.
3.4. DHCPv6 Virtual Subnet Selection Option
The format of the DHCPv6 Virtual Subnet Selection option is shown
below. This option may be included by a client or relay-agent (or
both).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_VSS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | VSS Information ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_VSS (TBD).
option-len The number of octets in the option, minimum 1.
Type and VSS Information -- see Section 3.5
3.5. Virtual Subnet Selection Type and Information
All of the (sub)options defined above carry identical payloads,
consisting of a type and additional VSS information as follows:
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Type VSS Information format:
0 NVT ASCII VPN identifier
1 RFC2685 VPN-ID
2-254 Unassigned
255 Global, default VPN.
o Type 0 -- Network Virtual Terminal (NVT) ASCII VPN identifier
Indicates that the VSS information consists of an NVT ASCII
string. It MUST NOT be terminated with a zero byte.
o Type 1 -- RFC2685 VPN-ID
Indicates that the VSS information consists of an RFC2685 VPN-ID
[RFC2685], which is defined to be 7 octets in length.
o Type 255 -- Global, default VPN
Indicates that there is no explicit, non-default VSS information
but rather that this option references the normal, global,
default address space. In this case, there MUST NOT be any VSS
Information included in the VSS option or sub-option and the
length of the option or sub-option MUST be 1.
All other values of the Type field are unassigned.
4. Overview of Virtual Subnet Selection Usage
At the highest level, the VSS option or sub-option determines the VPN
on which a DHCP client is supposed to receive an IP address. How the
option or sub-option is entered and processed is discussed below, but
the point of all of the discussion is to determine the VPN on which
the DHCP client resides. This will affect a relay agent, in that it
will have to ensure that DHCP packets sent to and received from the
DHCP client flow over the correct VPN. This will affect the DHCP
server in that it determines the IP address space used for the IP
address allocation.
A DHCP server has as part of its configuration some IP address space
from which it allocates IP addresses to DHCP clients. These
allocations are typically for a limited time, and thus the DHCP
client gets a lease on the IP address. In the absence of any VPN
information, the IP address space is in the global or default VPN
used throughout the Internet. When a DHCP server deals with VPN
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information, each VPN defines a new address space inside the server,
one distinct from the global or default IP address space. A server
which supports the VSS option or sub-option thereby supports
allocation of IP addresses from multiple different VPNs. Supporting
IP address allocation from multiple different VPNs means that the
DHCP server must be prepared to configure multiple different address
spaces (one per distinct VPN) and allocate IP addresses from these
different address spaces.
These address spaces are typically independent, so that the same IP
address (consisting of the same string of bytes) could be allocated
to one client in the global, default VPN, and to a different client
residing in a different VPN. There is no conflict in this
allocation, since the clients have essentially different addresses,
even though these addresses consist of the same string of bytes,
because the IPv4 or IPv6 address is qualified by the VPN.
Thus a VSS option or sub-option is a way of signaling the use of a
VPN other than the global or default VPN. The next question is: who
decides what VPN a DHCP client should be using?
There are three entities which can either insert a VSS option or
sub-option into a DHCPv4 packet or DHCPv6 message; a DHCP client, a
relay agent, or a DHCPv4 or DHCPv6 server. While all of these
entities could include a different VSS option or sub-option in every
request or response, this situation is neither typical nor useful.
There are two known paradigms for use of the VSS option or sub-
option, which are discussed below.
4.1. VPN assignment by the DHCP relay agent
The typical use of the VSS option or sub-option is for the relay
agent to know the VPN on which the DHCP client is operating. The
DHCP client itself does not, in this approach, know the VPN on which
it resides. The relay agent is responsible for mediating the access
between the VPN on which the DHCP client resides and the DHCP server.
In this situation, the relay agent will insert two DHCPv4 relay-
agent-information sub-options (one VSS sub-option, and one VSS-
Control sub-option) into the relay-agent-information option or a
DHCPv6 VSS option into the Relay-forward message of every request it
forwards from the DHCP client. The server will use the DHCPv6 VSS
option or DHCPv4 VSS sub-option to determine the VPN on which the
client resides, and use that VPN information to select the address
space within its configuration from which to allocate an IP address
to the DHCP client.
When, using this approach, a DHCPv4 relay agent inserts a VSS sub-
option into the relay-agent-information option it MUST also insert a
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VSS-Control sub-option into the relay-agent-information-option. This
is to allow determination of whether or not the DHCPv4 server
actually processes the VSS information provided by the DHCPv4 relay
agent. If the DHCPv4 server supports the VSS capabilities described
in this document, it will remove the VSS-Control sub-option from the
relay-agent-information option that it returns to the DHCPv4 relay
agent. See Section 5 for more information.
In this approach, the relay agent might also send a VSS option or
sub-option in either a DHCPv4 or DHCPv6 Leasequery request, but in
this case, it would use the VSS option in the Leasequery request to
select the correct address space for the Leasequery. In this
approach, the relay agent would be acting as a DHCP client from a
Leasequery standpoint, but it would not be as if a DHCP client were
sending in a VSS option in a standard DHCP address allocation
request, say a DHCPDISCOVER.
In this approach, only one relay agent would mediate the VPN access
for the DHCP client to the DHCP server, and it would be the relay
agent which inserts the VSS information into the request packet and
would remove it prior to forwarding the response packet on.
In the diagram below is an example of a DHCPv4 client, DHCPv4 relay
agent, and DHCPv4 server. The DHCPv6 situation is similar, but uses
the DHCPv6 VSS option.
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DHCPv4
DHCPv4 Relay DHCPv4
Client Agent Server
| | |
| >--DHCPDISCOVER--> | |
| on VRF "abc" | |
| | >--DHCPDISCOVER----> |
| | relay-agent-info: |
| | VSS type VRF:"abc"|
| | VSS-Control |
| | |
| | <----DHCPOFFER-----< |
| | relay-agent-info: |
| | VSS type VRF:"abc"|
| | |
| <---DHCPOFFER----< | |
| on VRF "abc" | |
| | |
| >--DHCPREQUEST---> | |
| on VRF "abc" | |
| | >--DHCPREQUEST-----> |
| | relay-agent-info: |
| | VSS type VRF:"abc"|
| | VSS-Control |
| | |
| | <----DHCPACK-------< |
| | relay-agent-info: |
| | VSS type VRF:"abc"|
| | |
| <---DHCPACK------< | |
| on VRF "abc" | |
| | |
... ... ...
Figure 4.1-1: DHCPv4 - Relay Agent knows VPN
The DHCP server would know that it should respond to VPN information
specified in a VSS option or sub-option, and it would be configured
with appropriate VPN address spaces to service the projected client
requirements. Thus, in this common approach, the DHCP client knows
nothing of any VPN access, the relay agent has been configured in
some way that allows it to determine the VPN of the DHCP client and
transmit that using a VSS option or sub-option to the DHCP server,
and the DHCP server responds to the VPN specified by the relay agent.
There is no conflict between different entities trying to specify
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different VSS information -- each entity knows its role through
policy or configuration external to this document.
If any mis-configuration exists, it SHOULD result in a DHCP client
being unable to acquire an IP address. For instance, a relay agent
which supports VPN access SHOULD couple transmission of VSS options
or sub-options to the configuration of VPN support, and not allow one
without the other.
It is important to ensure that the relay agent and DHCP server both
support the VSS option and sub-option (for DHCPv4) or the VSS option
(for DHCPv6). Deploying DHCPv4 relay agents which support and emit
VSS sub-options in concert with DHCPv4 servers which do not support
the VSS option or sub-option as defined in this document SHOULD NOT
be done, as such an ensemble will not operate correctly. Should this
situation occur, however, the relay agent can detect the problem
(since the VSS-Control sub-option will appear in the packets it
receives from the DHCPv4 server, indicating the server did not
effectively process the VSS sub-option), and it can issue appropriate
diagnostic messages.
4.2. VPN assignment by the DHCP server
In this approach, the DHCP server would be configured in some way to
know the VPN on which a particular DHCP client should be given
access. The DHCP server would in this case include the VSS sub-
option in the relay-agent-information option for DHCPv4 or the VSS
option in the Relay-reply message for DHCPv6. The relay agent
responsible for mediating VPN access would use this information to
select the correct VPN for the DHCP client. In the unusal event that
there were more than one relay agent involved in this transaction,
some external configuration or policy would be needed to inform the
DHCPv6 server into which Relay-reply message the VSS option should
go.
Once the relay agent has placed the DHCP client into the proper VPN,
it SHOULD begin including VSS information in requests that it
forwards to the DHCP server. Since this information does not
conflict with the DHCP server's idea of the proper VPN for the
client, everything works correctly.
The diagram below shows this approach using DHCPv4. The DHCPv6
situation is similar, but uses the DHCPv6 VSS option instead.
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DHCPv4
DHCPv4 Relay DHCPv4
Client Agent Server
| | |
| >--DHCPDISCOVER--> | |
| on unknown VPN | |
| | >--DHCPDISCOVER----> |
| | |
| | <----DHCPOFFER-----< |
| | relay-agent-info: |
| | VSS type VRF:"abc"|
| | |
| <---DHCPOFFER----< | |
| on VRF "abc" | |
| | |
| >--DHCPREQUEST---> | |
| on VRF "abc" | |
| | >--DHCPREQUEST-----> |
| | relay-agent-info: |
| | VSS type VRF:"abc"|
| | VSS-Control |
| | |
| | <----DHCPACK-------< |
| | relay-agent-info: |
| | VSS type VRF:"abc"|
| | |
| <---DHCPACK------< | |
| on VRF "abc" | |
| | |
| | |
... ... ...
Figure 4.2-1: DHCPv4 - DHCPv4 Server knows VPN
In this approach, the DHCP client is again unaware of any VPN
activity. In this case, however, the DHCP server knows the VPN for
the client, and the relay agent responds to the VSS information
specified by the DHCP server. Similar to the previous approach, each
entity knows its role through a means external to this document and
no two entities try to specify VSS information in conflict.
It is important that both the relay agent as well as the DHCP server
both support the VSS option and sub-option (for DHCPv4) and the VSS
option (for DHCPv6). Deploying and configuring VPN support in one
element and not in the other is not a practical approach.
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4.3. Required Support
DHCP relay agents and servers MUST support the approach discussed in
Section 4.1. DHCP relay agents and server SHOULD support the
approach discussed in Section 4.2. DHCP relay agents and servers
SHOULD NOT be configured to operate with both approaches
simultaneously.
4.4. Alternative VPN assignment approaches
There are many other approaches which can be created with multiple
relay agents each inserting VSS information into different Relay-
forward messages, relay agent VSS information conflicting with client
VSS information, or DHCP server VSS information conflicting with
relay agent and client VSS information. Since these approaches do
not describe situations that are useful today, specifying precisely
how to resolve all of these conflicts is unlikely to be valuable in
the event that these approaches actually become practical in the
future.
The current use of the VSS option and sub-option require that each
entity knows the part that it plays in dealing with VPN data. Each
entity -- client, relay agent or agents, and server -- SHOULD know
through some policy or configuration beyond the scope of this
document whether it is responsible for specifying VPN information
using the VSS option or sub-option or responsible for responding to
VSS information specified by another entity, or simply ignoring any
VSS information which it might see.
Some simple conflict resolution approaches are discussed below, in
the hopes that they will cover simple cases that may arise from
situations beyond those envisioned today. However, for more complex
situations, or simple situations where appropriate conflict
resolution strategies differ from those discussed in this document, a
document detailing the usage situations and appropriate conflict
resolution strategies SHOULD be created and submitted for discussion
and approval.
5. Relay Agent Behavior
Implementers MAY provide a policy or configuration capability to
enable or disable VSS support.
A relay agent which receives a DHCP request from a DHCP client on a
VPN SHOULD include Virtual Subnet Selection information in the DHCP
packet prior to forwarding the packet on to the DHCP server unless
inhibited from doing so by configuration information or policy to the
contrary.
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In this situation, a DHCPv4 relay agent MUST include a DHCPv4 VSS
sub-option in a relay-agent-information option [RFC3046], while a
DHCPv6 relay agent MUST include a DHCPv6 VSS option in the Relay-
forward message.
The value placed in the Virtual Subnet Selection sub-option or option
would typically 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. In some cases, the information in
the VSS sub-option or option might be an index into some internal
table held in the relay agent, though this document places no
requirement on a relay agent to have any such internal state.
A DHCPv4 relay agent MUST, in addition, include a DHCPv4 VSS-Control
sub-option (which has a length of zero) in the relay-agent-
information option [RFC3046] whenever it includes a VSS sub-option in
the relay-agent-information option. The inclusion of the VSS sub-
option and the VSS-Control sub-option in the relay-agent-information
option will allow the DHCPv4 relay agent to determine whether the
DHCPv4 server actually processed the information in the VSS sub-
option when it receives the relay-agent-information option in the
reply from the DHCPv4 server.
The reason to include this additional VSS DHCPv4 sub-option is that
[RFC3046] specifies (essentially) that a DHCPv4 server should copy
all sub-options that it receives in a relay-agent-information option
in a request into a corresponding relay-agent-information option in
the response. Thus, a server that didn't support the DHCPv4 VSS
sub-option would normally just copy it to the response packet,
leaving the relay agent to wonder if in fact the DHCPv4 server
actually used the VSS information when processing the request.
To alleviate this potential confusion, a DHCPv4 relay agent instead
sends in two sub-options: one VSS sub-option, and one VSS-Control
sub-option. If both sub-options appear in the response from the
DHCPv4 server, then the DHCPv4 relay agent MUST assume that the
DHCPv4 server did not act on the VSS information in the VSS sub-
option. If only the VSS sub-option appears in the response from the
DHCPv4 server and no VSS-Control sub-option appears in the response
from the DHCPv4 server, then the relay agent SHOULD assume that the
DHCPv4 server acted successfully on the VSS sub-option.
Anytime a relay agent places a VSS option or sub-option in a DHCP
request, it SHOULD send it only to a DHCP server which supports the
VSS option or sub-option, and it MUST check the response to determine
if the DHCP server actually honored the requested VSS information.
In the DHCPv6 case, the appearance of the option in the Relay-reply
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packet indicates that the DHCPv6 server understood and acted upon the
contents of the VSS option in the Relay-forward packet. In the
DHCPv4 case, as discussed above, the appearance of the VSS sub-option
without the appearance of a VSS-Control sub-option indicates that the
DHCPv4 server successfully acted upon the VSS sub-option.
This document does not create a requirement that a relay agent
remember the contents of a VSS DHCPv4 sub-option or VSS DHCPv6 option
sent to a DHCP server. In many cases, the relay agent may simply use
the value of the VSS returned by the DHCP server to forward the
response to the DHCP client. If the VSS information, the IP address
allocated, and the VPN capabilities of the relay agent all
interoperate correctly, then the DHCP client will receive a working
IP address. Alternatively, if any of these items don't interoperate
with the others, the DHCP client will not receive a working address.
Note that in some environments a relay agent may choose to always
place a VSS option or sub-option into packets and messages that it
forwards in order to forestall any attempt by a relay agent closer to
the client or the client itself to specify VSS information. In this
case, a type field of 255 is used to denote the global, default VPN.
When the type field of 255 is used, there MUST NOT be any additional
VSS information in the VSS option or sub-option. In the DHCPv4 case,
an additional VSS-Control sub-option would be required, as discussed
above.
5.1. VPN assignment by the DHCP server
In some cases, a DHCP server may use the Virtual Subnet Selection
sub-option or 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 or option with the
appropriate information to the relay agent in the relay-agent-
information option for DHCPv4 or the Relay-reply message in DHCPv6.
If the relay agent cannot respond correctly to the DHCP server's
requirement to place the DHCP client into that VPN (perhaps because
it has not been configured with a VPN that matches the VSS
information received from the DHCP server) it MUST drop the packet
and not send it to the DHCP client.
In this situation, once the relay agent has placed the DHCP client
into the VPN specified by the DHCP server, it will insert a VSS
option or sub-option when forwarding packets from the client. The
DHCP server in normal operation will echo this VSS information into
the outgoing replies.
In the event that the relay agent doesn't include VSS information on
subsequent requests after the DHCP server has included VSS
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information in a reply to the relay agent, the DHCP server can
conclude that the relay agent doesn't support VSS processing, and the
DHCP server SHOULD stop processing this transaction and not respond
to the request.
5.2. DHCP Leasequery
Sometimes a relay-agent needs to submit a DHCP Leasequery [RFC4388]
[RFC5007] packet to the DHCP server in order to recover information
about existing DHCP allocated IP addresses on other than the normal,
global VPN. In the context of a DHCP Leasequery the relay agent is a
direct client of the DHCP server and is not relaying a packet for
another DHCP client. Thus, the instructions in Section 6 on Client
Behavior should be followed to include the necessary VSS information.
6. Client Behavior
Typically, DHCPv4 and DHCPv6 clients have no interaction with VSS
options or sub-options. The VSS information is handled by exchanges
between a DHCPv4 or DHCPv6 relay agent and the corresponding DHCPv4
or DHCPv6 server.
However, there are times when an entity is acting as a DHCPv4 or
DHCPv6 client in that it is communicating directly with a DHCPv4 or
DHCPv6 server. In these instances -- where communications is
occurring without employing the DHCPv4 relay-agent-information option
or the DHCPv6 Relay-forward or Relay-reply messages, the entity is
acting as a DHCPv4 or DHCPv6 client with regard to its communication
with the DHCPv4 or DHCPv6 server, but not necessarily as a DHCP
client who is requesting a DHCPv4 or DHCPv6 address for its own use.
The client, in this context, may be requesting an IP address for
another entity, thus acting as a DHCP proxy. The client may be
requesting information about another client-to-address binding, using
the DHCPv4 [RFC4388] or DHCPv6 [RFC5007] Leasequery protocol.
In the rest of this section, the term "client" refers to an entity
communicating VSS information directly to a DHCPv4 or DHCPv6 server
without using the DHCPv4 relay-agent-information option or the DHCPv6
Relay-forward or Relay-reply messages, and there is no requirement
that such a client is a traditional DHCPv4 or DHCPv6 client
requesting an IP address binding for itself.
A DHCPv4 or DHCPv6 client will employ the VSS option to communicate
VSS information to their respective servers. This information MUST
be included in every message concerning any IP address on a different
VPN than the global or default VPN. A DHCPv4 client will place the
DHCPv4 VSS option in its packets, and a DHCPv6 client will place the
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DHCPv6 VSS option in its messages.
A DHCPv6 client that needs to place a VSS option into a DHCPv6
message SHOULD place a single VSS option into the DHCPv6 message at
the same level as the Client Identifier option. A DHCPv6 client MUST
NOT include different VSS options in the same DHCPv6 message.
Note that, as mentioned in Section 1, throughout this document when a
DHCPv6 address is indicated the same information applies to DHCPv6
Prefix Delegation [RFC3633] as well.
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. Thus, a client which places this option in a packet and
doesn't receive it or receives a different value in a returning
packet SHOULD drop the packet since the IP address that was allocated
will not be in the requested VPN.
Clients should be aware that some DHCP servers will return a VSS
option with different values than that which was sent in. In
addition, a client may receive a response from a DHCP server with a
VSS option when none was sent in by the Client.
Note that when sending a DHCP Leasequery request, a relay agent is
acting as a DHCP client and so it SHOULD include the respective
DHCPv4 or DHCPv6 VSS option in its DHCPv4 or DHCPv6 Leasequery packet
if the DHCP Leasequery request is generated for other than the
default, global VPN. It SHOULD NOT include a DHCPv4 sub-option in
this case.
7. Server Behavior
A DHCP server receiving the VSS option or sub-option SHOULD allocate
an IP address (or use the VSS information to access an already
allocated IP address) from the VPN specified by the included VSS
information.
In the case where the type field of the VSS option or sub-option is
255, the VSS option denotes the global, default VPN. In this case,
there is no explicit VSS information beyond the type field.
This document does not prescribe any particular address allocation
policy. A DHCP server may choose to attempt to allocate an address
using the VSS information and, if this is impossible, to not allocate
an address. Alternatively, a DHCP server may choose to attempt
address allocation based on the VSS information and, if that is not
possible, it may fall back to allocating an address on the global or
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default VPN. This, of course, is also the apparent behavior of any
DHCP server which doesn't implement support for the VSS option and
sub-option. Thus, DHCP clients and relay agents SHOULD be prepared
for either of these alternatives.
In some cases, a DHCP server may use the Virtual Subnet Selection
sub-option or 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 or option with the
appropriate information to the relay agent in the relay-agent-
information option for DHCPv4 or the Relay-reply message in DHCPv6.
In this situation, the relay agent will place the client in the
proper VPN, and then it will insert a VSS option or sub-option in
subsequent forwarded requests. The DHCP server will see this VSS
information and since it doesn't conflict in any way with the
server's notion of the VPN on which the client is supposed to reside,
it will process the requests based on the VPN specified in the VSS
option or sub-option, and echo the same VSS information in the
outgoing replies.
The relay agent receiving a reply containing a VSS option should
support the VSS option. Otherwise the relay agent will end up
attempting to use the address as though it were a global address.
Should this happen, the subsequent DHCPREQUEST will not contain any
VSS information, in which case the DHCP server SHOULD NOT respond
with a DHCPACK.
If a server uses a different VPN than what was specified in the VSS
option or sub-option, it SHOULD send back the VPN information using
the same type as the received type. It MAY send back a different type
if it is not possible to use the same type (such as the RFC2685 VPN-
ID if no ASCII VPN identifier exists).
A server which receives a VSS sub-option in the DHCPv4 relay-agent-
information option and does not receive a VSS-Control sub-option in
the relay-agent-information option MUST process the information
specified in the VSS sub-option in the same fashion as it would have
if it received both sub-options.
7.1. Returning the DHCPv4 or DHCPv6 Option
DHCPv4 or DHCPv6 servers receiving a VSS option (for sub-option
processing, see below) MUST return an instance of this option in the
reply packet or message if the server successfully uses this option
to allocate an IP address, and it MUST NOT include an instance of
this option if the server is unable to support, is not configured to
support, or does not implement support for VSS information in general
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or the requested VPN in particular.
If they echo the option (based on the criteria above), servers SHOULD
return an exact copy of the option unless they desire to change the
VPN on which a client was configured.
The appearance of the DHCPv4 VSS option code in the DHCPv4 Parameter
Request List option [RFC2132] should not change the processing or
decision to return or not return the VSS option as specified in this
document. The appearance of the DHCPv6 VSS option in the OPTION_ORO
[RFC3315] or the OPTION_ERO [RFC4994] should not change the
processing or decision to return (or not to return) the VSS option as
specified in this document.
7.2. Returning the DHCPv4 Sub-Option
The case of the DHCPv4 sub-option is a bit more complicated. Note
that [RFC3046] specifies that a DHCPv4 server which supports the
relay-agent-information option SHALL copy all sub-options received in
a relay-agent-information option into any outgoing relay-agent-
information option. Thus, the default behavior for any DHCPv4 server
is to return any VSS sub-option received to the relay agent whether
or not the DHCPv4 server understands the VSS sub-option.
In order to distinguish a DHCPv4 server which is simply copying
relay-agent-information option sub-options from an incoming to an
outgoing relay-agent-informaion option from one which successfully
acted upon the information in the VSS sub-option, DHCPv4 relay agents
MUST include a VSS-Control sub-option in the relay-agent-information
any time that it includes a VSS sub-option in the relay-agent-
information option.
A DHCPv4 server which does not support the VSS sub-option will copy
both sub-options into the outgoing relay-agent-information option,
thus signalling to the DHCPv4 relay agent that it did not understand
the VSS sub-option.
A DHCPv4 server which supports the VSS sub-option:
o MUST copy the VSS sub-option into the outgoing relay-agent-
information option
o MUST NOT copy the VSS-Control sub-option into the outgoing
relay-agent-information option
Moreover, if a server uses different VSS information to allocate an
IP address than it receives in a particular DHCPv4 sub-option, it
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MUST include that alternative VSS information in the VSS sub-option
that it returns to the DHCPv4 relay agent instead of the original VSS
information it was given.
If a DHCPv4 server supports this sub-option and for some reason
(perhaps administrative control) does not honor this sub-option from
the request then it MUST NOT echo either sub-option into the outgoing
relay-agent-information option.
7.3. Making sense of conflicting VSS information
It is possible for a DHCPv4 server to receive both a VSS option and
VSS sub-options in the same packet. Likewise, a DHCPv6 server can
receive multiple VSS options in nested Relay-forward messages as well
as in the client message itself. In either of these cases, the VSS
information from the relay agent closest to the DHCP server SHOULD be
used in preference to all other VSS information received. In the
DHCPv4 case, this means that the VSS sub-option takes precedence over
the VSS option, and in the DHCPv6 case, this means that the VSS
option from the outer-most Relay-forward message in which a VSS
option appears takes precedence.
The reasoning behind this approach is that the relay-agent closer to
the DHCP server is almost certainly more trusted than the DHCP client
or more distant relay agents, and therefore information in the
relay-agent-information option or the Relay-forward message is more
likely to be correct.
In general, relay agents SHOULD be aware through configuration or
policy external to this document whether or not they should be
including VSS information in packets that they forward and so there
should not be conflicts among relay agent specified VSS information.
In these situations where multiple VSS option or sub-options appear
in the incoming packet or message, when the DHCP server constructs
the response to be sent to the DHCP client or relay agent, all
existing VSS options or sub-options MUST be replicated in the
appropriate places in the response and MUST contain only the VSS
information that was used by the DHCP server to allocate the IP
address (with, of course, the exception of a DHCPv4 relay-agent-
information sub-option VSS-Control).
8. Updates to RFC 3046
This document updates the specification of the Relay Agent
Information option in RFC 3046 as follows:
Change the first sentence, second paragraph, section 2.2 of RFC 3046:
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o OLD:
DHCP servers claiming to support the Relay Agent Information
option SHALL echo the entire contents of the Relay Agent
Information option in all replies.
o NEW:
DHCP servers claiming to support the Relay Agent Information
option SHALL echo the entire contents of the Relay Agent
Information option in all replies, except if otherwise specified
in the definition of specific Relay Agent Information sub-
options.
9. Security
Message authentication in DHCPv4 for intradomain use where the out-
of-band exchange of a shared secret is feasible is defined in
[RFC3118]. Potential exposures to attack are discussed in Section 7
of the DHCP protocol specification in [RFC2131].
Implementations should consider using the DHCPv4 Authentication
option [RFC3118] to protect DHCPv4 client access in order to provide
a higher level of security if it is deemed necessary in their
environment.
Message authentication in DHCPv4 relay agents as defined in [RFC4030]
should be considered for DHCPv4 relay agents employing this sub-
option. Potential exposures to attack are discussed in Section 7 of
the DHCP protocol specification in [RFC2131].
For DHCPv6 use of the VSS option, the "Security Considerations"
Section of [RFC3315] details the general threats to DHCPv6, and thus
to messages using the VSS option. The "Authentication of DHCP
Messages" Section of [RFC3315] describes securing communication
between relay agents and servers, as well as clients and servers.
The VSS option could be used by a client in order to obtain an IP
address from any VPN. 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.
A DHCP server that implements these options and sub-option should be
aware of this possibility and use whatever techniques that can be
devised to prevent such an attack. Information such as the giaddr in
DHCPv4 or link address in the Relay-forward DHCPv6 message might be
used to detect and prevent this sort of attack.
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One possible defense would be for the DHCP relay to insert a VSS
option or sub-option to override the DHCP client's VSS option.
Servers that implement the VSS option and 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 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.
10. IANA Considerations
IANA is requested to assign DHCPv4 option number 221 for the DHCPv4
VSS option defined in Section 3.1, in accordance with [RFC3942].
IANA is requested to assign sub-option number 151 for the DHCPv4 VSS
sub-option defined in Section 3.2 from the DHCP Relay Agent Sub-
options space [RFC3046], in accordance with the spirit of [RFC3942].
While [RFC3942] doesn't explicitly mention the sub-option space for
the DHCP Relay Agent Information option [RFC3046], sub-option 151 is
already in use by existing implementations of this sub-option and the
current draft is essentially upward compatible with these current
implementations.
IANA is requested to assign the value of TBD for the DHCPv4 VSS-
Control sub-option defined in Section 3.3.
IANA is requested to assign the value of TBD for the DHCPv6 VSS
option defined in Section 3.4 from the DHCPv6 option registry.
The type byte defined in Section 3.5 defines a number space for which
IANA is to create and maintain a new sub-registry entitled "VSS Type
values". This sub-registry needs to be related to both the DHCPv4
and DHCPv6 VSS options and the DHCPv4 relay-agent-information option
sub-option (all defined by this document), since the type byte in
these two options and one sub-option MUST have identical definitions.
New values for the type byte may only be defined by IETF Consensus,
as described in [RFC5226]. Basically, this means that they are
defined by RFCs approved by the IESG.
11. Acknowledgments
Bernie Volz recommended consolidation of the DHCPv4 option and sub-
option drafts after extensive review of the former drafts, and
provided valuable assistance in structuring and reviewing this
document. Alper Yegin expressed interest in the DHCPv6 VSS option,
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resulting in this combined draft covering all three areas. Alfred
Hoenes provided assistance with editorial review as well as raising
substantive protocol issues. David Hankins and Bernie Volz each
raised important protocol issues which resulted in a clarified
document. Josh Littlefield provided editorial assistance. Several
IESG reviewers took the time to substantially review this document,
resulting in much increased clarity.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC2685] Fox, B., Gleeson, B., "Virtual Private Networks
Identifier", RFC 2685, September 1999.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
3046, January 2001.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and
M. Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633, December
2003.
[RFC4994] Zeng, S., Volz, B., Kinnear, K. and J. Brzozowski, "DHCPv6
Relay Agent Echo Request Option", RFC 4994, September 2007.
12.2. Informative References
[RFC951] Croft, B. and J. Gilmore, "Bootstrap Protocol", RFC 951,
September 1985.
[RFC1542] Wimer, W., "Clarifications and Extensions for the Bootstrap
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Protocol", RFC 1542, October 1993.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
Messages", RFC 3118, June 2001.
[RFC3942] Volz, B., "Reclassifying Dynamic Host Configuration
Protocol version 4 (DHCPv4) Options", RFC 3942, November 2004.
[RFC4030] Stapp, M. and T. Lemon, "The Authentication Suboption for
the Dynamic Host Configuration Protocol (DHCP) Relay Agent
Option", RFC 4030, March 2005.
[RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration
Protocol (DHCP) Leasequery", RFC 4388, February 2006.
[RFC5007] Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng, "DHCPv6
Leasequery", RFC 5007, September 2007.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
Authors' Addresses
Kim Kinnear
Cisco Systems
1414 Massachusetts Ave.
Boxborough, Massachusetts 01719
Phone: (978) 936-0000
EMail: kkinnear@cisco.com
Richard Johnson
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
Phone: (408) 526-4000
EMail: raj@cisco.com
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Mark Stapp
Cisco Systems
1414 Massachusetts Ave.
Boxborough, Massachusetts 01719
Phone: (978) 936-0000
EMail: mjs@cisco.com
Jay Kumarasamy
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