One document matched: draft-sheffer-ipsec-secure-beacon-00.txt
Network Working Group Y. Sheffer
Internet-Draft Check Point
Expires: December 16, 2006 June 14, 2006
Secure Beacon: Securely Detecting a Trusted Network
draft-sheffer-ipsec-secure-beacon-00
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Remote access clients, in particular IPsec-based ones, are heavily
deployed in enterprise environments. In many enterprises the
security policy allows remote-access clients to switch to unprotected
operation when entering the trusted network. This document specifies
a method that lets a client detect this situation in a secure manner,
with the help of a security gateway. We propose a minor extension to
IKEv2 to achieve this goal.
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Table of Contents
1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Client Mobility . . . . . . . . . . . . . . . . . . . . . 5
2.3. Alternative Solutions . . . . . . . . . . . . . . . . . . 5
3. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. IKE Network-Detection . . . . . . . . . . . . . . . . . . 6
3.1.1. The IKE_SA_INIT Exchange . . . . . . . . . . . . . . . 6
3.1.2. The IKE_AUTH Exchange . . . . . . . . . . . . . . . . 7
3.2. IKE Notify Payloads . . . . . . . . . . . . . . . . . . . 7
3.2.1. SECURE_NETWORK_DETECT . . . . . . . . . . . . . . . . 7
3.2.2. SECURE_NETWORK_DETECTED . . . . . . . . . . . . . . . 7
3.3. Detecting Movement . . . . . . . . . . . . . . . . . . . . 8
3.4. The Gateway's Decision . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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1. Requirements Notation
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 [RFC2119].
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2. Introduction
The IKE and IPsec protocols are often used for remote-access clients.
IKE version 2 [RFC4306] provides enhanced support for remote-access
clients through the use of EAP. In many cases, IPsec clients need to
be "turned off" when the client roams into the internal, or "trusted"
network of an enterprise. This operation is very sensitive, since an
adversary may use this mechanism to force the client to send
unprotected packets into the network. This document defines an
extension to IKEv2 where the client contacts a trusted gateway, the
gateway detects that the client is located in a trusted network, and
delivers an indication to the client in a secure manner. An
important property of this protocol is that the exchange may
terminate early, if the client and the server agree that IPsec is not
required; otherwise the protocol will "fall through" into a standard
IKEv2 exchange, generating IKE and Child security associations.
Unfortunately at the time of writing, there is no IETF work group
chartered with IPsec. We encourage discussion of this draft on the
IPsec mailing list, https://www1.ietf.org/mailman/listinfo/ipsec.
2.1. Goals
The proposed protocol should fulfill the following goals.
o Security, in particular the protocol should not adversely affect
the security of IKE.
o Robustness: the protocol should fall back into a full IKE exchange
if any error is detected.
o Performance: minimize the number of exchanges and the CPU effort
expanded, whether the client is in the trusted or untrusted
network.
o Usability: the user should not be required to perform any action
unless this is required for security. We avoid sending the
client's identity, because this normally requires input from the
user.
o Simplicity: the protocol should deal with the case of "simple"
networks, meaning networks where the internal network is wholly
trusted. It does not need to cover more complex topologies.
o Extensibility: however, the base protocol can be extended, e.g. to
handle more complex networks.
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2.2. Client Mobility
The current version of this protocol does not apply to clients using
MOBIKE [I-D.ietf-mobike-protocol]. We plan to add such support in a
future version. A possible solution is to allow the client to detect
a trusted network each time it sends an UPDATE_SA_ADDRESSES
notification.
2.3. Alternative Solutions
There are several alternatives for providing the functionality
discussed here.
o Several proposals related to Mobile IP, such as [I-D.ietf-mip4-
vpn-problem-solution], rely on secure connectivity to the Home
Agent, which is assumed to be in the trusted network. This
solution obviously can only be applied in a Mobile IP setting.
o Some proprietary solutions rely on secure connectivity to other
"internal" hosts, for example the Windows Domain Controller.
o Another solution we have considered is to open a dedicated, short-
lived TLS connection into the security gateway. This would enable
the client to authenticate the gateway. However an IPsec gateway
should not be assumed to implement TLS.
o Lastly, we considered a new protocol, possibly derived from IKE.
A separate protocol offers modularity as its main benefit.
However we have chosen to reuse IKE itself, where the exchange can
be completed as a full IKE exchange. This results in fewer
exchanges, and possibly in a simpler implementation.
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3. Protocol Details
The following sections describe the protocol, first at the exchange
level and then at the payload level. Following that, we discuss two
central issues: how the client detects that it has moved, so that
this protocol can be run, and how the gateway can make the decision
whether the client is in the trusted or untrusted network.
3.1. IKE Network-Detection
To summarize, we add an IKE notification to message #1 of the
protocol, and another to message #2. However, the protocol is only
terminated after the initiator has authenticated the responder, i.e.
after message #4. It is important to note that the initiator's
identity may not be authenticated if the protocol is terminated
early.
3.1.1. The IKE_SA_INIT Exchange
The IKE_SA_INIT exchange is modified as follows:
Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni, N1 -->
<-- HDR, SAr1, KEr, Nr, N2, [CERTREQ]
All payloads, with the exception of the notifications, have their
usual semantics. The first notification, N1, is of type
SECURE_NETWORK_DETECT. It denotes to the responder that it SHOULD
respond with a second notification (N2), which is of type
SECURE_NETWORK_DETECTED. Both notifications are defined in
Section 3.2. Note that both notifications are sent in the clear.
Following the first message, there are three options:
o If there was no response after the normal retransmissions, the
client SHOULD assume it is on an untrusted network, and is
experiencing connectivity problems. For example, the IKE port may
be blocked.
o Otherwise, a response was received. If N2 is not received, or if
it is received but explicitly specifies that the initiator is in
an untrusted network, the protocol continues according to standard
IKE rules. This would be the case if the responder does not
understand the SECURE_NETWORK_DETECT notification.
o If N2 indicated that the initiator is in the trusted network, the
protocol continues as detailed in Section 3.1.2 below.
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3.1.2. The IKE_AUTH Exchange
The initiator now responds with a truncated IKE_AUTH exchange
HDR, SK {[IDi, CERT,] [CERTREQ,] [IDr,] [AUTH]} -->
The initiator sends the AUTH payload only if it can be authenticated
in message #2, i.e. if it uses a shared secret or certificate, rather
than EAP. Even if the initiator normally authenticates using one of
these methods, it MAY omit both IDi and AUTH, in order to avoid user
interaction. If AUTH is included, then the responder MUST
authenticate the initiator.
The responder replies with:
<-- HDR, SK {IDr, [CERT,] AUTH}
The initiator MUST now validate the identity of the responder as
defined in [RFC4306], and following that, MUST terminate the
protocol. Obviously in this case, no Child SA is created and
therefore no IPsec-protected traffic will be sent. Moreover, no
long-term IKE SA is created, and both parties SHOULD delete their IKE
SAs.
It is RECOMMENDED that the client display a message to the user at
this point, announcing that it has moved into unprotected mode.
3.2. IKE Notify Payloads
We define two new notify payload types, SECURE_NETWORK_DETECT and
SECURE_NETWORK_DETECTED.
3.2.1. SECURE_NETWORK_DETECT
This notification type has the value [TBD-BY-IANA1]. It contains no
data.
3.2.2. SECURE_NETWORK_DETECTED
This notification type has the value [TBD-BY-IANA2].
This notify payload includes a single 1-octet data item. It has the
value 0 if the responder believes that the initiator is coming from
an insecure network, or if the responder cannot determine where the
initiator is coming from. It has the value 1 if the responder
believes that the initiator is coming from a secure network.
Implementations MAY include additional data in this notify payload,
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however this usage SHOULD be signaled with a Vendor ID payload. Such
additional data MUST be ignored by the receiver if not understood.
3.3. Detecting Movement
Mobility detection is outside the scope of this document. The
procedures involved are best described in [RFC4436] for IPv4. The
DNA procedures SHOULD be followed, so that the client can employ the
mechanism defined here whenever it suspects that it has moved into a
new network, particularly from a trusted to an untrusted network.
3.4. The Gateway's Decision
The gateway MUST be configured to make a correct decision regarding
the client's location. Typically, the gateway would only detect
clients connecting through the trusted network if their IKE packets
arrive from a trusted physical network interface. Determining which
network or network type is considered trusted is left to local
policy.
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4. IANA Considerations
This document does not create any new namespaces to be maintained by
IANA, but it requires new values in namespaces that have been defined
in the IKEv2 base specification.
This document defines several new IKEv2 notifications whose values
are to be allocated from the "IKEv2 Notify Message Types" namespace.
Notify Messages - Error Types Value
----------------------------- -----
None
Notify Messages - Status Types Value
------------------------------ -----
SECURE_NETWORK_DETECT TBD-BY-IANA1 (16396..40959)
SECURE_NETWORK_DETECTED TBD-BY-IANA2 (16396..40959)
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5. Security Considerations
The proposed solution is very sensitive, since it may cause a host to
switch from protected to unprotected communication. Following are
the threats that we have identified.
1. The notifications are sent in the clear. A passive attacker will
learn whether the responder is receiving traffic over a secure or
insecure interface. This is information that the attacker is
probably able to obtain otherwise.
2. An active attacker may be able to change either or both
notifications. The first notification N1 does not carry any
data, so it can at worst be deleted. In this case the protocol
will revert to normal IKE.
3. An active attacker's change to the N2 notification (or deletion
of N2) will be detected since IKE message #2 is authenticated and
integrity-protected. Therefore this attack is only equivalent to
a DoS attack on IKE.
4. This protocol can be defeated by an active attacker who can
inject packets into the trusted network and relay the responses
to such packets back into the untrusted network. Such an
attacker will be able to cheat the client into believing that it
is on the trusted network. We believe we do not have to address
this threat.
5. This protocol MUST NOT be used if the network can change the path
between the client and the security gateway without the client's
awareness, causing its security properties to change. That is,
if the network can route traffic sometimes over a trusted path
and sometimes over an untrusted one, without nofifying the end-
point. Such a situation might be possible in incorrectly
configured Mobile IP deployments, e.g. where the same Home Agent
is shared between a trusted Wi-Fi access network and an untrusted
one, and where the IPsec layer is not informed of the
connectivity changes.
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6. Change Log
6.1. -00
Initial version.
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7. Acknowledgements
I would like to thank Yoav Nir and Ariel Shaqed for their many useful
comments.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC4436] Aboba, B., Carlson, J., and S. Cheshire, "Detecting
Network Attachment in IPv4 (DNAv4)", RFC 4436, March 2006.
8.2. Informative References
[I-D.ietf-mip4-vpn-problem-solution]
Vaarala, S. and E. Klovning, "Mobile IPv4 Traversal Across
IPsec-based VPN Gateways",
draft-ietf-mip4-vpn-problem-solution-02 (work in
progress), November 2005.
[I-D.ietf-mobike-protocol]
Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", draft-ietf-mobike-protocol-08 (work in
progress), February 2006.
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Author's Address
Yaron Sheffer
Check Point Software Technologies Ltd.
3A Jabotinsky St.
Ramat Gan 52520
Israel
Email: yaronf at checkpoint dot com
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