Internet DRAFT - draft-ietf-dhc-dhcpv6-clarify-auth
draft-ietf-dhc-dhcpv6-clarify-auth
IETF dhc Working Group T. Jinmei
Internet-Draft Toshiba
Expires: December 28, 2006 June 26, 2006
Clarifications on DHCPv6 Authentication
draft-ietf-dhc-dhcpv6-clarify-auth-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes issues about the authentication mechanism of
the Dynamic Host Configuration Protocol for IP version 6 that were
identified from implementation experiences. It also tries to propose
resolutions to some of the issues.
1. Introduction
Several questions arose on the authentication mechanism of DHCPv6
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[RFC3315] from implementation experiences, particularly on its
delayed authentication protocol. Some of the questions may require a
change or addition to the current protocol, and one of them may even
cause discussions on a security threat.
This document describes the issues based on the questions and
proposes resolutions, hoping the resolutions will be merged in if
valid and accepted, to the next version of the base specification.
2. Usage with Information-Request
According to [RFC3315], it seems possible to use the authentication
mechanism for Information-request and Reply exchanges. The RFC says
in Section 21.4.4.4 as follows:
If the server has selected a key for the client in a previous
message exchange (see section 21.4.5.1), the client MUST use the
same key to generate the authentication information throughout the
session.
However, this description is not really clear. Section 21.4.5.1,
which is referred from the above part, actually describes the case of
Solicit and Advertise exchange:
21.4.5.1. Receiving Solicit Messages and Sending Advertise
Messages
The server selects a key for the client and includes
authentication information in the Advertise message returned to
the client as specified in section 21.4. [...]
It does not necessarily mean contradiction because the client and the
server may have exchanged Solicit and Advertise with authentication
before starting the Information-request and Reply exchange. But it
then restricts the usage scenario of the authentication mechanism for
Information-request and Reply exchanges. In particular, this
assumption prohibits the use of the mechanism with the "stateless"
service using DHCPv6 [RFC3736]. Whereas the specification allows an
implementation that only supports the stateless service and does not
support Solicit and Advertise messages, the authentication mechanism
depends on Solicit and Advertise exchanges.
This fact can (partly) invalidate a security consideration in
[RFC3736]:
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Authenticated DHCP, as described in sections 21 and 22.11 of the
DHCP specification [1], can be used to avoid attacks mounted
through the stateless DHCP service.
(where [1] refers to [RFC3315].) In fact, as was just shown above,
authenticated DHCP cannot be used unless the implementations also
support Solicit and Advertise messages (or the entire [RFC3315] in
general).
It should also be noted that [RFC3315] does not define what the
server should do when it receives an Information-request message
containing an authentication option; Section 21.4.5.2 excludes the
Information-request message.
2.1. Suggested Resolution
Considering the fact that [RFC3736] allows implementations that only
support the subset of the full specification [RFC3315], it should
make sense to define the authentication usage for Information-request
and Reply exchanges separately.
One significant difference between Information-request and other
"stateful" cases is that there is no explicit notion of "session" in
the former. In some cases, however, the same client and server may
exchange Information-request and Reply multiple times, where the
entire exchanges can be regarded as a "session". For example, the
client may want to get different configuration information in
multiple exchanges. Also, if the client and the server use the
Information Refresh Time Option [RFC4242], they will restart
exchanges when the refresh time expires.
A naive implementation of keeping a "session" in the server will
decrease an advantage of the [RFC3736] usage that the server can run
in a stateless fashion without any client-specific state.
Specifically, the server may have to maintain the following three
types of state per client:
o the authentication key shared between the server and the client
o replay detection information for messages to the client (such as
the replay detection value sent most recently)
o replay detection information for messages from the client (such as
the replay detection value received most recently)
It is possible for the server to have different keys for different
clients without having per-client information by the method described
in Appendix A of [RFC3118]. In this approach the server only
maintains a single master key and creates the key for a particular
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client by computing some one-way hash digest based on the master key
and the client's DUID. Since an Information-request message to be
authenticated must have a Client Identifier option as specified in
Section 18.1.5 of [RFC3315], this approach should work well.
The server can also avoid a replay attack using an old message sent
from the server without maintaining per client state for replay
detection information about messages sent to the client if the server
has a source of replay protection value that monotonically increases.
For example, system timestamp can be used for this purpose.
Without keeping the replay detection information for messages from
the client, the server may be vulnerable to a replay attack from a
malicious client. This should be relatively a minor issue because a
"stateless" server usually provides the same information for all
clients without consuming any of its resource. When the malicious
client can get an old valid message, e.g., by snooping the traffic,
it would also be able to get and use the response; it does not have
to mount the replay attack.
There is still a subtle case to be considered. If the server uses a
monotonically increasing counter for stateless replay detection
information to clients and the server does not maintain per client
replay detection information from the client, a malicious client can
reuse a valid Information-request message to get a reusable valid
Response message. The malicious client will then be able to mount a
replay attack on the client later.
The proposed revision of Section 21.4.4.4 is therefore as follows:
21.4.4.4. Sending Information-request Messages
When the client sends an Information-request message and wishes to
use authentication, it includes an Authentication option with the
desired protocol, algorithm and RDM as described in section 21.4.
The client does not include any replay detection or authentication
information in the Authentication option.
If the client authenticated exchanges of Information-request and
Reply in the past, the client MAY reuse the same key used in the
previous exchanges to generate the authentication information. In
this case, the client generates authentication information for the
Information-request message as described in section 21.4.
Note that the keys used for these exchanges are separately managed
from the keys used for the other exchanges beginning with the
Solicit message when the two types of exchanges run concurrently,
while the two keys may happen to be the same. For example, replay
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detection should be performed separately, and validation failure
for one type of exchanges does not affect the other.
Section 21.4.4.5 will also need to be revised. However, since this
section has a separate issue per se as will be discussed in
Section 5, further details on this are not discussed here.
The server side behavior needs to be described, too. Along with the
above change to Section 21.4.4.4, a new proposed subsection of
Section 21.4.5 should be added as follows:
21.4.5.x. Receiving Information-request Messages and Sending
Reply Messages
If the Information-request message includes an authentication
option without authentication information, the server selects a
key for the client and includes authentication information in the
Reply message returned to the client as specified in section 21.4.
If the key is selected from pre-configured information for the
client maintained in the server, the server MUST record the
identifier of the key selected for the client so that it can
validate further Information-request messages from the client if
the client reuses the same key for the future exchanges.
The server MAY alternatively use a stateless method such as the
one described in Appendix A of [RFC3118]. Then the server MUST
consistently use the same key for the client to validate further
Information-request messages from the client.
When the server uses such a stateless method for key utilization,
it may also seek to avoid having per client state for replay
detection. For outbound messages, it is easy when the server has
a source of monotonically increasing values such as system
timestamp; however, it is difficult if not impossible to refuse
inbound replay messages without per client state. The server MAY
skip the replay detection for inbound Information-request messages
if the benefit of being stateless outweighs the risk of replay
attacks for inbound messages. Care should be taken when this
relaxation applies because if the server also uses a stateless
method for outbound messages, a malicious client may be able to
get a valid reusable response by reusing an old, legitimate
Information-request message.
If the Information-request message includes an authentication
option with authentication information, the server uses the key
identified in the message and validates the message as specified
in section 21.4.2. If the message fails to pass the validation
test, or the key identified by the authentication information of
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the message is not identical to the key that the server used in
the previous exchange (when it has recorded the key), the server
MUST discard the message and MAY choose to log the validation
failure.
If the message passes the validation test, the server responds
with a Reply message as described in section 18.2.5. The server
MUST include authentication information generated using the key
just selected or identified in the received message, as specified
in section 21.4.
Finally, if the server previously recorded the key but receives an
Information-request message without including an authentication
option, the server MUST accept the message and respond with a
Reply message without including authentication information. The
server SHOULD then remove the recorded information.
Note that the keys used for these exchanges are separately managed
from the keys used for the other exchanges beginning with the
Solicit message when the two types of exchanges run concurrently
(See Section 21.4.4.4).
3. What If Replay Is Detected
It is not clear what the receiver should do when an attempt of replay
attack is detected from either Section 21.3 or Section 21.4.2 of
[RFC3315].
3.1. Suggested Resolution
It should be natural to discard a DHCP message containing an
authentication option whose replay detection field indicates a replay
attack.
Instead of concentrating on this particular case, we propose to
revise the entire second paragraph of Section 21.4.2 as follows:
To validate an incoming message, the receiver first checks that
the value in the replay detection field is acceptable according to
the replay detection method specified by the RDM field. If no
replay is detected, then the receiver computes the MAC as
described in [8]. The entire DHCP message (setting the MAC field
of the authentication option to 0) is used as input to the HMAC-
MD5 computation function. If the MAC computed by the receiver
matches the MAC contained in the authentication option, the
message regarded as valid. If the above procedure fails at any
stage, the receiver MUST discard the DHCP message.
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4. Inconsistent Behavior for Unauthenticated Messages
[RFC3315] says in Section 21.4.2 (Message Validation) as follows:
If the MAC computed by the receiver does not match the MAC
contained in the authentication option, the receiver MUST discard
the DHCP message.
On the other hand, Section 21.4.4.2 allows the client to respond to
an Advertise even if it fails to authenticate the message:
Client behavior, if no Advertise messages include authentication
information or pass the validation test, is controlled by local
policy on the client. According to client policy, the client MAY
choose to respond to an Advertise message that has not been
authenticated.
This seems to say, for example, that the client MAY accept an
Advertise message based on its local policy, even if the MAC computed
by the receiver does not match the MAC contained in the
authentication option. Apparently this contradicts the requirement
in Section 21.4.2.
4.1. Suggested Resolution
There seems to be no valid reason for accepting an Advertise message
if it fails validation. On the other hand, it may make sense in some
cases that the client accepts messages that do not include an
authentication option or even messages with an authentication option
which specifies a key the client does not know.
As a related terminology issue, [RFC3315] uses the phrase of
"unauthenticated message(s)" in Sections 21.4.4.2 and 21.4.4.5
without formally defining the term. Based on the above discussion,
the most appropriate definition of this term is the acceptable types
of messages. Specifically, a message that fails to pass the
validation test should not be regarded as an "unauthenticated"
message.
The suggested change to Section 21.4.4.2 is thus as follows.
The client validates any Advertise messages containing an
Authentication option specifying the delayed authentication
protocol using the validation test described in section 21.4.2.
Client behavior, if all Advertise messages are "unauthenticated",
is controlled by local policy on the client, where an
unauthenticated message means a message that does not include an
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authentication option or a message with an authentication option
which specifies a key the client does not know. According to
client policy, the client MAY choose to respond to an
unauthenticated Advertise message.
[...]
A client MUST be configurable to discard unauthenticated messages,
and SHOULD be configured by default to discard unauthenticated
messages if the client has been configured with an authentication
key or other authentication information. A client MAY choose to
differentiate between unauthenticated Advertise messages with no
authentication information and unauthenticated Advertise messages
that specifies a key the client does not know; for example, a
client might accept the former and discard the latter. If a
client does accept an unauthenticated message, the client SHOULD
inform any local users and SHOULD log the event.
The second paragraph of Section 21.4.4.5 also needs a change. But,
again, we will discuss this case in Section 5.
5. Possibility of DoS Attack
Section 21.4.4.5 of the RFC says as follows:
If the Reply fails to pass the validation test, the client MUST
restart the DHCP configuration process by sending a Solicit
message.
The purpose of this specification is probably to avoid a deadlock
scenario when the server suddenly reboots forgetting the
authentication key and/or the replay detection counter.
However, this behavior can easily cause denial of service (DoS)
attacks; the attacker can simply send an invalid Reply message at
some valid timing and can invalidate configuration information of the
client or can prevent the client from configuring itself.
As a side issue, this section seems to not consider Information-
request and Reply exchanges.
5.1. Discussion on Resolution
Even if a Reply message does not pass the validation tests, it is
probably reasonable to wait a certain period for an authenticated
one. Additionally, if the Reply message is a response to Release,
the client will not have to restart the configuration process with
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Solicit. It can simply quit the session after the waiting period.
The appropriate waiting period would be the first timeout for the
message, since in the intended scenario described above the
legitimate (but without knowing a valid key) server should be working
and respond within the timeout period.
Reply messages to Information-request will need a separate
consideration. According to the resolution (to a different issue)
suggested in Section 2.1, the client may or may not reuse the key for
previous exchanges. If the client does not reuse the key, or if this
is the first time the client sends an Information-request message,
there should be no other behavior than simply discarding the message
and waiting for a valid response (usual timeout and resend will
apply). Otherwise, the appropriate behavior would be similar to the
case described in the previous paragraph. That is, the client should
wait for a while and start new exchanges without including
authentication information with the reused key.
5.2. Suggested Resolution
The suggested change to Section 21.4.4.5 based on the above analysis
is as follows. It includes resolutions to the issues discussed in
Section 2.1 and Section 4.1.
For Reply to a message other than Information-request, if the
client authenticated the Advertise it accepted, the client MUST
validate the associated Reply message from the server. The client
MUST discard the Reply if the message fails to pass the validation
test and MAY log the validation failure. Then the client MUST
wait until the corresponding timeout expires as specified in
Section 14 as if it did not receive any Reply. If the client
cannot receive a valid Reply within the first timeout period, the
client MUST restart the DHCP configuration process by sending a
Solicit message or the client MUST simply quit the configuration
process if the Reply should be a response to a Release message.
If the client accepted an unauthenticated Advertise message that
did not include authentication information or did not pass the
validation test, the client MAY accept an unauthenticated Reply
message from the server.
If the client sent an Information-request message including an
authentication option, the client MUST validate the associated
Reply message from the server. The client MUST discard the Reply
if the message fails to pass the validation test and MAY log the
validation failure. If the client reuses the key used in previous
exchanges and authenticated the Information-request message with
the key, the client MUST wait until the corresponding timeout
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expires. If the client cannot receive a valid Reply within the
first timeout period, the client MUST restart the configuration
process by restarting exchanges of Information-request and Reply
without reusing the previous key.
The client MAY choose to accept an unauthenticated Reply message
to an Information request message. All the discussions and
behaviors described in Section 21.4.4.2 should simply apply to
this case.
6. Lack of Authentication from Client
It is not clear what the server should do when the client does not
include an authentication option while the server has previously sent
authentication information in the same session.
A proposal for the Information-request message was provided in
Section 2.1. For messages other than Information-request, the lack
of an authentication information would mean the Advertise message
sent from the server was "unauthenticated" to the client and the
client chose to accept that message. Thus, the server should
basically accept that message, while it may still want to reject the
message if the server uses the authentication information to
authenticate the client.
The proposed change to Section 21.4.5.2 based on the above discussion
is to add the following paragraph at the end of the section.
If the message does not include an authentication option while the
server included an authentication option in previous messages of
the session, the server SHOULD accept the message. In this case,
the server MAY skip including an authentication option in the
Reply message. The server MAY still choose to discard the
received message in this case based on its local policy.
7. Key Consistency
[RFC3315] requests in Section 21.4.4.3 that the client use the same
key used by the server to generate the authentication information.
However, it is not clear from the RFC what the server should do if
the client breaks this rule. It says in Section 21.4.5.2 that
If the message [...] or the server does not know the key
identified by the 'key ID' field, the server MUST discard the
message and MAY choose to log the validation failure.
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It is not clear whether "does not know the key" means a different key
from the one the server specified in the Advertise message. If this
is the intent, this sentence should be clarified as follows:
If the message [...] or the key identified by the authentication
information of the message is not identical to the key that the
server has been using in the session, the server MUST discard the
message and MAY choose to log the validation failure.
8. Security Considerations
This document specifically talks about security issues for DHCPv6.
It also points out a possibility of DoS attacks, and proposes a
resolution to prevent the attacks.
9. Acknowledgements
Christian Strauf, Stig Venaas and Bernie Volz reviewed a preliminary
version of this document, and provided specific suggestions and
further clarifications.
10. IANA Considerations
This document has no actions for IANA.
11. References
11.1. Normative References
[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.
[RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", RFC 3736, April 2004.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
Messages", RFC 3118, June 2001.
11.2. Informative References
[RFC4242] Venaas, S., Chown, T., and B. Volz, "Information Refresh
Time Option for Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 4242, November 2005.
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Appendix A. CHANGE HISTORY
Changes since draft-jinmei-dhc-dhcpv6-clarify-auth-00.txt are:
o Loosened the description of Information-request and Reply
exchanges so that the server can skip maintaining per-client
information.
o Changed the lifetime option to Information Refresh Time Option,
according to the change of the document.
o Clarified the definition of "unauthenticated" messages so that
those would not include messages that failed the validation, and
revise the specification using the term. Section 4 of the
previous version was removed accordingly.
o Provided specific suggestion to solve denial of service attacks.
o Changed the draft name to an IETF dhc working group document.
Changes since draft-ietf-dhc-dhcpv6-clarify-auth-00.txt are:
o Updated the considerations and recommendation on the server
behavior for Information-request and Reply exchanges based on
stateless key utilization described in [RFC3118].
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Author's Address
Tatuya Jinmei
Corporate Research & Development Center, Toshiba Corporation
1 Komukai Toshiba-cho, Saiwai-ku
Kawasaki-shi, Kanagawa 212-8582
Japan
Phone: +81 44-549-2230
Email: jinmei@isl.rdc.toshiba.co.jp
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